timing problem

[cantcatch06]

so i've got an 85 model 318 with with 308 heads on it a edelbrock performer intake,Carter AFB, 904 Auto,spitfire headers, and 2.5" dual exhaust, and 2.23 rear gears stock roller cam in a 66 barracuda. I went to check the timing today to see if i could get a bit more pep out of the car and noticed that the mechanical advance on the distributor was only giving me 17* advance at 2500 RPM. Which combined with the 2* initial brought me to 19* total timing. I've heard that these motor like 30-35* total timing so i went back and set my initial to 15* and drove the car and it's not pinging under load and cranks just fine. I'm reading 24 inches vacuum at idle in gear, which sounds like too much vacuum. So I'm beginning to think that i have something wrong because 15* initial. timing seems like alot. I rechecked my timing mark on my balancer and it is correct and used 2 different timing lights and vacuum guages thinking that i had a faulty tool. So my question is
1) what kind of vacuum should i be reading in gear with a stock cam
2) am i running too much initial timing
3) where can i get the springs to recurve the distrubitor to get more mechanical advance if necessary

 

[dgc333]

The factory springs won't give you full advance until to close to 4000 rpm. If you get the two spring MP kit and replace the heavy spring with one of the springs from the kit you will have all your timing in around 2500 rpm. Using both of the springs from the kit and you will have it all in around 1200 rpm.

The amount of initial timing is a function of the amount of overlap in your cam. More over lap requires more initial. Up to 20 degrees BTDC is not unusual with an aggressive cam. The way to find what the engine likes is to increase the timing in small increments while maintaining a constant rpm and adjust for max manifold vacuum.

On a stock distributor you may find that the engine wants that 15 degrees you have and that you will have more than the 35-36 total the engine wants. The quick way to check the amount of timing in the distributor is to glue a cheap protractor to an old rotor and tape a piece of wire to the housing as a pointer. Twist the rotor against the spring an measure the advance. Because the cam turns a 1/2 the speed of the crank you need to multiple the number you read by 2 to get crank degrees. If you have to much total you will need to restrict it by welding or epoxing the slots to limit the timing.

 

[juvat]

those intial timing settings from the factory were likely based on emissions and your engine could use more initial advance than they put in the haynes manual. what I do is disconnect the vaccuum hose from the distributor, hold the engine at 4.5k rpm so the full mechanical advance is all in, then check that the timing is at 36*. if your initial is at 15-16* that sounds okay to me, mine is at 18* but i have a bigger cam. don't forget to hook your vaccuum hose back up to a constant manifold vacuum and re-set your idle speed. if it pings under full throttle, back your total timing back to about 34* and recheck. if it pings under full throttle again, back it off another 2* and retry until you get it to not ping. if it pings under partial throttle and not full throttle you could just lower your vacuum advance canister some, but i don't think you'd have pinging problems anyway with that stock cam and compression. if your engine is pinging with 36* timing you probably have carbon build up and could use some higher octane fuel. if your engine kicks back cranking it when it's fully warmed up that would be a reason to retard it a bit.

 

[cantcatch06]

ok that makes a bit more sense now. i've noticed the car pinging at higher rpms under load so i'm going to back it off some more and see what i can get. Going to try and put some better fuel in it first though to see if i can kill the pinging.

 

[crackedback]

I'd do the protractor thing or put the timing light on it and keep increasing RPM until the advance stops and note the amount of advance and RPM. Then do a little math to find mechanical in the distributor. You might have to limit the mechanical.

Make sure you set timing with vacuum adv disconnected and the port at carb plugged.

There's usually an allen/hex key adjuster in the vacuum canister to adjust amount of vac adv available. A little trick is get your initial set, then hook the vacuum adv hose to a direct vacuum source, note the change in timing. That's the amount of vacuum advance in the canister. Twist the adjuster either direction to get the amount of adv you want coming in from vacuum.

Dave and Juvat have you headed the right direction.