Marine application - 1980 Mercrusier 260 idle OK under load, high in neutral

[CanyonDoug]

Besides being a newb here, I am no mechanic but trying to see what's up before I bite the bullet on a re-build, that may not be the problem.

Have changed fuel separator and cleaned filter at carb.  When cold, engine idles at about 600... as it warms it gets to 1200-1300.... only because when I set the idle while in gear, in the water with a warm engine at 650-700.  Have tried doing what i could with richness mixture, about 3.5 turns out on each.  Guessing something to do with idle circuit only, because seems to run smoothly under acceleration and at wot.  at about mid-range I did try quickly nailing it and there seemed to be a momentary hesitation but then took right off.... boat is new to me, so haven't really been nailing it other than smoothly bringing up to speed.  Does top out nicely at around 4400 - 4500 and runs strong at that speed... runs cool now that I had an impeller put in the drive unit.

What  do i look for to figure out if this is a carb issue?  Seems no vac advance on these.  No idea about timing and dwell, don't even have a light to check but can get it done.  Just don't want to throw parts at it until I am fairly confident I am going to solve the issue.   Previous owner said engine had been rebuilt prior to him buying it... with about 15 hours.  But, no paperwork to prove it. And I suppose that doesn't mean the carb was touched.  Might very well need a going-over, but would be disappointed to do that and have the problem remain.  Thinking this is either an idle circuit issue, or maybe vacuum leak elsewhere...?

[Cliff Ruggles]

What is the carburetor part number?

[CanyonDoug]

Cliff... 2 a.m. ?!!  Talk abot burning the midnight oil.... I am most appreciative.  I will most assuredly be dismounting and sending yo this thing anyway just to know it was done and done right, but at the moment wish to understand more about what is or is not happening and is it related to a necessary re-build or something else.  Soooo, many thanks for taking time to respond.

Carb number is, as it appears on the side of the carb:
17053288
0829

Looking forward to learning your thoughts.

- Doug

[Cliff Ruggles]

It's a little later than 2am here on the East Coast!

Most likely the "3" is an "8" in the part number.

We do a LOT of Marine carburetors these days, and there is no wait time for them......Cliff

[CanyonDoug]

Cliff,

Since I no nothing really about the history of this engine/carb, other than a claim it was rebuilt, do you have any comment/suggestion as to the symptom I described?  Sound like an internal issue or do you think I should chase something else first.... like vacuum, or....??  In another forum someone suggested adjusting the barrel connection on the linkage after cranking the idle screw up.  Might help me balance the issue, or is that more of a cover-up of an underlying issue in your opinion.  And, of course I understand you cannot actually diagnose something not in front of you, but perhaps the high idle in neutral rings a familiar bell based on past experiences.

I did send in a form for quote and will be sending it your way, but I may also need to address something else that could affect even a fresh re-built carb.

Thanks again, Doug

[Cliff Ruggles]

Being "rebuilt" more times than not isn't telling us anything, and it's likely to be as bad as, or even worse than before it was touched!

Right to start with, especially with Marine carbs, folks do NOT use the correct Marine components, which REQUIRE the high flow N/S assembly, special accl pump, etc.

99 out of 100 times you'll fine the power piston hanger arms bent, so the job is doomed there as well.

Marine carbs also live in a crummy environment, wet all the time, and the fuel sits for weeks, months and even years w/o being freshened up, so they get to digest a lot of stale fuel and water, plus a good amount of sediment and other debris.

Marine carbs also have very tiny idle tubes, not being emission calibrated, so they are almost always "varnished" up, and often plugged solid, so even IF everything is done correctly with good parts, it's still not going to work well, if at all.....Cliff

[CanyonDoug]

Cliff, help.  Carb just got back today (UPS took their sweet time)... doesn't appear to be any gasket, but instead, two, thicker, spacers...?one is marked 1140, the other is 2269-something... If I try to use these in place of a gasket, the mounting studs are way too short... Seem to be adding 1/2 or 5?8" to overall thickness.

If that is the intention, then I guess i need to get new studs of the proper length. Can you advise what size studs?  The two in the back are firmly screwed in.. not sure if I should try taking then out with a small pipe wrench or vise grips... heat them... or maybe I just accidentally got the wrong stuff???  Was really hoping to mount it, dial it in and splash it, but I am kinda dead in the water.  Hope you can reply tonight...
Many thanks,

Doug

[Cliff Ruggles]

Those are the two most common gaskets used with Marine applications.  Unfortunately they don't specify anyplace what gasket goes under the carb for any particular Marine set-up. 

We can supply a thinner gasket if that is what you need.  Do you have a picture of the old one or know what thickness it is?.....Cliff

[CanyonDoug]

Worked great... One of your gaskets was like the Mercury one. Runs beautifully!