Pilot jet sizing - how can I tell?

[yeadon_m]

Folks,

Posted on a UK forum also, but good to be open to widest range of inputs.

I'm almost embarrassed to have to ask, as I 'should' know VM carbs by now, but....

On my GT550B, I have 30 pilots fitted, as thats what was in them when I got it (also in the 550A I sold, both have / had Nikasil / Euro cylinders). Reiner says thats correct according to the stampings on the carbs. Pilot screws 1 1/2 turns out per the manual.

I'm just not sure if my pilots are right-sized. The web indicates a range from 25, through 30 and some users even go to 32.5.

Symptoms:
-starts cold dead easy, and also when hot (no throttle required at all).
-idles well, and does not 'hang up' after throttle blipping, nor dip too far on RPM. All sounds OK, no?
-However, off-idle on the road I feel it distinctly lacks power, but if I crack the throttle a little more so I'm onto the NJs/JNs and a bit onto mains, it fair surges away (as far as running in RPM limits allow).
-and, if I turn the air screws IN it keeps running all the way down to....fully closed. It really shouldn't. I can stop the 380 like this, and also the 550A. My notes are unclear on this bike before rebuilding, drat!
-if I open the air screws it improves to about 1.5 turns (sounds great doesn't it) but if I go to 1 3/4 and certainly 2...it stops stone dead. It really shouldn't.

But I cannot work out what this is telling me. If PJs were too big I should be able to wind OUT the air screws until they fall out and it should just keep running faster and faster. If they were too small it wouldn't run well at stock settings of air screw. I think at any size I should be able to stop it by closing them completely, but it ignores me. I have some 32.5 pilots so I might try these empirically but would love any advice / deductions.

I will be having the carbs out anyway as the LH cylinder idles a bit cooler and while it may be carb balance (or ignition - will check through again) I want to double check pilot circuits once more.

Cheers,
Mike

[tz375]

Every engine is a little bit different, but the symptoms you describe suggest that the pilot jets are slightly small. Try 32.5 and even 35.0 to be sure, but I suspect that 32.5 will do the job. The other possibility is that it needs smaller slide cutaways, but pilot jets are cheaper, so try them first.