Uh-Roh- valve stuck?

[powerplay230]

Uh roh.. I have not used the boat in almost 5 months due to lack of time and too many things going on. Off work this afternoon and it was beautiful out so thought hey why I don't go clean up the boat and make sure batteries are charged and everything is ready to go for the weekend. Good news was that 1 battery had plenty of juice and engine fired up right away.
Well of course where there is good news there is also rotten news. Upon starting engine started back firing through the carb and making a way too loud banging noise. Shut it down after 10 seconds and take a look at everything, yep all looks ok. Start it back up and yep still doing it, it sounds like 1 cylinder backfiring about every 1 1/2 seconds. At first I was hoping the banging was the spring loaded Stainless Marine flappers I have in the exhaust. Now I'm having a nightmare of a stuck valve- what would that sound like? First steps to check it out- pull valve covers and see if one appears to not be moving? pull plugs and run compression test? Have never had anything like this happen so am a little lost- engine is a 502 18 months old with about 50 hours on it.
Any advice (besides buying lotto ticket on way home)
Thanks,
Rick
:confused:

[canuck1]

Pull the covers and check the springs/valves. could be cam lobe or rocker arm. you could also pull 1 plug wire at a time to find which cylinder it is

[Fiat48]

Pull valve covers and turn the egine over and watch all valve train action. Also check for broken valve spring. Start there.

[powerplay230]

Hope this isn't too dumb a ? but are roller cams less likely to have a cam lobe knocked off. If it is a stuck valve shouldn't I pull both heads and go through them or just the one with the stuck or by now bent one (I'm not an optimist today). Anyone know if the 502 magnums came with stainless valves?
:eek!: < ready to move away from the gulf and find a big lake somewhere...
Thanks Guys!

[Fiat48]

Don't paint the devil on the wall yet. Get those valve covers off.
Generally a roller cam failure is a lifter failure. The roller breaks and suddenly you got a whole bunch of valve lash.

[powerplay230]

It just hurts a little bit more when everyone at marina is going aahhhh those batteries aren't even going to turn on the radio. Then I get that s**T eating grin when it fires up as soon I turned the Key. Of course my jaw dropped before I could make any smart comments to the naysayers.. Friday will have to sneak out of work so I can go pull the covers and all the plugs out and start looking at some of the basics. Hmmm wonder if the wife will fall for it's time for the 502/502 conversion haha.
Thanks Fiat!
Rick

[powerplay230]

Today only had a little bit of time so pullled plugs, #4 was really fouled bad. Did compression test and #4 was only at 110lbs and #5 was between 130-135lbs. so now I only know which cylinder is definately hanging up. To get valve cover off I need to pull exhaust manifold on that side and didn't have time so will get into over weekend. Saltwater boat, always flushed and thru-hulls so thinking since it sat quite a while something corroded enough to hang up valve. Will find out soon hopefully, just bites as spring break is just beginning and I won't be able to look at the bikini's from this vantage point.:) Still hoping its nothing too big but with my luck I almost already plan on pulling both heads to clean up and do complete valve job. If I get that far is it worth putting a new cam grind in- 502, sq ports, world products intake, mercruiser manifolds running to 4" thru hulls, still running Thunderbolt ignition and Bravo 1 drive, Thinking if I have to pay to go through heads what's another 500 for cam if it will make a difference, right now stock cam that cam in 450hs/502.
Thanks all,
rick

[Dave C]

Are they sticking?
if so, try squirting some ATF down the valve guides to see if it will free up. If not pump some compressed air into the cylinder to see if its leaking, around the valves. (see other thread about leak down testing)
Good luck

[powerplay230]

Pretty sure on #4 something is causing it to hang up and not close fully. I think if it was a lifter, cam lobe or anything like that it wouldn't open but would be sealed pretty good for compression test. air test for a "leak up test" is a good idea, but now I think it's a matter of finding out what is making it not close all the way, what might have hurt the other side (#5 a little) and making sure valve isn't bent. Tried pouring some oil thru carb without any result. Haven't had time to do much else yet and to get valve cover off I have to drop exhaust manifold down out of the way. If nothing else is obvious will try some ATF or marvel or something down the valve guide before pulling heads off. Don't want to spend a lot of money on it right now but don't think I would be comfortable running it long even if the ATF worked, and wifey just found out so she isn't too happy. She was just getting ready to buy new furniture and play "Trading Spaces" not me spend money on the boat. Hopefully will find out more by Sunday night or Monday..... Thanks for the Tips!
Rick
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