working great and then.. BAM!

Hello again, so my 351 C was working like a top after taking it to garage and having timing set and carb set up. Now only one week later it is back to back firing through the carb, which I guess is technically not a backfire, but I still call it that due to the hair loss on one side of my head from flame! If you guys recall I put the new cam and lifters and Edelbrock intake on my 73 vert. I had a time getting the dizzy right so I ended up taking to a garage to get timing set and then it worked great, now once again it is back to hesitating and backfiring through carb when you stomp on it under load, but when I was playing around with choke yesterday.. BAM... a big ball of fire comes out of carb and singes my hair on one side...... now I had to march into house to explain to my wife how I still know what I am doing despite the funny smell of burning hair. After doing much research on the internet I learned that in order to backfire through carb, it is a spark issue?

Is your distributor gear in good shape? Double check to make sure the distributor hold down is not loose or possibly bad threads. It sounds to me that your timing is jumping somehow. Another possible issue is your balancer. Sometimes the rubber wears out between the inner and outer sections of the balancer and it can rotate and through off your timing setting. I had this happen on my 390 and it drove me crazy. Usually if it is the balancer you will not be able to get the timing set right at idle. Just a couple of thoughts.

Unfortunately if the timing is right and it is still backfiring, you may have worn a lobe off of your cam. Clevelands are notorious for being hard on new cams if everything isn't broken in right due to the angles and loading on the cam and lifters created by the canted valve arrangement. Not saying that this is the only potential cause, but I would be concerned after a recent cam change and the prior troubles.

I would pull the valve covers and check for a loose rocker arm, then rotate the engine to observe rocker arm movement.

check the coil condition and the wiring condition, though I suspect you already know they are good from your prior issues as well. Looke at the cap for cracks or other damage.

.. BAM... a big ball of fire comes out of carb and singes my hair on one side...... now I had to march into house to explain to my wife how I still know what I am doing despite the funny smell of burning hair.

Click to expand...

:worthlesswithoutpics:

have you checked spark plugs and made sure you got good juice. Sometimes a bad plug will not fire for a few rotations then boom it lights up. Take your plugs out and see what they look like. A spark plug can tell you loads about how an engine is running. "rich,lean"

And found this for you to.

The spark/ignition timing has to be changing. Now the question is why? I'm just throwing out some random possibilities.

If the distributor is loose it can rotate, grab the distributor and try to twist it. It should not move. If it does figure out why if not move on to the next thing. It may be the distributor gear moving. Look at the roll pin that retains the gear, if it is sheared the engine will not hold time. If the balancer ring is moving it will make the timing look like it has moved but will not actually change the timing. The advance plate could stick too far advanced and backfire on start-up. I'm not sure anything else inside the top end of the distributor can move or cause enough timing change to cause a backfire.

I had a similiar prblem with mine. I had to keep changeing the timing because it would start running like ccrap and backfiring. I pulled the dizzy and found the roll pin was sheared and the gear was walking on the shaft. Ordered a new msd probillet and no troubles.

Yes, I had to change the balancer on the front of the crank upon close inspection, apparantly I ruined it by using the wrong puller to remove it during the rebuild(live and learn). Now I reset the timing, (I learned how and bought a timing light), it seems to like 14 degrees best. But once again, it runs great but has a new problem... a small surge in the car, when you are going about 50 mph. It is very slight but bugs me just the same lol!

Ted. Ahappy mustanger in Canada!

Yes, I had to change the balancer on the front of the crank upon close inspection, apparantly I ruined it by using the wrong puller to remove it during the rebuild(live and learn). Now I reset the timing, (I learned how and bought a timing light), it seems to like 14 degrees best. But once again, it runs great but has a new problem... a small surge in the car, when you are going about 50 mph. It is very slight but bugs me just the same lol!

Ted. Ahappy mustanger in Canada!

Click to expand...

How big is the cam? I could cause a small surge at certain speeds/rpms

If it is an older Holley, your backfires could have blown a power valve. The simple test is to turn in the idle mixture screws until the car dies. COUNT THE TURNS! If the screws bottom and the car does not die, then the power valve is blown.

If you have not adjusted these, the method I prefer is to put a vacuum gauge on the engine and adjust for maximum vacuum at idle and then turn them in a 1/4 turn. Keep in mind these only affect idle mixture and once you are driving are irrelevant to your air to fuel ratio

If it is an older Holley, your backfires could have blown a power valve. The simple test is to turn in the idle mixture screws until the car dies. COUNT THE TURNS! If the screws bottom and the car does not die, then the power valve is blown.

If you have not adjusted these, the method I prefer is to put a vacuum gauge on the engine and adjust for maximum vacuum at idle and then turn them in a 1/4 turn. Keep in mind these only affect idle mixture and once you are driving are irrelevant to your air to fuel ratio

Click to expand...

Hi, it is a new carb. (prior to the backfires),

You've put your initial timing at 14 degrees but do you know what your total timing is with full vacuum advance applied? Excessive advance at cruise can cause the surge you describe. At cruise your distributor is applying full mechanical and vacuum advance. If your distributor is giving you more than about 48 degrees total advance at cruise you need to limit your mechanical and/or vacuum advance to reduce total timing. Lean cruise A/F mixture can make it surge as well.

When I took mine in for initial timing after the first fire-up and after replacing the distributor, the builder (who builds engines for the strip) pulled the vacuum hose off the bottom of the dizzy and threw it across the parking lot. Then he took a pair of pliers and broke the vacuum nipple off the dizzy and said, "You can't time these Clevelands properly with the vacuum advance connect, and they won't run for shit if you connect it.

I've put about 60 miles on it since then, and it runs and starts like a champ. Food for thought, maybe?

for the dragstrip maybe, for the street-I would use a different mechanic.

Agree to checking total timing. You can put a tape on the balancer and check timing at 3000+ rpm's.

Mine is set to get 38 degrees total advance. I set mine in this manner. Another method is to use a vacuum gauge and tune to produce maximum vacuum at idle. Vacuum advance should connect to manifold vacuum not ported vacuum. Ported vacuum was for emissions and does not help at idle where you need it.

New Holley has blowout valve protection and should be fine.