General timing questions(please help)


If a car with a HEI distributer sits over a couple of years, can the timing go out and need adjusting. Also if A car will run for a minute then backfire out the carb, can that mean that either the pickup coil, or the ignition coil is going out? I think the carburetor is in good shape, so I’m guessing it is the timing. Am I right?


If the car has been sitting for a long time… the valve stems may have a little rust on them and be sticking or pieces of carbon are being knocked off the piston and getting caught in the exhaust valve. My guess is that it will quit backfiring after you have it running for awhile. The other possibility is a cracked cap… Gl…


Old/stale gas can also cause this.



Timing doesn’t change all by itself.

The carburetor could have plenty of problems after sitting for a couple years, including dried and leaking gaskets and seals, a dried out accelerator pump or it may be full of varnish. If you’re trying to run this thing on the same fuel that has been sitting for years, there’s your problem right there.


My vote is for stale gas and a bad carburetor accelerator pump circuit.


Vacuum leaks from a split hose can happen too. They usually split at the ends. That includes the PCV hose. Was it your car? Do you know how it ran before it was parked, or did you take somebody’s word for it? What engine, what car? A manual trans will cause the wires to the pickup to wear out and fray. It doesn’t always happen, but I have seen it. If your vacuum advance doesn’t hold vacuum or doesn’t move the breaker plate, then it needs attention too. An EGR valve that stays open can cause problems. A V8 can have the 5 and 7 wires crossed or arcing between one another.


I have checked for vacuum leaks, and split ends. There are none. It ran excellent before it was parked. I siphoned all of the bad gas out of the car and put fuel stabilizer and techron with the new gas. The cap is not cracked, and the carburetor is a newly rebuilt carter 4bbl. The engine is a 350, with a 350 transmission. There is no egr valve, and the vacuum advance works. Any other Ideas?


enough bad gas in the lines? how long have you run it after you put in new gas?


The old HEI distributor had internal mechanisms for vacuum advance and mechanical advance. Take off the distributor cap and make sure that both are working.


How bad is the “backfire out the carb”? Does it do it when trying to start or ever once and awhile after car has started? If it backfires when starting I suspect you may have a clogged exhaust system. If it is ever once and awhile then I agree with the other posters. Do you know if this car has a belt drive smog pump? A bad valve or vaccum line to it can cause backfireing. Can’t remember if it backfires through the carb or exhaust. Maybe some other poster can tell us.

Just courious what year, make, model of car is this?

You should beable to run a compression test to see if all cylinders are about the same compression. You can also use a vaccum gage to see check many problems also.


The car is a 1970 Monte Carlo. The HEI distributer about 4 years old. It is and MSD. There was about 2 or 3 quarts of bad gas, but I siphoned all of it out. It will run for anywhere between 30 seconds and 2 minutes before it will start backfiring through the carburetor pretty bad until it is turned off or dies. The exhaust system is completely fine except the bottom of the headers before the collectors are dented badly. The car ran with them in the same condition for a while though. This car does not have a smog pump of any kind. the only real vacuum lines used are for the brake booster, and for the pcv valve.


What are the chances that it is the pickup coil or the ignition coil in the distributer?


I guess you will have to pull the plugs and take a look. If any of them are damaged or covered with deposits, you will have some kind of clue. If your ignition wires have been sitting, the insides sometimes swell from the reaction to the moisture that got into the ends. It’s easy to guess but sometimes work has to be done. You should be able to adjust the timing to find out if that is the problem. Put a timing light on each wire (yes, one at a time) and see if your spark is consistent.


Thanks for the info. I have replaced the spark plugs so I can’t check the old ones out. I will however use the timing light idea.


You can’t rule out the possiblity that the timing is off. If there was enough slack in the chain and the engine backfired it could have caused the timing to jump. I have seen this happen many times.

Get #1 cylinder on TDC and see where the timing marks line up.


If I were to check the timing and it was off, Would that mean that I need a new timing chain?


If the timing was off you would need to pull the timing cover off to inspect the gears and chain. Chances are the nylon coating on the cam gear has hardened, cracked, and fallen apart. When that happens there is ennough slack in the chain to allow it to jump and retard the timing.