1988 40hp low rpm

[dschroeder]

had the 3 carbs just rebuilt, on the water at full throttle its only getting 1600rpm with a 13" pitch prop. starts great. great throttle response and dosen't stutter at all when i snap the throttle open on the water-muffs. started it on the driveway and misted starting fluid into the carbs rpm sagged dramatically on the bottom 2 carbs, but no change at all on the top carb. I'm getting spark to all the cylinders, grounded each plug against the block and had the girlfriend bump the starter.

fuel is fresh, new tank and lines with a fuel/water seperator filter installed

this engine had sat for about 5 years before I bought it, compression is good. drained fuel out of all 3 carb bowls so its getting fuel. pulled off the fuel filter on the engine, and found some small black particles in the center of the filter.

where do I go from here?

 

[jeffersondodson561]

Re: 1988 40hp low rpm

Fuel pump replaced? Should be after sitting that long. Put remote control in WOT postion, pull off airbox and check that throttle plates are in the horizontal position. Did they doe a sync/link?

 

[dschroeder]

Re: 1988 40hp low rpm

throttle plates do open wide and move together. didn't do the fuel pump though

 

[99yam40]

Re: 1988 40hp low rpm

If you have good compression and spark on all cylinders then I think you have a problem with the top carb jets/passages being plugged.
Would not be the first time the carbs were not cleaned good enough or trash from lines or fuel pump came through and plugged the carbs because they were not replaced or cleaned at the same time as the carbs.

 

[dschroeder]

Re: 1988 40hp low rpm

whats the easiest way to clean them?

 

[jeffersondodson561]

Re: 1988 40hp low rpm

I perfer ultrasonic cleaner, HF has one for about $70, half/simple green half/water. Heats up the solvent too, run the cycles about four times (half to rotate the carb every cycle because the solution does not totally cover the carb) but I have never had a problem and I don't have to disassemble the carbs jets, then air dry (compressor nozzle). Everyone could use an ultrasonic cleaner I brought mine up to my Dad's and put some of his irrigation heads in it and they came out good as new. Saved him about 60 bucks, it will pay for itself in a couple of uses.

 

[dschroeder]

Re: 1988 40hp low rpm

well, ran the boat on the driveway this am...sprayed carb cleaner in each carb (1 second burst) and it flooded the engine out, flywheel can be turned by hand, not sure how tight this engine should be turning it by hand, starts quick great throttle response, no soot on plugs or in the prop hub from exhaust. just falls flat on its face once it gets in the water.

 

[jeffersondodson561]

Re: 1988 40hp low rpm

well, ran the boat on the driveway this am...sprayed carb cleaner in each carb (1 second burst)

You mean like 1 mississippi... or 1; you just need to spray barely a burst, if rpms drop you most likely have proper air/fuel mix, no change combustion chamber problem (spark/compression/too rich), if rpms increase you have a lean condition.

 

[dschroeder]

Re: 1988 40hp low rpm

turns out head gasket is split between #1 and #2 . compression was 110/125/125 and leakdown was 30% 10% 10% so should be back together mid week so hopefully that fixes it

 

[99yam40]

Re: 1988 40hp low rpm

this engine had sat for about 5 years before I bought it, compression is good.

I thought you said compression was good.
What happened to cause the blown gasket? could have been running lean and overheating
Usually over heating is what I have seen.
Make sure the head is flat and true

 

[dschroeder]

Re: 1988 40hp low rpm

first mechanic that looked at it said "compression is good" never gave me the numbers. not sure what caused it. previous owner just idled around in it, cylinder heads were carboned up pretty bad

 

[dschroeder]

Re: 1988 40hp low rpm

final issue found, cdi box won't advance timing, its stuck at 7deg before tdc so that part will be here tues and should be running right