Hitchcocks Forum

HI all, I've just bought a 2006 bullet, its fitted with a mikuni 37mm carb, has anyone advise as to what the float hight should be. The bike was running perfect, it stopped yesterday and now I cannot get it started, I will do comp test tonight, i have a good spark, fuel to the carb but the spark plug remains dry, have I missed something. Kind regards, Andy.

In my experience, float heights don't alter themselves. One of the last things I'd suspect. That said, easy to check on a mikuni with a bit of clear hose on the drain stub. Not sure of the exact height it's supposed to be but usually somewhere int he vicinity of the flange height.

If there is fuel getting to the carb, check the main jet. Also check there really is sufficient fuel getting to the carb by cracking open the drain screw. Make sure the throttle needle hasn't come off the circlip.

Also check for cracks/splits in the inlet manifold rubber.

Have done all of that, its a new carb that came on the bike, bloke I bought it off told me he fitted it but could not get it to run reliably, I've fitted some new jets, what I don't know is if anyones changed the float hight, thats the reason for the post, kind regards.

Buck wrote: ↑

Thu Mar 26, 2020 4:09 pm

The bike was running perfect, it stopped yesterday and now I cannot get it started,

So not float height. Unless someone sneaked in and bent the tang on your floats when you weren't looking, the problem is elsewhere.

Dry plugs imply a lack of fuel or a lack of vacuum. Floats don't adjust themselves.

Are you sure it's a 37mm carb that seems wrong
I believe the 500 was fitted with a 28mm carb
Unless you have done a massive amount of tuning work I wouldn't have thought a 500 would pull enough air through to get fuel up the jets

Take care all Dai

yes its 37 mm, ive done no work to the bike its how I bought it, looking through paperwork that came with the bike work has been done, comp test reading is 52 psi, cylinder head holds petrol which makes me think valve seats are ok, piston doesn't slap around and the bore looks good, have I missed something. Andy.

37mm does seem may too big but you say it was running OK?
I wouldn't think a normal 500 would produce enough vacuum for a 37mm
Something odd going on there.

Compression test at 52 is low
Try a wet test see what it reads then
If it's higher you have a piston bore or ring problem
If it stays the same you have a valve or head problem

Pete Snidal manual states 110 for compression test give or take 5psi

Hi, check the decompressor is sticking open. Vince

You say that it was running perfectly so it is highly unlikely to be the float height, as suggested it is not going to adjust itself unless something has broken.

Sometimes spark plugs just give up for no aparrent reason, so it si worthwhile checking over the ign system and trying a new plug.

Have you checked valve clearances? - if the adjusters were not tigthtened propoerley they can adjust themselves and cause running problems.

It does seem odd that one minute it is running perfectly and then stops, have you checked fuel supply and jets for any blockages?