The Stovebolt Site Forums The Shop Area Engine Shop Is it to far advanced or retarded if it pings, and is tach needed to set timing?

My GMC 302 six starts very easy but pings on acceleration. Does this mean it is too far advance or retarded. The motor is not broken in so I'm worried about damage to the cylinder walls. I guestimated the RPMs when setting the timing. Do I need a tach to get it right?

And what the hey is a Petronix?

Thanks guys

Well, First Lets Answer A Few Questions.Does The Truck Have Good Fresh Gas??Is Your 302 A Fresh Rebuild??Is Your Vacuum Advance Plugged When You Time It??What Are You Setting The Timming At?? And To Answer Your Question On The Ping.You Must First Answer These Questions.If You Answer YES On All The Above The Answer Is Yes It To Far Advanced.10-4 GMCJOHN

Whats your compression ratio? Pinging can be from too much timing, or too little octane for compression ratio. You running high-octane gas? Rule of thumb is the octane you need is 10 times the compression ratio. If you've got 9:1 compression, you need 90 octane gas.

Timing should be set with vacuum advance disconnected and vacuum source plugged. With motor at slow idle, set base timing for about 10 degrees BTDC and see how it runs. Is your advance mechanism free, lubed and working correctly? The mechanical advance mechanism is what controls spark advance above idle. Too much and she'll ping and detonate, to little and she'll run like crap.

Petronix is a replacement for points. It fits inside the stock dist in place of points, and uses a magnet and hall-effect sensor with signal amplifier to trigger coil. Costs about 80-100 bucks for the system, hook up 2 to 4 wires and go. I've got one in a 235, and its great.

You also may need to limit the travel of the vac advance to stop it from pinging. Some of the replacement vac advances today allow too much advance.

Thanks guys for the primer on timing. You've answered my questions but I can't answer many of yours. I gotta confess that my mechanic actually timed this particular truck and my original post was based on past experience with my old GMC from 20 years ago.

It does have fresh gas in a vatted gas tank, don't know compression ratio, it has about 80 miles since a complete rebuild (which I don't know much about since this motor is an Ebay adventure), I'm thinking the idle was high (1200 RPM per Tach)when they set it because I had to back off the idle screw. I assume the vacuum advance was disconnected and the source plugged cause even though this guy is kind of "shade tree" (which is a good thing), he's cheap.

Anyway, you've addressed my main concerns which were: I don't need a tach to set timing, just low idle. Pinging means it may be too far advanced. A Petronix point replacement is worth getting.

Thanks again

I don't think a tac is necessary. I usually don't use a timing light. Started "timing by ear" about 25 years ago and used my light very little since. Try it, drive it and if necessary readjust it, this is easy when you know the vehicles.