Engine stop on sudden acceleration

Hello all,

i need your help because i don't find the problem after a frech rebuilt on my carburator vm26ss on my kz1000 ltb 78 b2.
Carb is on specs and i have a correct idle but the engine stop if i try to do a sharp acceleration.

Do you have an idea please ?

Best,
Fox


 

If I understand correctly what your describing it pretty normal. From a normal idle speed if you quickly twist the throttle your likely to stall the engine. Reason is you creating an extremely lean condition, a large volume of air is pulled into the engine and there isn’t time for fuel to be pulled in with it. 

Is the airbox installed?

Hello,

i rebuilt the carbs because of eratic idle with those specs :

main jet : 115
pilot jet : 17.5

and now :

main jet : 105
pilot jet : 15

i sync the carbs with the solid wire 0.7mm (engine side of the carbs)

I din't have this trottle problem before and i don't know if it's normal, if it's a carbs problem, or a cables problem.

The bike is on specs with 2 on 4 and original air box installed.

wrote:

Hello,

i rebuilt the carbs because of eratic idle with those specs :

main jet : 115
pilot jet : 17.5

and now :

main jet : 105
pilot jet : 15

i sync the carbs with the solid wire 0.7mm (engine side of the carbs)

I din't have this trottle problem before and i don't know if it's normal, if it's a carbs problem, or a cables problem.

The bike is on specs with 4 on 2 and original air box installed.
 

It may help to go back to your slightly larger jets, I'm seconding that it sounds like a result of being too lean - may help to roll on the throttle more smoothly.

Also, vacuum-sycing your carburetors may help somewhat - matching the butterfly valves with wire is a good start but there's variation between cylinders.

Best I can discover is you are correct in the jet size. The A model with the VM26SS had 107.5 mains and 17.5 pilots. The B1 supplement say the VM26SS had 105 mains and 15 pilots. The B2 supplement doesn’t mention jets size. I assume That means no change over the previous model. 

The .07 wire bench sync is only to get the bike to start. You will need to vacuum sync them. Its the only way to get even air and fuel into each cylinder. The carbs are then Balanced. Don’t expect the stock engine to rev with the quick snap off idle of the throttle. If you were talking a built engine that was designed for performance low to mid range then I’d expect better. Emissions were really eating performance during the late 70’s. You should be able to crack the throttle off idle with a usual twist speed and have the engine respond. 

Other than that how is the rideability, if it accelerates hard from low rpm, and pulls strong through each gear then I’d say your good. Have you set valve lash, set points dwell, timing. Carb float levels. Do you have a good fully charged battery. Is primary voltage to the coils within 1 volt or so of battery. Lots of thing need to be correct for the bike to respond at its best. You can’t blame performance on one discipline, unless you know all are in spec. 

If you have an erratic idle problem it may be due to air entering through the intake manifold boots.  Try spraying some carburetor cleaner on each one and listen for a change in rpm.  If you hear the rpm rise it means that carb boot is not properly sealed.

You also may have leakage through your choke plungers.  

Your former carburetor setup was identical to the 1977 version of those carbs, except the needle jet in 1977 was one size larger.  I would go back to the 115 mains and 17.5 pilots because you had no throttle problem with them.  

Do your carbs have an accelerator pump underneath #2 carb??

I see better now.

Ok... i will go back to older specs tomorrow. I had Better performances before.
Then i will check the manifold boots.

The only other problem that have the bike is i can't really more start with the kickstarter. (or after 15 times)
3 years ago it started perfectly but now I start with the electric starter...
I will go for new coils i think.

Before changing your coils, test them for correct resistance.  And check your battery to see if it has the correct voltage.

Have you changed the jet needle position?  I should modify my previous comment slightly.  The needle jet was used in the 1976 version of those carburetors with a needle jet that was one size larger.  You could probably get the same effect from your set by raising the needles to the #2 notch from the top.

Do your carbs have an accelerator pump under #2 carb?

Hello,

do you mean the needle jet is one size bigger ?
because the jet needle is the same one than i have.

No pump on those carbs.

Are the valve clearances properly set?  If the valves are tight the bike will struggle to start