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Old July 20th, 2021, 11:00 AM   #1
Sandr
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Symptoms of a bad Fuel screw Adjustment

I was wondering if anyone could explain the symptoms of a bad Fuel Screw Adjustment, How would the bike run at different RPM's if they are too tight, or too loose.
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Old July 20th, 2021, 12:16 PM   #2
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The mixture screw adjusts idle mixture (closed throttle) only. Starting and idling are affected by the adjustment of these two screws of the two carburetors. They should be adjusted for fastest idle speed, plus maybe a tiny bit richer (more counterclockwise).
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Old July 20th, 2021, 01:17 PM   #3
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I'm personally curious about why the question ❓ us the OP having issues?
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Old July 20th, 2021, 01:28 PM   #4
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I'm personally curious about why the question ❓ us the OP having issues?
Trying to understand my bike more i suppose. After a war with my carburator i finally got my bike running again. The bike is a little jumpy at low speeds, but on Highway speed it runs perfectly. i suspect a minor tuning is in order.

hard to describe but say around 20-30 mph, or say 15-20% throttle the bike doesnt hold a consistant speed. almost like its pulsing . Before swaping out a hose on the bike wich finally led to the fix, i was playing around with my mix screws, and didnt know if a bad mix could lead to these problems. Also to note the bike runs better on choke then without making me suspect im too lean or rich
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Old July 20th, 2021, 02:25 PM   #5
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That's into the needle-needlejet circuit. Mine was exactly like that when I got it, with surging at steady running speeds. I shimmed the needles something like 1/2 a millimeter and it cured it. When Ducatiman restored my carbs to new condition he asked if I wanted to keep the shims and I said no. When I got them back and installed them, EVERYTHING about the carbs and fueling was perfect and no shims were needed. I assume there was some varnish or other obstruction in the needle jet circuit that he removed.
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Old July 21st, 2021, 07:24 AM   #6
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Start with your Idle Mixture Screws about 2.5 turns out. That should get you close if everything else is right.

Set it precisely after a ride, when the engine is at operating temp.

The adjustment can change, depending on air temp and fuel type (% of ethanol).

As TJ said - highest RPMs + about 1/8 richer (out) is my method. Reset the idle speed to about 1300 when you are done.

Also sync the carbs if you haven't, then redo the mixture setting.

Lots of info in this section -https://faq.ninja250.org/wiki/Intake
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Old July 21st, 2021, 09:02 AM   #7
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thanks for the heads up guys! ill give it a go ill play with them some more! I'll look to send em his way towards the end of the season so i can take advantage of the gas mileage while i still can. My truck is killing me with like 12 MPG lolol
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Old July 21st, 2021, 09:06 AM   #8
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Quote:
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I'll look to send em his way towards the end of the season so i can take advantage of the gas mileage while i still can.
He turns them around very quickly. I don't think it was a week between when I took mine out and when I was putting them back in.
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Old July 21st, 2021, 09:35 AM   #9
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thanks for the heads up guys! ill give it a go ill play with them some more! I'll look to send em his way towards the end of the season so i can take advantage of the gas mileage while i still can. My truck is killing me with like 12 MPG lolol
Sending them to ducatiman will end any concern about carb issues for you.
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Old July 21st, 2021, 10:14 AM   #10
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Thanks guys! i just reached out will be doing this
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Old August 24th, 2021, 07:42 PM   #11
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I've noticed a slight surging on my Ninja as well, it has a after market muffler
on it but everything but the gearing in the rear is original....
those air screws are next to impossible to adjust... so I suspect them not being perfect..... but that wouldn't effect the surging in and about 4 grand... and that is what I am experiencing. I'm wondering if Jims shimming up the jet needles would cure this ?
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Old August 24th, 2021, 08:20 PM   #12
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I'm wondering if Jims shimming up the jet needles would cure this ?
I shimmed them a little to put a band aid on the problem. When Ducatiman rebuilt the carbs I asked him to leave out the shims and it ran perfectly without them. I mean perfectly.
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Old August 24th, 2021, 11:03 PM   #13
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understood ! I'm thinking my carb cleaning wasn't as good as it could have been now...
I have to take the right side fairing off to replace the crushed turn signal I did today... so I might go ahead and pull the other side and pull the carbs too....
.....I doubt it though I got alot of yard stuff I gott'a do coming tomorrow
I have a pallet load of 2' pavers to put in as our patio, so I'll be busy with that
... the bike can wait. ( it's a toy after all ! LOL)
I did buy a rear lowering kit for the thing but it won't be here for a month or two.... it's fully adjustable so I can set it at the height I want.
that was one of the reasons I dropped the bike today... it's too tall for my short legs ! LOL.... that and not having been on a bike in such a long time... but I'll never admit that ! LOL
at least the scratches aren't real deep ! LOL
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Old August 25th, 2021, 06:06 AM   #14
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Quote:
Originally Posted by Triple Jim View Post
I shimmed them a little to put a band aid on the problem. When Ducatiman rebuilt the carbs I asked him to leave out the shims and it ran perfectly without them. I mean perfectly.
People usually go way overboard with the shims, and wonder why it runs worse.
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Old August 25th, 2021, 06:28 AM   #15
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indeed... I've never shimmed a needle yet in all my years simply because that would mess up the 14:1 fuel to air ratio that the carb was designed to deliver
...but if it's not quite right maybe a band-aid would work ...LOL
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Old August 25th, 2021, 08:06 AM   #16
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Yeah, band-aids don't really "fix" actual problems, just cover up symptoms.

Due to increased use of ethanol petrol thanks to corn-lobby, there's more issues with corrosion in carbs. In addition to previous dried petrol deposits. Spray carb cleaners no longer work due to removal of chlorinated compounds.

These bikes ran perfectly when new leaving showroom floor. There's absolutely zero reason they won't run like that again if carbs are factory-fresh clean. In order to restore carbs to that condition, you'll need to do:

- complete disassembly to every last nut, bolt, O-ring and individual components
- scrub all parts by hand with various solvents (PEA-based ones work best)
- scrub petrol passages from tank to intake-valves with brushes of matching sizes
- ultrasonic soak
- micro soda-blast
- replace all rubbers, float-valves, float-bowl seals, fuel-rail O-rings, slide-diaphragms if needed
- re-assemble

If you don't have all this equipment and don't want to buy them all to restore one set of carbs to factory-fresh clean, then you won't get bike that runs like brand-new. Easier to send them to someone who has this equipment and experience to do restoration work to factory-fresh clean condition...
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Old August 25th, 2021, 09:39 AM   #17
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agreed there is no substitute for (nor compensating for) *truly* clean, well set up carbs. Imperative starting point. Ditto petcock.
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Old August 25th, 2021, 11:25 AM   #18
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Quote:
Originally Posted by DannoXYZ View Post
(SNIP)

These bikes ran perfectly when new leaving showroom floor. There's absolutely zero reason they won't run like that again if carbs are factory-fresh clean. In order to restore carbs to that condition, you'll need to do:

- complete disassembly to every last nut, bolt, O-ring and individual components
- scrub all parts by hand with various solvents (PEA-based ones work best)
- scrub petrol passages from tank to intake-valves with brushes of matching sizes
- ultrasonic soak
- micro soda-blast
- replace all rubbers, float-valves, float-bowl seals, fuel-rail O-rings, slide-diaphragms if needed
- re-assemble

If you don't have all this equipment and don't want to buy them all to restore one set of carbs to factory-fresh clean, then you won't get bike that runs like brand-new. Easier to send them to someone who has this equipment and experience to do restoration work to factory-fresh clean condition...
Most people (shops too) don't have the skills or tools to do a proper carb cleaning and rebuild.

Luckily, we have ducatiman...
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Old August 29th, 2021, 05:30 PM   #19
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to answer the original question, symptoms of a bad fuel adjustment (screw)
I would have to say this, most times it is difficult to tell if their out of adjustment with one another, But in extreme cases, it's fairly easy... you can hear it as it idles
in the case of my 250R it was very apparent as it would not idle on both cylinders when the engine was cold... In most cases though you can hear it as one cylinder will be heavier than the other, the boom is a bit louder or stronger than the next boom.... in most cases this is all you have to go by until you put the wrenches to it.
keep in mind the fuel air screws do Not effect the running of the engine above an idle
if it's idling at 800RPM the most it will effect the running is about 1200 rpm
lean Fuel air screw adjustment will result in a Sogg when cracking the throttle open.
if they are too rich in adjustment it will smoke black smoke at idle. which is bad as it will foul the spark plugs quickly.... You are supposed to adjust the air fuel screws when the engine is warm and ready to go... not cold !
and you adjust then to the fastest idle you can get by continuously dropping the idle speed ( knob on the side) to as slow an idle as you can get, say 500 RPM ... no lower than that. if you adjust the left cylinder to max RPM then you need to drop the idle to 500~800 RPM and adjust the right cylinder the same way, once your sure you have the maximum RPM on each air screw then you should turn them out about 1/8th turn each More... this will set the Fuel/Air adjustment just right for Idle. it should take any throttle change with out stumbling , it will grab the RPM's fast and without hesitation if it's done right....
this of course assumes the carbs are clean and in good working order, if the bike has been setting all winter with gas in it and it doesn't want to idle like it did before you parked it chances are real good you have to clean the carburetors to get it working properly again... using a gasoline stabilizer is a real good idea in the winter but better yet I like to shut off the fuel and run the bike till it is out of fuel in the carburetors as well... but with the vacuum petcock that is impossible to do...you can't shut off the fuel if you wanted to. only by pulling the vacuum line to the petcock and plugging it
can you actually turn off the gas... but if your vacuum diaphragm leaks gas to the carbs like mine does anyway, that isn't going to help at all.
Up to this date I have always just started every motor on the property at least every 4 to 6 weeks during the winter, I also like to add a little chainsaw gas ( gasoline and 2 cycle oil mix) to the engine's gas tank to help stabilize the fuel, this has never failed me
and I've not had to clean the carbs on anything in the 5 years I've been here. but I do start the motors about every month and run them till their good and warm then turn them off. this is what I do... I don't buy fuel stabilizers, I just add chainsaw gas to the tank.... if your worried about a catalytic converter perhaps you shouldn't do that !!!!
because chainsaw gas and catalytic converters don't mix well, it'll plug them up fast !
....
But the Fuel Air screws under the carb are for Idle Only, they don't adjust the speed of the idle, they adjust the Fuel / Air mixture, the speed is adjusted by changing the butter fly stop screw ( the knob on the Ninja.)
....
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Old August 29th, 2021, 06:52 PM   #20
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Quote:
Originally Posted by Bob KellyIII View Post
...
....
But the Fuel Air screws under the carb are for Idle Only, they don't adjust the speed of the idle, they adjust the Fuel / Air mixture, the speed is adjusted by changing the butter fly stop screw ( the knob on the Ninja.)
....
Bob...
Yup right on! You'll want to set each screw for fastest idle-speed with each cylinder on hot engine (then +1/16-1/8). Then re-adjust idle-knob afterwards for factory idle-speed. Done!

Any other issues after that in off-idle transition or mid-range is due to insufficiently cleaned secret hidden passages in carbs. Jets are just first 5% of carb-cleaning, there's plenty of passages that flow petrol before and after jets that needs attention as well.

These nifty little suckers really help!

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