My motorhome (with a 460 Ford) is backfiring through the carburetor when I accelerate from a stop. It was discussed on another thread (I don't know how to link it) ... Tonight I did a compression check and one of 8 cylinders has NO compression, 6 cylinders average 135 to 140 one at 125 and one at 0. Burnt valve? Stuck (open) valve? I didn't put any oil in the cylinder for a recheck I figured NO compression, why bother. The backfire indicates valve ...Right? I mentioned on the other thread that I was getting oil into the breather filter inside the air cleaner too, another indication of valve? If it's a stuck valve can I see it by taking the valve cover off...Wouldn't I also hear the rocker tap? What's funny is how well it idles (slight miss) on 7 cylinders. Appreciate any help. Thanks in advance.
First thing..pull the head over the cyl with zero compression. You may get lucky with a burned valve or a head gasket. More likely, you have a broken ring on that cylinder or a hole in the piston. Even if you have a toasted piston, don't freak. Pull the pan, pull the rod and piston. Buy a new piston and rings and stick it back in the hole. Of course, once you figure out what happened, you need to figure out WHY it happened, so it won't happen again. Good luck, -Abone.
Yes you could see a stuck valve if you remove the valve cover. You can also hook up an air hose to the bad cylinder thru the spark plug hole. Make sure the valves are in the closed position. Listen for the air leak. If it's coming thru the carb, it's the intake valve. If it's coming thru the exhaust , it's the exhaust valve. If it seems to be going into the crankcase it's religious piston (Holy).
If it's backfiring through the carb, you could very well have a stuck intake valve...try what Fast Fords said too
I've seen zero compression and a burnt valve more than a few times ... Only way to know for sure is to check ... I usually use Fast Fords method.
If this motorhome has sat for a spell - THEN HAS BEEN STARTED UP-....it can have a chunk of carbon debris ,stuck in a valve or even two,keeping them from sealing,resulting in very low compression.... Spray water from a spray bolttle such as old Windex bottle INTO the carb as engine is revved at about 2500 RPM speed, [that will steam and dislodge the debris]- if that were the case......
Somewhere, back in my memory, is this big engine had some plastic somewhere in the valve train that broke off and traveled throughout the engine. Could stick in the oil pump, or be sucked into the engine. Maybe one of the Ford guys will clarify. I remember a Mark four that had this problem.
Timing chain gears may be plastic (probably nylon) coated. It breaks down after a while, causes slop, and jumps time. The particles wind up in the oil pan. Sounds like a bad ring or a stuck valve.
The Jalopy Journal
