91 mercury 90 hp

[philipjames5k5]

I have got the oil injection dissabled and I am running 50:1 premix. The motor idles fine but begins to miss at about half throttle.Compression is 90 on all three and carbs have been cleaned. The top carb was running lean about a month ago so I cleaned the carb and now the plug is beggining to tuen darker. Any ideas to trouble shoot my miss fire?

 

[philipjames5k5]

Re: 91 mercury 90 hp

Oh also when I trim the motor up after running it for a bit then lower it there is fuel leaking from my airbox probly a 1/4 cup of fuel. Could this be a sign of a carb not functioning properly that would cause my motor to miss

 

[philipjames5k5]

Re: 91 mercury 90 hp

Anyone?

 

[Silvertip]

Re: 91 mercury 90 hp

Carburetors have a float bowl that is full of fuel when the engine is in the vertical position. When you tilt it full up you essentially are tilting a coffee cup full of fuel so it will run over. don't tilt it so high.

 

[philipjames5k5]

Re: 91 mercury 90 hp

Ok so that has nothing to do with the miss under load?

 

[philipjames5k5]

Re: 91 mercury 90 hp

Anyone???

 

[CharlieB]

Re: 91 mercury 90 hp

Carb float adjustment is critical to proper running, too high and that carb will flood, allowing that spark plug to foul.

Aging spark plugs with carbon accumulation can seem to run fine at lower speeds then begin to mis-fire at higher speeds when the load is increased.

First inspect the spark plugs, if carboned replace them, second, open the airbox, trim the motor level, with the motor OFF, open the throttle wide, while looking down the carb throats pump the primer bulb and see if any fuel is leaking in any carb, if so double check all float levels. while the carbs are off, turn them upside down and blow into the fuel inlet to ensure the needle is sealing.

 

[Texasmark]

Re: 91 mercury 90 hp

When your carb floats are functioning properly that engine can be tilted all the way up with out a problem. I have an '02 version and it doesn't.

Mark

 

[philipjames5k5]

Re: 91 mercury 90 hp

Ok ill take them off and make sure the floats are working properly. Anything else that I can check besides plugs?

 

[Texasmark]

Re: 91 mercury 90 hp

Ok ill take them off and make sure the floats are working properly. Anything else that I can check besides plugs?

No. Take the airbox cover off and tilt the engine all the way up. The one dribbling gas out the venturi is your smoking gun.

Mark

 

[philipjames5k5]

Re: 91 mercury 90 hp

So you what your telling me is the one that is leaking is probably causing my missfire

 

[quicktach]

Re: 91 mercury 90 hp

Missfire could be a carb being too lean BUT I suspect an electrical issue. All plugs firing with good blue spark ? All new clean plugs ? Get the CDI book and start electrical testing.

 

[philipjames5k5]

Re: 91 mercury 90 hp

TexasMark - is it possible that the leaking carb is the result of a miss fire I am confused on what you mean by its my smoking gun.
Quicktach - I did a quick google search, I cant seem to find a CDI book for my engine, but i am getting a blue spark when holding the plug about 1/2 inch from the block. I know in order for it to miss its got to be air fuel or spark. How can I first figure out which one is missing? it idles fine, starts right up, works fine at quarter throttle but misses instantly at half throttle and anything over it. I do not want mess anything up by running it so any pointers on how to figure out what cylinder is missing? Thanks for all the help so far guys.

 

[Texasmark]

Re: 91 mercury 90 hp

I directly answered the airbox question, second one from the top. Once you get up there in rpms you are working more off the high speed jets or you have a wiring issue under the flywheel....chafing. But I don't see chafing being a problem in that wide of a band of operation, but a high speed jet clogged could.

Check for your smoking gun first. Smoking gun being the term for he carb that has fuel leaking out the venturi when you tilt all the way up. Then right the engine and go back in and drop your bowls. In the bottom of the bowl is a brass plug....high speed jet. Check for plugging.

I just redid my '02 with the 2B2 carbs. In the bottom of the bowl was the plug as I mentioned. Upon inspection the bowl "appeared" to be clean, no nothing but alum and the plug. However, when I hit the plug with air, blowing from the center bowl direction to the outside of the bowl, a plug of black something or other blew out. That was my problem for high speed troubles.

HTH,
Mark

 

[philipjames5k5]

Re: 91 mercury 90 hp

Ok co thanks for clearing that up for me