adjusted the carb and advanced the timing a hair but still having trouble

[travisr1988]

Hi guys, I'm new to carbs as they predated me, I adjusted the carb as my dad instructed me to on the phone and got it running good, took it around the block and only had 1 stumble incident when gradually accelerating. So after a couple hours of it sitting I decided I'd see if it'd do it again, well as soon as I put it in gear it sputtered and stalled, so I started it again and it idled okay, temp was good, so I put it in gear again and it stalled again. So to fix this problem I adjusted the carb again until it didn't stall, but it dropped idle alot still, so I advanced the timing a hair. Now it doesn't stall when I put it in gear but it does stumble in drive when I initially take off and then runs fine...until I shut it off after driving a few miles, at which point it diesels for a second then dies. The carb is a 80s era quadrajet off of a 81-87 model c20, my truck is a 79 c20, the only difference between the original carb and this one is this one doesn't have a vacuum port for one of the canister lines, which is rotten anyway and my dad says isn't necessary and won't effect the way it runs. my truck does not have an egr or the stuff for it. As of now after it stumbles or cuts out when I touch the throttle in drive the first time it runs good, great response, gets up and goes alot better, shifts much smoother, etc. But how can I solve the issue with it cutting out/stumbling off idle at first and dieseling when shut off after a few miles? Also it smells after driving it a few miles now too. The only way I can describe the smell is spent gas and it comes from the front of the truck. The old carb was replaced because the fuel inlet was stripped and dripping gas on the intake.

The truck is a 1979 c20, 350 with an rv cam, quadrajet 4 barrel backed by a th400. it has glasspacks which are dumped straight back under the bed, which seem to pop more now. like pop pop pop bang pop bang bang pop...

Oh and my gas mileage is terrible...like 8-9mpg mixed with p235/85r16 tires.

[jaredts]

Likely a combination of issues you need to work out.
Did you install a new gasket for this carb.?  Is it the thick gasket designed for quadrajets?
You should check for vacuum leaks.  Dieseling can be from the idle screw set high to compensate for vacuum leaks.
Have you driven it until its up to temp.?  Does it mainly give you trouble cold or also after up to temp.?
What does the crumbling vacuum hose go to on the canister?  Is it the one labeled carburetor on your canister?  I think an 81-87 should have every hookup needed for your canister.
Have you googled quadrajet adjustment procedure?  There is a methodical process and order to getting one just right.
Both the new and old carb.'s are electric choke?  Did you adjust the choke and verify that there is power with the engine running and none when it is not?
Can you take some pics?

[Fairlane514]

Try setting the idle mixture screws at 3 turns out...this is a starting point.  It sounds like you may have too much timing in it, if it diesels. Does it also grunt when you try to start it after its hot?  Do you know what your initial timing is?  I would think you would be fine at 12 degrees advanced.  Are you also using vacuum advance at idle? 

How old is the carburetor as far as a rebuild?  May be your accelerator pump is bad and not giving enough of a fuel shot to come off of idle.

If you have a vacuum gauge you can also set the idle mixture screws to the highest vacuum reading.

[travisr1988]

my dad just rebuilt the carb and yes we used all the right parts including the correct gasket. the idle screw was backed off all the way, I turned it in half a turn which picked up the idle slightly. It stopped dieseling so far now that I've driven it more, it never gets  to 195* it stops at the first line on the temp gauge b/c the previous owner installed a 160* tstat. The rebuild is only a month old, now I can't get it to reproduce the symptoms but I know as soon as I think it's good it'll do it again. It starts right up unless it sits for a couple hours in which case it cranks some, my dad thoroughly inspected the carb and choke and found no issues, but he only did a quick tune as he had to pack for his trip to cali for a funeral. So I drove it like that until he came back and he's been having me mess with it since. Both carbs are electric choke, the crumbled line is from the driver side of the canister. I have noticed that the oil pressure drops to 0 at the same time it runs rough and drops the idle, it isn't burning much oil at all, and when I think it's all good it starts to show the symptoms again slowly. It's bugging me, I've always had EFI cars so the whole carb thing is new to me, however I looked up the procedures and they match what my dad told me and I followed them exactly.

[Fairlane514]

If you truly have 0 oil pressure, I would stop driving the truck. 0 oil pressure means you have a bearing issue, or oil pump failure, at the least.  Also the pressure in your lifters fail and then the valves wont open properly and the engine runs terrible until you build up pressure again, such as revving it.

I am assuming you have a hydraulic cam.

[bake74]

If you truly have 0 oil pressure, I would stop driving the truck. 0 oil pressure means you have a bearing issue, or oil pump failure, at the least.  Also the pressure in your lifters fail and then the valves wont open properly and the engine runs terrible until you build up pressure again, such as revving it.

I am assuming you have a hydraulic cam.

     DANGER, DANGER WILL ROBINSON, find out why you have oil pressure and it drops off to zero before doing anything else.

[78 Chevyrado]

     DANGER, DANGER WILL ROBINSON, find out why you have oil pressure and it drops off to zero before doing anything else.
[/quote]

NOTHING matters before this is fixed.

[jaredts]

Sounds like I'm in the minority here, but I'm not sure the oil pressure would be my focus.  It's not good, but you're saying it drops to zero when it stumbles badly and the idle drops very low.  If your engine is stumbling and at a very low idle rpm you won't have oil pressure.  What's the pressure when its idling properly?

[bake74]

Sounds like I'm in the minority here, but I'm not sure the oil pressure would be my focus.  It's not good, but you're saying it drops to zero when it stumbles badly and the idle drops very low.  If your engine is stumbling and at a very low idle rpm you won't have oil pressure.  What's the pressure when its idling properly?

     Those two are connected, they both need to be looked at, but if it is dropping to zero a lot it should be looked at before driven too much or engine damage is more than likely going to happen.

[travisr1988]

When it idles properly the pressure sits a little over a quarter of the way up, so probably 25-30psi