Guys,
I'm having trouble figuring out what the initial timing supposed to be on my 390 engine. I'm running a comp cam XE256H cam with stock heads and exhaust. I originally set my timing at 6 degrees BTC. I got 19.5 mpg but each time I kicked it down hard I got black smoke out the exhaust pipe on the initial start. I also was pulling 14 - 15 inches manifold pressure at 650 rpm. Acceleration was smooth so I didn't mess with it.
I started playing with it today and bumped the timing up to 20 degrees BTC. I'm getting 18 inches manifold pressure at 650 rpm and it starts better than it did before. I also have no black smoke coming out the exhaust when I first get on it. During the start of hard acceleration you feel an extra kick after about a second that you didn't before though. It winds up tighter and quicker than before and seems to go better. I'm using a stock distributor and Holley 570 4 barrel carb.
Does 20 degees BTC sound like too much.
Does anyone know what initial timing should be, I understand the cam has 4 degrees already built into it.
Thanks for any education I can get from you guys.
Jim Nolan

 

Cam timing and ignition timing are two seperate, but related settings. The cam has 4 degrees built in as you say, so installing the cam "straight up" with cam gear index pointing to crank gear index mark will result in 4 degrees cam advance relative to the crank.
Ignition timing is when the spark is sent to the spark plug relative to how close the piston is relative to top dead center. Setting the ignition timing at 20 degrees advance means the spark is firing 20 degrees before the piston is at top dead center. Assuming instant and perfect ignition burn the piston would be pushing up against the flame front and compression as it pushes down against the piston. This is not a good thing when taken to extreme.
The ignition timing needs to be set with the distributor vacuum disconnected at the distributor, and plugged off.
In my opinion 14-20 degrees of advance is way too much. At your altitude, Indiana, I would use maybe 10 degrees at most, the lower the altitude the less you can use without risking detonation. You want a quick and complete burn of the combustion gases, the more efficient the burn the less distributor advance you will need. Next time you tear the engine apart, set the piston to cylinder head quench at 0.04 inch to help agitate the mixture as the piston approaches top dead center, and you won't need so much distributor advance. Hope this helps.

 

FEmtnmax

Changed the timing to 10 degrees initial. Had 30 degrees total and best estimate 38 degrees with vacuum hooked up. Smoother at low speeds. Haven't tried going through gears yet. I called the Comp help line and told the guy about my concerns. He said my circumstance was the reason they didn't give advice on ignition timing. He said all engines have a sweet spot and the best thing I could do was advance the timing until it pinged and then back it off. He also advised me to check timing with the vacuum advance off and with it on to see what the weights did and what the total timing would be with the vacuum hooked up. Everything looks OK, I was just trying to figure out how to get rid of the black smoke when I first nailed it. I think I'm just dumping too much gas before the engine can catch up.
Thanks for the advice, Jim

 

To solve the black smoke issue will require you to adjust the idle mixture screws on your carb. Your Holley has them on each side of the front metering block, between the float bowl and the carb body. Turn the screws in a 1/16th turn at a time, pausing for 10 seconds or so after each adjustment, until idle speed quits increasing, then back out a 1/16th. Now back the idle stop screw down until your idle speed is about 750 to 600 RPM.