It's my first time posting so be gentile and excuse my spelling. I have a 73 highboy it has a 390 with a 428 crank. It's a mild build with I believe an rv cam and a holly street avenger 670 carb the ignition is a mallory mech advance. I bought the truck about 6 months ago and hardly drive it. I'm just starting to drive it more often and I'm having a hard time dialing it in so it runs good. Since it is a rebuild (I hope) the timing mark is a line of whiteout. However when I time it to the mark it runs like crap. It stutters at take off and when I shut the truck off it keeps running for a few seconds. Decided to time it with a vacume guage. I heard the more the better. If I advance the dist say 10 atdc the rpms go up and vac go from 17 to 20 before wires prevent me from advancing any more. It seems to run better now advanced that much from the mark but It think its detinating when I really get on it however it shuts right off no run on. What am I doing wrong and are the rpms supose to go up when you advance the dist. I thought they would go up until you find the sweet spot then go down once you past it. could I be a tooth off on the dist? AS you can see I'm no mechanic but hey I try. Any advice would be helpful thank you

 

Yes, the idle RPM will increase when you advance the timing (BTDC) up to a point. You won't really notice the RPM drop if you go too far unless you go WAY too far. It'll be hard to start and ping before you get to that point. For a quick setting, start where you were (closer to TDC) and advance it 'til the RPMs come up and it seems to be running easily. Then back it off until the RPMs just start to drop off a little. Snug it down there and try it. If you have any hesitation, try advancing it a little. If it pings under load or is hard to start when warm, back it off a little.
Hope this helps.

 

yes this does help, I think I have it advance just a little too much since it does try to ping under load. what kind of vacume reading should I get if the timing is right on.


thanks a lot

 

Your timing should be some where "before" top dead center. Anything less than about 10btdc (with vacuum advance disconnected) should be too little unless you have tons of comression. Which you might since the crank was changed. That's where the balancer should have most of the marks.

If it's running pretty good but pinging I would back it off a little at a time till the pinging stops.

What octane fuel are you running?

 

I'm running 91 but everyone is telling me I should be able to run the cheep stuff. The reason I started using 91 is before I advanced it every time I shut the truck off it would run on for a few second I assume thats because I had it backed off of the dist.

 

Run-on is usually caused by the idle speed being set too high.