9.9 Cutting out at low throttle (trolling speed)

[ctranter]

I have an early 90's mercury 9.9 outboard that constantly cuts out when the throttle is very low. I end up trolling too fast, or not trolling at all. I clean the spark plugs every season, and last season I had full maintenance done on it, yet it has always been a problem. Could it be caused by a problem in the gas line? Old gas? some other gas intake problem? Usually right before it happens, the engine revs to a high rpm. I have a feeling this is a common problem with old outboards, is there an easy fix?

[SlowPoke]

Leaking gas line I suspect. Replace the clamps on the hoses from crimp style to a proper hose clamp and inspect all the fitting up to the carb.

[fish-miester]

i have the same problem with mine in both forward and revers. idles beauty in neutral just doesnt like to idle in F or R tho if it starts to bog out or die I just rev it slightly and then it seems to smarten up and will go for a while in low rev..

[discophish]

Leaking gas line I suspect. Replace the clamps on the hoses from crimp style to a proper hose clamp and inspect all the fitting up to the carb.

 

I would suspect this too. I had the same problem with my 9.9. Sounds totally gas related. The vacuum in your gas line is probably leaking somewhere ... check your line all the way to the tank. Could also be a torn diaphragm on your bulb. I had that problem too! Both caused the motor to respond as yours is currently operating. Good luck with the fix.

[DanD]

Usually right before it happens, the engine revs to a high rpm.

 

Could also be a weak fuel pump that at idle it just doesn’t suck hard enough to keep the carb’s float bowl full of fuel?

The next time it starts playing its game of that high rev, try pumping the in-line primer bulb and see if it comes back to life?

If so make sure the lines are not leaking as suggested if they’re ok maybe have a vacuum test done on the pump. I can’t remember what the spec is for sure but 4-6 inches of vacuum comes to mind; regardless that would be more then enough to move the fuel from the tank into the carb.

 

Dan.

[ctranter]

Thx for the advice gents, Ill give it a try this weekend and report back

[tjsa]

 

Could it be caused by a problem in the gas line? Old gas? some other gas intake problem? Usually right before it happens, the engine revs to a high rpm.

 

 

Gotta be air getting in the fuel line leaning the mixture out somewhere, pinhole leak, bad clamp. My 3.3 outboard with internal tank does this exact thing when it runs out of gas. The fuel delivery system is not constantly pressurized(although mine is gravity feed), it gets sucked in from the tank and lines. You provide enough pressure squeezing the bulb to get it started, then the motor sucks what it needs to operate. That is my guess anyway.

[Muskieman]

SEAFOAM!!

 

Randy

[fish_fishburn]

I have run into to this problem a few times over the years and both times I changed the fuel line connector(where the fuel line connects to the motor) and problem solved. Its a cheap part but overlooked often.

[Fisherman]

I have an early 90's mercury 9.9 outboard that constantly cuts out when the throttle is very low. Usually right before it happens, the engine revs to a high rpm.

 

An engine that revs up just before quitting exhibits a lean run, either by crud in the low speed circuit,(dirt,gum,varnish)that requires cleaning and a Seafoam treat and/or sucking air through a hose leak. Big Question, does the rubber gas line from the tank and primer bulb stay full of gas or do you have to pump it up again? Check the "o" rings on either end of the rubber hose, some have them and they get brittle and crack.

[camillj]

Yep .. sounds like air in the gas line to me ... I messed around alot with that and finally switched to a new tank with new bulb/line/float and viola the problem disappeared ...try a new tank first before messing around with the motor ...especially if it revs up before it dies...

[thalweg]

Oxygen(air), spark, fuel. Check each individually and always the easiest things first...ie fuel filter, plugs etc. Could be as simple as a rich/lean setting to as complicated as a bad magneto. Before changing anything...verify it is indeed the problem.

Edited July 6, 2010 by waterrunner

[Big Cliff]

Reving high just before it dies is really bad for the engine, it is running lean so don't keep doing it, get it fixed before you end up with a much bigger problem!

 

It could be a few things and they have already been covered here but it is without a doubt a fuel supply issue and the first thing I would be doing is checking the fuel lines.

[Sinker]

yep, go get a new fuel line, and a can of seafoam.

 

S.

[fishermccann]

yep, go get a new fuel line, and a can of seafoam.

 

S.

Seafoam X 3 .