'73 340 4bbl Hesitation • The Dodge Challenger Message Board

by **Rob** » 12 Dec 2001 14:34

I'm having a problem stopping my engine from back firing when accelerating have tried setting the timing at 12 Degrees BTDC as recommended by a fellow mopar guy. Then tried changing one degree at a time until I was advanced at 4 degrees TDC, still the problem has not gone away. The car idles good and when I accelerate slowly I have no problem. But when I accelerate quickly it back fires and stalls. The distributor, the plugs, the EI Module, wires, rotor and cap are all new. Still the problem exist. So my question is before I take everything apart. Can it possibly be the CAM or the timing chain or possibly my carb. The carter which was originally on the car is gone, replaced with a holley 4bbl about 3 years ago.. Any ideas?

Rob

by **dave-r** » 12 Dec 2001 15:00

12 degrees BTDC is about right. How are you measuring it?

A BOG and a stall would most likely be the carb but a backfire has to be the spark or valve timing I would have thought. Also make sure the plug wires are separated correctly.

by **Rob** » 12 Dec 2001 21:22

I will check the plug wires first. One little thing I didn't mention. When I had the car at 12 BTDC it was running ok. The problem really didn't start getting bad till I adjusted the accelerater pump. I know they are not in the least related. But it seems everytime I touch something, something else goes wrong.I'm checking the timing with a timing light, why do you have any other suggestions?
Rob

by **dave-r** » 12 Dec 2001 22:07

Well put it back to 12 degrees BTDC and leave it there for a start.

Is the vacuum advance disconnected and the carb port for it plugged when you are setting the timing?
Is the vacuum advance connected to the correct vacuum port? You don't want manifold vacuum. You want the port that feeds from above the throttle plates.

Did you adjust the pump shot richer or leaner? Adjust it so that there is just enough so that any bog just disappears.

by **Christer** » 13 Dec 2001 11:24

When I listen to your story, I come to the conclusion that there is something wrong with the acceleror pump or that it is wrongly adjusted. Maybe it needs a rebuild (leaking membrane)? Maybe the carburetor is set to lean overall? We call this problem that the engine "dry-bangs" here in Sweden (to lean mixture).

by **dave-r** » 13 Dec 2001 12:31

I tend to agree with you the more I think about it Christer. That is why I asked which way he adjusted the pump shot.

by **Rob** » 18 Dec 2001 15:44

ok.. I think I finally got it, but I still don't have the answer WHY?. OK I readjusted the timing to 12 BTDC still had the problem.. SO I road tested it and kept backing it up (the timing that is) Finally after getting to 32 BTDC the Hesitation stopped for the most part. I can feel it a little when the car is cold. But warmed up it seems to be running great. I'm not exactly an expert in motor repair so, I'm having a hard time figuring out why 32 BTDC works for my motor. Any ideas on whats causing the problem or is it not a problem?
Rob

by **dave-r** » 18 Dec 2001 19:21

Take it to someone that knows what they are doing is the short answer.

If I asked you to check if the timing marks are correct at true TDC on the compression stroke would you know what I mean or be able to do that?

For a start if it is BEFORE TOP DEAD CENTER you are ADVANCING THE TIMING not retarding it or "backing off" as you put it. No way will an engine run with the timing advanced to 32 degrees INITIAL. There is only 45 degrees between the firing of each spark plug. At 32 degrees BTDC the piston would only be just over half way up the cylinder. And thats before and distributor advance! At 4000 rpm the spark plug would be firing when the piston was only JUST starting the compression stroke from the bottom!

So either your timing marks are wrong or you are doing something wrong yourself such as using the wrong spark plug lead as the source for your timing light or using a dial-back type light without understanding its operation.

One of the things I am going to do when I get my web site updated is to a series of photos showing simple tasks like this for people with little or no mechanical experience. That will be too late for you though Rob so get it to a tune-up shop before you do any damage!

by **Rob** » 21 Dec 2001 15:07

Dave.. Thanks for the advice.. I think I have learn here reading the boards than anyplace else.. As far as the timing marks go, I know for sure the balancer on my motor is not the correct one, because when I bought the car it had a wax pencil mark for TDC, I have a new Balancer on the way, I think I'll change that first before I seek professional help. BTW the cars running great still, Maybe I'm a genuis and I don't know it..lol... Have a good one dave and Thanks again...
Rob

by **dave-r** » 21 Dec 2001 20:23

Ah!
Well that will not help for sure!
Let us know what it was when you get it sorted Rob.
Good luck.