Need help with surging problem at 1/4 throttle

[toddb3]

Hello all,

I've got an 86 Mercruiser 170hp that is actually running pretty well with the exception of a overheating problem that I fixed recently via updated aluminum exhaust manifold, new riser, exhaust bellows, and impeller. I took it out on the lake today and it ran like a charm, no more overheating but after aprox. 20 minutes of running 1/4 throttle to 3/4 throttle I stopped for a break and let it idle for a couple of minutes. After I resumed the engine started making an intermittant poping noise, sounded like it was coming from the air cleaner then the engine started surging. I ran it up to 1/2 throttle and no more poping and only the occasional surge. As soon as I throttled back to 1/4 again it would pop. What is your first guess? The engine starts easily and doesnt deisel when I cut it off.

 

[lilmandavis]

Re: Need help with surging problem at 1/4 throttle

Could be a few different things but I would start with a fuel problem. Maybe some water in there somewhere.

 

[Capt. Valk]

Re: Need help with surging problem at 1/4 throttle

If its popping (backfiring) out the carb then it is firing while the intake valve is still open. Could be a stuck valve or lifter, a little chunk o' carbon stuck under the intake valve, or a timing problem. The surging usually also means a lack of fuel pressure or flow. I would check the fuel filter/water seperator for water, replace plugs, wires, cap, check the timing. Ya hear any tapping at other rpm's? This could be a stuck hydraulic lifter if ya have em. Some Marvel Mystery oil in your oil may free it up if so. If not and ya have a carb and not fuel injection, it could even be the carb screwing up as it comes out of the idle circuit.

 

[toddb3]

Re: Need help with surging problem at 1/4 throttle

Thanks for the replies, I plan on replacing the fuel filter this week and checking the timing. I've got new plugs, wires and distributor cap so that shouldnt be the case. Probably a stupid question but is it OK to run a fuel system cleaner like "STP Complete Fuel System Cleaner" in a boat engine?

Thanks again

 

[cooter2506]

Re: Need help with surging problem at 1/4 throttle

popping thru the carb is usually a lean fuel mixture so I would think you have some water in your fule seperator. remove and replace and test again.

 

[Haut Medoc]

Re: Need help with surging problem at 1/4 throttle

I'm with the water in fuel theory.....
Change the separartor.....​

 

[thrasher]

Re: Need help with surging problem at 1/4 throttle

STP fuel cleaner will not harm anything. Your boat engine is the same basic engine that is in a car. Most people seem to recomend Sea Foam over STP, but any fuel cleaner will work and will not damage the engine.

Gary

 

[toddb3]

Re: Need help with surging problem at 1/4 throttle

Thanks again for all the advice, I found out this morning that my boat doesnt even have a water seperator! Just a clear inline fuel filter like you would see on a muscle-car. Any tips on an affordable seperator?

 

[thrasher]

Re: Need help with surging problem at 1/4 throttle

My boat also does NOT have a water seperator, its 16 years old and has not had a problem. I give it a dose of sea foam every now and then as that is suppose to help mix the water in with the gas and let the motor burn it off (obviously with a slightly lean condition!).
I have not looked at water seperators much, the few I have seen all sold for over $200 and I couldn't justify the cost (I only boat on small lakes with the kids).

Gary

 

[wallydc2]

Re: Need help with surging problem at 1/4 throttle

Wally world sells fuel water separators for $25.00, and every one needs one installed.

 

[thrasher]

Re: Need help with surging problem at 1/4 throttle

I have never heard of wally world, but will take a look. Thanks for the info.

Gary

 

[Capt. Valk]

Re: Need help with surging problem at 1/4 throttle

You should really have a water seperator. You shur there isnt a water trap built ibto you fuel filter? There would be a drain on the bottom. You could take the fuel line off and stick it in a clean glass jar. Pump some fuel into it. Any water will settle to the botton.

 

[thrasher]

Re: Need help with surging problem at 1/4 throttle

My fuel filter is a paper element that is fitted to the mechanical fuel pump. There is definately no water drain or fuel seperator. The boat is a 1991 V6 4.3L mercruiser Glastron and runs great. I like the idea of a seperator, just the cost was far more than I could stomach.. I will try to find wally world.. Now back to Todds problem (I didn't mean to hijack this thread!).

Gary

 

[toddb3]

Re: Need help with surging problem at 1/4 throttle

Well I checked through my fuel pump and carb last night and no water trap, evidently the seperator was an option when the boat was made. I have the inline fuel filter that I spoke about earlier and a small screen filter in the carb inlet. I did find some evidence of water in the fuel and luckily I only have about 5 gallons left in a 12 gallon tank. I'll get some sea foam and add fill the tank and hopefully run it out this weekend. I definetely plan on keeping the tank full for now on and using Stabil. On another subject, is Sea Foam pretty good at removing carbon build-up on intake valves by just adding it to the fuel tank? I read somewhere that people actually pour it into the carb air intake?

Thanks

 

[Capt. Valk]

Re: Need help with surging problem at 1/4 throttle

I was looking on West Marine and I think they may have a spin on fuel filter/water seperater conversion for your motor.