Broke down at rest area

[akeagle]

Originally Posted by shootist

Gm high energy distributors all, and I mean all, eventually short the distributor cap inside. Look for cracks and brown residue. Put on a new cap, coil and rotor first. Odds are it will fix the backfiring. Proceed with plug wires and plugs from there if not. frequently the whole center of the cap will be burnt away under the coil and the stupid things would still run , the spark jumping well over an inch to the rotor.

That just reminded me of an old issue with conventional ignitions called carbon tracking inside of the distributor cap. I don't remember the cause(s), but under certain conditions carbon tracks can form and create electrically conductive tracks between spark plug and/or coil terminals inside the cap.

That could easily have the effect of totally messing up the timing of getting the right ignition pulse to the right cylinder at the right time.

Arguably popping the distributor cap off and inspecting the inside for carbon tracking should be one of the first things to check in this case.

[153stars]

Someone could have a pair plug wires / firing order wrong as well as timing. How's the idle smooth ? Excessive popping when letting off happens burnt exhaust valves. Should be big distributor cap HEI it's not too hard to unhook the spring loaded screws and flip it over to inspect it, the carbon brush and rotor without removing any or just a wire or two. Scrape off deposits look for arching tracks wipe it out good with dry towel. Look and listen plug wire jumping to ground we used to look when it was dark .

[George Schweikle]

Another first post, then no response. With all due respect, it sounds like there is not much mechanical knowledge, so detailed troubleshooting advice may not have much value. Other than calling a mobile mechanic, driving home with the blown muffler may be the best plan. Then, finding someone that knows how to work on an engine.

Good luck, and please post back here with any news.

[TMyers17]

If you own a 35 year old RV, you need to know the workings and mechanics of it. Swapping mufflers was a big waste of money.

Unburnt fuel cause backfires. I suspect the carburetor is dumping fuel down the intake. Pull the sparkplugs and see if they are soot black.

The choke could be sticking. In 1985, they were still controlled by a round spring in the intake. When the motor heated up, the spring pulled the choke open.

A shorting plug wire. If its dragging a cylinder, raw fuel is pumping into the exhaust.

Timing could be off. Also, as mentioned, switch the distributor from points to electronic ignition.

My money is on the Carburetor.

Edit: If it is the carburetor and it's a Rochester Q-Jet, the best fix for it is a new Holley.

[MandyRH]

It's a 454. Chassis I believe is a p30. So far we have replaced: spark plugs and wires, cap and rotor and fuel filter. We have checked the fuel pump, vacuum advance and looked for anything possibly clogging it up. It's weird that it's only doing this on the driver side. Boyfriend is starting to think it's a lifter or valve issue. And believe me when I say we would have drove it home without a muffler already if there wasn't a battery issue too. Something is draining it and we haven't been able to figure it out yet. But we did manage to get it going at almost 3:00 this morning. Still not home. But we are getting there!

[Alan_Hepburn]

Originally Posted by baraff

THIS^^^
I would add, a 1985 vintage engine is likely to have points and condenser and not electronic ignition.

Our old 1984 C30 pickup with the 454 engine did, indeed, have electronic ignition - no points in the distributor that I recall.

[LETMGROW]

Originally Posted by MandyRH

It's a 454. Chassis I believe is a p30. So far we have replaced: spark plugs and wires, cap and rotor and fuel filter. We have checked the fuel pump, vacuum advance and looked for anything possibly clogging it up. It's weird that it's only doing this on the driver side. Boyfriend is starting to think it's a lifter or valve issue. And believe me when I say we would have drove it home without a muffler already if there wasn't a battery issue too. Something is draining it and we haven't been able to figure it out yet. But we did manage to get it going at almost 3:00 this morning. Still not home. But we are getting there!

Low voltage could lead to a miss fire and a back fire. Weak spark not totally igniting the fuel charge.

BTW, GM has been electronic ignition since 1974.
Lynn

[TonyMac]

Originally Posted by Alan_Hepburn

Our old 1984 C30 pickup with the 454 engine did, indeed, have electronic ignition - no points in the distributor that I recall.

I can assure all that my 1974 Corvette has electronic ignition original from the factory. I'm certain about every other GM vehicle had it shortly thereafter.

[TonyMac]

Your engine is carbureted. Does it have an air pump on the engine? If so, can you disconnect it?

[Roy-c]

Drive it home the best you can and then find a mechanic that is older than 50 to look at it. You are in over your head and are thinking about tearing into the engine mechanicals on a guess. You need to get competent help. You have already thrown parts ( money ) at it and that hasn't solved the problem. I don't mean to sound brusque but that is the bottom line.

[Down South]

I'm with Roy-c. Changing parts and hoping you changed the right one gets expensive. Get a mechanic familiar with that engine.

[153stars]

If it's true dual exhaust it's only dumping excess fuel and or post cylinder combustion at that bank blowing its muffler.
Pretty sure there isn't and air pump on an 86.
Checking choke is pretty easy. But the Quadrajet kind of always have choke partially closed and will open with higher throttle and load. It should kind of float when off cold choke 5/8" to 3/4" gap open at idle. If you tie it open it will have poor vacuum and power lighter acceleration .
Old trick to check bad exhaust valve/s is to put dollar bill at tail pipe ,if tries to pucker it in shows bad exhaust valve/s. Has to be warm engine at low idle and good exhaust piping.

[MandyRH]

I don't consider it "throwing/wasting" money on replacing a part that needs replaced. And it's not that we're incompetent or in over our heads. We we're stuck in the middle of nowhere with limited resources and tools. Was hoping someone had a quick fix to get us home. But we got here finally. Thank you for taking the time to give your advice. I know you didn't have to.

[THenne1713]

Suspicion of an engine MISS and raw gas going into Hot exhaust; Many small engines/ mowers have solenoid to shut off gas leaving carb when engine turned off to prevent muffler backfire. With yours, being 1985, carburetor (vs Injectors), float valve sticking open/ or set too high can cause FLOODING, as can trash in fuel/ carb needle/ seat? Hope this helps? Is backfire on shutdown, or always while driving? LEAN can also cause backfire, and can be as simple as clogged fuel filter or failing/ intermittent electric fuel pump? Just listening to exhaust and/ or holding hand to exhaust pipe outlet can often detect engine miss. LUCK