Need you guys assistance. I'm getting a backfire at cruise or deceleration. Now engine is a 305 sbc New 1406 carb Non-Hei Dizzy New Pertronix ignitor New Plugs New Wires 8.5mm Plugs gapped at .040 Compression test shows between 115-120 on all cylinders. Found a slightly bent pushrod and replaced it on cylinder 5. I removed a spark plug wire from the number 1 cylinder while running and there was no change in idle speed. Same results on 4 and 6. #1 plug fouls up bad the rest look ok. Help me figure this out please. Engine sounds good at idle and pulls hard, but the backfire is annoying and I know something is wrong. I bought plugs from local store and did not tell them it a non-HEI Dizzy, would that make a difference? Thanks
You can get a bad plugs from the store, I would swap another plug in to that hole and see what you get , and look at the dizzy cap you didn't say anything about it's condition and you may also have to do a leak down test.
Bent pushrod may be a clue. Could be a valve not seating properly and/or sticking closed. Backfire out the exhaust would indicate an exhaust valve. My main question would be why did the pusrod bend? Difficult to diagnose via internet. But, I suspect that is where you problem originates. Others will chime in with other thoughts. Keep us posted if and when you find the problem.
Sounds like premature destination (a cylinder firing while the exhaust valve is still partially open or a cylinder with no spark and the fuel from that cylinder is exiting through the exhaust port and burning as the next cylinder fired. Pull each plug site one at a time while running. The one you pull that does not change the idle is the bad one
Your comment of now a 305 - is this a new install of an unknown engine? Ok, without seeing and hearing it In general, backfiring on deceleration (as opposed to acceleration) is generally caused by a lean condition in the pilot circuit. What happens is that the mixture leans out enough to where is fails to ignite consistently. This, in turn allows some unburned fuel to get into the exhaust pipes. Then, when the engine does fire, these unburned gasses are ignited in the exhaust pipe, causing the backfire. Then, the classic diagnosis is too lean a fuel mixture. The real mystery is where that lean condition is coming from. The wired thing is your #1 fouling would indicate otherwise or a completely separate problem. It could be the #1 cylinder charge is lighting off in the exhaust ? Idk and Internet diagnosis is difficult. 115 seems low but if its even across all 8 there's no single smoking gun issue to fix. Since you say it idles well and pulls well I don't think back fire out the exhaust is anymore than a tuning issue. But when you say 1,4,6 have no change when pulled i cant see how it idles well. Some info is skewed or missing here.
yes. Doesn't sound like a damaged engine component but more like a tuning/timing or ignition component problem
Dizzy cap had a small amount of corrosion. Swapped plugs no change. Pulled wires while running and it appears 1,7,4,6 are dead. Posted using the Full Custom H.A.M.B. App!
Unseated, worn or broken ring. if it's only one and is indead oil fowled then chances are it nor worn.
Ok so please expound on your comment of idles well? If 4/8 cylinders are dead it's not going to run well. If the cap is corroded, Change it and the rotor.
A rich gas fouled plug is often mistaken for oil fouled and if it's been running rich or dragging a cylinder the oil will smell gassy adding to the confusion.
Again, this particular cylinder would show low compression. I'm not there doing the compression test but he said it was 115-120 on all 8 cylinders. To me that low # being even indicates a well worn engine but still has some life left in her. Perfect candidate for "restore engine treatment"
This is a new install of an unknown engine. Idles well meaning rpms are for the most steady. Posted using the Full Custom H.A.M.B. App!
I think we have here either some bad info or bad testing methods. Re do that compression test, and redefine runs well please. This will go 6 pages and lead no where, just like the "second opinion" thread did.
There's no way it can run well and steady if 4 cylinders show no change in rpm when cancelled or pulled.
OK so I'll head to the store and grab a cap and rotor. If that's not the fix what else should i look at to restore the dead cylinders Posted using the Full Custom H.A.M.B. App!
I was told there was a small hole in the exhaust before the muffler but they said it wouldn't do anything. Posted using the Full Custom H.A.M.B. App!
Since its a new install of an unknown engine you don't have the luxury of being able to take anything for granted. If it don't run perfect you'll have to go back to basics and of square 1 and work your way up. 1st is to run a good compression test. If you look at your symptoms, you can see how they lead to an internal mechanical issue but your test results rule those out. You need good results on a valid test to proceed up the hierarchy of problem diagnosis. There are other causes that are a bit more obscure but just as viable. Cap and rotor is cheap and if its visually not passing inspection you'll need one anyway. General tune up stuff is just that. You can't tune an engine that needs routine tune up parts.
Maybe intake gasket leaking on the inside and sucking oil from the lifter valley and oil fouling the plug.
Compression came out the same they were all close to 120. Double checked Spark plug wiring. Rocker arms appear to moving pretty good so not sure about the cam? Store was closed so no cap and rotor tonight. Will get it tomorrow. Posted using the Full Custom H.A.M.B. App!
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