[70bird]

Ok, my flywheel will be in tomorrow and I am devising a plan of attack on tuning.

The crate HO 502 in my bird occasionally coughs (off idle) and fouls a plug. Do you think this is a timing issue? After a munute or so the plug will clean itself. The cam in this engine is 211/230 at .05, .51/.54 lift, not sure of LSA. The carb is a 4150 double pumper 750 w/ milled choke horn. The exhaust fumes will suffocate one quickly. I've jetted down but experience ping at crusing speed. This is an 8.75:1 cr engine, and the ping occured with 91 octain.
I just remembered something though. The guy I bought the car from was running a standard rotation water pump in reverse (with serpentine setup) which resulted in an operating temp of 210*. He said the guys at the local GM dept. said that is normal for the 502. BS! I put a high flow, reverse rotation pump on it and it runs at 180* in +90* weather. I didn't get a chance to drive it much after the pump switch before the T10 gave and then the Centerforce dual friction gave after the TKO 600 was installed.

ANYWAY:

Someone suggested that a timing issue is being crutched by overjetting. Once stated, it makes very good sence.

The engine idles rough (which sounds good/mean) but I don't think the present cam warrants as rough an idle as exists.

I plan on working on the timing first off. I'm not sure of what initial timing to start at. I usually set the initial timing for max vacuum at idle.

I'm not sure how soon the adjustable vacuum advance kicks in. I plan on finding out as soon as I get the bird running again (Possibly Thursday night, sorry neighbors). Will too much vacuum advance at cruse speed (1500-2000 rpm) cause detonation? If I remember correctly, vacuum at cruse is about 14" hg. The cars been out of commision for a year now.

OK, any suggestions.

[Rick WI]

Buy a timing light and set the base timing correctly to the recommended specs. This tool will also allow you to verify vac advance function as well as total timing. A vac gauge is not a timing tool.

Once timing is set correctly then you can tune the carb idle mixture screws. Thats what you can use the vac gauge for, not setting timing.

[70bird]

The timing is supposed to be set at 36* @ 3600 rpm w/ vacuum disconnected. Can anybody guestimate where that would put my initial timing, or is it not that easy?

[Rick WI]

You should set the base to 10 with vac disconnected and see where the total ends up at. You will be within 2-3 degrees at this setting, if not right on.

[70bird]

Base is at 10 @ idle. At 3600 rpm total is at about 45* when it is supposed to be at 36*. I'm pretty sure I checked it with the vacuum disconnected.

What would cause this?

I'll check it again on thursday after I get done going through the carb.

[TurboS10]

Sounds to me like you have a smog distributor with a ton of advance since they would run those engines with very slow timing at idle. You need to be around 20 degrees intial with 34-36 total. It will make the engine much more responsive. I would also adjust the idle mixture. If you have bad exhaust fumes with a cam that small, you are flooding the piss out of that engine.

Get the mixture right and get a decent distributor with tunable mechanical curve, and your problems will go away.

Chris

[70bird]

The distributor was built by The Carb Shop (about 5 years ago/and only has about 3000 miles on it) and is a billet unit. Is there any way I can rebuild the distributor to different advance specs?

[TurboS10]

It should be adjustable. 35 degrees of mechanical advance is WAY too much. I would adjust down to 15 like I mentioned earlier. It will make the car more responsive, start easier, and have better street manors.

Chris

[Rick WI]

You should be able to get a different bushing to dial in the mechanical. Your best bet would be to call The Carb Shop up. Verify it again to make sure the vac advance was disconnected. 45 would make sense if you had it connected.

[70bird]

Originally Posted by TurboS10

It should be adjustable. 35 degrees of mechanical advance is WAY too much. I would adjust down to 15 like I mentioned earlier. It will make the car more responsive, start easier, and have better street manors.

Chris

36* @ 3600 rpm is what gm recomends for this motor. I'd like to get the motor to run how it was designed to and then go from there.

The motor starts easy but as for street manors, they don't exist. The car wants to excelerate anywhere belowe 60 mph or 2000 rpm. Throttle response is instintanious. I couldn't imagine it being any better.

I ordered new brass floats (the secondary nitrophyl quit floating), needle & seats, and gaskets from Summit today. They should be here Wednesday. I'll let you know how the Thursday morning tune goes.

[TurboS10]

36 degrees total is fine. What I am telling you is that you have 35 degrees mechanical advance for a total of 45(initial plus mechanical) from what you are telling us. Like Rick said, you should be able to fix this. I would recommend taking it down to around 15-20 mechanical advance so you can set initial higher. The higher initial advance is what will help response.

Chris

[70bird]

Now I see what your saying. Thanks for the clarification.

[machine shop tom]

If you are running an intake manifold with a blocked off heat riser, or gaskets that do the same thing, that may be the cause of a cold plug fouling problem.

tom

[70bird]

If your talking about the exhaust crossover that goes under the carb, I have that open with the little metal tabs that restrict it to about a 3/4 " diameter hole. What is the exhaust crossover for anyways? Isn't a cooler mixture better?