Motor Backfires Thru Exhaust When Trying To Start

[Jim Coleman]

The 350 chevy engine that came with my Arntz is giving me problems.

I was told it is a rebuilt 70's 350 with a 30-30 solid lift cam.

I brought it up to tdc , and ran the plug wires according to the firing order.

Tried to start it and it is backfiring thru the exhaust. The new 1980's Holley
started leaking gas, so I removed it and installed a 750 Edelbrock that I know
was a good carb as I took it off a running engine.

My question is this: I was told that backfires thru the intake are usually ignition related and that backfires thru the exhaust are usually fuel related.

I haven't tried to start it with the Edelbrock on it yet, thought I'd get some guidance here first.

Thanks,

Jim

[bobcowan]

If it backfires through the carb, it means an intake valve is open when a plug is firing.

Through the exhaust is essentially the same. OR, there's so much fuel in the exhaust port that it's lit off by a hot spot.

[snapier]

It sounds like that the distributer could be 180 degrees out...

[Gaz64]

Backfires can still be wrong ignition timing out the exhaust.

Either 360 crank degrees out, or leads wrong direction on cap.

Verify 10 BTDC, with a spark plug out feeling for compression as you rotate towards TDC.

Remove distributor, refit with rotor where you want NO.1.

Gary

[Jim Coleman]

Originally Posted by snapier

It sounds like that the distributer could be 180 degrees out...

Can you tell me how to fix it?

Thanks,

Jim

[t walgamuth]

If there is a plug fouled the fuel mixture from that goes unignited into the header to be set off by flames from another cylinder that is firing. So If I am correct I believe you can get this even if the timing is correct.

[Jim Coleman]

The plugs are all brand new, and I don't think the motor wasn't turned over enough to foul a plug.

I think my balancer or indicator is lying to me about true TDC. Am going to try a piston stop and rotate the motor
by hand and find both TDC's then divide the marks and find the real TDC and see if that solves the problem.

If it doesn't, I'll have to drop the oil pan and pull the timing cover and check the alignment marks. (hope I don't have
to do that).

Thanks,

Jim

[LG57]

Also check for corrosion inside the distributor cap. Mine was hard to start and would occasionally backfire thru the exhaust. New cap fixed that.

[Jim Coleman]

Thanks.

The distributor is a new one with the module on top of the cap.

Jim

[nathanmargolis]

Not an ASE mechanic but it could be all things mentioned with not being distributor 180 out because the car was running fine.
Does the Cobra run? If so, start it, heat it up, and then touch each header tube right off the head to locate a possible dead-weak cylinder. A few drops of water a sizzle will tell you if the hole is running. That way you can isolate the cylinder if there is a problem.
To me a pop through the carb could be a bent pushrod and easy identified by pulling the valve covers. Worse case scenario is a flat cam lobe.
Most times you can see a flat came lobe by lack of movement in rocker arms.
Probably just a simple tune-up problem though and somehow I bet it could be a bent exhaust pushrod. Good luck in your diagnostics.

[jhv48]

Just for clarification, the number one cylinder is the front right cylinder looking back from the front of the car. Also, the distributor turns clockwise. My bet is you have a plug wire switched.
Firing order is 18436572. Cylinders are numbered 1357 on the right side, front to back, and 2468 on the left bank, front to back.

[Gaz64]

Originally Posted by jhv48

Just for clarification, the number one cylinder is the front right cylinder looking back from the front of the car. Also, the distributor turns clockwise. My bet is you have a plug wire switched.
Firing order is 18436572. Cylinders are numbered 1357 on the right side, front to back, and 2468 on the left bank, front to back.

Sorry Jim, but my pet hate on parts identification.

Many parts are related to the drivers seating position.

The LEFT bank is near the LEFT FRONT GUARD.

So in a small block CHEV, the LEFT bank is 1,3,5,7, and the RIGHT bank is 2,4,6,8.

Gary

[Jim Coleman]

Thanks, Guys --

The wires are installed correctly now- one was crossed.

The 350 came with the car, and I installed some better dress up parts on it.

It has never been started since I got it until now. I was told the motor was rebuilt
many years ago, and it did look like the block had been tanked.

Anyway, I'm still trying to get the thing timed correctly. Thought I had it, it tries to start but just doesn't get there yet.

Thanks,

Jim

[blykins]

You can't put it on TDC and then point the rotor at #1 plug wire. It will be too retarded. Crank a hand-full of advance in it and I bet it will start.

The way I time a brand new engine is this way (adjusted for being a Chevy):

Pull the driver's side valve cover off. Roll the engine over by hand and watch the rocker arms on #1. Roll the engine over until you see the exhaust valve on #1 open then close. At this time the intake valve should be opening as well. Keep rolling the engine over until the intake valve opens, then starts to close. At this time, keep rolling the engine over and be watching the timing pointer and the balancer. When you get to 30° BTDC, stop there and point the rotor at the #1 plug wire on the distributor cap.

I guarantee you that if the engine has spark and fire, then it will start very quickly. Once it starts, set your timing with a timing light.

[Jdata]

I don't think the distributor is 180 out. His timing sounds like it's off. Hopefully, you don't have a stuck or hung exhaust valve.

[joyridin']

If the timing is correct, it sounds more like somebody cranked the rocker arms down too tight and the valves are not adjusted correctly. I would back all the rockers off and check if the valve adjustment is correct.

[Jim Coleman]

Originally Posted by blykins

You can't put it on TDC and then point the rotor at #1 plug wire. It will be too retarded. Crank a hand-full of advance in it and I bet it will start.

The way I time a brand new engine is this way (adjusted for being a Chevy):

Pull the driver's side valve cover off. Roll the engine over by hand and watch the rocker arms on #1. Roll the engine over until you see the exhaust valve on #1 open then close. At this time the intake valve should be opening as well. Keep rolling the engine over until the intake valve opens, then starts to close. At this time, keep rolling the engine over and be watching the timing pointer and the balancer. When you get to 30° BTDC, stop there and point the rotor at the #1 plug wire on the distributor cap.

I guarantee you that if the engine has spark and fire, then it will start very quickly. Once it starts, set your timing with a timing light.

OK, after taking the carb apart and cleaning it finally got it to squirt fuel,
then the connector on the coil on top of the distributor was loose, and I couldn't get it to fire. FINALLY got everything together, and started the process you gave me. My timing indicator only shows a max OF 16 BTDC,
so I guestimated where 30 would be.

Bottom line, it won't start - is backfiring thru the exhaust and now also back firing thru the carb.

Next I'm going to check the valves and see if they're correctly set.

Thanks,

Jim

[Gaz64]

How long has it been since this engine ran?

How old is the fuel?

Gary

[Jim Coleman]

The motor was rebuilt in the 80's.

The fuel came out of the pump at the gas station yesterday.

[Gaz64]

Ok, so the fuel is fresh.
When did the engine run last?
Gary