1996 90HP Mercury 2-Stroke - Plug Read Help Diag

[Idunnomyname]

Hi there. I used to have an account but had to register a new one because it's been so long since I posted on here. Thanks in advance for any help/replies here!

I'm helping by brother-in-law sell his boat and working to get it running perfect before I list it for sale. The boat has a 96 model 90HP 2-stroke Mercury. Been sitting for a while un-used before be brought it to me.

What do these plugs tell you? Left-most plug in the pic is the top cylinder. Right-most plug is the bottom cyl. (it's dry compared to the others)

Here is a brief history. When he brought me the boat, it would run for about 30 seconds, then die until I re-primed it. I replaced the fuel bulb and the fuel pump (old pump was awful - pic attached). Now it runs continuously but won't achieve full power/rpm. Compression is 90/90/88-ish. I removed and cleaned the 3 carbs, re-assembled, and it runs better now, but still doesn't "scream" like it should. I don't have a tach on it but my guess is I'm getting no more than about 4300-4500 RPM. Top speed is 31-32mph and it should be about 40-41mph.

Thanks again! Eager to hopefully figure this out. To me, it seams something is preventing the bottom cylinder from getting enough fuel but I'm stumped as to what that might be at this point. Any idea's? Suggestions? Insights? Cheers!

 

[racerone]

Does spark jump a gap of 3/8" on the three leads , yes or no ?-----Possibly running on 2 cylinders.

 

[Chris1956]

The bottom spark plug is pretty clean. It should look like the others.

Make sure it is getting spark and fuel from the carb. If you see beads of water on it, suspect the crank seal.

 

[airshot]

From the looks of the pump diaphram, are there pieces of diaphram clogging the fuel lines?? Comp numbers seem low. As stated check spark gap jump.

 

[Idunnomyname]

From the looks of the pump diaphram, are there pieces of diaphram clogging the fuel lines?? Comp numbers seem low. As stated check spark gap jump.

Hadn't thought of that but I'll check. Thanks!

 

[Idunnomyname]

The bottom spark plug is pretty clean. It should look like the others.

Make sure it is getting spark and fuel from the carb. If you see beads of water on it, suspect the crank seal.

Thanks. I'll keep digging and see if I can verify fuel is getting down to that bottom carb and check for spark off that plug wire. But... if it weren't getting spark, wouldn't that plug we soaking wet vs. clean like it is?

 

[Idunnomyname]

Does spark jump a gap of 3/8" on the three leads , yes or no ?-----Possibly running on 2 cylinders.

I'll check. But.... if it weren't getting spark on that bottom cylinder, wouldn't the plug be soaked vs. clean and dry?

 

[Chris1956]

Possibly, however, it is a simple test to see if you have good spark, and if so, rule out that issue.

 

[Idunnomyname]

Possibly, however, it is a simple test to see if you have good spark, and if so, rule out that issue.

Touche - Spark tester ordered from Amazon. Couldn't find my old one. Thanks again.

 

[QBhoy]

I had one of these. Was a 96. Boat sat unused for ages. Previous owner was told the engine was done. I bought the boat and engine cheap. Compression was over 100 on top two and 50 or something like that on bottom. Maybe less. Can’t remember. I left her with 2 stroke oil and penetrating oil down her bores over night. Turned her over by hand a few times over the next day or so. Another test and all three were over 100 and close.
Had a sticking ring.

 

[Idunnomyname]

I'll check. But.... if it weren't getting spark on that bottom cylinder, wouldn't the plug be soaked vs. clean and dry?

Well.... I'm glad you suggested this! What do you know. Top 2 cylinders have excellent crisp spark at 3/8" even up to 1/2" using this new spark tester I bought. The bottom cylinder won't jump across beyond about 1/8". What does that tell you? Thank you again for suggesting this.

BTW.. I re-checked compression w/ my son at the controls so I could watch the compression gauge and capture the readings before my old leaky gauge dropped down. All 3 cylinders are reading 115-120 psi at their peak (before the gauges leaks).

Also - I removed the bottom carb again (from the cylinder with the dry clean plug) and confirmed there is no blockage in the line running to it and the needle valve is *not* sticking closed.

It would appear the issue is the coil servicing the bottom cylinder but I'm still puzzled how that could result in a clean and dry spark plug. Thoughts?

 

[racerone]

No surprise to me on the spark.----Fuel is not burning so plug stays clean !