5.7LX miss/backfire at WOT

[nickdcj7]

I took the boat out fishing on Sunday. It was just me and the father-in-law, had about 1/4 tank of gas. I knew it was going to be a PITA to get started because it hasn't run for 3-4 weeks. I finally got her started, and of course it was running like crap for the first 10 minutes, but once out on the river everything smoothed out a bit. I wasn't really planning on cruising, we were just trolling for stripers. On the way back, it was running well and since my wife wasn't there to nag me for going too fast, i decided to open her up. Popped right up on plane and ran up to 30 mph at 4650 rpm, into the tide and wind. About 20-30 seconds after she topped out, I heard some slight missing, then maybe another 10 seconds, she started to backfire. I cut the throttle after the second pop, backed it off to 3800 and everything was fine. Ran it up a second time, and I pulled back as soon as the missing started, same deal.

I'm just wondering where to start looking for the problem. I'm hoping to put the for sale sign up on her this week, and I want to get this sorted out right quick.

Here's what I can tell you:
Carb was rebuilt this year
Plugs, wires, cap and rotor were new mid-season last year, and Timing was done at that time
Gas is maybe a month old, but it only got filled twice this year, both times at between 1/2 and 1/4 tank, and I started the season with a 1/2 tank.

Any ideas would be appreciated.

Thanks!

 

[scoflaw]

Why would it be hard to start ? Choke issue? Starving for fuel, like in plugged fuel filter? Shouldn't run like crap for 10 minutes { choke}. Probably running out of fuel after that WOT 20 second run.

 

[nickdcj7]

Why would it be hard to start ? Choke issue? Starving for fuel, like in plugged fuel filter? Shouldn't run like crap for 10 minutes { choke}. Probably running out of fuel after that WOT 20 second run.

I'm assuming it was hard to start because the carb dried out from sitting. I also attributed the rough running to the same. It went away once everything warmed up. My motorcycles and jeeps used to do the same thing if they sat for too long. The choke linkage does need to be adjusted, but I have it turned out so that it's staying all the way open when hot. I'm thinking it's a fuel issue. I only have maybe 20 hours since the filter was changed in the spring, but I guess that's as good a place to start as any.

 

[Lazy Daze]

Check that all the plug wires are seated well and making good contact at both ends, along with the coil wire. But a few years ago I had what sounds like an identical situation to yours - would only backfire from about 3,800 and on up. Below that I could run all day. The culprit turned out to be the ignition sensor (under the distributor cap). I'm pretty sure that your engine uses the same sensor as my '99 did.

 

[CliffA]

could it be possible that you were hitting the rev limiter?.....if so the ECM would retard the ignition timing to keep the engine rpm's below what it thinks is a safe max....this sometimes sounds like the engine is missing or backfiring....

cliff

 

[nickdcj7]

Check that all the plug wires are seated well and making good contact at both ends, along with the coil wire. But a few years ago I had what sounds like an identical situation to yours - would only backfire from about 3,800 and on up. Below that I could run all day. The culprit turned out to be the ignition sensor (under the distributor cap). I'm pretty sure that your engine uses the same sensor as my '99 did.

I'll see if the book has any way to check that short of replacing it. Mine stops missing as soon as I pull back the throttle though, even if I keep it up even higher than 3800.

 

[cp_71]

I'll see if the book has any way to check that short of replacing it. Mine stops missing as soon as I pull back the throttle though, even if I keep it up even higher than 3800.

When were the risers and manifolds last replaced? A sticking exhaust valve in an SBC, either the number 7 or 8 cylinder, will cause that symptom you're describing. The sticking valve is usually a result of small amounts of water leaking back due to a riser on the way out. MMO may help free up the valve but check the exhaust and replace as needed first.