BS34 cold-starts with choke-ON, 10 min later DIES

BRAND-spanking-newly-refurbished BS34s on my 79 KZ1000 ST Shaft. (sorry I wanted to take audio of it and post, but I was so tired from a week of dealing with a previous head ache I just put on my old vm28 pumpers so I could at least ride).

BS34 setup: 140 main jets and 42.5 pilot jets is what I'm told carb-guy put in.
MODS: Slash-cut 4-2 stock pipes (meaning no baffle, just cut near rear-footrest)
& Pod filters
ALTITUDE: Albuquerque, NM (elevation 5312 ft)
SITUATION: Bought carbs by-mail. Don't want to send them back and wait for work.

-> When I started up with full-choke, engine ran pretty stable (no backfiring, seemed to be stable without any misfires or pops etc). Actually sounded VERY strong and steady, with even, solid drumbeats.
-> After about 10 minutes, maybe 15 but no later, it died. (stopped).
-> Turned over fine, but would NOT start at all afterward.
-> Gave up, next day came back when engine was completely cold, tried to start - Same as above, until about 10 or 15 min.

I attempted to keep the engine on by holding the accelerator handle long enough to adjust the idle screw (the big center bolt which actuates the mechanism shared by the throttle cable), - I thought it was working, but it would die eventually, unless it went high enough to be in the 3K or higher range. Of course that freaked me out, so I immediately killed it.

After a good 'walk-away,' I found on one of these forums, after reading probably 100+ articles on carbs, :



I took notice of it because the author cites different aspects of a carburetor working at different RPMs. This is when I sort of had a 'brain-light' turn-on. I thought, 'Well, since it was staying on at higher RPMs, that means the Main Jet, Jet Needle etc, are all fine, or at least okay-for-now. What I have to look at is the Pilot Air Screw and Jet.'

1) Is that a correct evaluation?

Of course I also realize that Airleaks must be sorted-out. I DID NOT do an air-leak test, and I admit, I let my frustration get the best of me, and just took them off to attack it later.

If correct, what should I do first? Adjust the screws? What do I do with those screws? I did the 'Check rotations-in' & I wrote those down: They were all turned in 2 turns, so I marked on the eyelets where they were and put them back in their location-of-turns-back-out.

My carbs and their mixture screws are:


I haven't been able to find any thread which actually explains specifically HOW these screws function - Does tightening them INCREASE or DECREASE fuel?

I'm thinking that the engine running WITH choke, implies that it needs MORE gas, ie, more 'gas' in the mixture-screw's fuel/air ratio, and if that doesn't solve the problem, I'm going to need a larger pilot jet. Okay I'm done yapping. Anyone who can tell me what to try that I could possibly try now, is greatly, greatly appreciated.

mikoarts wrote: ...started up with full-choke, engine ran pretty stable (no backfiring, seemed to be stable without any misfires or pops etc). Actually sounded VERY strong and steady, with even, solid drumbeats.
-> After about 10 minutes, maybe 15 but no later, it died. (stopped).
-> Turned over fine, but would NOT start at all afterward.
-> Gave up, next day came back when engine was completely cold, tried to start - Same as above, until about 10 or 15 min....

Might be caused by too tight valve clearances.
Valves may marginally seal on a cold engine and then begin leaking as the engine warms.

Good Fortune!

Your lo-end is WAY too rich.