70 hp 2 stroke low throttle issue

[medicineman]

Just put my project 1987 70 hp etlh 2 stroke engine in the water for the first time this weekend. only history i knew on the engine was that it came off of a 16 foot bass tracker and had sat unused several years. i cleaned the carbs (three times actually as i missed a jet the first two atempts!), put on new plugs and a new fuel pump and replaced some of the fuel lines. i put the engine on a double decker 28 foot pontoon.....so a much heavier boat. I put a new water pump in it and changed the prop to a very low pitch prop appropriate for a pontoon. it has the auto oil mixing yamaha blend but to make sure it was working properly i also used 50:1 mix in the first 12 gallon tank. i ran the engine pretty easy and watched the oil consumption, and it used 7 ounces oil in approximately 4 to 5 gallons of fuel on the variable mixing system----- less oil than 50:1, but i had read that less oil is used in that system than manual mixing----hoping that is appropriate!. now to the small problem i have. It cranks and idles fine. when i put it in gear it does not cut off, but it runs a little rough---almost like it is running on two cylinders until i give it about half throttle. then it clears up and sounds great. if i lower the throttle, it will seem to still run well. If it stop and let it idle, it will do the same thing all over again with the roughness. The idle circuit carburetor adjustment screws were set before i cleaned the carbs at factory settings----1 7/8 turns and i returned them to the same setting after cleanings. The fuel tank has been diluted down to approximately 100:1 now and of course it is getting oil from the yamaha precision blend system also. I doubt that that tiny extra amount of oil is causing my issue, but since it will be a month before i get to ride the pontoon again I would appreciate some opinions on what to do next. I also am wondering if due to the extra weight of this boat over what it was originally on if perhaps i need to open the idle jet screws maybe 1/4 turn each. Since the engine is running well at 1/2 to 2/3 throttle-----actually about the time it kind of comes on plane, or as much plane as a pontoon does, I want to go slow with changes and not cause other issues.

[TownsendsFJR1300]

Can you post a video of it running both smooth and rough?

IF there is an issue, sounds like an idle circuit still slightly dirty.
With the carb clean, did you PULL all the jets and physically look thru them? Also, did you check the float levels?

Did you do a compression test on the engine since the issue arose?

I'd also peek at the spark plugs, they should look pretty much the same...

[99yam40]

how about the fuel pump leaking gas into #3 crankcase.
Just something to look at

[medicineman]

i did a compression check before i used the engine and they were all 130-`135. i did pull all jets and cleaned them well. I did not measure float levels. could see thru all holes. have not pulled plugs as they only have a couple of hours.run time. will not be back to boat for month so can not do video.

[medicineman]

how about the fuel pump leaking gas into #3 crankcase.
Just something to look at

what would be the best way to check that out? I put a new fuel pump on it-----i could put the old one back on i guess.

[99yam40]

what would be the best way to check that out? I put a new fuel pump on it-----i could put the old one back on i guess.

I was just saying an old fuel pump could be leaking, I would not expect a new one would be

[TownsendsFJR1300]

The fuel pump would be a prime candidate but you replaced it. You could/should check it, but it's probably fine.

The spark plugs should show something amiss...

If you can, when you bring the boat in for the day, run it where it's "mis-firing" for a bit. Then shut down, trailer/dock, etc.
Then check the spark plugs... If one's mis-firing, flooding, lean, it should look different from the others...

You can adjust those idle screws just a tad, just try to keep them within spec. I kinda doubt that's the issue if it runs noticeably rough for that large RPM range..

[medicineman]

The fuel pump would be a prime candidate but you replaced it. You could/should check it, but it's probably fine.

The spark plugs should show something amiss...

If you can, when you bring the boat in for the day, run it where it's "mis-firing" for a bit. Then shut down, trailer/dock, etc.
Then check the spark plugs... If one's mis-firing, flooding, lean, it should look different from the others...

You can adjust those idle screws just a tad, just try to keep them within spec. I kinda doubt that's the issue if it runs noticeably rough for that large RPM range..

I will check the plugs out as you suggest next trip. I would not say it is a wide rpm range----even thou i am giving it half throttle or so the rpms are not picking up very much. seems to struggle a bit to get rpms up enough to make boat plane out, but once rpms pick up it does perfect. but even with roughness, has never once stalled.

[99yam40]

too much fuel to a cylinder is bad enough, but not enough can be very bad.
need to find out what is wrong before you cause major problems.

Took me a long time to figure out someone had drilled out the main jets in my C40

test everything you can to eliminate what you can.

used motors can be a pain