I'm needing to set the timing on our '79 C60 straight truck... However, there's a catch. I have no accurate timing marks!

It's got a pretty standard bone-stock smallblock 350 with HEI igniton, but the marks on the balancer have "slipped" (the balancer uses a rubber bushing to mount up to the crank, and it rotated a long time ago). I have all the equipment for conventional timing, but with those marks messed up it isn't going to do me much good.

What I was wondering is if anyone has any experience with setting ignition timing with a vacuum gauge? After doing a little research it seems that it's a very common practice for people who have engines that cannot be timed conventionally for whatever reason, and some even claim it does a better job than normal timing with a light. However, there's also just as many people who gripe and complain and say that the method is inaccurate. Any insights on this?

Basically the process is to disconnect the vacuum advance, set the engine at a nice idle, advance the timing to achieve maximum vacuum reading, then retard it back about 1 to 2 inches of vacuum.

 

yep, it works.
the old timers in my shop used to do this on vans when the timing marks were not accessible.

 

Well, one big ?. Are you saying the line on the balancer is, gone, or the timing tab? It's keyed to the crank. If it is running, then you have it somewhat right. You should always disconnect the vacuum to the distributor when setting the advance. Mechanical advance should be around 30 deg at 3000 rpm. I always just used a match book to set .017" point gap at 700 rpm or so. With that said, yes you can set the timming with vacuum, however I don't suggest that since altitude will have a different effect on carburation.

 

Old age and treachery....

 

It's been a while since I've fiddled with it, but what most likely happened is that the outer ring of the balancer moved a little bit on the inner hub of the balancer. Yes, the balancer is keyed to the crank, but the rubber ring between the two parts of the balancer can slip depending on the conditions...

 

Sounds like you got a handle on it

 

Oh, a couple other things while on the subject...

1) Does ambient (weather) temperature have a major effect on setting timing?
2) Does changing the intake or exhaust systems screw up correctly set timing?

Just curious as the weather outside is pretty chilly these days, and we may or may not be changing the exhaust setup someday in the future.

 

extreme temp may effect the carb mixture setting but not timing. set it up to stock

 

You may have a larger manifold pressure with cold air, but not enough to make a difference. As tech2 says, set it up to stock.

 

that said...the old timers i was refering to are now retired or passed on and I am now the old timer.