Getting the Ninja going again !

[Bob KellyIII]

Ok I am trying to get my 2012 Ninja 250R running right again.....
I sent the carbs to Ducattiman and had him clean them real good and check them over.... he said they will be real good runners !
I got a bunch of things done to the bike in the interim term of Gordon working on the carbs, like adjusting the valves the exhaust valves were a tiny bit too tight so I got the shims and got them set correctly, I have double and tripple checked the valve timing and the cams are spot on.
but the bike will start and idle but it will not take throttle even when warm
the only way I can get it to gain RPM is to use the choke and that raises the RPM to around 3,000 RPM usually but as soon as I crack the throttle it just sucks air and dies.... if I play with it at the bottom edge of increasing the throttle I can usually get it to rev up past 6k RPM and then it runs great it will go to 9k RPM from there with no problem
but starting it and giving it throttle to raise the RPM makes it bog down
if I let up (shut off the throttle to idle) it will usually catch it's self and recover and continue to idle.... but trying to get it to rev up is near impossible.... it looks like when the choke is off that the flat spot is between idle and 6,000 to 7,000 RPM..... this is something i have never encountered in all my years..... I am assuming the carburetors have been cleaned correctly as I have never heard any bad things about Gordon's work ...
which leads me to installation... I must have hooked up the hoses wrong or something... but I have been through the hose diagrams so many times I am blue in the face ! they are hooked up right ! .... but if they were it would run right !.....
I have a California model and yes it has hoses and smog crap all over it
....... I can't find the problem !
so I am looking for help does anyone have any suggestions as to where I might look to find the problem .... let's assume I screwed up putting it together..... because that is probably the case....

I think it is sucking too much air from somewhere causing it to be too lean off idle.... Looking at the hose chart in the appendix of the manual I saw there a vacuum line directly from the carburetor goes to a vacuum operated switch above the head mounted to the frame.... I think I will take that hose off and plug it and see if it runs better.... that is about all I can think of at the moment....if it runs better and takes throttle like it should that vacuum operated switch is likely bad.
....
at this stage I will entertain any suggestions no matter how outlandish ! HAHAHA


Bob......

[Shafer]

Hope you figure it out. I know you have been hunting this gremlin for awhile.

Have you looked for any air leaks between the carb and air box?

I got these for my pregen ( not sure if they fit yours) because I was sick of the spring clamp, just don't over tighten.

Roadformer 1.75" Worm Gear Hose Clamp - SAE 28 Size, Full Stainless Steel with 1/2" Band Working Range 33mm - 57mm Duct Clamp Pipe Clamp Worm Drive Hose Clamp Fuel Line Clamp (10 Pack, 33mm - 57mm) https://a.co/d/hPyVWJE

[ducatiman]

a few in process pics....you tell me, think they're clean?

2 immediate questions.....on install, carbs are fully plugged into their forward rubber manifolds? Actually felt with a "click" when fully installed.
And airbox side rubbers are completely sealed? No areas could be leaking, for sure?

[Bob KellyIII]

LOL Yah Gordon ! those look cleaner than I've ever seen them before...even after I cleaned them myself.... I actually DO have faith in your work buddy infact your the best out there in carb cleaning... that is WHY i sent them to you in the first place ! you do fantastic work ! so if the carbs are not the problem what else could be ?
... carbs are fully seated I did feel them go in to their seat their clamped down, fully seated.... I spreyed WD-40 around the boots when it was idling and NO change in RPM
so, no air leaks there.
it's got to be a massive air leak in the smog control crap ..... that is about all it can be
...so I will see if I can find the leak in there....
....
Bob.......

[Triple Jim]

Air leaks tend to cause the worst symptoms when vacuum is high, which is at idle and low throttle. If it's OK at idle, but you have a dead spot from 6,000 to 7,000 rpm with the throttle wide open, I'd be surprised if an air leak is the cause.

[Bob KellyIII]

Thanks Jim.... the dead spot is from Idle UP to 6,000 ~ 7,000 RPM everything from idle UP to 6k is dead it is almost impossable to get the RPM's up that high only choke and babying it at the edge of more throttle for a few minutes will get it above that
it's a real big sogy spot right in mid range.... the bike is not rideable like this at all
i can't even fake it lol.....
so yes it sounds like a big vacuum leak it starts where vacuum is highest and then lets off when vacuum declines at higher RPM so yah your right !
....
I'm thinking the vacuum switch must be stuck open.... it was working when I parked it but it's not working now I think so a quick check of that switch would be to pull the vacuum line to it and plug that line and see if it runs better.... if so that is the problem
...I'll do that in a few minutes when it warms up a bit.... we had a trace of snow last night and my fan on the pellet stove quit last night.... so I had to replace it this morning
to get heat in here....luckily i had the forethought to buy one just after I bought the stove....now I need another one !
but it is below freezing out side right now.... so I'll wait till around noon to test that valve.

....
Bob.......

[Bob KellyIII]

strange ! I got a notification that Jim responded here but it didn't show up on the forum
weird !
....
This thread is located at:
https://www.ninjette.org/forums/show...0&goto=newpost

Here is the message that has just been posted:
***************
If this problem happens with the throttle wide open, I doubt if it's caused by a vacuum leak. If it happens at part throttle, it might be.
***************

....Not at WOT.... only on trying to open the throttle where the vacuum is at it's highest
of course when you open the throttle the vacuum drops dramatically... but it is fairly certain that when the throttle is opened the reason for the Sog or bog is because it is too lean to ignite...so the engine stumbles and dies..... pointing to a big air leak somewhere....
I'll check that smog valve ...I bet that is it ! i'll let ya know if that fixes it...I'll go test that now....

....
Bob.....

[Bob KellyIII]

Well.... I went out there and pulled off the main vacuum line to the smog crap ... pluged it with a ball point pen cap.... fired it up and there is no change it still will not take throttle without falling on it's face....
I did get it above it by using the choke and babying it till it finally took off and got above 7,000 RPM.... but it is really hard to get it there.
....
I'm completely stumped !

if it's not the rats nest of vacuum lines then it has to be carbs !
unless you guys have a better idea !
Bob.......

[thedrewski86]

If there's a rat's nest... I'm starting there

[ducatiman]

i'll extend myself here...Bob, I offer you a zero charge 2nd look at carbs. You ship them to me, I'll return them, get them back out by the very next day.

[Bob KellyIII]

here's a question for you guys.... if I plug off the vacuum ports on the carbs
they should still work right ? ( I mean the ones to the smog crap) I think there are 2 lines ....
....
No need for that Gordon but I do appreciate the offer !
if you remember a short while back a fella with 2 Ninja's had the same problem like this on BOTH ninjas.... it turned out that the part under the emulsifier tube was Upside down.... and that causes his problem
I think that is so far unlikely that Gordon could have made such a mistake it's unreal ..... but he is Human and Humans do make mistakes so that is a possibility. not a big possibility, but a possibility nonetheless.
....
I think it more likely it's hooked up wrong.... but how is the big question
... what I need is a diagram of hose connections to the carbs
and the one in the Appendix of the manual is almost impossible to see
their drawing over the photo type pictures is not as clear as a drawing would be.....
....
but I have traced each line back to where it is supposed to connect...I THINK
and it doesn't work so I need to do something different.

....
Bob.....

[Shafer]

Originally Posted by Bob KellyIII

here's a question for you guys.... if I plug off the vacuum ports on the carbs
they should still work right ? ( I mean the ones to the smog crap) I think there are 2 lines ....
....
No need for that Gordon but I do appreciate the offer !
if you remember a short while back a fella with 2 Ninja's had the same problem like this on BOTH ninjas.... it turned out that the part under the emulsifier tube was Upside down.... and that causes his problem
I think that is so far unlikely that Gordon could have made such a mistake it's unreal ..... but he is Human and Humans do make mistakes so that is a possibility. not a big possibility, but a possibility nonetheless.
....
I think it more likely it's hooked up wrong.... but how is the big question
... what I need is a diagram of hose connections to the carbs
and the one in the Appendix of the manual is almost impossible to see
their drawing over the photo type pictures is not as clear as a drawing would be.....
....
but I have traced each line back to where it is supposed to connect...I THINK
and it doesn't work so I need to do something different.

....
Bob.....

IMG_7340.jpg

Oh I hope I'm not going to be known as the upside-down part guy now, haha

I'd try one final leak test but under higher RPM. I'd also recommended using starter fluid with a straw in the nozzle. I've had better luck with starter fluid than WD finding tiny leaks.

I wonder if you have a tiny crack in one of the boots. It's a pain but sometimes its better to start over and try mounting the carb again with a fresh seating starting by mounting to the air box first and working your way to the engine side.

I don't much about your gen bike but I read a lot of threads during my hunt for my simular issue. Of the threads I read some had a issue with the snokal blockage or not connected or something.

Jay

[Bob KellyIII]

well, I think I may well have found it....
I started the ninja and as it was idling on choke warming up I took a can of carb cleaner
and spreyed the boots ... no change the RPM started to raise as the bike warmed up so I turned off the choke and the idle dropped real low but still idled....
I thin soaked the carbs.... both sides no change then spreyed inbetween both carbs and the RPM raised quite a bit...... AHH HA !!!! i found the air leak ! it's between the carbs !
.....
so the carbs have to come off !
unless there is a vent hole between the carbs that cross over pipe is leaking....
....so that is something at least
I at least have a direction to go in now ! .... but I am not going to do it today...
.....
Bob......

[Bob KellyIII]

I just got the carbs back out , but I think what I'll do instead of taking the carbs apart and seperating them .... I think I will just put Shoe-goo on that connector between the carbs
(which looks perfect I might add) and just cover the entire joint so it can't suck air from the outside.... easier than loosing the carb sync and a perminant fix sense the carbs are super clean to begin with and I am sure the shoe-goo will stick to everything very well.
.....
My question is what is that cross over tube for ? I have seen them on other bikes and usually their for balance between the carburetors but I am not sure on this beast. it should be for balance purposes as well .... but the thing about balance tubes is it will not balance the carbs completely it is only for slight imbalance kind'a a fine tune for imbalance.... carb sync makes up the majority of carb balance but the balance tube does have it's function it makes for a fine running engine 99% of the time.
a mechanical carb balance is never spot on.... the minute changes in the metal alone from changes in tempiture are enough to throw it off so they devised a way to internally balance the carbs and it works very well.... so it is needed !
the insert tube that Gordon put in there looks right and it is seated.... perhaps it was just a milimeter too short or something I don't know.... I sujest gordon checks that on his builds now ! the O ring kits he got may well be faulty because of that !
.....
but I will just cover the entire connection with shoe -goo and put the carbs back in
.... I did look down the throat of the carb for the tip of the spacer to protrude into the throat like it should , that sits on top of the emulsifier tube.... they look fine !
....
I am hoping that I found the problem I won't know till I put it back together and try it again....
....
Bob.......

[ducatiman]

"on that connector between the carbs"

Are u referring to the upper, black VENT tube?

[DannoXYZ]

Bob you dorkus!!!

Didn't you take ANY pictures before ripping it apart so you can put it back together??

1. internal fuel filtre - did you put it back in correctly? Remove fuel-hose and blow-compressed air through it to clean out. I've seen 2-3 of these filtres wadded up inside blocking flow. Straighten out hose so you can see light through it to verify it's clear? Leave out this filtre for now.



2. hose diagram - Technology is your friend, let's use it:

- click on down-arrow next to SEARCH in menu-bar above
- click on Advanced Search
- entre in Key Words = carb hoses
- entre in User Name = dannoxyz
- click on Search Now button below
- click on topic = Petcock vacuum hose questions

3. vacuum hoses -

Originally Posted by Bob KellyIII

here's a question for you guys.... if I plug off the vacuum ports on the carbs
they should still work right ? ( I mean the ones to the smog crap) I think there are 2 lines ....

So what about non-CA bikes? Do they just have NOTHING connected to 2 vacuum ports on carbs? If you have both ports capped off, where are you getting vaccum to open petcock? If petcock doesn't open and let fuel flow into your carbs, what happens when you try to start and run engine?

Here's what you do:

- disconnect vaccum hose going to PAIR valve sitting between tank and head, that's it, disabled. Leave everything else alone, do not touch anything connected to emissions equipment

- right-carb vacuum-port goes directly to coast-enricher. Cap off T-fitting in middle that used to connect to PAIR valve in front

- left-carb vaccum-port goes directly to petcock. DONE!

Switch hoses on carb-ports if T is on left-carb vacuum port. You want direct uninterrupted hose directly to petcock. Next up is petcock flow-test if this doesn't solve it.

Also how old is petrol in that tank?

[ducatiman]

dannoxyz posted:
"Didn't you take ANY pictures before ripping it apart so you can put it back together??"

Fully agreed. In addition...marking all cam timing points with (white or yellow) paintpen PRIOR to taking apart.
Even vac hoses can be marked using differing # of dots to each.

I use "dot system" to mark timing belts on my Ducatis...CAN'T screw it up (and 1 tooth off = disaster)

Welcome back, danno!

[DannoXYZ]

Originally Posted by ducatiman

"on that connector between the carbs"

Are u referring to the upper, black VENT tube?

Yeah! Do you have drain hose connected to T on vent-tube going down to rear-shock?


If you're spraying into open vent-tube, that would force fuel into carbs and certainly change idle. Even though there's no leak.

[DannoXYZ]

Originally Posted by ducatiman

dannoxyz posted:
"Didn't you take ANY pictures before ripping it apart so you can put it back together??"

Fully agreed. In addition...marking all cam timing points with (white or yellow) paintpen PRIOR to taking apart.
Even vac hoses can be marked using differing # of dots to each.

I use "dot system" to mark timing belts on my Ducatis...CAN'T screw it up (and 1 tooth off = disaster)

Welcome back, danno!

I was just checking to see what's new. Couldn't let Bob drown like this!

[Bob KellyIII]

Howdy Danno ! good to see you here again it's been a while !
yes like a dummy I didn't really take pictures before disassembly ,,,and I will not make that mistake again ! (lesson learned)
....
What I am refuring to is the cross over pipe at the back of the carbs just under the yellow fuel line in the picture above... there is a connector between the carbs and I think that was sucking air that changed the idle when I spreyed the carbs.
.... Gordon if that is just a vent tube to the carb bowls it wouldn't change the idle would it ? it has to be a balance tube to effect the idle !
....

Danno has given me alot to read up on again so I will get that done and then get back here.....
thanks Danno I really appreciate your help !
Bob........

[ducatiman]

Is the tube (circled in yellow) what you are referring to?

"I think that was sucking air that changed the idle when I spreyed the carbs."

inferring you've plugged its centrally exposed "connector" (running vertically in the pic) ???? Is that a correct assumption i'm making?

[Bob KellyIII]

I don't know for sure Gordon but I don't think so, that looks to be the vent I think....it is high and to the front of the carbs (when I sprayed it was down past the caps of the CV portion down on the outside body of both carbs...so if a connector between the carbs is leaking it is probably the back one by the bowls...)
I am not as stupid as I thought I was as I do have pictures of the hookup of the hoses before removal and cleaning and I am certain now that I had them hooked up correctly... however your not supposed to raise the idle if you sprey
carb cleaner between the carbs.... and I only have that as a reference..."between the carbs" i have no idea if its in front or in back or dead center I just know that it's between the carbs...I first thought that it must be the connectors between the carbs as that is the most likely area to leak suction
....I went out there and EYE balled the carbs and I did not see 2 connectors between the carbs just one at the bottom back.... that is the one I was going to Shoe-goo ! rather than split them....
so I don't actually know if it is leaking at that point or not..... probably but that is all....
........
this carb cluster has 2 cross over connectors to join the carbs together one at the top that I call the carb vent tube ( gas poured out of it when I turned them upside down) and the other at the bottom back by the bowl
there are 2 vacuum hose connections there one from the venturi area that goes to the top of the decell valve and the other by the back of the throat of the carb to the underside of the decell valve both are hooked up and in place from Gordon's rebuild . nothing looks bad or out of place.
the bottom connector is obviously the gas feed to that Right side carb....i didn't see that earlier... so shoe-goo isn't going to help it at all.... and it's not leaking fuel anyway. .... that only leaves the top vent tube if it is indeed a vent tube open to outside air it shouldn't matter in the least.
and spreying carb cleaner on it may well result in a faster idle.... so that is a bust....
or it could be sucking air from the choke area....but the choke operates perfectly every time so that is unlikely if it did suck air from there I doubt it would be enough to give me a humgous flat spot from everything from idle to 6 grand.... that is a big air leak not a tiny seep
....so I pulled the carbs off and found nothing.....
....
i did notice that the carb vent tube does rotate easily the small amount it can move.......
I do not want to split the carbs
....
Danno there is no inline fuel filters in the fuel line , I put a proper inline filter on shortly after I got it and cleaned the carbs myself ...
I am almost positive the vacuum lines were all connected correctly
..... However I am now running the bike on a axillary fuel tank and have the gas tank lines all plugged as well as the one to the petcock
....but it would not take throttle at all.
...I am at a loss here. from what I know it should run and run better than it ever did.... but it will not take throttle
....
Bob.

[Bob KellyIII]

Originally Posted by ducatiman

Is the tube (circled in yellow) what you are referring to?

"I think that was sucking air that changed the idle when I spreyed the carbs."

inferring you've plugged its centrally exposed "connector" (running vertically in the pic) ???? Is that a correct assumption i'm making?

Um no.... I just left the hose on it when I sprayed between the carbs and got an increase of RPM..... so yes that is the vent connector between the carbs that I refured to in my last post....
I cannot get to it without splitting the carbs and sense it is Just a vent
I cannot see how it would effect the running of the engine.
...
Bob....

[DannoXYZ]

Do petcock flow-test next:

- unplug fuel-hose from carb
- aim hose into measuring cup
- crank engine for 10-sec

How much petrol did you collect?


Also post photos from different angles so we can see how vacuum-lines are connected.

[Bob KellyIII]

I gott'a get the carbs back on Danno and then I will....
the fuel flows great so I don't think that is it, when the tank is on... right now I am using a aux tank to feed it and the flow is a solid stream from a 1/4" line
( more than plenty!) I'll probably put the carbs back on tomorrow
without doing anything to them....I took them off because of the raise in RPM when spraying the carb cleaner between the carbs.... but I can't see anything wrong there.... it all looks perfect ! their like brand new carbs so I think it's something other than carburation....
if I could get the big air box off so I could get my hand behind the carb I could tell more by putting my hand over the carb throats but I have to disassemble the back half of the bike to do that !
.....
I do NOT think it is fuel flow as I held it at 8kRPM for a few seconds maybe 30 seconds... if it was low on fuel in the float bowls it would have staggered and died then but it cleared out and purred like a kitten at that RPM
I let it idle and then could not get it back to high RPM without the choke
.... by then I was disgusted and called it quits for the night
....
after I assemble it again I will post a few pictures so you can see the hoses.
.....
Bob.......

[Bob KellyIII]

Originally Posted by DannoXYZ

Yeah! Do you have drain hose connected to T on vent-tube going down to rear-shock?


If you're spraying into open vent-tube, that would force fuel into carbs and certainly change idle. Even though there's no leak.

yes the vent hose between the carb goes to a hose that goes over the air box and down in front of the battery it's just a vent....
that hose was connected when I sprayed the carb cleaner between the carbs and got the raise in RPM.... so I don't know how that could have effected the RPM..... but I am assuming that it was the cause of the raise in RPM simply because I can find nothing wrong
.....
Bob.....

[DannoXYZ]

Leave air-box off when you put carbs back on. Wedge some wood or something to keep it spaced away from carbs. Then we can see the slide action.

[ducatiman]

Well? Anything happening @bobKellyIII ?

Or did you take this offline now?

[Bob KellyIII]

Sorry guys I got derailed.... and haven't decided what to do about the Ninja......
I have the carbs on the bench ready to go back in....
but look at it from my prospective... I took the carbs out and found nothing wrong
( i didn't split them) so I haven't changed a darn thing. if I put the carbs back in there it will undoubtedly do the same thing....
my only recorse is to tear into the carbs or send them back to Gordon....
.....
I do NOT understand this problem at all.... I've never had a bike do this in my life
it is a new thing for me and I don't know how to handle it
.....
if you look at the symptoms that should tell you what it is doing...(should)
there is a major flat spot just above idle , which means the fuel /air ratio at that area
is either too lean or too rich.. sense it is not belching blue smoke I'ed say it is too lean.
so a lean mixture off idle would be something amiss in the carburetor OR a major vacuum leak, both of which have been ruled out. but it has to be SOMETHING !
....
the Parts on top of the emulsifier tubes are protruding into the carb throat like they should be ( one of the reasons I wanted to take the carbs out just to check)
....
I can't find anything wrong !
...
the only thing I can do now is re install the carbs and see if it runs right.... which it won't, because I didn't find anything wrong, so why reinstall ?
...
so it's send the carbs back to Gordon OR tear into them myself.
because it is not going to run right like this !
....
Bob......

[ducatiman]

send them back, i'll carefully inspect all circuits, fuel levels, etc etc

[Bob KellyIII]

Ok I think I will on monday I'll have to make a special trip but it's worth it to me to get this darn thing running right again.
I want 100 main jets in this thing this time and get rid of all the other generic parts in the carbs ! I'll pay for that ! i want them back to stock not just re-assembled !
.....
Bob.........

[Bob KellyIII]

here is a few pictures of the muffler on the Ninja
....
Bob.......

[ducatiman]

"the muffler"

singular, as in its got a 2 into 1 system on it!

you've never referred to a non stock system on your 250! I had no idea that was on there

[Triple Jim]

Originally Posted by ducatiman

"the muffler"

singular, as in its got a 2 into 1 system on it!

you've never referred to a non stock system on your 250! I had no idea that was on there

Newgens came with a 2 into 1 system, but that muffler is not stock. Bob will have to tell you if the whole system is aftermarket or not.

[ducatiman]

In that case chances are the PO changed some stuff in the airbox as well. We'll need to confirm that too, and confirm it is a CA model as well.

Bob also provided the following info: "it does have an after market pipe on it so the flow out is un restricted and I know that will effect the burn"

I'm now thinking some non-standard jetting mods will be necessary. Welcome to "the chase".

[Triple Jim]

Well, a dead spot at mid throttle might call for the addition of needle shims. It's easy to try.

[Bob KellyIII]

the exhaust system that is on the bike now came with the bike when I bought it, I did manage to get the stock muffler from the guy , but I have never had it on the bike.
I am almost positive that the hedders are stock and just the muffler was changed
... I did buy a large box of extra plastics for the bike that the guy has replaced
because of scratches, which was a smart move on his part or I would not have paid as much as I did for the bike ($4000.00)
the bike has a dead battery and would not run , I told him I didn't have a truck but if he would bring it to me and I liked the look of it I'ed buy it for his asking price...
(i was getting desperate, I had been without a bike for at least 2 years by then! )
....
it IS a california Model and still has all the smogg **** on it.
....
the Air cleaner as far as I know is totally stock I've had it out, cleaned it and put it back in... it looks stock to me, a foam filter square incert in the rt.side of the bike.
i believe he told me he put a Jet kit in it and the muffler, other than that it was a stock bike.
I expect the "Jet Kit" was a cheepo Chinese Kit from E-Bay
the plug color is black.... or super dark grey.... (it does not use any oil )
the bike has always been hard to start.... when cold....cleaning the carbs 3 times finally got the bike to start fairly well... but it will not start without the choke when cold and it takes a extreamily long time to warm up
.....
so the bike has not run correctly sense I got it as it is running too rich
....
I did finally manage to Isolate the Smog crap on this thing and make sure the carbs were not sucking air from a vacuum line somewhere.... and it still did the same thing
that is it would bogg soon as you cracked the throttle ,same thing with it revving to 3,000RPM with the choke on soon as you try to give it gas it falls flat on it's face ...
I did get it to High RPM a few times though and the point to where it boggs is around 6,000~7,000 RPM above that point it throttles ok up or down in RPM. any thing below 6,000 RPM if you try to give it throttle it just boggs and tries to die.
........(and that is with NO vacuum lines attached , so I am sure it's the carbs)

...so I will box the carbs up and send them back to Gordon on Monday or Tuesday
coming up in a few days...
...
it's 70 degrees here today ! spring is finally here ! and the apple trees are budding out real fast !...
....
I got the XR650L Running really well now, it starts with a touch of the button now never makes a full revolution .it is burning a Tan color but being a air cooled bike I would like it a bit darker color... so that is in the works....
....
it won't be long and we'll be looking for ways to get cool again it's going to be a hot one! as summer usually is here.
.....
Bob.........

[Bob KellyIII]

Jim: needle shims means the carbs have to come out.... getting the carbs out on that bike is Not EASY ! it's only easy if the carbs are already OUT.... LOL !!!!!
....
Needle shims have never worked for me... they introduce a flat spot in almost every case
when I have tried to use a shim... you mess with the fuel to air ratio when you Shim it
changing it from 14:1 to 12:1 or some thing like that and that is not a good idea to do.
changing the Main jet is by far better if you want to get more or less fuel in it. because it will increase or decrease the voluum but keep the fuel /air ratio the same. in almost every case a flat spot in the throttle opening is due to something else and not the jet needle. they put alot of time and effort to keep the fuel /air ratio correct throughout the RPM Range so the stock carb is usually spot on.... BUT it may well be spot on in a very lean or Rich way throughout the span of RPM and this is not uncommon at all especially with mods to intake and exhaust systems on the bike. if it comes to the point of shimming a needle I have to think something else is very wrong in the system.... it is designed to put out 14:1 fuel air ratio throughout the RPM range and that is a very hard trick to accomplish.... changing the main jet changes that 14:1 ratio through the entire range.... a larger main jet will lower the ratio from like 14:1 to 12:1 and a smaller main jet will increase it like from 14:1 to 16:1.... so In Most cases there is never a need to change the main jet at all... but when changes are made then adjustments have to be made....
for instance: on my XR650L I have been trying to get the Fuel/air ratio Down because it runs very lean for a air cooled bike
it needs a richer mixture to help cool the air cooled engine... so I have increased the main jet size from 150 to 160 and it only changed it a little I may need to go more to something like 165 this will change the fuel /air ratio to something like 12:1 ( which is my target) this will use alot more gasoline but the bike will run alot cooler in the hot summer !
I had a flat spot on the XR650L just above idle but that was due to a too small of a slow speed jet anything above 3,000 RPM was perfect lean but perfect ,.... no flat spots in acceleration just one coming off idle.. it had a 50 slow speed jet in it and I put a 55 in it's place and re adjusted the fuel air screw.... the hesitation coming off idle is now gone....
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Yes in some cases shimming works great, but I have never found an instance where it helped me...... EVER !
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my opinion, take it for what it's worth ! ....I never shim the needles ! LOL
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it has been argued that shimming the needle is the same as changing the main jet but I disagree because it changes the fuel air ratio not the volume of fuel going into the engine, there is a Suttle difference. (6 of one half a dozen of another right? ) it still changes the amount of fuel going into the engine so what ever trips your trigger !!!!!


Bob.....

[Bob KellyIII]

Ok Gordon the carbs are in 3 plastic bags with card board spacers to prevent damage from rough handling , I put them in a medium sized flat rate box and sent them about 10 minutes ago..... so they are on their way !
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Bob...........

[ducatiman]

acknowledged