still tuning the smallblock. help please

[Benrs86]

now by playing with the timing i can get this smallblock with hei and a quadrajet to idle at 600 (idles rough though) and runs good. the problem is it runs so rough when i put it in gear it dies. also when im rolling up to a stoplight with my foot on the brake it dies. what do i do? what vacuum source does the vacuum advance pull from? there are 3 small ports on the front of my qjet and one on the passanger side. i am going to go redo the timing right now. someone told me if i disable the mechanical and vacuum advance it should be timed at 4 - 6 degrees btdc. is that right? i really need to get this running good to day. any help is appreciated, Ben

[67ChevyRedneck]

That is correct on the timing. If your truck has a lot of HP you can bump it up to 8, but I'd leave it around 6. My truck is stock 350 set at 6 and it runs awesome. Not sure on Q-jet, but the vac adv on edelbrock is on the pass side on the front of the carb. Maybe Q-jet is the same? It does matter which it goes to. Some are full time vac and some only part time, during acceleration. You want a part-time port for vac adv.

[bigjimzlll]

I would start at 10 degrees....if you hook the vacuum advance to the full manifold vacuum(with engine running its a port thats is sucking at idle) it will smooth out the idle.

[Huck]

I have personally seen 350's run off both full vacuum and ported vacuum (0 at idle). Kind depends on the distributor--HEI vs point disy. I would first ensure no vacuum leaks as this will drive you nuts. Then set your timing @ .06 advance when warm and vac advance plugged. Next would be to use a vacuum gauge to adjust your timing. Your idle mix screws should be out about 1.5-2 turns.

[StingRay]

Vacuum advance can run off of either a full vacuum port or a timed port. Depending on your particular engine specs, how you drive and what your performance/economy expectations are can all be factors in your decision. With my 350 for the last 12 years I ran timed vacuum port to the vacuum advance and ran as much as 20 degrees static, restriced the vacuum advance and had it cut out a bit sooner and ran stock hei mechanical advance with a bit lighter springs. It was a stock compression engine with 76cc heads and mild RV cam. I got a good as 20 Miles per US gallon at 70 MPH with a 3.73 rear end and a TH350 tranny. The engine had been gone through and had composite head gaskets so the compression was only in the low 8's. Jetting, operating temp, loading, ambient air temp and driving style can all be factors on what you can get away with. When I had to tow or it got real hot I usually set the timing back to between 8 and 10. I seldom ever had my truck idling at 600 (generally when it was real warm out). Usually about 650 was the best. Any slower it would die, any faster and it was uncomfortable at a stop light holding it back with the brakes.

You can tell what ports you have on your carb by hooking up a vacuum gauge and seeing what the vacuum is at idle, off idle and low throttle. Timed won't cut in until partial throttle but will reach full vacuum. Full vacuum ports will be full at idle. Some ports don't get full vacuum so be careful. With an rv cam you may see anywhere from 18 to 22 inches of vacuum at idle. With my speedpro CS1028R cam that was 214 duration 443 lift and 112 lobe seperation I had about 18" vacuum at idle and just a slight lope at 650 rpm. All around a nice cam for my daily driver.

My engine had a qjet on a performer 2101 and ran stock rams horns with 2" exhaust and turbo type mufflers. I also run an msd and a hei blaster coil.

The engine is apart now and will go back together bluprinted and balanced with hypereutectic flat tops and otherwise basically the same set up for cam, induction and exhaust. With the flat tops I will probably only be able to get away with 8 or 10 degrees static.

[Benrs86]

thanks for the advice. it is running and idleing decent. i think the timing is good. when the engine is hot the choke should be open all the time right? i dont have an egr valve on my exhaust so i disconnected the choke linkage on the intake that is operated by heat. so now when i start it the choke is closed with the fast idle on, then i tap the gas and it goes to slow idle and runs. its like 20 degrees here and it all works ok. but once the engine heats up, it gets worse. i live 5 miles from sioux falls. by the time i get to sioux falls the first stop light i stop at will kill the engine. everytime i let off the gas and press the brake to stop the engine dies. what do i do about that? im sure i could have a lot of different problems, but what should i check? thanks for the help, Ben

[StingRay]

Are you saying that you disconected the choke linkage from the manifold? If it is operated by a divorced choke coil on the manifold then the rod between the coiled spring on the manifold and the choke mechanism must be hooked up or the auto choke must be converted to manual. If the carb is newer and has the black pod on the side with an elctrically heated coil then it must be hooked to 12v. If its age is in between and has the black pod that works with tubing between the choke boss on the manifold and the integrated choke housing then the tubing must be in place. I am confused about what an egr valve and choke linkage hook up have to do with eachother.

With a properly functioning choke and a good running tuned small block with a q-jet when it gets colder and in particular cold and damp they have a tendency to ice up the idle circuits if the heat stove/pipe and valve are not working corectly. I have to run a stock air cleaner with tube and the heat stove on the drivers exhaust manifold. The valve on the air cleaner must be hooked up to vacuum and be functioning properly as well or you will have just the kind of symptoms you described. Does it improve if you shut the truck off and let it heat soak the carb a bit? If so then it is a freeze up issue. If not probably a choke issue

[Benrs86]

i always thought the egr valve on the exhaust manifold closed to force hot air through the bypass on the intake, and when the intake had exhaust running through it the spring would heat up, and open the choke. maybe i am way off?

what i did was disconnect the linkage, and this way the choke shuts when i start the engine, and when i rev it once the fast idle turns off, and the choke opens. and whenever it runs after that it has been open. but the engine still dies when i slow down or stop, and idle with it in gear. it is already idling fast so i dont want to speed up the idle. is my idle mixture off and thats why it idles bad in gear? thanks, Ben

[StingRay]

The valve on the exhaust manifold is called a heat riser valve. The stock 67-72 valves were a buterfly type of arrangement with a bimetal coil spring to activate them. Later engines had a vacuum actuated unit. You can get away fine without a heat riser valve but you will need the stock type of air cleaner with heat stove, pipe and working valve on the air cleaner all properly hooked up for some kinds of cold damp conditions. If you have disabled the heat riser make darn sure it is locked in the open position.

Does it improve if you shut it down and let the engine heat soak up into the carb for a while? If so then it's carb icing. Also check for vacuum leaks. Cold tends to lean things out and a vacuum leak can be the straw that's braking the camels back. Have you set your idle mixture screws? What kind of intake are you running? Some early cast q-jet intakes had the heat riser passages right to the bottom of the carb and without a metal plate would spit exhaust back in through the carb and the engine won't run worth a crap.

[Benrs86]

thanks for the help stingray. its still the same, i just played iwth the idle mixture screws a while, that helped, but it still dies when i put it in gear. there is no heat riser valve at all on the exhaust. its about 20 degrees outside and i drive it around, stopping and going testing it, and it always dies when i put it in gear. when i pull it into the garage it works fine though. this makes me think that you are right about me needed a air cleaner with a heat stove. i have one, but there is no room, so i have a 9 inch mr gasket round thing. the problem is this engine is in a 87 s10 so i had to cut the tin heat thing off the manifold for room, i had to get rid of the heat riser for room, and i dont have enough room for a big air cleaner with the valve on it. i think that is my problem. are there any other solutions? thanks very much for the help. thanks, Ben

[tom hand]

Originally Posted by Benrs86

now by playing with the timing i can get this smallblock with hei and a quadrajet to idle at 600 (idles rough though) and runs good. the problem is it runs so rough when i put it in gear it dies.

If I understand you right you are setting it at 600 in park...it should be 600 in gear which is about 900 in park

[StingRay]

Oh gosh ya idle settings are in gear. What Tom says for sure. A heat stove and pipe set up are only required in certain conditions and like I said once it's warmed up shutting it down to heat soak the carb would cure it for a while if it were carb icing.