Hoping this doesn't come under jetting...

I have noticed my MC21 is slow to return to idle when revved. It takes a good five to ten seconds after the throttle is blipped when warm to return to idle. I've heard this could be a symptom of the idle mixture screw being adjusted too lean, is this correct? After I got it tuned by a professional tuner, the dyno sheet he gave me says he went 2.5 turns out on the air screws. This seems a bit far to me. Possibly the cause of the problem?

I have heard the idle screw being mentioned in plural form, as in there are two of them. However, I have only been able to locate one (which is hard to get at) on the right carb. Can anybody explain where the other is hiding?

Thanks.
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MC21 Honda NSR250. TYGA 300 conversion.

Well, I've found the hidden idle screw as I had to remove the airbox anyway. Right smack bang in between the carbs. Bad placement, but as it's the same location on both sides I should have guessed. I'll fiddle with the idle mixture soon, after I put my engine mounts, exhaust mounts and pipes on.

Air leak is possibility but if you are lucky, as I was, it was a matter of the idle being set a bit too high. Should be 1250. If it is higher, try setting it down. Good luck.
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-Jim
'93 MC21

It does idle, just after the throttle is blipped it takes its time as it slowly goes back to idle.

The idle screw is on the left hand side of the carbs when sitting on the bike (on mc18's anyway). I think you have it confused with the carb balance screw. Which can affect idle as well. There have been a few threads on this topic with one of the solutions being the carbs out of balance.

Here is a picture of where the idle screw goes. I have taken it out to use on a different set of carbs, but you can see where it presses on the throttle butterfly, the more you screw it in the more it forces the butterfly's open and vice versa.

I meant air screw, not the idle adjuster.

Ahh sorry! My mistake!

When it is idling, what are the RPM?
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-Jim
'93 MC21

The air screw should be close to 1.5 turns out , if there is not an air leak you may have to go up 1 size on the pirate jet .

Pirate jet?....said with japanese accent??
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it wouldn't be fun if it was easy!

sounds lean for sure take it back to the tuner as it sux paying for a tune thats not done correctly and if he's any good he will be more than happy to fix it for free but of cause make sure to check for an fix all the little things like air leaks first.
did ya get a dyno read out while you were there it may have a air/fuel ratio graph at the bottom also id love to know how much the big bad 300 puts out .

I thought so. I'd prefer not to have to waste time taking it back to him, though he should have done the job properly, so I should make it his problem considering the money I paid...

However, if it can be solved by adjusting the fuel screw while I've got it apart, I'll give that a shot. 2.5 turns out sounds a bit much...

Oh, and it only turned out 65 SAE HP on a dynojet I've since adjusted the power valves (they were out by a fair margin) and am about to fix the exhaust leak. I'm hoping that'll pick up a good 5hp

Double check all the pipes from the solenoids to carbs , check all the wiring connections are in and set the idle as recomended , the airscrew does nothing after 2 turns out . If the problem still exists try bigger pirate jets daniel son . What was done to the carbs , airbox while it was aprt ? Nothing wrong with 65 just bolting on hp !

ha ONLY 65 with the power valves out of adjustment and air leaks thats the second funniest thing i heard tonight "you got to be happy with that"!
by the way funniest thimg i heard tonight was my misses trying to tell me she was watching dancing with the stars so i might only catch the end of the GP .
dam she's a funny girl guess you got to love em for trying!
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"Batmans a scientist"

Yup, only 65 Dynojet horses. Been told they're not real rear wheel horses, but "SAE corrected" or something...

Air leaks, what makes you suppose it is air leaks, rather than just ill tune?

Actually, I did just fix an air leak - one of the hoses from the carb connecting it to the airbox wasn't on properly, perhaps that was doing it? The airbox has had the partition removed and four holes drilled in the top. Carb is stock.

lol at the misses.