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1983 18hp mercury. Carburetor issues.

D

Dan.G

New member
Hi new here, I have a 18hp mercury that I'm having surging at wot and running a bit rough. Had mechanic do full service carbys ect but still same problem. Checked and replaced all the usual plugs hoses, leads, coils.
Can someone tell me what this air jet is actually for?
I think the main fuel jet is supposed to be .052 or 1.3ml but is actually .0571 or 1.45 ml could this be the problem??? And somthing to do with that air jet on the left because when I put a smaller air jet in that one circled it ran better but still surged a little at WOT. The first pic is that jet which was 3ml and I went to 2ml.
 

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Squeezing primer doesn't help or change when surging.
But holding finger over that air jet on the left does help stop it for some reason that's why I tried making it smaller did help quite a bit???
 
Sorry----I guess I missed running with another tank in post #1-----What other information can you pass on before I make more wild trouble shooting suggestions ?
 
I Really feel like it is a fuel air mixture problem.
Mabe float level but have followed manual to make sure its right.
Wondering if having that larger fuel jet could cause issues with that carby?
And someone must know what that air jet is on the left in the picture.
Link to parts diagram above its number 32
 
Mechanic said compression was very good but the plugs don't look even.
First pic old plugs second pic new plug after 30 min
 
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That is a backdraft jet, it reduces pressure in bowl to lean out the midrange for fuel economy. You got a carb problem if you got to cover it up...take some grease and seal up top cover of carb and retest.
 
Thankyou for your reply I'll try that first
Do you think that because the main jet is bigger than it should be its running to rich and that by making the backdraft et smaller it leans it out? Could it be a solution as I can't find any replacement main fuel jets anywhere.
 
Thankyou for your reply I'll try that first
Do you think that because the main jet is bigger than it should be its running to rich and that by making the backdraft et smaller it leans it out? Could it be a solution as I can't find any replacement main fuel jets anywhere.
 
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