Periods of Intermittent Missing

[GTP1960]

I'd suggest first removing the dome & piston, to check that the piston moves absolutely freely in the dome. You shouldn't even hear a rubbing noise. Also pull the jet out and check that it is clear inside. It's possible (tho unlikely) that there is a bit of FOD (foreign object or debris) in the bottom of the jet that sometimes jams the needle. Then center the jet as Charley suggests. In my experience, jet centering problems are consistent, not "sometimes".

But it's definitely not right and worth attending to before investigating anything else.

If you do find that the piston rubs on the dome, try swapping pistons between domes. They are supposed to be a matched pair, but sometimes people get them mixed up. I believe there is a Charlie Twist video on the University Motors site that addresses this, but don't have the link handy. He also shows how to do the "drop test", to check how well matched they are.

BTW, the jet centering needs to be "right on the money". If it is just slightly off, the needle and jet will wear from rubbing on each other, which messes up the mixture profile (making it go rich at idle) even after you correct the centering. Since the mixture is adjusted at idle, but the adjustment affects the entire range, you can wind up with the mixture going too lean under cruise conditions. Which in turn can cause poor performance and overheating.

Also, I've discovered that on my carbs, there is enough play in the connection of the dome to the body to affect the jet centering. To make the dome attachment repeatable, I install the 3 screws loosely, then gently turn the dome clockwise while snugging the screws down. This might not apply to all carbs, but certainly won't hurt.

PS, to check the centering, lift the piston a bit and let it fall. It should land with a clearly audible 'click'. If not, the centering is probably off (although it could be binding as above). I prefer to do this test with the jet in the full up position (mixture nut full lean) to help ensure a bit of clearance with the jet in the normal position. But, it makes the centering adjustment really finicky.

PPS, the "SU tool kit" includes a centering tool that is junk in my opinion. It centers the jet to the body, not the needle. If the hole in the body could be made perfectly centered to the needle, then there would be no point in providing an adjustment! But some people seem to like the tool, so maybe their carb bodies just happen to have the hole in the right place.