4.3 Mercruiser hesitates and carb backfires when full throttle from stop

[jdelisle]

I have a 4.3 mercruiser, carbrueted with thunderbolt 5 ignition. This issue started the first time using the boat this season.
I took the boat out on the lake and it seemed to be running good while cruising around at 3k rpm in cold water. The engine temp went to 160 before t-stat opening then dropped to 120-130 and stayed there. Then after a few minutes driving at 6 mph motor stalled. Got it going and made it to the dock. A mechanic I knew happened to be at the dock and he quickly looked at the boat and gunned the engine several times and it didn't stall. He left then I gunned it and it hesitated and carb backfired. The doesn't happen every time but maybe every three times. I added sea form and sprayed carb cleaner in the carb. Removed the gas strainer on carb it was clean and sprayed carb cleaner in carb though gas line.
At the end of season I change oil and filter, oil and water separator, plugs, replace 160 thermostat with 140.
Any ideas, I'm leaning to a carb rebuild but want to run it on the lake a couple of more times to get the sea form through the carb.The boat idles smoothly at 850rpm's and back fire doesn't happen every time.

 

[Scott Danforth]

pull your carb
dismantle your carb
clean your carb
buy new gaskets for your carb
re-assemble your carb

no magic potion in a can will clean your carb

 

[poconojoe]

Could it be bad gas? Or moisture from storage? Maybe run a tank or two so you're running fresh gas. How about some marine stabil? I add it to every tank.

 

[Rick Stephens]

Needs a carb rebuild. Scott is correct, you cannot clear the plugged ports in your carb from the outside. Just wasting money putting Seafoam in it.

 

[jdelisle]

I'll give it one more beat run tonight then rebuild carb. The backfiring or popping is carb issue?

 

[Oshkosh1]

First; Is it a true "back" fire...which is coming "back" through the carb or an "after" fire which comes out the exhaust and is usually indicative of timing or an excessively rich condition in which a bit of raw fuel is detonated past the exhaust valve in the manifold or pipe.?

IF you're sure the timing is good and your cap/rotor/plugs all look good then the carb is the issue and needs to be rebuilt as stated. No magic potion will substitute for physically cleaning the tiny passages. Buy a welding tip cleaning tool with tiny files or use some wire to get the smallest passages clean. It doesn't take more than one of them to block off to cause some pretty maddening issues!

At WORST...you'll then have a rebuilt carb, which is NEVER at bad thing!

Well worth the time and effort. Plenty of YouTube assistance as well!

Oh...and YES, the carb most definitely can be the culprit in this case...

 

[Scott Danforth]

its usually referred to as a lean sneeze. means your motor is running out of fuel at upper RPM's

 

[jdelisle]

Th back fire is definitely from the carb. I took off the flame arrestor and saw the puff coming out of the carb. I've cleaned many small engine carbs and I'm sure it can't be that difficult with aid from utube..
I'll update everyone when job is complete.

 

[nola mike]

Actually, wouldn't jump to a carb rebuild quite yet. Pull your fuel/water separator and see if you have water in there--that will cause those symptoms as well.

 

[jdelisle]

I'll look at the fuel water separator before picking up the gaskets for the carb.

 

[jdelisle]

Spratyed carb cleaner in fuel intake and let it sit overnight. Ran boat on lake and had hesitation but no carb pop. Then ran it at 3k rpm then 4k for about half hour.
Hesitation cleared and boat running fine except for tach which will be another post. Even a blind squirrel can find the nut.

Thanks everyone

 

[Oshkosh1]

Spratyed carb cleaner in fuel intake and let it sit overnight. Ran boat on lake and had hesitation but no carb pop. Then ran it at 3k rpm then 4k for about half hour.
Hesitation cleared and boat running fine except for tach which will be another post. Even a blind squirrel can find the nut.

Thanks everyone

Good to hear.

Yeah...sometimes contrary to the CW the "easy" fix will sometimes work. Sometimes they're permanent, sometimes not. IF it's an old carb I'd still look into rebuilding it when you have a couple of hours though. If indeed you cleared a little gum out, there's probably more somewhere else waiting to cause another issue. May not be right away...but when you say it still has a hesitation...it may be.

In general, and throughout the forums I belong to, I usually assume(I know, I know)the OP has at least checked the basics(fuel/air/spark/cap/wires/rotor/firing order/vacuum leaks) on engine issues and things like F/W strainers on boats. Usually they have but at times we all at times just need the "oh yeah...I forgot about that" simple suggestion.

Enjoy the boat!

 

[jdelisle]

thank you

 

[Bondo]

Ayuh,.... Have ya examined the contents of the fuel filter yet for anything but clean fresh gasoline,..??

I use a quart freezer baggie,....
Crack the filter loose, slip the baggie 'round it, spin it off, seal the bag, 'n dump it out into the baggie,...
Water or crud is easily visible,...

 

[Wnafabrication]

What a great idea! Something so simple but I’ve never thought of that! Good for any time the oil filter comes off!