Rebuilt 327 (275 hp) stock ,q-jet and manifold ,has 4" of vacuum.Timing is 10*btc, compression is 140 to 150 psi.There is no sheet metal baffle under the carb, could this be the problem?

missing the metal baffle should not be your problem, sure sounds like a vacuum leak for sure, check the carb mounting area, vacuum hoses to pwr brakes, intake gaskets, was the block decked during the rebuild? was the intake angle milled if the block was decked? did you actually verify TDC to insure the timing mark on the harmonic balancer ring did not slip?

George,As already mentioned the metal baffle under the carb may cause the low vacuum so listen & check for that.

Where are you checking the idle vacuum,it must be checked from a full on vacuum source like where the power brake booster or vac line from auto trans hooks to the intake. If you hook it to a ported port off the carb it may only allow partial vacuum reading at idle so keep this in mind too.

Also,something thats often overlooked is even though your running a basically stock motor if your running a very mild aftermarket perf cam(Not a stock from GM Cam) 10 deg initital timing is very retarded and can very well cause your problem.

Most mild (with smooth stock type idle)to fairly hot perf cams require a min of 16-18 deg initial timing due to them having more duration then the stock GM cams and in that case the 10 deg initial your running is too retarded which can cause low idle vacuum.

So if you dont find any vacuum leak/s between carb & intake,where power brake boosted hooks on both ends,dont hear a vacuum leak at/in brake booster due to bad booster,or maybe the modulator line cam off trans causing vac leak then try advancing the initial timing to approx 16-18 deg to see how that helps out esp if you dont know if your cam is a stock GM or a mild aftermarket cam.

If its a stock gm cam as soon as you advance the timing to to approx 18 deg initial it will run like crap but if its an aftermarket mild perf cam it will repsond well to the additional initial timing and in that case leave it there.

Make sure the motor is idling low like approx 550-600 rpms when checking the initial timing to ensure the mech advance is not starting to activate which will mess up the initial timing setting.

lastly make sure the vac adv hose is unhooked & plugged when checking/setting the timing and only hook it back up after all the timing adj are completed. Don't forget to raise the idle back up to where it was prior to checking/setting the timing too.

Let us know how you make out.

Scott

Scott, I have checked vacuum at every port including at the intake port. At idle I hear no leak but I hear a whistling sound at the carb when I Rev the motor up. The cam is not stock but very mild (437 lift).I will advance the timing tomorrow and see what happens. Thanks for your help

George,since you have a mild perf cam you definately need to bump up the initial timing to 16-18 deg. Make sure to have a low idle (550-600 rpm)and vac adv hose plugged when doing this.

Also,you mentioned you may have heard a slight vac leak in carb when hitting the throttle,this may just be the normal increase in motor vacuum noise through the carb when the motor is pulling more air in that your hearing but if its not that your carb could have an internal vacuum leak. If your carb had an internal vac leak the motor would very likely not run correctly at all or at least not good at certain throttle positions depending on where or what was causing the vac leak in the carb and in that case it would need to be checked out & or rblt. But a q-jet is a great carb for your mild sbc so get it fixed it thats whats required.

scott

Update: I installed the heat shield under the carb,no change. I move the timing up to 16*btc and the vacuum went up to 6". I have now notice the motor has a bad miss. Plugs,wires, rotor and cap are new(HEI dist). Carb is a new re-man.I have pulled the plug wires off one at a time with the motor running, no change.

You need to do the plug wire thing again. If the car is running on 7 cylinders and you remove a wire from a running cylinder such that you then have only 6 cylinders firing there WILL be a noticible change in RPM. Either that or your motor is shaking so badly that it really is hard to tell the difference in which case the cause should be relatively easy to find. If thats the situation then something else is grossly wrong and should be obvious.

George,

careful on that timing. advancing it that much may push the total advance up to a level that cause your engine to be pinging. You need to verify the total before driving it around with the timing that far advanced.

Has it ever run right since it was rebuilt? When did it start running bad?

Is your engine idling too fast and rough with poor idle quality, like it dies or tries to die when you put it in gear? if you have a big enough vacuum leak to cause it to have 4-6 inches of vac you have a large vacuum leak that should be findable.

If you engine is performing normally there's something goofy about the vacuum gauge or the vacuum source.

Are the #5 and #7 plugs wires reversed? how about #4 and #8?

Update: I changed the plugs and the miss is gone. plugs are fouling from the motor running so rich. I have had three carbs on the car, no change.I had the idle screw almost all the way in for it to idle on all carbs.My next move is to take the intake off.I talk to a friend yesterday who had a summary problem and found the intake cracked on the bottom.
Tom a bought the car with the motor apart. This is a new rebuild.It's run like this from the start.
Thanks Everyone for your help

one thing you may want to double check is the angle at which the intake manifold and head mate. I had a intake that was angle milled and created a slight gap underneath that allowed vacuum to be sucked up into the engine from the lifter area. so if your engine has been recently rebuilt that may account for your problem especially with the plugs fouling, which may be drawing up a bit of oil. it would also show very little vacuum no matter where you hook up your guage.