33ss 8.1 crusader engines fluctuating rpms HELP

[mrterry72@gmail.com]

On my 33ss we ran about 3 hours and the starboard engine rpm went from 3000 to 2500 back to 3000 then to 1700 rpm pack to 3000 and so on .. I suspected the fuel as the boat had set for about 2 years . I have an aux valve to cross feed off the port tank so I switched the starboard to the port tank jand everything ran fine .. I ended up running the port tank dry and had to switch both engines to the starboard tank and sure enough both engines rpms now fluctuated .. I checked the water separators and only got gas . I'm a long way from home any suggestions.. total newb here ..

[tomschauer]

Sounds like you figured it out. Bad gas in that tank.

[bud37]

Also ....check the anti syphon valve on that tank.

[waybomb]

And the vent

[mrterry72@gmail.com]

Can you please tell me where the vent and the siphon value is ..

[buster53]

You’re going to have to climb in the engine room and look at the top of the fuel tanks. Should have 3 hoses coming out of the top…large fuel fill hose, small vent hose routed to the side of the boat and the fuel hose going to the engine. The fuel hose will have an anti siphon valve either at the top of the tank or just slightly inside. I had real problems with my genset a couple years ago. Turned out to be the anti siphon valve.

One way to eliminate the vent as a possible problem…next time you run the boat, take off the fuel fill cap. If the engine all of a sudden starts running good, you have a blocked or partially blocked vent.

[mrterry72@gmail.com]

Thanks I will tomorrow

[tomschauer]

I don't think its the anti syphon valve. you have one for each engine on each tank. The odds of them both failing at the same time are pretty slim.
Vent is a possibility, but if you ran three hours before the problem, most likely not the vent.
Another possibility is a bunch of junk in the tank, clogging the inlet screens. If this is the case, if you shut down the engines for a half hour or so, then they run fine for awhile before acting up it could be tank crud.
I am sticking with bad gas.

[buster53]

I don't think its the anti syphon valve. you have one for each engine on each tank. The odds of them both failing at the same time are pretty slim.
Vent is a possibility, but if you ran three hours before the problem, most likely not the vent.
Another possibility is a bunch of junk in the tank, clogging the inlet screens. If this is the case, if you shut down the engines for a half hour or so, then they run fine for awhile before acting up it could be tank crud.
I am sticking with bad gas.

Re-read the OP’s first post…he only had problems with the starboard tank. When he ran both engines on the port tank, no problems. Sounds very much like bad gas or anti siphon valve….or both.

[mrterry72@gmail.com]

So we let the boat set over night and the boat ran fine for about an hour each off there own tank then the same thing the starboard engine started to have the fluctuating rpms again I opened the gas cap on the starboard side and it did nothing ran every thing off the port and we ran fine for almost 2 hours then both engines had a hiccup and dropped about 300 rpms I opened the gas cap on both engines and opened the hatch to the engine room and the engines seemed to stabilize.. ran the rest of the day on the port no issues.. also this boat sat for two years . It had about 1/4 of gas in both engines. 2 years a go I have the tanks and gas cleaned and filtered . Just before this trip I topped it off with fresh gas and fuel treatment. . I thank it could be this pick up screen your speaking of but where is it and how do I access it thanks ..