Driving me crazy

[JOE BYRNE]

I read every thread but need some more advice. I have a 350 with 86,000 miles on it. I have always had a build-up of black (not wet) build up on the inside of my rear pipes (more on the passenger side). I do not seem to smoke upon start up. I do notice that if I get on it in 2nd and 3rd that it appears to be black out the rear view. Normal driving I am not noticing it. Also on deceleration I am not seeing anything. I do use oil depending on how I drive it. If im easy on it I don't seem to use that much. If I get on it from time to time I will go through a quart after 400 or 500 hundred miles. My compression test shows about 160 in each cylinder. My plugs are not wet although the passenger side rear is somewhat darker than the others. I put a rebuilt Rodchester on last summer and has run well although mixture screws seem to do nothing. I can turn them in and engine idles just about the same at around 900 too 1000 rpm. I do have a rear main seal that is seaping that could be part of the oil cunsumption under heavy acceleration. All threads say blue oil-balck rich. I have my new valve seals and am ready but am not completly sure that is the problem. It has eldebrock heads and manifold with headers. If it was not for the black stuff i would not even think twice about it since it has always run well. I have had it for almost two years now with this aggervating problem....ideas?????

 

[LT4man]

You hit the nail on the head!

Mixture screws that do not affect idle are not working properly.

What is your vacuum at idle? At what RPM?

The answers to those questions will allow me to troubleshoot your problem.

Save The Wave!

 

[JOE BYRNE]

I have never checked it and am not sure how to. I had the same problem with the other carb on it (black junk) but never tried the mixture adjustments on it. I wont be up much longer since 6am comes fast but I will get back in the am from work.

 

[LT4man]

You need to borrow a vacuum gauge from Murray's, Auto Zone or Pep Boys.

You will need to find a vacuum port on your manifold that has full manifold vacuum (between 15 and 22 inches of vacuum).

Then tell me the amount of vacuum on the gauge.

I can then tell you how to adjust the mixture.

Save The Wave!

 

[wishuwerehere82]

Hi Joe,

I don't want to sidetrack you from the advice Lt4 man is going to help you with, but I have gone through this stuff before and having an abundance of advice that points to the same solution is always better, IMHO.

Since you are on the track of this, and the weather is cooperating at the moment, here are some things you can try to pinpoint the problem.

Valve stem seals:
If you first start the engine, or rev the engine under load after idling for 30 seconds and see a puff of blue smoke out the exhaust, that's a sign your valve stem seals are bad. If not, don't worry about it. A compression test will not bear out a stem seal leak since the valves are closed when you do the test. Since you are getting good compression, your valve seats and rings are in good shape.

Idle mixture adjustment:
You can use either a Tachometer or a Vacuum gauge to do this.
The engine should be fully warmed and the choke wide open.
Set your idle down to around 600 RPM using the idle speed stop screw on the throttle linkage, then turn the idle mixture needle valve screws in one at a time until the RPM or Vacuum drops and the engine starts idling rough.
Then back the needle valves out until you reach maximum RPM or Vacuum. Alternate back and forth between the two sides until you reach maximum total RPM or Vacuum. Then re-adjust your RPM's to the idle speed you like(usually 650-700 RPM).

Now, if you can't adjust the mixture and nothing changes when you do the procedure, there are some steps you need to take to determine where the problem is.

If your idle passages are clogged with varnish the mixture screws will not affect the mixture. The idle will only be controllable with the idle speed stop screw on the throttle linkage. You will likely experience stalling and a very bad idle since the engine will be running lean at idle.
Unscrew and remove the idle screw needle valve, spring, and squirt some carb cleaner into the passage where the needle valve is, and wipe the varnish off the needle seat and needle valve with a cloth on the end of a small wooden dowel.
Don't use a metal tool because it can scratch the seat. Spray some more carb cleaner until you can see the spray coming out into the primary carb throat on the side you are doing. Then replace the spring and needle valve and screw it in until it bottoms out, then unscrew it 1-1/2 turns to set the initial idle mixture.
Then do the other side.


Then redo the mixture adjustment procedure to reset the mixtures on both sides.
I hope this helps.

 

[JOE BYRNE]

I will stop and pick one up on the way home tonight...and will get back...thanks

 

[JOE BYRNE]

I also might add that this is a 73 that has no emmisions on it.....EGR notta..... taken off before I bought it.would that make any difference? It threw out black smoke on heavy acceleration with the old q-jet also. I only replaced it because it was fairly old and seaping a bit

 

[ChuckG]

You hit the nail on the head!

Mixture screws that do not affect idle are not working properly.

What is your vacuum at idle? At what RPM?

The answers to those questions will allow me to troubleshoot your problem.

Save The Wave!

I'm no engine guru, but black, sooty exhausts coupled with idle screws that do not work at all point to a rich running condition. Somethin' ain't right with the carb. Your car should stall out if you turn the idle mix screws all the way in. Chuck

 

[JOE BYRNE]

Off manifold vacuum 24, off PCV 25, vacuum advance 10 to 12 at idle up the 20 at 1/4 acceleration...idle mix screws consist of a screw, spring but no o rings

 

[glen242]

Off manifold vacuum 24, off PCV 25, vacuum advance 10 to 12 at idle up the 20 at 1/4 acceleration...idle mix screws consist of a screw, spring but no o rings

That is really high manifold and pcv vacuum. I would check your gauge. Using ported vacuum on the vacuum advance?

 

[JOE BYRNE]

high or not just trying to get to the bottom of running rich....air/fuel miture screws are not functioning ...

 

[wishuwerehere82]

Everything you always wanted to know about the Quadrajet carburetor.
http://www.florida4x4.com/tech/quadrajet/index.php

And I was wrong, there are no O-rings. Just a spring.

 

[JOE BYRNE]

Thanks pete

 

[wishuwerehere82]

Also, see the last line in this one. If all else fails, contact Lars.
http://www.florida4x4.com/tech/quadrajet/qtune.pdf

 

[JOE BYRNE]

got it ..thanks

 

[JOE BYRNE]

got it ...thanks

 

[LT4man]

Sent you an email.

I think you will get to the bottom of this.

Your Vette sounds sweet. What a cool body style! A real Vette!

Save The Wave!

 

[JOE BYRNE]

Thank you