Merc cruiser 5 0 backfire

[QBhoy]

Don't see popping/backfire when rev limiter is reached. It kills spark a bit

TBV limiter is in the 5K rpm range

A friends mid to late 90’s 4.3 carb did just that when it was limiting. Literally a popping sounds. Not the usual electronic type the efi or mpi might sound like. Surprised me too. Known others since to do it too. Like a rapid popping sound for sure. Was about 4900/5000 on his questionable dash tacho. But me around there somewhere you’d think.

 

[Lancerdad34]

Got it!

 

[QBhoy]

mine will seem like a popping for sure when i hit it. If you don’t know what it is or not experienced could be confused with a back fire easily. Big difference between a lean condition is the boat will run perfectly until 4900 + depending on tach accuracy

Definitely Scott. Very easily confused with a carb backfire or lean condition. Sounds exactly like that.

 

[QBhoy]

the rev limiter is set at 5200 or so

at what RPM are you getting the backfiring thru the carb?

as stated above, most likely a lean sneeze.

what were the compression numbers?
what is the fuel pressure to the carb/
what were the results of checking the fuel filter contents?

Think it might be a little less than that. Not sure. If it’s rev range is the usual 4400-4800. Around 5000 or close to it I think others have popped at that I’ve been on similar. Weird for sure the first time I heard it. Not like the usual limiter on later models like mine.

Got it!

let us know how you get on. The guys may well be right with it being other than the limiter. But 100% some of these things defo sound like a popping from carb when they hit it. If you are anything near or over 5000 rpm…that’s exactly what it is.

 

[alldodge]

A friends mid to late 90’s 4.3 carb did just that when it was limiting. Literally a popping sounds. Not the usual electronic type the efi or mpi might sound like. Surprised me too. Known others since to do it too. Like a rapid popping sound for sure. Was about 4900/5000 on his questionable dash tacho. But me around there somewhere you’d think.

Agree to disagree
All good

 

[Lancerdad34]

Think it might be a little less than that. Not sure. If it’s rev range is the usual 4400-4800. Around 5000 or close to it I think others have popped at that I’ve been on similar. Weird for sure the first time I heard it. Not like the usual limiter on later models like mine.

let us know how you get on. The guys may well be right with it being other than the limiter. But 100% some of these things defo sound like a popping from carb when they hit it. If you are anything near or over 5000 rpm…that’s exactly what it is.

4800 when it happens if I back off a little it goes away.

 

[Scott06]

4800 when it happens if I back off a little it goes away.

So it is more rpm related than I have it at WOT but am at 4k rpm and accelerating ? If so then I would think you are tapping the rev limiter. I am a life long motor head and it surprised me the first time I hit it on my 21 ft'er w/5.0.

If you see it at lower RPMs when under full throttle acceleration then I would think this is more fuel delivery related. Reading your sparkplugs immediately after a good WOT run would confirm if you are lean...

 

[Lancerdad34]

So it is more rpm related than I have it at WOT but am at 4k rpm and accelerating ? If so then I would think you are tapping the rev limiter. I am a life long motor head and it surprised me the first time I hit it on my 21 ft'er w/5.0.

If you see it at lower RPMs when under full throttle acceleration then I would think this is more fuel delivery related. Reading your sparkplugs immediately after a good WOT run would confirm if you are lean...

Thanks for the reply!