old k5 ignition timing

[supernaut]

I have conflicting timing info in my chilton manual and under the hood on my truck. '75 k5 w/ 350 4 barrel and HEI. I just fixed (well mostly) a rough running problem. test drive elicited some back fires through the carb, especially at higher (that means above 2500 in my 150k mileage beast) rpm's. i would think that since the vacuum advance (hopefully anyway) is advancing as rpm's increase that this means my base timing is too far advanced.

What timing are you guys running?

 

[MTMike]

Are you getting any pre-det (ping)?

If your stock or near stock, you should be running 8-10deg of static advance at idle with the vacuum advance disconnected & plugged. Then you should have mechanical advance (springs & weights) in the distributor that progressively bring in another 22 - 26 degrees starting around 1300-1500 rpms and peaking at around 3000rpms. That's just "standard" and you may need to fine tune for your engine and fuel grade needs. Bascially, you want as much advance throughout the RPM range as you can get without any predetonation.

Vacuum advance only works at light or no throttle, (coasting, idling, etc) and is primarily there for added fuel economy. When you're on the throttle the vacuum advance is literally doing nothing. When you're setting & adjusting timing, it's a very good idea to disconnect the vacuum advance & plug the vacuum port on the carb.

Good luck...

Mike

 

[MTMike]

Here's a really good article to read...

http://chevyhiperformance.com/howto/97438/

Edit: and another...
http://sporttruck.com/techarticles/33703/index5.html

 

[supernaut]

Thanks Mike. I was scratching my head thinking that if the vacuum advance is being subjected to manifold vacuum then why am i getting advance at higher rpm's? Maniflod vacuum dies off really quickly as the rpm's increase if i remember right.

I am very near stock, i'll adjust between 8 - 10 degrees and see how it performs. I havent' been getting any pre-detonation that causes the pinging.

I will definitely kill the vacuum advance while setting my timing tonight and thanks for the links, i'll read those for sure!

 

[MTMike]

Thanks Mike. I was scratching my head thinking that if the vacuum advance is being subjected to manifold vacuum then why am i getting advance at higher rpm's? Maniflod vacuum dies off really quickly as the rpm's increase if i remember right.

I am very near stock, i'll adjust between 8 - 10 degrees and see how it performs. I havent' been getting any pre-detonation that causes the pinging.

I will definitely kill the vacuum advance while setting my timing tonight and thanks for the links, i'll read those for sure!

No problem. if 8-10 doesn't work, you can even try 11 or 12. My old 79 had some serious timing problems, so I spent a LOT of time trying to get it just right.