Power loss problem (vacuum leak?)

[WallyMaster]

I have a 1981 9.9 Marioner (the Japanese mariner model). Starts and idles great. When open up to full throttle, goes at full speed for about 15 seconds, then bogs and reduces to about 70% power. I have done the following and the problem still persists:

1. Got a new fuel tank, hose, bulb, and fuel hose connectors.
2. Replaced fuel pump diaphrams.
3. Suspecting possible overheating, I pulled the t-stat and tested it in a pot of water. Opens at about 150 degrees. So that's ok. Also, there is a good water stream out tell-tale.
4. Removed carb and cleaned it out with carb cleaner and compressed air. Checked float level and it was right on spec.

Last time out (after having done all of the above), when it started bogging, I pulled the choke out just a bit -- not all the way or it stalls... just out about about 1/4 inch. That did it. The power increased back to full speed. Clearly, then, there is some fuel starvation or excessive leaning-out. Since the fuel pump diaphrams and everything upstream of it have been replaced, I am suspecting a possible vacuum leak at or near the carb. Sound reasonable? if so, how would I check for that? Anything else it could be?

 

[Yepblaze]

Re: Power loss problem (vacuum leak?)

Pull carb again and manually probe the jets.

I've had a piece of plastic get in a main jet and remain unseen until I actually poked the jet and it fell out.

Good thing I was wearing my reading glasses that day, or I would have never even seen it.

 

[WallyMaster]

Re: Power loss problem (vacuum leak?)

OK, pulled carb again and manually poked through the jets and passages. Re-installed and tested. Problem persists. So, I'm back to the theory of a vacuum leak 'somewhere' near the carb... but how would I locate such a leak? Unless there is something else it could be??

 

[achris]

Re: Power loss problem (vacuum leak?)

I've seen the same problem caused by a leaky crankshaft seal. That would be considered a vacuum leak... Check that the crankcase hoses are all ok.. Not sure about that specific model, but some have check valves in the line... Check they are all ok.

Chris.....

 

[WallyMaster]

Re: Power loss problem (vacuum leak?)

How would I test for a bad crankshaft seal? Would a bad lower seal be revealed by fuel/oil leaking down the shaft (with the lower unit pulled), and a bad upper seal by fuel/oil in the area of the breaker points? I guess I am thinking that a bad seal should reveal itself in SOME leaking at least somewhere (or not??)

 

[gss036]

Re: Power loss problem (vacuum leak?)

Try squeezing your primer bulb and see if that helps. Hope you replaced the fuel line/bulb w/OEM. Some/most of the after market bulbs do not work well. Make sure your fuel supply is not too far from the motor. If possible raise your portable fuel tank above or as high as you can above the engine to see if that helps, if it does, then I would be looking at connections. Crankshaft seals can cause that type of problem, but I would try others things before digging into the engine. On auto engine, we spray wd-40 or similar product around the gasket areas and if it sucks it in and stops the surge/inceases the speed, that's an indication of the problem. You actually can spray the whole engine, but I would do it slow to see where you can judge the effects. I would not use gasoline for this test..