Topic: Almost zero vacuum and it runs-kind of. ????

[Magpie]

For a 1978 CB750F. I'm using a Carbtune to balance the carbs and it is showing no vacuum at idle and very litle at higher rpms. I have the original carb manifolds and they are not rock hard. I have installed the correct way I believe. The carbs have been gone through. The engine will start with full choke but dies when I give it a bit of throttle. I can very gently increase the revs and it will keep running. I'm thinking new manifolds and accelerator pump diaghram piece. Any other hints you may have? Thanks, Cliff.

[Don R]

  I'd guess the pilot jets aren't feeding any fuel.

[BenelliSEI]

Have you done a compression and leakdown tests? I once had this issue and it was just the motor was done and actually making no “suction”......

Incorrectly adjusted valve clearances can cause the same symptom.

[Magpie]

I have adjusted the tappets to spec and the compression is just under 150 psi. I'm soaking the pilots in Honda Combustion Cleaner and will be sure they are ultra clean before installing them. I made sure the accelerator pump is working fine and the diaphram is in good shape. So, I'll reassemble everything and install the carbs and see what happens. One thing I did find when I put the carbs on my workbench was this spring. It mustof got dislodged somehow and luckily stayed with them and fell out when I put the carbs down. It appears to be part of the accelerator pump system. It will be interesting to see what it does when I put the carbs back on the bike. Thanks for all your comments.