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Mariner/Yamaha 55hp 2-stroke won't run good (Video)

R

Royall

Member
Hi there,

Just bought this old 2 stroke engine from the 80's. It's a Mariner/Yamaha 55hp. Got it running, but it doesn't run good.

I had to turn the gas screw on the carbs all the way in to get it running idle, but still bad.

Here is a video:

What do you guys think?
Is one cilinder not running?

Cheers
 
Just tested the engine if it runs without one of the plug caps and yes. Engine runs the same without the bottom plug cap.

So it's running on 1 cylinder... I checked the bottom spark plug and it is pitch black?

Swapped the spark plugs but then the engine won't run.. Edit: Engine does run when sparks are swapped
 
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Compression is 110 psi on lower and 120 on upper cilinder.

Hooked the lower cil spark plug cable to my spark gab tester and it does spark when running the motor on the upper cilinder, but it's a rather thin white spark.

Also tried swapping the spark plugs again and the engine now also runs when swapping plugs. So it's not a faulty spark plug.
 
racerone said:
Does the spark jump a gap of 1 cm (3/8" ) on both leads when cranking it over , yes or no ?
Click to expand...

Im not sure as it isnt an electrical start motor. Have to pull and watch, but if im right it is the same...
Bottom spark is easily tested as I can run the engine on the upper cilinder and watch the lower spark while it's running.
 
Sorry,---- I was looking for a simple YES or NO on this.-----With clear answers you will get more tips.------As it is now I can not be of further assistance
 
Can you take a video that just shows one of the carburetors? Is there only one adjustment screw?

Sounds like one of the plugs is getting fouled because the air/fuel mixture is incorrect.
 
View attachment 20212
Found a problem in the bottom carburetor. The pilot jet tube has a big crack.

I removed the carb to check it out. Tried soldering the crack but the solder wouldn't stick and I made it much worse as you can see :p

View attachment 20213

I bought a brass tube at my local hobby store and tried to copy it. Couldn't really get the exact same diameter but got a close one, the tube is only a little thicker. Drilled out a bit on one end so the jet screw would fit but I couldn't replicate the holes at the bottom like the original because I don't have a table drill... The thing is I also can't figure out what they are for because they end up inside the carb wall as explained below:

3.jpg
Now I tried running the engine with the new made pilot and the cilinder fired in idle! So my guess is I did well. The only downside is that the engine is stuttering now when I let go of the throttle after revving up. (It's probably because the engine is now running on 2 cilinders and is out of sync or something)

Could that be because I didn't replicate the tiny holes? Also tried and tested each cilinder individual and I was able to run on the bottom cilinder only but it's having a hard time. Have to give it more gas, but too much gas will stall the engine while running on the top one only allows me to throttle it up just fine.

I tried playing with the air mixture screws but I also can't find that sweet spot.
 
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Your arrow appears to be pointing to the main jet and the pilot jet.. they're close to each other. The main jet is the bottom-most hole.
 
Royall said:
Hi there,

Just bought this old 2 stroke engine from the 80's. It's a Mariner/Yamaha 55hp. Got it running, but it doesn't run good.

I had to turn the gas screw on the carbs all the way in to get it running idle, but still bad.

Here is a video:

What do you guys think?
Is one cilinder not running?

Cheers
Click to expand...

oops.

that was engine of my first boat.

I wonder did you clean up the carb.
 

This is how the engine runs atm. Recreated the pilot jet from a brass tube which made it run on two cylinders again.

It runs a lot better but also has some nasty hiccups when you let go of the gas...
 
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