I've been working on my first bobber build, an 86 Sportster 1100 for the last year and a half, photo attached.

The last few months it seems that everything that could cause a problem has! I've overcome them all, except one, which is stopping me getting the bike all legal tegel and on the road !!

When I give it a rev it's takes maybe 10 to 15 seconds to return to idle, sometimes longer and sometimes quicker.

Also seems to backfire through the carb every now and then.

It's has a Mikuni carb stamped with – TO 3 1 - in one spot and just below stamped T40 6P.

I though it might be an air leak, so I've replaced the seals between the intake manifold and head, also the flange between the carb and intake. Any idea's on how to further eliminate the problem as an air leak?

Also checked the ignition timing as per the Clymer manual that I have. Do you think it could be a timing problem? I've tried my best to get the timing mark into the centre of the inspection hole. It never seems to be consistent though, when the strobe light flashes the timing mark is never in exactly the same spot, jumps around a little bit. Is that normal?
 

Nice ride. It's going to take a lot of un-modding to get it on the road though. Id be looking at what KiwiDave said - the fact that it doesn't happen all the time makes me think frayed cable though.

Yep I've checked the obvious stuff over and over again. Been trying to solve this problem for a few weeks now, think it might be over my head.

Kiwidave - Do you know anyone in Auckland that knows their shit about Mikuni carbs?

I had a similar thing happen when i changed my bars recently. I have a 2001 softail but the bars i fitted also fit modern bikes with fly by wire and thus have the end of the bars cut to fit this (assuming it was for this anyway). Turns out that when the cuts were made the ends of the bar was slightly out of shape (slightly higher at end)

Anyway short story. filed the end of the bars so all straight (didnt have to take much off) and now works a treat

So is the problem that when you rev it up the handlebar throttle dies not return to the zero point straight away? First thing I would check is to remove the cables at the carby end and make 100% sure that there is no binding at all from the throttle perspective.... 9 times out of 10 when I have this problem it's the adjuster nuts at the handlebar end that need tweaking.