My 454 Chev is backfiring if you give to much gas peddle . While driving, if you push the gas peddle down past a point it will backfire thru the carb and sometimes out the tail pipe. 4 mechanics gave me 4 different reasons. I replaced the carb , spark plug wires, spark plugs, mufflers, fuel pump, and fuel filters. Putting in shop Monday to make sure the distributer is functioning properly. You can get the speed up to about 50 without backfiring if you watch the gas peddle pressure. Any ideas

Sometimes the power valve in the Q jet will stick. This can be remedied by a dose of carb cleaner

A weak coil will fail under higher load, especially when hot. Aftermarket coils more often than Delco.

It is unlikely for the fuel level to go down, but low fuel level in the bowl will do what you have.

The little filter in the carb inlet can restrict fuel flow, as can any debris in the inlet passage.

Try driving it with the gas cap off.

Hi,
Try running your rig up a hill and down a hill, trying to put a load on the engine, even when going downhill.

If the downhill run is OK, and the uphill run is not, I would vote for the electric fuel pump in the gas tank. Been there, done that.

There have been a few cases of this happening. The pump wasn't designed for a 30 year service, and other brands of coaches have disintegrated before reaching this point.

This problem has stumped many mechanics and owners before. Fuel pressure looks good when standing still, but add the inertia of the fuel and 25 feet of tubing, and issues with the electric pump become evident.

happy motoring!
Matt

I put an oversize fuel pump on the exterior of the gas tank (pump on it was not strong enough). Would it have had a pump on the inside of the tank if the past owner had a pump on the outside when he replaced the tank?
It will backfire hot or cold.

I replaced the carb, spark plug wires, and spark plugs and had it was timed. It ran great for 2 days then started it again.

The pump on the outside of the tank makes me suspicious that the earlier owner had a fuel delivery problem and didn't resolve it. What do you know about the new tank? When I replace the pump I also replaced the pickup tube assembly (Spectra FG12X - part number seared in to my memory).... Was it a new tank, or another used tank that the prior owner used?

It sounds like you are doing the right things with getting mechanics involved and looking at "the usual suspects"... But this problem you have might be something like a fuel delivery one.

Other guys here have experience with fuel pressure gauges; and also warnings!

What year is your rig? Does your fuel tank also supply a generator? Any problems with fuel to that?

Happy Motoring!
Matt

quote:

Originally posted by Jerry Campsey:
I put an oversize fuel pump on the exterior of the gas tank (pump on it was not strong enough). Would it have had a pump on the inside of the tank if the past owner had a pump on the outside when he replaced the tank?
It will backfire hot or cold.

!986 GM started putting pumps in the tank. They were trying to fix a vapor lock problem. they rerouted fuel lines to keep them away from exhaust. I think there is a PDF on here somewhere about this. Your coach being a 1985 probably dose not have an in tank pump.

I had almost the identical problem. mine ended up,being a bad connection on the distributor where the 12v power went into it to power it. The previous owner has removed the factory connection and the connector he put on would come loose...drove me crazy foe a year. this can blow out and burn the coil in the distributor. Also it seems like I burned up a couple of coils because I did not put them in correctly.

If you are having a backfire, it probably means you are losing power to the distributor intermittently and when you get back, it fires the unburied fuel...mine would do THE VERY SAME THING

Here is the PDF on the vapor lock issues. Vapor lock PDF

looks like all thats left is the distributor, everything has been replaced. I know some guys have added a fuel pressure gauge to help diagnose fuel delivery problems. Could be your centrifugal springs on your distributor aren't functioning properly causing timing issues, definitely check that 12 volt connection as suggested. Is your new carb a shop rebuild or a remanufactured one?

Good luck and let us know what they find Monday.

I've dealt with that issue a few times. I've found that OEM spark plug wires are worth the cost. A bad distributor bearing was the culprit on one engine. Also - this is simple - check to see the wires go to the correct plug.

I owe someone a steak at the Barth Rally. Who ever told me to remove the gas cap and try it. To this point solved the problem. I removed the gas cap and tried to make it backfire but I could not get it to. Then I put the cap back on and it started backfiring again. Tomorrow I will have the new cap attach the trailer and go to Palo Duro Canyon and see if it will backfire. Fingers crossed.

Fantastic!

Sorry no steak. Went out put her nose in to a 50 mph wind and started up hill and the backfiring started again. Continued with no gas cap and no air cleaner. In shop now tring to find why. Has OEM plugs and a new Eldebrock carb. Generator runs on same tank with no problems.

Certainly sounds like the distributor advance isn't working right.