Diagnosing a running problem, and setting the carb right

[Robby]

Another instalment in the ongoing saga of one man from London and a CB250RS from Grimsby. For reference, the carb is a keihin slide carb with an accelerator pump. None of that CV nonsense or diagragms to worry about.

Last weekend I fixed my exhaust problem (thanks Pete, I owe you spanner bitch time) and also found that my inlet rubber was split all the way round. Surprised the bike still ran fairly well.
I rebuild the carb with a rebuild kit (jets, needles, seals and clip). This was the carb that had just been ultrasonically cleaned. It didn't work, so I transferred the rebuild parts into the other carb after a good clean. It now runs fairly well.

Fairly well being the key word.

I think it's a bit lean still. When decelerating with the throttle closed it pops and bangs a fair bit, but with the throttle open a little it behaves. This is leading me to the idle mix screw, which is a screw head on the bottom of the idle mix needle at the front of the carb. It's currently set to the factory setting, but the little passeway may be slightly blocked.
The float height may be between 0.5 and 1.0mm low as well.
the third possible problem lies in the float bowl. The float bowl has a brass tube set into it that extends up into the carb body. I'm not sure what it does, but it may be slightly blocked. Going to swap that for the known good one this weekend too.

The immediate question relates to the idle mix screw. I assume that I make the mixture richer by screwing it out (anti-clockwise), which will move open up the hole the needle plugs and let more fuel through - this is assuming it controls fuel and not air, but the hole looks too small to be an air hole.

Other problem is around normal running. If I whack the throttle wide open from low revs, the bike coughs and does nothing much, and will eventually die if I leave the throttle open. I'm assuming from this that either the brass tube/jet pressed into the float bowl comes into play on wide open throttle, or having the float height off by a tiny amount cocks it up.

I should mention that the float bowl fitted is the one that was ultrasonically cleaned, but the body fitted is the one that just worked and hasn't been ultrasonically clean. When I got the bowl back from cleaning there was a lot of fluffy crap in the long brass jet and in the drain screw cavity, there may still be some in the brass jet.

What does the brass jet do, and does having the float height out a little make a huge difference?

[richbike]

this is handy for basic carbs
https://www.motorcyclecarbs.com/carbs101.pdf

I just burnt 3 hours upping my main jet slightly and changing the needle.
then I took it for a run
then I burnt another hour returning it to stock...slightly better low end but crap wide open

have you done the tedious but obligatory plug chop?

[evoboy]

The brass tube is normally to do with the 'start up system/circuit' ( if the RS carb is similar to the one im thinking of!).

On my x7, it disturbs the fuel in the float bowl and feeds the start up circuits in both carbs. ( twin mikuni 26mm if i recall )
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Suzuki GT250 x7------- Fazer 600------CB250RS------Aprilia Rally 70----- Bandit 600

APT Motorcycles

[prawny1]

I find filling a tub with carb cleaner and submerging the carb helps to soften stuburn deposits, wynns carb cleaner is pretty good and comes with a straw spray nozzle.

If you fill up an airtight tub you can keep the cleaner for use again later, I use a metal automotive paint tin with the locking clips refitted to stop it evaporiting, at £5 a tin for decent carb cleaner its a shame to let it just dissapear.

I use paint spray gun cleaning brushes to get into air galleries and a selection of wooden and plastic toothpics to clear jet holes as well as the careful use of drill bits (never to clear metering orifices).

A lot of the time the main nozzle on the carb cleaner can, can be made to screw into jet holes and pilot screw holes letting you get good pressure inside the gallery, this lets you see how well the holes flow and if and any are blocked.

[paddlesat16]

Get yourself a colourtune, this works wonders on getting the mixture correct etc.
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I once saw a bloke shagging a donkey.... in Saudi arabia. Theory Test Passed 26/10/09 Mod 1 Passed 26/4/10, Mod 2 Passed 7/6/10 Current Bikes Suzuki GSX1400 K5, Aprillia Pegaso 650, Suzuki DR350T.

[Odie]

EASY TEST to set up your mixture screws.

If at 4,000rpm; (steady state riding)
lean surging happens you need to richen the mixture (going outward 1/4 turn at a time.

A rich problem gets worse as the bike heats, lean gets better as the bike heats up.

Now to do the test: Here’s how it works (and it will get you dead spot on!)

Set your idle below 1,000 (6-700rpm works good)
1) Blip the throttle quick, lightly. If the idles rises up then drops below the set point you selected (6-700), you are too RICH, you need to go IN with your mixture screws 1/2 turn at a time.

2) If you blip the throttle and the needle rises, but then hangs for a second before dropping to the perfect set point of 6-700rpm you are too LEAN, you need more fuel, go OUT 1/2 turn at at a time with your mixture screws.

Now use your head. If you turn the mixture screws 1/2 turn one-way or the other and the problem goes back and forth from lean to rich, you need to turn less.......try 1/4 turn.

Once done, you should be able to set your idle back to normal, give a blip and have it rise fast, and drop fast right to the perfect spot. Now your mixture screws are set

If you run into problems and it just won't work. Make sure you have not intake leaks, bad exhaust leaks. Intake leaks (carbs to motor) cause the most frustration.
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You are so ugly when you were born the doctor said "I?m gonna drop it, if it falls is a rat, if it flies is a bat."

[Odie]

Also try this:

Follow steps in order....First, dial in:

1. Top end (full throttle / 7.5k to redline - Best Main Jet be selected before starting step 2!
Select Best Main Jet
To get the best, most even top end power (full throttle/after 7500 rpm), select the main jet that produces the highest top speed / pulls hardest at high rpm.
If the bike pulls harder at high rpm when cold and less hard when fully warmed up, the main jet is too large. Install a smaller main jet and retest until you find the main jet that pulls the hardest at high rpm when fully warmed up. This must be done first - before moving on to the other tuning ranges.
If the bike doesn't pull well at high rpm when cold and gets only slightly better when fully warmed up, the main jet is too small.
In order to properly tune the midrange and low rpm carburetion, THE MAIN JET MUST FIRST BE PROPERLY SELECTED after 10 to 15 minutes of hard use!
Do not pay too much attention to the low-end richness when you are changing main jets - you still need to be using the main jets that produce the best power at high rpm. You will deal with the low-end / cruise later - after step 2.

2. Midrange (full throttle /5k-7k)
Select best needle clip position
To get the best power at full throttle / 5k-7k rpm, after you have already selected the best main jet,
If the engine pulls better on a full throttle roll-on starting at <3k, when cool but soft when at full operating temperature, it is too rich in the midrange and the needle should be lowered.
If the engine pulls better when fully warmed up but still not great between 5k-7k, try raising the needle to richen 5k-7k.
If the engine pulls equally well between 5k-7k when cooler as compared to fully warmed up, the needle height is probably properly set.
Do not pay too much attention to the low-end richness when you are changing needle clip positions - you still need to be using the clip position that produces the best full throttle / 5k-7k power in conjunction with the main jets that produce the best power at high rpm. You will deal with the low-end / cruise next.

3. Low end (full throttle / 2k-3k)
Float height (AKA fuel level & how to..)
To get best low-end power, set float height (fuel level) so that the engine will accept full throttle, without missing or stumbling, in 2nd gear from 2.5k to 3k rpm at minimum.
Float heights, unless otherwise specified in the installation guide, are measured from the "gasket surface" of the carb body to the highest part of the top of the float - with the float tang touching but not compressing the float valve spring.
If the engine has a "wet" rhythmic, soggy area at full throttle / 3k-4k rpm, that gets worse as the engine heats up, lower the fuel level by resetting the float height 1mm greater (if the original was 13mm - go to 14mm). This will lower the fuel level, making full throttle / 2k-3k rpm leaner.
If the engine is "dry" and flat between 2k to 3k rpm, raise the fuel level.
Example: change float height from 15mm to 14mm to richen up that area.
REMEMBER, since the main jet WILL affect low speed operation, the MAIN JET has to be within 1 or 2 sizes of correct before final float setting.
Warning: If the engine is left with the fuel level too high,, the engine may foul plugs on the street and will be "soft" and boggy at part throttle operation. Adjust Floats to raise/ lower the Fuel Level.
Base settings are usually given if a particular application has a history of fuel level criticalness. The Fuel level height in the float bowl affects full throttle/low rpm and, also, richness or leanness at cruise/low rpm.
Reference: a bike that runs cleanly at small throttle openings when cold, but starts to show signs of richness as it heats up to full operating temperature, will usually be leaned out enough to be correct if the fuel level is LOWERED 1mm. Check out and RESET all: Suzuki (all), Yamaha (all) and Kawasaki (if low speed problems occur). Needless to say, FUEL LEVEL IS EXTREMELY IMPORTANT!!!
If there are low-end richness problems, even after lowering the fuel level much more than 1.5mm from our initial settings, also check for needle wear and needle jet (part of the emulsion tube). See Worn Needle and Worn Needle Jet diagram. It is VERY common for the brass needle jets (in the top of the "emulsion tube") in 36mm, 38mm and 40mm Mikuni CV carbs to wear out in as little as 5,000 miles. Check them for "oblong" wear - the needle jet orifice starts out round!

4. Idle and low rpm cruise
Fuel Screw setting (AKA mixture screws)
There is usually a machined brass or aluminum cap over the fuel screws on all but newer Honda. It's about the diameter of a pencil. Cap removal details. Newer Honda carbs use a special "D" shaped driver, usually supplied in the carb re-build kit.
Set for smoothest idle and 2nd gear, 4k rpm, steady state cruise operation. Set mixture screws at recommended settings, as a starting point. For smoothest idle, 2nd gear 4000 rpm steady state cruise , and 1/8 throttle high rpm operation.
Pilot fuel mixture screw settings, float level AND pilot jet size are the primary sources of mixture delivery during 4000 rpm steady state cruise operation.
If lean surging is encountered, richen mixture screws (turn out) in 1/2 turn increments. Alternative pilot jets are supplied when normally required.
Pilot fuel mixture screw settings, float level and pilot jet size also affect high-rpm, 0 to 1/8 throttle maneuvers. Too lean, will cause surging problems when the engine is operated at high rpm at small throttle openings! Opening the mixture screws and/or increasing pilot jet size will usually cure the problem.
NOTE: A rich problem gets worse as the engine heats up.
If the throttle is lightly "blipped" at idle, and the rpm drops below the set idle speed, then rises up to the set idle speed, the low speed mixture screws are probably set too rich: try 1/2 turn in, to lean the idle mixture.
NOTE: A lean problem gets better as the engine heats up.
If the throttle is lightly "blipped" at idle, and the rpm "hangs up" before dropping to the set idle speed, and there are no intake leaks and the idle speed is set at less than 1000 rpm, the mixture screws are probably too lean: try 1/2 turn out, to richen mixture. Be sure there are no intake leaks and the idle speed is set at less than 1000 rpm
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You are so ugly when you were born the doctor said "I?m gonna drop it, if it falls is a rat, if it flies is a bat."

[Robby]

Sorted. Getting the float height spot on, fitting the good float bowl and giving the idle mix screw an extra half turn out has hit the sport. Lots of carb cleaner sprayed into everything using the nozzle tube too. I tend to find that carb cleaner itself is pretty crap, but if you can spray it into something with the nozzle tube then it does some good.

[Mudskipper]

Posting purely so I can find this thread when I take my carb apart.
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CBR125|||GSXR400|||CBR400|||CBR400|||CB250RS|||GSXR750|||CB250RS
"You're clumsy, you eat too much and you behave like a 12 year old boy. But you know what? Every once in a while, you find a thumb."