496 backfire
 

My friend's fairly new 496 started backfiring through the exahust yesterday anytime the rpms went over 4000. We plan to check the codes later this morning with a scan tool but does anyone have any thoughts on this? Would it be worth checking the fuel pressure?

You sure its coming out the exhaust and not the flame arrestor? We had a cracked spark plug, which caused a misfire to come through the arrestor screen.

Had the same issue on a pair of hp500's. Let my son help on the tune up. and he cracked a couple on install. The back fire was thru the flame arrestor, thuogh it sounded like exhaust when running.

I believe I have the exact same problem. Ran fine last weekend but this Friday went out and started missing/backfiring at high rpm but only under load. Revs up fine and smooth in neutral. Runs perfect cruising around. Looking forward to some advice also.

Yeah, sounds just my dad's 496 in his Sunsation 288. He just replaced the plugs and it was fine. It was backfiring through the arrestor only under load. Change the plugs and I'll bet you'll be fine.

Here is a site that talks about the same thing you might be having, It turns out the $18.00 spark plugs that I believe you need to run gives out also, But from what I have been told it is not safe to run any other brand of spark plugs in that engine, Or you may have even a bigger problem. Read the post from "Gfinishline" It sound like he found it and after reading that post I have read about three other engines doing about the same things as yours, So It might be it worth looking at anyways, Good luck with it.http://www.performanceboats.com/html...ad.php?t=37580 http://www.performanceboats.com/html...ad.php?t=37171 http://forums.iboats.com/showthread....pi+spark+plugs

It sounds to me like it would be a broken valve spring on an exhaust valve.I would think that motor would have dual springs and maybe one is broken.Or the valve is hanging up in the guide at higher rpm's.If timing is all working correctly that"s about all I would think could make it backfire out the exhaust.Spark plugs fire when both valves are closed so a bad plug would make the motor miss and not fire out of time.

I think I would have to agree with Pickeljim on this one.
If the engine is truly backfiring or popping out the exhaust only at above 4000 rpms, I would suspect a weak or broken valve spring on an exhaust valve on that side of the engine. If the cylinder does not seal completely during combustion with an exhaust valve hanging open, it will combust out the exhaust system with a pop or backfire.
I also have seen a cylinder not firing and spitting a lot of unburned fuel into the exhaust where it lights of from the other cylinders hot exhaust which could be caused by a bad plug, but the engine should be missing from that plug under load.
I have seen 496's go into guardian at 4000 rpms and spit and backfire if the throttle is held WOT, but that will show up on the scanner. Good luck finding the problem. My suggestion, figure out which side is backfiring and then look for exhaust valve spring issues under that side valve cover.
I don't think spark plugs are the issue here.

Best Regards,
Ray @ Raylar

If it was valvetrain related why would it not do it in neutral? I can hold mine at 5000 rpm in neutral steady and it runs perfect. And it runs perfect almost all the way up to full throttle.

I did have a poly lock back off and actually tighten, causeing zero + lash. It ran fine at idle to about 4 grand. Then would pop out the exhaust. So must confirm if it is out the exhaust or flame arrestor.