Hi there, I am running my electronic HEI 14 degrees before TDC on my 73 351 C. Where does everyone else run there timing at? 14 seems to be where it likes to run. Will it do any damage to engine running at 14? I THINK it is supposed to be at 10?
The OVERKILL and confusing guide to timing adjustment.
This guide will help determine not only what your engine timing should be, but how to determine if your vacuum advance and centrifugal advance are working properly. Springs in the vacuum and centrifugal advance can wear out(crummy over the counter parts) or get stuck, preventing the engine timing from being correct.
It also helps if you have a VACUUM GUN which will allow you to pull varying inches of vacuum on your distributor so you can curve your distributors VACUUM and CENTRIFUGAL advance. You can also measure the vacuum your engine is pulling. Logging your engines vacuum from time to time can help provide clues as to performance issues down the road. For example, if you have 20 inches of vacuum at the manifold today, but 12 inches 2 years from now, you have developed a serious vacuum leak, or your engine simply isn't sucking like a typical Chevy.
In any case,
0. Get under the car with Whiteout and mark a line at 0 and at the 10 BTDC mark on your damper. You may have to turn the motor over to see the marks from the ground.
BTDC is BELOW the TC(0 degree mark) on the damper, or as I like to say, the side of the TC mark that goes to 30 degrees.
1. Remove the vacuum hose(s) at the distributor.
2. Take this opportunity to measure the vacuum with your vacuum gun by hooking up the gun and starting your car.
A stock cam will generally pull 20 inches of vacuum near idle. A performance cam will only pull 12-17. If you are using a performance cam and not had your distributor curved to your cam, your vacuum pod may need to be adjusted/set for the lack of vacuum at idle. This is shown later.
Also, the vacuum should generally be steady. A jumping needle indicates a problem cylinder.
3. According to the specification bookelt, 1973 351C, timing should according to engine decal. On my 1973 351C 2v, the decal said:
6 BTDC at 600 RPM.
At 600 RPM the centrifugal advance should BARELY be active(maybe 2 degrees), giving you a true timing reading. You will probably have to decrease your engine idle at the carb to do this.
4. After setting the timing, you should pull on the throttle to increase your RPM to see at what RPM your timing changes. The table below details the increase in timing you should see based on the increase in RPM.
If you do not get a good match to the table, you need to perhaps look into your distributor and free up/grease friction points. Or you may need to change the advance springs.
5. Now, you can hook your vacuum gun to the distributor vacuum pod, set your engine to 1000RPM and start pulling vacuum.
Confirm your timing changes match the table below. (Because you are operating with the distributor on a car, you must factor in the the centrifugal advance. For example, if your centrifugal advance increased timing by 5 degrees at 1000 rpm, TOTAL timing you should see at 5 inches of vacuum should be 5 degrees more than what the book shows, because the book is assuming centrifugal advance has been disabled on the test stand)
If you do not get a good match to the table, you may need to adjust the pod. Or if you do not get any advance at all, you may need to replace your vacuum pod.
Good luck.