Mercruiser 4.3 odd running condition

[sledmonster]

Good day all,

1994 Bayliner 2252, well maintained 4.3 and Alpha Gen II. Running 15p prop and will pull up to 4400-4600 WOT @ 27knots.

I have an odd condition that I can't diagnose. I have strong spark, good timing, good fuel pressure and supply.

- The condition is that while coming on step or once cruising at around 3900-4100rpm and at close to 3/5 - 3/4 throttle I will

have what I feel is a single cylinder or only one at a time either; missing, backfiring. It is an "audible" and "feel" in the boat.

definetly miss firing and backfiring thru either intake or exaust and sounds to me like both. If I do nothing it will continue to

happen with no input, I can either drop a bit on the throttle and it will go away or I can apply more throttle and "power thru it".

I can essentially duplicate the problem at will (same throttle and RPM).

My thoughts - head gasket, floating valve, aspirating water through a failed gasket?


Any thoughts or Ideas would be helpful. Also this will not happen on muffs with no load.

 

[alldodge]

I'm thinking fuel but what is your serial number?

 

[sledmonster]

Engine Serial Number 0D824060

 

[sledmonster]

I was wondering what adjustment the 2brl carb might have for higher rpm.

Also thinking maybe carb icing?

 

[alldodge]

I think its the carb getting lean which causes the backfire. Its at a transition point and most likely caused by the channels, power jet and or adjustments. Has the filter in the carb been changed?

 

[sledmonster]

Carb filter has been removed and cleaned, fuel is new, ran of a daytank as well to confirm fuel flow and supply. So if its fuel its likely a carb adjustment issue.

 

[alldodge]

Most likely it just needs cleaning and adjusting
The power jet circuit may be the issue

https://www.mercruiserparts.com/bam/subassembly/31484/1619/80

 

[sledmonster]

Do you know if the "power jet" has any adjustment or moving parts? Also any base settings known would be helpful. And to clarify I can get through this rough patch with more throttle and rpm. During rough operation it seems to hunt/surge combined with backfiring sounds, again this is all at very high rpm so it has been hard to sort out. And if I just avoid letting her go slowly through this range I can avoid it ( as in a faster acceleration through this range) will avoid it.

I will look again into fuelling.

 

[alldodge]

Do you know if the "power jet" has any adjustment or moving parts?

No adjustment for the power valve, it works off of vacuum produced by the motor. If its a bit clogged then motor does get enough fuel when transitioning up in rpm

The only issue with letting it go is if it gets lean enough to start preignition. If preignition starts it can melt a piston.

 

[QBhoy]

Crap fuel would be most likely. Perhaps a small chance you may be hitting the rpm limiter too and your tach is off. But I’d think fuel issue is the answer

 

[Bondo]

The only issue with letting it go is if it gets lean enough to start preignition. If preignition starts it can melt a piston.

Ayuh,...... Lean Detonation can completely destroy a motor in minutes,......

Do a carb Rebuild,.... After it's tanked, 'n blown clean of course,......

 

[sledmonster]

I will look at the carb closer as a possible culprit. Fuel is fresh, definetly not a rev limiter as I can pull another 500 rpm above this point with smooth and clean power. Also timing light confirmed rpm as well. I never run it in this condition more than a second until I can slide through it. I can cruise at 3700rpm at 24 knots for as long as I want with no symtoms, only when i ask for more power and throttle does it show up.

 

[achris]

Used to have exactly the same on my old 4.3LX. How old are the spark plugs? I had it happen every 2 or 3 years. New set of plugs cleared the problem every time.

Chris......