Sticking valve

"I've been following some

"I've been following some of the posts on here concerning backfiring through the carbs and also knocking noises in the 350 crusader engines. I've tried several of the suggestions found here but to no avail. Curious though, I'm beginning to think that what I am hearing is not a backfire at all. I say this because I started the engine with the flame arrestor off and brought the RPM's up to where I heard the noise I thought was a backfire. I held my hand above the carb and could feel or see nothing that would be described as backfire. If a valve were sticking or something to that effect could that be mistaken for a backfire?"

"Is it a rythmic "pfff, pf

"Is it a rythmic "pfff, pfff, pfff"? A fairly minor exhaust gasket leak sounds like a little explosion. Exhaust manifold gaskets, intake manifold crossover exhaust, and the choke riser are areas of concern."

"A sticking intake can cause a

"A sticking intake can cause an intermittent backfire. Without knowing what you heard, it's hard to do more than guess. a bit more info may help: things like one engine or both, idle or running, hot or cold, etc.....

I can tell you that 'noise sources' are tough to find on a boat, especially driveline related ones. Best thing to do is to get someone to man the helm while you do the investigating.

If you suspect sticky valves, a vacuum gauge would be a good investment. There have been several threads where links to good "how-to" sites were posted."

If you suspect a sticking valv

If you suspect a sticking valve it might be due to condensation on the stem causing rust. If you take the valve cover off you could operate the valve by tapping it with a hammer mildly which will cause it to operate and return faster than the cam will operate it. I've cleaned several up this way.

A worn cam lobe can cause a co

A worn cam lobe can cause a constant backfire through the carburetor at higher speeds and under acceleration...Idle is usually not affected...check for equal lift on all rocker arms.

Re: A worn cam lobe can cause a co

bobby c is right about the flat lobes and chevrolet did have a problem with this in the early 80s - but any engine that runs at low rpms (like idling) stands a chance of loosing a lobe or 2 . the use of an aftermarket oil supplement is a good idea for boat engines that see a lot of low rpms .