1979 Buick Quadrajet Carb Question

[Jolly_John]

Hi. This question is in reference to the stock Quadrajet carb found on the 1979 Buick Riviera, with 350 Olds engine (no computer controls of any kind).

I have adjusted the idle mixture screws on the freshly rebuilt stock Quadrajet carb using a vacuum gauge. The vacuum at idle peaks at 18", when both idle mixture screws are 4-1/4 turns out. Does this seem like too many turns out? If so, I thought this might indicate a problem elsewhere (plugged fuel filter, low float level, etc.) Any ideas will be appreciated. Thanks! John

[Larry Schramm]

What is the engine timing?

[Guest kaycee]

Do you have a vacuum leak ( carb. to manifold, intake manifold gasket, or loose or off vacuum hose?)

kaycee

[Jolly_John]

Hi, Guys. Thanks for posting your thoughts. Because of your questions, can I assume you feel that 4-1/4 turns out on the idle mixture screws is unusual?

Larry, the timing is set per the manual: 20 degrees before TDC at 1,100 rpm, with the vac. advance disconnected.

Kaycee: I have looked for vacuum leaks, since I also suspected that. I have not discovered any.....but, I will go back and check everything.

Does anyone have a specific comment based on their experience, as to the 4-1/4 turns out on the idle mixture screws being unusual (or not) for this Quadrajet? Thanks. John

[Larry Schramm]

I checked with a friend of mine that worked for Rochester Products to confirm my thoughts and he confirmed that 4 1/2 is way too far out. Something is not right. The aproximate setting would be about 1-1/2 +- 1/2. I would look for vacuum leaks with a can of wd-40.

Maybe a dumb thought, but are you sure that you have the correct carb for the engine/car? Might want to double check the part number and application.

[Guest]

Just out of curiosity from this thread I went out and checked my 350" and 455" Pontiac's and they are set at about 1- 1/4 to 1- 3/4. I don't think I would check for leaks on mine with W-D40, even if you can wipe it off (which you can't in all places) it still attracts dust and dirt to your engine like a magnet. In the industry we used to use freon gas but that among other things is illegal these days. You could use water in a large surrenge, I have several and one I would use has a long curved tip You could also use a bottle of 02 with a small diameter hose. Make sure in all the usual places like vacuum hoses ( and where they lead to example; vacuum modulator diaphram for the A/T) and all those hoses for heater/ A/C. Throttle shafts should be checked because the shafts are only supported by a aluminum base plate and they wear out. Fortunatly they make bushing kits for the plates. Loose shafts really play havoc trying to set mixtures.

BTW Q Jet, IMO is one of the best carbs ever made.

As a former tech I have a large collection of forceps that I got from a sergical supply to one by one pinch vacuum lines to check for leaks. I also have many dental tools too. A couple of friends of mine were over one day while I was working on a race car, one a dentist and the other a sports medicine sergeon and both said they felt right at home in my tool box.

Don

Edited October 12, 2010 by helfen (see edit history)

[Jolly_John]

Definitely the right carb number for the car, Larry. And Don, thanks for going to the trouble of checking the idle mixture screw settings on your two cars. That pretty much confirms that things are wacko on the Riviera in question.

I will search more diligently for a vacuum leak in the next few days. A dumb question.....using Don's water bottle technique: does the water momentarily seal the vacuum leak, causing the engine to speed up....or does the water get sucked in by the vac. leak and cause the engine to stumble? In other words, what should I be looking for (with the engine speed or whatever), when the water hits the vac. leak? Thank you again for the help, guys. Best, John

[Guest kaycee]

It'll stumble and you'll probably have some white exhaust smoke. If needed, block off some of the vacuum hoses off using tape, etc., (one at a time) and see if it makes a difference. I think it's probably the carb to manifold gasket or possibly the intake manifold gasket. Please keep us posted.

kaycee

[Guest carlnut]

I have seen the brake booster cause a vac leak and you can not find it with any kind of spray or otherwise. If it is real bad you can hear it hissing where the brake pedal rod enters the booster under the dash. Plug the lines one by one as somebody else said and see what happens. Also, you can find a vac leak with an unlit propane torch. Another way is a 3 ft length of garden hose. Put one end to your good ear and search for the leak with the other end. You may be shocked at what all you can hear this way. This is old school methods but it works.

[Guest carlnut]

Also, I meant to say that the heater/air cond vac circuit is always suspect. I worked on GM stuff for a living in the 70's and have seen the a/c controls or tubing or vac canister leak many times.

[Guest]

Also, I meant to say that the heater/air cond vac circuit is always suspect. I worked on GM stuff for a living in the 70's and have seen the a/c controls or tubing or vac canister leak many times.

That is why I have all those forceps . Isolate every vacuum hose at the manifold and carb base. Forceps lock in various positions. Sometimes while dealing with a hard to find leak you may have ten or so on which makes it look like your doing open heart sergery. They really work good for this kind of job.

Don