I have a 94 S10 with a 355 and a 650 holley. I've had to do a few things to it recently because of letting the truck sit for so long without running. I seem to have it running fairly decent (idles at 600 rpm) but noticed when tuning the carb its only pulling a maximum of 14 to 15.5 inches of vacuum at 1000 rpm. I was told its suppose to be around 20. I have plugged all vacuum ports at the carb and still get the same reading. Should the engine be pulling aroung 20 inches and if so what else could I check?

thanks for any help.

 

Ignition timing

 

thxs. will check the timing. you would agree then that 20 in @ 1000rpms is what I should aim for?

 

The main thing you should look for is that it is steady. But also listen to Mdtahoe and tell us what your timing is set at.

 

checked the timing: it is currently at 7 degrees advance

 

Does the vac go up as you turn the distributor. If the balancer is spun the timing light would read 7 but it would not truly be 7. Lean fuel mixture can also cause low vac. Jumped timing chain would do it also. Along with restricted exhaust.

 

vacuum, as well as rpms, goes up when turning the distributor counter clockwise. I assumed the higher rpm was causing the higher vacuum numbers and thus was not indicative of finding a fix. engine exaust is open headers so there should not be a restriction problem. The motor was built by a local machine shop approx. 6 years ago and sense that time it has done a lot of sitting still. probably have a maximum of 200 miles on it sense built. I did notice today while checking the timing that while at 1000 rpms. the timing advance is the same whether the vacuum line to distributor is hooked up or not. but with the hose hooked up, as soon as I blip the throttle the advance increases 15 to 20 degrees or so, cant be exact as the mark leaves the little timing indicator plate at the balancer.

 

Turn the distributor untill the vac is as close to 20 as you can get it, then see how it runs. If it does not ping and has plenty of power leave it there.

 

thanks for all the help guys. I'm starting to think that it is either a vacuum leak (somewhere south of the carb) or my vacuum advance not operating properly. from 2600 rpms on up the engine is really flat. ( it feels like my dually diesel on the interstate when I'm trying to pass somebody)