Questionhello, before i start just want to thank you for the service that is much needed and that you provide, thanks. I have a 72 chrysler moterhome. sat for five years. told it ran well, not positive. it backfires on the right side, the #1 cylinder, you can here it and after having the thing running for some time and the plugs already changed, I did a compression test and that plug was wet, but like new. the parts that have put in are, ( plugs, wires, points eliminator kit, coil was leaking and got replaced, had to add a ballist resistor for the point kit, will have a carb kit in by tommarro night, and bought a vaccumm gauge for testing). The compression test was only done on the side that was backfiring, it is a start and the other is a pain to get to alone and have to crawl back out every time to complete the test, the readings are as follows, 110, 121, 110, and 115. after a ounce of oil in each cylinder it was 124, 136, 130, and 116. the last was the plug that was clean still. I am not experienced enough to tell but i am wondering if this is due to the valves not seating. cam wear. there is obviouse sighns of rings going, and I have no reference as to what the compression should be. the gauge had no hesitation which would indicate a sticking valve. any thoughts would be much appriciated. thanks for your time. Neal
AnswerThe MOPAR RB block engines (413, 440) are about as bulletproof as ever came down the pike, so I doubt there is anything seriously wrong, as long as the valves are all opening and closing. Your compression readings are in the normal range - I'd have to know your altitude to be more specific about the ideal readings, but this engine should run just fine with those readings.
I don't know for sure what you mean by "Backfire". In 10 years of doing this, I've learned everyone has a different definition. If you mean that there were flames or an explosion sound coming back up through the carburetor, that is what I call a backfire, and that is always caused by an intake valve hanging open when that cylinder's spark plug goes off. Since you suspect one cylinder of doing that, try pulling that spark plug wire off and run the engine that way. It will miss, of course, but if the "backfire" is gone, you've found the problem-- the intake valve is closing too slowly. This is usually caused by sticky crud on the valve stem. If this is your situation, you can remove that side valve cover, and squirt some really powerful solvent down inside the valve seal for that valve - then crank the engine a bit to circulate it and hopefully disolve enough of the crud to get it moving freely again. If not, I'm afraid it's "off with her head" and pull the valves for a good cleaning, and new stem seals and gaskets.
If this is an intermittent problem, I'd try just changing the oil to a very high detergent rated (Diesel spec) oil like Chevron RPM Delo 400 15W40 and then just running the vehicle on that for a few thousand miles. It wouldn't hurt to add a detergent booster like CD-2 to the oil for some help in cleaning things up.
Getting the engine good and warm and letting it work hard pulling a load for a while is the best way to circulate the lubricant and detergent.
Problems like yours are common if the engine has been run on really stale gas. The valves get sluggish due to years of sitting with crud on the stems, in addition to the sticky varnish that the stale gas brings to the party.
You should always drain stale gas from a vehicle that has been stored - don't try to just dilute it with fresh stuff - drain out ALL the old sticky stuff and run the engine on fresh new gas - failing to do that is the main cause of sticky valves.
This can cause backfires if an intake is sluggish in closing when the engine is at running speeds. The compression check wouldn't catch this, neccesarily, because the engine is turning slow enough that the valve might have time to close. Since you didn't pull all the plugs to make the compression test, the readings would be a bit low anyway.
I've also heard people refer to a pop out of the exhaust system as a "backfire" - that is another problem entirely - it is an intermittent ignition system - some spark plug is not firing consistently, and when it does fire, BOOOM goes the accumulated fuel/air mixture in the exhaust system.
Once the original ignition system has been modified, I'll check out of the discussion - I don't know what you have there, and I don't know anything about them anyway. I like things I can fix myself by the side of the road (and I've had to do it many times, as we travel in Mexico quite a bit) - the electronic systems are great when they work, and it's tow truck time when they don't.
Good Luck - I hope this helps.
Dick
