oversized main jets, problem?

Cross checking with chart in merc factory manual, the carbs on my 88' V6 merc (W22-1 22-2 22-3) have main jet orfices of 0.76. Checking what I have, they are all in the range 0.80-0.84. What can this do besides burn excessive amounts of fuel. I suppose bigger is better than not big enough. This engine was bought rebuilt, do think the mech's motivation was to avoid eating pistons or damaging cylinders due to lean running conds?<br /><br />-andy

Re: oversized main jets, problem?

its common. I would not switch them back unless there is a running issue. those motors ran on the lean side when new.

Re: oversized main jets, problem?

Thanks rodbolt-<br />update, the idle jets are just undersized. They are 0.04 undersized. I imagine that this is too ok, even though they are small rather than large? <br /><br />I will probably put in the proper size over the winter.<br /><br />-Thanks<br />Andy

Re: oversized main jets, problem?

.04 or .004?<br />you will find that quite often the "wrong" jets were factory installed as a running change. sometimes it does not get into the manual. we usually reccomend a .002" increase for the first oversize aand a .002" increase for the next as well. idle air or idle fuel jets get played with to allow for a better idle occasionally. your spark plugs after a few hours will tell you if the jetting is over rich.but with that motor I would take over rich to somewhat lean anyday.

Re: oversized main jets, problem?

Rodbolt, thanks once again-<br />I meant .004" undersized (should be .046", I have .042"). My plugs don't foul out to the point that the engine won't run, but they have an black oily flim on them consistently. I don't know if this is normal. However, I also recently discovered that when the powerhead was rebuilt the mech didn't put in any t-stats. I've heard that a stuck open t-stat (or no tstat at all) can cause excessive fouling, maybe this is the cause of the black plug. I've installed some tstats. My engine idles great though.<br /><br />-Thanks<br />Andy<br /><br />P.S. what did you mean when you said "we usually reccomend a .002" increase for the first oversize aand a .002" increase for the next". What's the first oversize, what's the next?

Re: oversized main jets, problem?

Having the Idle jets undersized will richen the Idle curcuit. The Idle curcuit is just backward from the main, as they are air jets. Im sure you knew this though.<br /><br />Also, what HP engine is this, my 150 has 66s as main.

Re: oversized main jets, problem?

It's a 200hp.

Re: oversized main jets, problem?

by oversize I was refering to piston size.<br /> on a merc we always richen the jetting after boreing a cyl.

Re: oversized main jets, problem?

I see rodbolt. Thanks for the clarification. It seems to work just fine. But now I know if I ever run into this again. This is the type of info that can only be gained from years of experience or many years of shop work, which you deleiver. Now to my poppit/diaphragm problem