Any suggestions will help!...... Just had the motor rebuilt from the rooter to the tooter, lol... new Qjet , replaced the intake manifold and gaskets, but i still have ruff idle on start up and when i cover the carb with my hand the car runs like it should but soon as i release the pressure it running ruff, this problem has been happening for at least a year. What i need to do.ray:
You have a vacuum leak. The most likely culprit is the carb. You can try spraying carb cleaner in and around the carb to pinpoint the source of the leak. Or you can swap another known good carb off another engine for testing purposes
Just grasping at straws here; you haven't said what carb on what manifold, so I'll assume it's a newer Q-Jet on the 66 nailhead Q-Jet manifold. Sorry if I'm wrong...If you are using the 66 Q-Jet manifold and a later carb, then your spacer and gaskets are wrong. A 68 or later carb will not fit on the earlier manifolds. Using gaskets from the newer carb doesn't work either as the base on the manifold is actually wider than later carbs. If this isn't the case, then check the steel plate to gasket fit. I've seen some cases where a paper gasket is required on both sides of the plate just to seal it up. Another source of a major vacuum leak is the Q-Jet itself. The base is very suseptible to warping if overtorqued.
If a vacuum leak isn't located... Tune it, Dial in the timing, idle screws and idle rpms with a vacuum gauge for highest steady vacuum... Assuming it's a correctly calibrated carb for your combo.. If it's an automatic, check that vacuum line going down to the trans, make sure it's connected at both ends and no cracks in the rubber.... made that mistake before....
If you cover the carb with your hand and it runs good , It's with out a doubt a vacuum leak!!! Spray all around the manifold and the base of the carb, Check all your vacuum lines, Brake booster .Also check the line(s) to the control for the a.c. if equiped
it is definetly running lean,,,and a vacume leak could very well be the cause,,,, that said,,, be aware that all nailheads tend to be ''cold natured'' on start up... because the intake is a ''divorced'' intake, it runs cooler than the later engines intakes do.... When you are looking for vac leaks it can drive you right up the wall....they can be at the trans vac modulator or anywhere in the line between the engine and the modulator.... they can be in the heater/ac vac lines or a combination of all of the above... if they are old and brittle replace all of them .... vacume leaks can be in the heater/air vacume motors.... and in the usual places , like under the carb,, and at the head/intake joint.... or in the vac brake booster.... a lot of guys over look the brake booster.... the test is to isolate those items by pulling the line and blocking the hole and see if the engine runs right....if it does,,, the problem is in that vac system.... but if a nailhead is tuned right , and timed right , and not off with the carb mixture, with the cat pee substitute we get for gas now days, the engine will still be grumpy untill it warms up some.......
He sprayed around the carb, no sign of a leak there. It has perfect vacume. How can i find out how to trace all these vacume lines is there a book to tell me where they are? Could it be the spacer plate between the carb and the intake manifold, do i really need it? He made paper gaskets, tried different combinations with gasket sealer. If its in the heads, what can i do to check them?
Hey Doc! Do you know of anyone around Tennessee that works on late model buicks that i can take my car too.
What Doc said in post #7. You do not have to chase down every item in the car's vacuum systems. Just unplug all vacuum lines from the engine and plug the vacuum ports at the engine. With all disconnected, if the engine is still running lean, the problem is in the carb or in it's installation. If the problem goes away with all systems plugged, then hook the vacuum lines back to the engine one at a time until you get to the system that causes the rough running. Air is definitely getting into the engine somewhere that it shouldn't.
Its 17 inches of mercury, not inch pounds. and we're talking about vacuum , which is not pressure. What is the reading with yout hand over the carb?
You both get an E for effort. By definition, a vacuum is a volume of space devoid of matter, which is never the case in an automotive intake manifold. In the automotive world and others, the word vacuum is used to describe any pressure below atmospheric pressure. So in this case, we're measuring a pressure in the intake manifold that is lower than atmospheric pressure. So there. :grin: Devon
:laugh::laugh::laugh: Once again, your absolutely right Devon! :beer Well, the OP better fix his car soon before he sucks all our atmosphere into his engine! :laugh:
