Bs34 cold start

I have Mikuni bs34 carbs on my kz750. Carbs are very well cleaned, pod filters, 140 mainjet and 40 pilot jet, pilot screw 2 1/2 turns out, fuel level is adjusted 3mm from carb body. But, bike wont start. It started once when i adjusted fuel levels very high, but it run rich. I dont twist throttle when I start, so there should be more vacuum for enrichmen circuit. After I tried to start, I removed carbs and noticed that fuel wont rise up from float bowl. I have checked that small brass tube which feeds fuel for enrichment circuit, and it is clean. What should I check next. Is it possible that those enrichmnet plungers leaks and makes vacuum weaker?

When you pulled your carbs apart for cleaning, did the choke parts look to be in good shape?
Did the bike start okay before?


Also -- are you running the stock airbox or pods? If you run pods, it would be easy to pull one or more off for a quick starter-fluid test, such as trying to get it started by spraying starter fluid into the back of the carbs?

Other things that will cause hard starting include valves being out of adjustment.
How many miles are on the bike? If the mileage is high, doing a valve clearance check will eliminate a possible contributor.

The starter fluid sprayed into the carbs will tell a tale. If you can get her going with starter fluid and she stays running, it could well mean your choke circuit is ineffective.


Even though you can't get it warmed up to do a compression test, you can do a 'cold' compression test and get an idea of the valves and the cylinder wear.

Never used those carbs before, there is pod filters, and top end is recently rebuilt, year ago i installed 810 wiseco pistons and week ago gpz 750 head, valves are checked works properly. Because mikunis wont work, I have to use now my keihin cv34 carbs, pods and dynojet kit. Bike is really fast, but I want to go faster, thats why i want bs34 mikunis. I tried starter fluid, and engine tried to start. Mikuni choke parts are mystery for me. What should i check?

By the way, when i go those carbs, the rod which pulls those four enrichment plungers was slightly bent. I straightened it. Could this be something which makes enrichment circuit work improperly?

If you can't get her going with starter fluid, the chokes might be an issue, but shoot -- it should start with starter fluid -- all the enrichener does is get you over the transition from cold to running -- that's also what the starter fluid does -- so if starter fluid doesn't get her going, sure the choke might have a problem but it sounds like there is something else going on there.

checked cold compression it is 140-150 psi. How much pilot jets affects starting? I mentioned that there is bleed type pilots, and there should be non bleed type. Pilot circuit picks fuel straight from float bowl.

vonbagh wrote: I have Mikuni bs34 carbs on my kz750. Carbs are very well cleaned, pod filters, 140 mainjet and 40 pilot jet, pilot screw 2 1/2 turns out, fuel level is adjusted 3mm from carb body. But, bike wont start. It started once when i adjusted fuel levels very high, but it run rich. I dont twist throttle when I start, so there should be more vacuum for enrichmen circuit. After I tried to start, I removed carbs and noticed that fuel wont rise up from float bowl. I have checked that small brass tube which feeds fuel for enrichment circuit, and it is clean. What should I check next. Is it possible that those enrichmnet plungers leaks and makes vacuum weaker?

How is the corresponding hole at the float bowl, for the starter circuit? The little brass tube goes into a bore in the bowl, there is an orifice at the bottom, and it must be clear to get fuel.
Are those BS34 carbs from a Kawasaki? Or something else? Do you know what model they are from?

Float bowl starter circuit hole is clean and open. I raised float levels little bit and bike started but very rich again. I tried to adjust pilot screws but it didnt help. I put keihins back and those works very fine. Maybe i should forget those mikunis and start looking round slide carbs. Dont know. Any ideas what carbs would work on my bike. I want neck breaking velocity :evil:

Those mikunis are from gpz750 1984 i think.

If the bike needs more fuel, than give it more fuel.

Turn those mixture screws out till she starts with full choke and increase the idle screw a couple turns.

If it still needs more fuel, then change the pilot up to a 42.5, but I doubt it will.

Do it.

Bike gets too much fuel already. Turning mixture screw leaner did not help. black sMoke from exhaust when revving, gas smell and black, sooty spark plugs => too rich. Carbs were synced quite well, but i mentioned that idle was not steady, revs went up and down. Maybe those choke plungers leaks and let too much fuel in?

YES! I got those mikunis working. I changed fuel bowl venting hoses and put 37,5 pilot jets. Now its running fine. There was first intake popping, but when I adjusted mixture screws popping was gone. Popping never came back, no matter how I adjusted mixture screw. Why that intake popping? Could it be too low operating temperature? Starting was bit harder with those smaller pilot jets.
Those carbs seems to hate me.

Why that intake popping?

To get this thread done...

It usually happens when there's a lean condition (ex: vacuum leak, lean adjustment) and a cold engine.
Problem usually gets away when engine reach optimal running temperature.

Or bad engine timing could cause this (Ex: spark plugs firing when intake valves are not closed yet!)

Hope this help!

Best regards,
laurent