Mine is a 73 f250, had it a year now. The motor is a 390 from the sixties, the parts guy tells me. I don't know its history. Bored .040". Automatic transmission. D2 heads, standard lifters and rocker arm shafts. Recent valve job and timing chain. Carter AFB carburetor, Fomoco distributor with points. Exhaust headers.

I think it has an aftermarket cam. It lopes. I've been fiddling with timing and carb settings (I put a kit in the carb) and I notice I get the best running with about 25* at 800 rpm, up to 45* at 3000. That is with mechanical advance only, vaccuum disconnected and plugged.

The exhaust is deafening (I like it) so I can't swear it is not pinging, but it feels just strong and eager, not stumbly. I did a quick check on the harmonic balancer using a stick in a sparkplug hole, and TDC seems correct.

My question is, could the cam justify this kind of timing?

 

A performance cam likes more initial timing generally. From 15 to 22 or so degrees at idle is common. Total mechanical advance should generally still be limited to 34-36 degree range at higher RPM both for max power and to prevent knock/ping. If it is pinging at higher RPM and you can't hear it you may find out with a ventilated piston, head gasket or bearing issue. Not fun.

I really think it would be worth full accurate verification of TDC as a start... even 5 degrees off could cause a problem and 10 degrees a big problem. The "positive stop" method is accurate and not too hard. You can google it.

Then set high RPM timing at 35 or so degrees mechanical. Dialing in the idle advance for 20 degrees (for example) may require adjusting the distributor total mechanical advance to keep around 35 degrees at high RPM with 20 degrees at idle. .

 

With 45 degrees total with vacuum advance disconnected I can assure you you are pinging (detonation) you may just not be able to hear it over the loud exhaust.

36-38 total is about max before detonation ruins your day, you can run as much initial timing as the big cam will tolerate but you want the total (mechanical) timing under 38 degrees.

Are you 100% positive the harmonic balancer has not slipped? and TDC is accurate?

 

Thanks blue and flat. After reading your responses I brought the advance back some for now to avoid damage. I will get the "positive stop" tool and check for the real TDC. The high initial advance is kinda fun so I will investigate shrinking the mechanical advance from the distributor. I appreciate your help, especially the numbers to aim for.

 

just to satisfy my curiosity, which plug wire are you using when checking the timing?

 

Passenger front plug wire.

 

thanks, just never know...