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Old November 21st, 2020, 12:55 PM   #1
clakmurrick
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Unhappy Horrible performance below 6K RPM, especially when warm. Video attached.

Hey all, this might be a cluster. Hopefully my ADHD mind won't screw this post up too much, I'll try to stay on track.

I had a buddy of mine help me find a 250R for dirt cheap. We checked it out and everything was fine, but it didn't run. The PO dropped the bike, didn't ride it for many months, and of course the carbs gummed up. We took the bike home and my buddy taught me how to rebuild the carbs. We cleaned everything thoroughly, including dipping it all in a tub of carb cleaner. I used little bread ties to poke the holes of all the jets and what not. I also removed the block plate for the idle mixture screws and adjusted them to 2.5 turns counter clockwise from the closed (tight, idk what to call it) position.

I should note that my friend convinced me to remove the stock airbox, so we replaced it with velocity stacks, and we removed the stock muffler and attached a SC Project exhaust.

In order to make use of these mods, we got a jet kit, which replaced the main and idle jet, and we also added washer spacers to the needles and cut the spring slightly, according to the instructions provided.

The bike has always ran a little odd, at first, when we had the modifications but no jet kit, the bike ran somewhat okay down low (sounding like it was breaking up), and it would not get above 7k RPM at all. We jetted the bike and then it was able to rev to redline and pull well. The only problem is that I still have horrible performance down low. The bike will idle and will start fine, especially when cold, but once the bike warms up, I can't make it through intersections without the bike bogging and dying on me. I have to rev it to 6k RPM minimum before taking off so it won't bog.

I have a video linked here that shows the behaviour once warmed, and this was an extreme event, because I literally could not rev past 3k. I will admit, that at this current time the idle mixture screws have been tightened almost all the way back, however I did this to test their function, and oddly it had no difference on the bike as far as I can tell since the bike was doing these same shenanigans before adjusting the idle screw.

Here's the video: https://www.youtube.com/watch?v=ovJGXEefjGw

I hope y'all will go easy on me, I learned my lesson to get the bike running right before modding it, my damn friend convinced me it would be alright!
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Old November 22nd, 2020, 06:45 PM   #2
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My guess is the carbs are not right.

Either not assembled correctly, not completely clean, damaged diaphragms, or jetting/adjustments are way off.

Put the airbox back on when you have the carbs off, and put the stock jets back in. It's never going to be a rocket off the line, but removing the airbox and using pod filters is going to make it flatter everywhere but on the very top.

Another thing it could be is a fuel restriction from junk in the tank, petcock screen, petcock, or inline filter at the carb inlet.

But I'm betting it's the carbs.
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Old November 22nd, 2020, 06:59 PM   #3
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Originally Posted by clakmurrick View Post
I should note that my friend convinced me to remove the stock airbox, so we replaced it with velocity stacks, and we removed the stock muffler and attached a SC Project exhaust.

In order to make use of these mods, we got a jet kit, which replaced the main and idle jet, and we also added washer spacers to the needles and cut the spring slightly, according to the instructions provided.
Do search here for "pod filters" and you'll find no one has been able to get them running properly. Causes mismatched fueling and issues in crosswinds. For no benefits in power whatsoever because stock airbox does not block any flow at all. It's same size as one on my CBR600RR which flows 5x more air. Most posts will be from people reverting back to factory airbox trying to track down OEM parts.

Over 30-years of racing Ninja 250s and not a single racer uses pod-filters. Even though it's allowed by rules, no one does it, because there's no benefits and too many downsides as you've found out.

Go back to 100% stock and your bike will run like fresh off showroom floor.
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Old November 22nd, 2020, 09:23 PM   #4
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oh yeah, low-end issues is most likely carb-related. Idle & pilot jets either clogged or wrong size for actual airflow. Which is same as stock, so stock size jets need to be used.
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Old December 4th, 2020, 06:34 PM   #5
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Originally Posted by DannoXYZ View Post
Do search here for "pod filters" and you'll find no one has been able to get them running properly. Causes mismatched fueling and issues in crosswinds. For no benefits in power whatsoever because stock airbox does not block any flow at all. It's same size as one on my CBR600RR which flows 5x more air. Most posts will be from people reverting back to factory airbox trying to track down OEM parts.

Over 30-years of racing Ninja 250s and not a single racer uses pod-filters. Even though it's allowed by rules, no one does it, because there's no benefits and too many downsides as you've found out.

Go back to 100% stock and your bike will run like fresh off showroom floor.
Haha I wouldn't doubt it, I wish I hadn't thrown the airbox away like an idiot. Now I gotta see if I can go get one. Why do I learn things the hard way
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Old December 6th, 2020, 09:02 PM   #6
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Haha I wouldn't doubt it, I wish I hadn't thrown the airbox away like an idiot. Now I gotta see if I can go get one. Why do I learn things the hard way
Now you know - never throw original parts away!

Even if you don't decide to reinstall them, they can add to the value of the cycle if you ever decide to part with it.
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Old March 10th, 2021, 04:59 PM   #7
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oh yeah, low-end issues is most likely carb-related. Idle & pilot jets either clogged or wrong size for actual airflow. Which is same as stock, so stock size jets need to be used.
When you said "which is same as stock," did you think that I left the pilot jet the stock size? I replaced the main jet with a 117.5 and the pilot jet with a 40. I replaced what I believe is the idle jet with a new OEM spec one. That should still be the same size. Sorry for the late response. I'm coming back to these threads now. The bike ran a lot better when it was cold, but now that it's hot again, it needs to be revved to 6k RPM to get up and go otherwise it bogs out. I want to keep trying with my setup and see if I can be successful, otherwise I might have to just buy an oem airbox and put the jets back to stock. I also need some springs for the slides since I cut those. I sure am learning a ton!
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Old March 11th, 2021, 08:33 AM   #8
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Originally Posted by clakmurrick View Post
When you said "which is same as stock," did you think that I left the pilot jet the stock size? I replaced the main jet with a 117.5 and the pilot jet with a 40. I replaced what I believe is the idle jet with a new OEM spec one. That should still be the same size. Sorry for the late response. I'm coming back to these threads now. The bike ran a lot better when it was cold, but now that it's hot again, it needs to be revved to 6k RPM to get up and go otherwise it bogs out. I want to keep trying with my setup and see if I can be successful, otherwise I might have to just buy an oem airbox and put the jets back to stock. I also need some springs for the slides since I cut those. I sure am learning a ton!
OK - from what you are telling us it's running significantly rich.

You don't need 40 Pilot Jets, but you may be able to use them if you can turn the Idle Mixture Screws in enough to get the proper mixture. You may be around 1 to 1.5 turns out with the larger Pilot Jets.

117.5 Main Jet is way rich. We ran pods and a leaky stock exhaust with 108s I believe - and it pulled strong all the way. Not a big fan of carb kits with larger everything.

Not sure who suggested cutting springs and slides, but that's not a typical carb mod from what I've seen.

As you can see, changing a bunch of stuff based on someone's suggestions can create all sorts of problems and most of the time doesn't really gain you any usable power.

A properly tuned 250 with the stock airbox will give you nice crisp low and mid-range performance. Opening things up usually flattens that out, and sometimes doesn't gain anything anywhere.

Proper carb tuning is critical. They are pretty close as is, but require precise adjustment of the idle mixture and maybe a shim or 2 on the needles to run much better. The Main Jets are usually overly rich, and more power can be gained by replacing them with the optimum size depending on other factors.

Final step is to make sure the carbs are synced properly.

Last futzed with by jkv45; March 11th, 2021 at 12:48 PM.
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Old March 11th, 2021, 11:23 AM   #9
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Just to throw some piece of data in here: ZZR 250 uses the same engine with different jets. Main jet standard is 108 with optionals going to 112
The pilot is 38.

I also ran it with one shim under the needle and it ran very rich to the point where I had to remove and clean the spark plugs after about 500km. It started running like crap. Removing the shim brought stuff back to normal. Mine uses stock 108

No other modifications are made, stock airbox , stock exhaust
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Old March 11th, 2021, 09:39 PM   #10
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Just to throw some piece of data in here: ZZR 250 uses the same engine with different jets. Main jet standard is 108 with optionals going to 112
The pilot is 38.

I also ran it with one shim under the needle and it ran very rich to the point where I had to remove and clean the spark plugs after about 500km. It started running like crap. Removing the shim brought stuff back to normal. Mine uses stock 108

No other modifications are made, stock airbox , stock exhaust
Cool, thanks for the data!
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Old March 11th, 2021, 09:41 PM   #11
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OK - from what you are telling us it's running significantly rich.

You don't need 40 Pilot Jets, but you may be able to use them if you can turn the Idle Mixture Screws in enough to get the proper mixture. You may be around 1 to 1.5 turns out with the larger Pilot Jets.

117.5 Main Jet is way rich. We ran pods and a leaky stock exhaust with 108s I believe - and it pulled strong all the way. Not a big fan of carb kits with larger everything.

Not sure who suggested cutting springs and slides, but that's not a typical carb mod from what I've seen.

As you can see, changing a bunch of stuff based on someone's suggestions can create all sorts of problems and most of the time doesn't really gain you any usable power.

A properly tuned 250 with the stock airbox will give you nice crisp low and mid-range performance. Opening things up usually flattens that out, and sometimes doesn't gain anything anywhere.

Proper carb tuning is critical. They are pretty close as is, but require precise adjustment of the idle mixture and maybe a shim or 2 on the needles to run much better. The Main Jets are usually overly rich, and more power can be gained by replacing them with the optimum size depending on other factors.

Final step is to make sure the carbs are synced properly.
Funny enough, the kit I got recommended cutting the spring, and so did my friend. The kit included a 112.5 number main jet, so I'll at least size down and see what happens. I have a tiny shim on the needles already, I'd estimate 2mm or less if I remember correctly. I'll try removing it, too. It's odd how well the bike seems to run above 6k RPM, even when hot outside. It's just below 6k RPM. When do the mains seem to kick on?
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Old March 12th, 2021, 06:56 AM   #12
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Originally Posted by clakmurrick View Post
Funny enough, the kit I got recommended cutting the spring, and so did my friend. The kit included a 112.5 number main jet, so I'll at least size down and see what happens. I have a tiny shim on the needles already, I'd estimate 2mm or less if I remember correctly. I'll try removing it, too. It's odd how well the bike seems to run above 6k RPM, even when hot outside. It's just below 6k RPM. When do the mains seem to kick on?
What kit was it? 112.5 is still to large IMO.

Honestly, as I've said, I don't care for any of the kits.

With all the changes it's hard to pinpoint which is causing the most problems, but going back to stock jets (stock mains are plenty big) and other modified parts would go a long ways to getting it running properly.

Do you know what the idle mixture screws are set at? That would be the first/simple thing to try to get it running better.

The Main Jet affects mixture as soon as you lift the slides and the needles pull out of the jet - so almost all the time except when the throttle is completely closed.

When making carb adjustments, think in terms of throttle position - 1/8, 1/4, 1/2, etc.

Having ridden a 250 with pods, I would get an original airbox and bring the carbs back to stock - with maybe just a small shim on the needles. The airbox makes it much more pleasant to ride normally.
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Old March 16th, 2021, 03:44 PM   #13
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What kit was it? 112.5 is still to large IMO.

Honestly, as I've said, I don't care for any of the kits.

With all the changes it's hard to pinpoint which is causing the most problems, but going back to stock jets (stock mains are plenty big) and other modified parts would go a long ways to getting it running properly.

Do you know what the idle mixture screws are set at? That would be the first/simple thing to try to get it running better.

The Main Jet affects mixture as soon as you lift the slides and the needles pull out of the jet - so almost all the time except when the throttle is completely closed.

When making carb adjustments, think in terms of throttle position - 1/8, 1/4, 1/2, etc.

Having ridden a 250 with pods, I would get an original airbox and bring the carbs back to stock - with maybe just a small shim on the needles. The airbox makes it much more pleasant to ride normally.
Hey there, I've made a change and I've come to report what my findings are. I downsized from the 117.5 to a 112.5. The bike seemed happier at idle. I adjusted the idle mixture screws to two full turns out, since revving it and returning to idle was the smoothest with this setting. Any less and it would hang a little, any more and it drooped below idle.

Once I got it on the road, I noticed it was not wanting to rev past 6k easily. The lower-rev range is better, but not great. Once I fought past the 6k area, probably 1/4-1/2 throttle, it would pick up and take off, but then close to redline it starts to hesitate. If I hold it there, it pulls, then hesitates, and then pulls again. I have not removed the spacers on the needles, only downsized the main jet. The pilot jets are still a 40, but I should be fine as long as I can screw the idle mixture screws in.
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Old March 16th, 2021, 04:30 PM   #14
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For your viewing pleasure. Check out these idle mixture screw positions!
  • Velocity stacks
  • Slip-on muffler
  • 112.5 main jet
  • 40 pilot jet

Ignore the gas gauge, it's unplugged. Also, even though it may seem to rev fine under little throttle, as soon as its under load, it's bog-city

Link to original page on YouTube.

Link to original page on YouTube.

Link to original page on YouTube.

Link to original page on YouTube.

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Old March 17th, 2021, 06:37 AM   #15
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As far as adjusting the idle mixture goes, that's not usually how I do it.

CV carbs are going to "bog" when you open them fast - no matter what. That’s especially true when not under load and with small displacement engines.

To adjust the idle mixture, first make sure the carbs are synced. Then warm the engine by riding a while.

Turn each adjustment screw to get the highest RPMs. After both are set at the max idle speed, I go out (richer) about 1/8 or so from there. Then adjust the hot idle speed to the correct RPM with the idle speed adjuster.

Going just slightly richer than the max RPM gives you a smoother transition as you open the throttle.

EDIT - Your Main Jet may still be too rich.

Last futzed with by jkv45; March 18th, 2021 at 06:45 AM.
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Old March 19th, 2021, 03:30 PM   #16
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As far as adjusting the idle mixture goes, that's not usually how I do it.

CV carbs are going to "bog" when you open them fast - no matter what. That’s especially true when not under load and with small displacement engines.

To adjust the idle mixture, first make sure the carbs are synced. Then warm the engine by riding a while.

Turn each adjustment screw to get the highest RPMs. After both are set at the max idle speed, I go out (richer) about 1/8 or so from there. Then adjust the hot idle speed to the correct RPM with the idle speed adjuster.

Going just slightly richer than the max RPM gives you a smoother transition as you open the throttle.

EDIT - Your Main Jet may still be too rich.
Good to know that CV carbs don't like it when you open the throttle fast haha, I don't have any baseline feeling for what a Ninja 250 feels like and how it responds. I'll take all the info I can get!

In the video, when I barely start opening the throttle slowly, I assume the main jet is kicking in almost instantly? As far as jet size goes, this is the smallest I have, unless I size down to the OEM one.

Also, on the idle mixture adjustment, what do you mean "
Turn each adjustment screw to get the highest RPMs"? Should I get the bike warm, and literally turn the screws until the RPMs raise, and find the highest point and leave it there? It's certainly different than my method.

Thanks!
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Old March 19th, 2021, 03:36 PM   #17
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The main jet does not start to flow "almost instantly". The slide has to be pretty far open for that to happen, and the diaphragm won't raise it until there is sufficient flow. Initially the pilot circuit is supplying fuel. As the throttle valve is opened further the needle jet circuit is active. Only after that does the main jet start to be the controlling fuel supplier.

Reread what JKV45 said about adjusting the idle mixture, you missed some of his points.
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Old March 20th, 2021, 05:50 PM   #18
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Good to know that CV carbs don't like it when you open the throttle fast haha, I don't have any baseline feeling for what a Ninja 250 feels like and how it responds. I'll take all the info I can get!

In the video, when I barely start opening the throttle slowly, I assume the main jet is kicking in almost instantly? As far as jet size goes, this is the smallest I have, unless I size down to the OEM one.

Also, on the idle mixture adjustment, what do you mean "
Turn each adjustment screw to get the highest RPMs"? Should I get the bike warm, and literally turn the screws until the RPMs raise, and find the highest point and leave it there? It's certainly different than my method.

Thanks!
If you have 108s, I would try them. In stock form, they are too rich - so it may be close. We ran them with pods and a leaky exhaust - and it ran strong on top.

For the idle mixture - yes - you are trying to find the highest RPM setting you can get by adjusting the idle mixture screws. I like to make it about 1/8-turn richer (out) from there. It will most likely be higher than it is now, so you just turn the idle speed screw to get it back to 1300 or so.

This diagram shows the different fuel circuits - https://jdjetting.com/wp-content/upl.../163996103.jpg - they overlap and blend together, so changing one affects a wider range than just one circuit.

Did the kit have different carb needles?
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Old March 23rd, 2021, 08:34 PM   #19
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If you have 108s, I would try them. In stock form, they are too rich - so it may be close. We ran them with pods and a leaky exhaust - and it ran strong on top.

For the idle mixture - yes - you are trying to find the highest RPM setting you can get by adjusting the idle mixture screws. I like to make it about 1/8-turn richer (out) from there. It will most likely be higher than it is now, so you just turn the idle speed screw to get it back to 1300 or so.

This diagram shows the different fuel circuits - https://jdjetting.com/wp-content/upl.../163996103.jpg - they overlap and blend together, so changing one affects a wider range than just one circuit.

Did the kit have different carb needles?
The kit had replacement needles, but they should have been identical to the OEM iirc. If it makes you happy, I'm ordering a stock airbox and I'll be getting the bike running in factory condition (with an exhaust still), before I continue. I'm sending off the carbs to ducatiman to see if he can check them out. I think my choke is clogged still and I'm sure there are other issues. Let's see what happens!
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Old March 24th, 2021, 06:06 AM   #20
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Kits are notorious for supplying not-quite-right parts. Needles need to be VERY accurately made. Even one thousandth of an inch in diameter different in one region of the needle will make a difference to performance. If this were mine, before I continued I'd get a pair of original needles. Even if that's not your whole problem, it is likely to mess you up for years to come anyway.
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Old March 24th, 2021, 01:24 PM   #21
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Originally Posted by clakmurrick View Post
The kit had replacement needles, but they should have been identical to the OEM iirc. If it makes you happy, I'm ordering a stock airbox and I'll be getting the bike running in factory condition (with an exhaust still), before I continue. I'm sending off the carbs to ducatiman to see if he can check them out. I think my choke is clogged still and I'm sure there are other issues. Let's see what happens!
Chances are they had a "richer" taper - which complicates the mixture issue.

"If it makes you happy..." - not sure what that means. Sounded sarcastic to me.

Having Gordon go through the carbs is a good idea. Send the original parts and needles with the carbs.
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Old April 2nd, 2021, 05:27 PM   #22
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Chances are they had a "richer" taper - which complicates the mixture issue.

"If it makes you happy..." - not sure what that means. Sounded sarcastic to me.

Having Gordon go through the carbs is a good idea. Send the original parts and needles with the carbs.
Haha, I did not mean for that to be sarcastic, I am happy I got a stock airbox. I got a mint condition one for literally $22 shipped. IDK why the seller made it a bid item, but no one needs them often, so I was the only bidder.

Anyways, Gordon hooked me up! He found a ton of problems with my carbs and went over them on the phone and what he could do to rectify the problems. The bike runs amazing now! I have everything stock except the exhaust, and the bike pulls amazing at all throttle positions and doesn't hesitate at all down low. Thanks Ducatiman!

If y'all are interested in what was wrong, here's his list he gave me:
I spent afternoon into evening completing your carbs. They came out really nice.

Quote:
Issues and resolutions:

*mesh filter found upwards in fuel hose (slightly crushed/ not filtering) creating an impediment to fuel flow, .......installed EX500 mesh filter into the fuel rail (much tighter fit)
*1 float assembly adjustment tab was mauled/bent....replaced float assembly
*float valves had visible wear “lines” ...replaced with new float valves
* float valve seats showed corrosive deposits.....cleaned
*choke fuel pickup points were both totally clogged......cleared them both
*aftermarket “jet kit needles” unmarked as to size......replaced with correct needles N9VU (LH carb) and N9VW (RH carb)
*oversized pilot jets #40.................... replaced with #38 pilot jets
*oversized #112 main jets .................replaced with #102 main jets
* aftermarket pilot screws from jet kits.................replaced with OEM Kawasaki/Keihin screws (with new orings) Set adjustment screws at 2.5 turns on LH and 1.75 on RH
*wet testing revealed perfect fuel levels, no leaking (internal or external)
*bench synched the throttle plates, estimated and set idle accordingly
*slide return springs were both cut....replaced with salvage used.
*cleaned slide surfaces....perfect action.
Thanks again to y'all for recommending him to me! Could have had this bike running perfect forever ago! Oh well, now I know who to hit up next time!

Here's a pic of my bike all complete! I need to do a few touch ups to some scuffs, otherwise it's perfect! Also yes, I have a few Miatas too haha

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Old April 3rd, 2021, 01:42 PM   #23
jkv45
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Name: Jay
Location: WI
Join Date: Jul 2013

Motorcycle(s): '06 SV650n, '00 Derbi GPR, '64 CA77 Dream 305, '70 CL450 Scrambler, numerous dirt bikes

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MOTM - Jun '18, Oct '16
Glad to hear all is well now.

It's no surprise to the regulars here that Gordon (ducatiman) did a fantastic job and got the carbs looking and working like new.

He's one of the few peope that really know their way around a set of carbs, and does whatever it takes to get everything right.
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