Engine backfiring (long)

[tchesney]

Was in Dallas TX this weekend for Cotton Bowl (GO REBELS!!) and stayed with a cousin that has a 65 Convertable that has been converted properly to a V8. Anyways, I want to make sure I am on track here. Every since he got the car back from the engine rebuild, the engine bogs bad off the line and when it gets to app 3500 rpm, all manner of misfireing and backfiring is going on. Engine revs slow also. In the past few months, (figuring it was fuel problem) he has replaced the intake and the carb. It now has a Edelbrock intake and a 600 cfm vacuum secondary Holley. I figured the fuel was not the problem, the ignition was. #1 problem found was the dist. is sitting in there one notch off on the cam gear. At TDC, the rotor is pointing out at the left (drivers) fender, not the #5 piston, as it should. The person that built the engine has offset the firing order by one position and marked it on the cap. OK, this is the same effect as pulling distributor and moving rotor back one notch, shouldn't affect the engine, BUT, also, I pulled the #1 spark plug wire and cranked engine and used a screwdriver to test the spark going to #1. It was very weak, very yellow, not blue strong like my MSD. This car has a late model dist. with a factory type idiot box for the ignition. I have advised him to install a MSD 6AL with a blaster coil and this should help tremendously. Engine has about 2000 miles on it and it hasn't ran right since he got it back. Also, I pulled the plugs and all of them are sooty and dry, pointing to the ignition probs.

Any other ideas guys?? He's flying me back out to Dallas in two weeks to install the new ignition and I want to make sure this is all I need to do. I simply think it's a case of weak spark.

 

[GaPonyFarm]

Was in Dallas TX this weekend for Cotton Bowl (GO REBELS!!) and stayed with a cousin that has a 65 Convertable that has been converted properly to a V8. Anyways, I want to make sure I am on track here. Every since he got the car back from the engine rebuild, the engine bogs bad off the line and when it gets to app 3500 rpm, all manner of misfireing and backfiring is going on. Engine revs slow also. In the past few months, (figuring it was fuel problem) he has replaced the intake and the carb. It now has a Edelbrock intake and a 600 cfm vacuum secondary Holley. I figured the fuel was not the problem, the ignition was. #1 problem found was the dist. is sitting in there one notch off on the cam gear. At TDC, the rotor is pointing out at the left (drivers) fender, not the #5 piston, as it should. The person that built the engine has offset the firing order by one position and marked it on the cap. OK, this is the same effect as pulling distributor and moving rotor back one notch, shouldn't affect the engine, BUT, also, I pulled the #1 spark plug wire and cranked engine and used a screwdriver to test the spark going to #1. It was very weak, very yellow, not blue strong like my MSD. This car has a late model dist. with a factory type idiot box for the ignition. I have advised him to install a MSD 6AL with a blaster coil and this should help tremendously. Engine has about 2000 miles on it and it hasn't ran right since he got it back. Also, I pulled the plugs and all of them are sooty and dry, pointing to the ignition probs.

Any other ideas guys?? He's flying me back out to Dallas in two weeks to install the new ignition and I want to make sure this is all I need to do. I simply think it's a case of weak spark.

I agree that the MSD ignition & coil will be an improvement...don't forget to add new wires.. and a MSD billet distributor would be an excellant investment. I'm not convinced the "screwdriver" test is an accurate measurement. His standard ignition WON'T be as hot as your MSD.

It sounds to me like its a timing problem. I'd pull the cover and check to be sure the timing marks are aligned and that the chain hasn't slipped, or the cam wasn't installed (degreed in) properly. I would check the timing issues before I invested in new electronics.

I can't believe he drove the car for 2000 miles with it backfiring!!!

 

[BigDaddyCee]

Some things I would check is to see where the backfire is coming from? Is it coming from the exhaust/tailpipe area or is it coming from the intake manifold area? With what you have described I would first check for correct gap on the spark plugs. Also I would check your plug wires and make sure that they are not touching one another and crossfiring. Not sure what v-8 you are running but if it is a 289/302 plugs 7 & 8 fire one after the other and if the wires are touching they can crossfire and ignite at the same time. Causing a backfire. Just a few thoughts.

 

[69stanger]

definately check the timing

Im not sure if this will help you or not but check the rocker arms. When I pulled my heads off and reassembled them again I torqued my rocker arms down way too much and my car was back-firing like crazy.

 

[tchesney]

Well, here's some other info. Just normal putting around, it doesn't backfire and spit and sputter. Only under full throttle does it do this. It is definatley backfiring thru the exhaust, not back thru the intake. I also suspect the cam might not have been installed properly, so will check that also. Also, fuel mileage is terrible, about 8 mpg. Engine is a late model 302, freshly rebuilt, (the 2000 miles is an estimate, prolly a lot less than that) and other clue, is that it runs hot, and when engine is cold, it doesn't backfire as bad under load, but when it gets to operating temp, it's getting worse.

At time of rebuild, the radiator was replaced with a new 4 core unit, fresh thermostat and water pump. Transmission is a built C4. The cooling system is sufficient, so this running hot is a timing issue I believe. Doesn't severly advanced timing cause overheating?

 

[BigDaddyCee]

Check all plugs to make sure none of them have cracked ceramic insulators or other damage. Any idea if it is a late model "5.0-HO" or is it a "NON-HO-5.0" Because they do have different firing orders. The HO motor has the same firing order as the 351W and the NON-HO motor is the same as the 289. Yes incorrect timing can cause overheating and excessive fuel consumption. I would get your distributor and timing straightened out first and then see how it is.

 

[68GEETEE]

Timing does cause an engine to run hot, but just for kicks have your friend measure how far away the fan is from the radiator. Mine was so far away that it couldnt be effective and my engine ran hot big time. We rebuilt heads, new radiator, therm, all along it was the freakin fan was too damn far to work right. The fan should be within an inch or inch an a half from the radiator. Another way to check is to look at the fan shroud and see how far in the fan blades are. The blades should be half covered by the shroud. Hope this helps some.