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Tell me about a Yamaha 4SG 4hp 2-stroke

D

dieselfuelonly

Member
I picked up this little 4SG tonight. It's in super nice condition, seems to run great, and I know very little about it!

From what I gather from the model # it was made in 1988. Code is "6E0 S" and serial is "094456". Before I take it out on the lake, a few questions:

On the front there is a sticker for 100:1 mix, any harm in running 50:1 TCW3 for a little extra insurance on an older engine?

I have some Quicksilver SAE90 I just put in my Evinrude, would it be OK to use this oil for the lower unit of this motor too?

I tried to find a service or owners manual but didn't have much luck. I have heard this is the same motor as a Mariner of the same size?

Thanks!

sHLjZWZ.jpg
 
Love the pipe wrench, in conjunction with an outboard!
I have a 1996 Yamaha 8hp I picked up for a cousin. It too (owners manual & cowl sticker), states 100:1
We run it at 50:1, with no issues. Back in the 80’s the EPA wanted 100:1 for pollutions reasons, Evinrude also had those 100:1 stickers for a few years, until they cooked some motors. A service bulletin changed it back to 50:1
The gear oil you have is good. The Yamaha manual calls for the same 90W oil.
If I was using this motor only for trolling, or low rpm operation, & running a bigger motor for main propulsion, I might run 75:1 or even 100:1.
Look up Leroy’s Ramblings - he kept two fuel tanks, 50:1 for hi power running, then switched to another tank,with less oil for hours of trolling.
50:1 might foul plugs quicker, but they are cheap.
 
Why not try 25/1 and see if it runs well. If so, better lubrication.

Then maybe try 12.5/1 and see if it runs well. If so, better lubrication.
 
My point is, Yamaha does not have any better metallurgy than other 2 -stroke manufacturers. They tried the 100:1, under pressure from the EPA.
It didn’t work. A service bulletin was issued somewhere around 1987. It involved removing the 100:1 sticker, & replacing it with a 50:1 sticker.
Why? Because they were ruining engines. A recommendation from 30 odd years ago just might have been superseded.
There are not nearly as many Yamaha small outboards on this Continent as there are JohnnyRudes. I spend quite a bit of time on the other side of the World, & over there they are far more common, & they don’t say to run them at 100:1.
Here is a Screen grab from the manual for the 8HP - (Note, the cylinders/pistons are the same metallurgy as the smaller motors), it specifies 50:1
Your post above is ridiculous, that analogy would see 0/1 as being the best lubrication, most critical thinking folks understand it wouldn’t work.
3652D5B3-E1C5-4335-9130-C7B4BD061858.jpg
 
Thanks all, I will try 50:1 and see how it goes. Makes life a little easier on me since that's what I'm mixing for the Evinrude as well. Will change lower unit oil tonight and hopefully take it out on the lake today or tomorrow and see how it does!


[edit]

Just changed the oil in the lower unit, came out nice and clear, looks like the previous owner really took care of this thing. Hopefully it'll run as good as it looks.
 
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Took it out on the lake yesterday, idles nice but I think I have some carb cleaning or adjustment to do. At WOT it would run too rich and bog. Bringing the throttle back to around 2/3 - 3/4 it smoothed out a lot. Got a rebuild kit on the way for the carb. Need to find a service manual for this thing.
 
After taking the carb apart, I found the fuel pump (it's part of the carb on this motor) had a torn diaphragm that was just dumping fuel into the carb which explained the rich running and crazy fuel consumption. Someone had attempted to remove the main jet and cracked the bottom part where a screwdriver bit can grab it, and there was what I'm assuming is an idle orifice (small brass piece that threads in the side of the carb) that was cracked in the body of the orifice as well. I ordered a rebuild kit for the carb, as well as a $60 clone carb off of Amazon. I put the clone carb on as it arrived first and the little motor runs fantastic with a tiny bit of mixture adjustment. I'm actually very impressed with the clone carb. I will rebuild the original carb and keep it on hand as well. I took it down to the coast last week with my Seabright 270 inflatable and believe it or not with just me in there it would get up on plane if I moved all my weight to the bow for a bit to help, as long as the wind wasn't against me. It would push me at about 12MPH. The original throttle cable was in rough shape and badly frayed at the end, I made a temporary throttle cable out of a modified lawnmower cable, and have a OEM one on the way. The only issue I had is after idling for a while once warm, it would stall out and take a bit of pulling to restart. I'm guessing a new spark plug will work wonders as who knows how fouled up it was after it was run extremely rich for a few hours with the leaking fuel pump diaphragm.

Flushed it out with fresh water when I got back from the coast, and am gonna take it out on the lake this upcoming weekend to putz around and do some fishing. I wasn't sure if I was going to keep this one long term or sell it, but after seeing how great it runs and the fact that it's so light and easy to carry around I'm definitely going to hang on to it. I would like to add an external fuel tank connector, since the internal tank has no way of showing fuel level.

 
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