1982 ADI Mercury 115 6 Cylinder pesky miss

[Kentuckylake94]

New to the forum, and running out of hair to pull. Full backstory on this engine. Original to the boat, which is a 1982 pro craft 1650v, serial of the engine, 06093360 model:115622
Engine had low compression on cylinder 3, I have touched many inboard engines, hardly any outboards. But I dug into the fun task of doing a ring rebuild on the engine. New gaskets all the way around, insured exhaust baffle plates were not warped and luckily had no bolts break off, Compression is currently 120 on every cylinder with a few cylinders within 3 psi. The engine has a new stator, cdi boxes, crank trigger, spark coming off of every plug wire, new plugs, carburetors have been rebuilt with oem kits, and yet still the engine has a sluggish miss at idle, and will run a 16 ft boat decently around 25-30 mph, studdering at any higher rpm. I took it to a shop with a guy I trust very well that tried doing a lync and sync on the carbs and timing. What he ended up telling me was that the bottom two cylinders were still misfiring even though it’s really hard to tell if you disconnect the plug wires to said cylinders. (Slight change in engine rpm) even though I have good compression. He’s stating I may have low vacuum on the bottom two cylinders. My next thoughts for diagnosing are to look into yet another compression test, and where I have the enrichment valve that is functional, take vacuum readings on each intake port. And check diaphragms on the fuel pump. Anything else you guys can think of?

 

[racerone]

If maximum speed is 30 MPH there is something wrong with the engine.------You might want to revisit carburetor work on bottom one.----Labarynth seals OK ?-----Bottom seals replaced ?-----Reed valves OK ?---------What sealer did you use on the crank case joint ?

 

[Kentuckylake94]

If maximum speed is 30 MPH there is something wrong with the engine.------You might want to revisit carburetor work on bottom one.----Labarynth seals OK ?-----Bottom seals replaced ?-----Reed valves OK ?---------What sealer did you use on the crank case joint ?

Thanks for the reply, fully aware with the low speed meaning something wrong with the engine, labrynth seals are good, bottom seal replaced, not a lot of carbon build up on the mid section where the powerhead connects, reeds we’re all in very good shape, and for the crankcase joint I used marine rtv, called many boat shops and did my research before taking on this rebuild. Thin layers by no means would I gob the stuff on, as far as the rebuild goes everything seems kosher and I have backtracked on the rebuild to check these issues. The miss almost acts like yes I will be visiting the bottom carb, or still so a spark issue. The miss is pretty prevalent when at idle speed in gear, or under a load, but not as prevalent at idle in neutral, motor bucks kicks. However, at 2000-3000 rpm on plane the motor runs pretty decent, I wouldn’t go so far to say 100%, maybe 80%, anything higher once on plane it starts to do a studdering miss almost like a too rich situation.

 

[racerone]

RTV is not to be used on this or any outboard motor !!-----Use only Loctite- 518 or BRP product called " gel seal " on this joint.

 

[Kentuckylake94]

I’ll go back and look at that.

 

[racerone]

The sealers mentioned have " low filler content " and most if not all will squeeze out.------This then results in a precision " metal to metal " joint.-----Then the bearings and reed blocks will be clamped up properly !

 

[Kentuckylake94]

The sealers mentioned have " low filler content " and most if not all will squeeze out.------This then results in a precision " metal to metal " joint.-----Then the bearings and reed blocks will be clamped up properly !

Ahh I understand, what would be my end result in that though? Low vacuum? Loss of pressure within the block? is there any sort of way to test for that?

 

[Chris1956]

Compare your spark plugs with each other. All should be brown or black. Any that are clean, like new can indicate water infiltration into that cylinder.

Your main jets in the carbs are installed and correct for the motor? It should not studder at 3000+ RPM. If so, make sure the high and low speed stator windings are wired to the switchboxes correctly.