Tohatsu m50d - Dragging piston with spark & Fuel

[Burgieman]

Hello all,

I have recently discovered my Tohatsu 50 two stroke is only running on the top two pistons. The third has no effect when I pull the coil from the plug.

So far I have confirmed:

1) Spark is good. New plugs gapped.
2) swapped coils and there was no change to dead piston
3) sprayed premix into carb no improvement. (#3 carb appears not to be sucking like #1 & #2
4). Engine dies if I cover up other carbs. #3 has no effect
5). pulled # 3 carb. It is very clean as are the other two.
6). Spark looks powerful and appear same as other good pistons
7). Ran Mercury Quicksilver Power tune
8). Compression for this motor says 102 for the 50hp.
My results warm:
#1- 110psi, #2- 105psi, #3- 95psi
9). Plugs in #1 and #2 seem normal, a little carbon, #3 seems lightly damp with oil or fuel...

Am I looking at stuck rings in #3? How should I proceed from here? I’m mechanical, have rebuilt heads on cars but don’t really want to tear into this unless I have to.

Engine say for five +years fogged and stored prior to purchase last year. Any next step advice would be very helpful.. was hoping to go out this season.

 

[MattFL]

Going on what you posted; compression on #3 should be "good enough" to fire I believe, and you have spark. The #3 not sucking like #1 and #2 is a big clue. Look down into the reed valve area, or take it apart if you can, and check if one of the reeds has failed or if something is stuck in it. Any problem with the reed valves could cause that cylinder to stop sucking air. Reed valves failing on boat motors is pretty rare, but maybe a critter died in the carburetor during storage and when you fired it up it got sucked in and is now holding a reed open or something.

 

[Burgieman]

Thanks Matt,
I did do an in experienced visual flashlight look in there. Reeds seemed in tact. But could still be an issue...

another tech mentioned the CDI possibly causing incorrect or inefficient spark. He suggested using a timing gun to confirm spark so I’ll do that shortly.

one other thing that came up about 20 mins before me throwing the towel in yesterday was that the Tach started to act erratically high and low readings that were clearly off.

 

[MattFL]

When you look at the reeds, look very closely at the sealing edges. Look for something small that might be holding one of the pedals slightly open, or any chips if it's plastic or rubber. The tach does sound like you may possibly have some electrical gremlins though. The timing light is good idea. To test the strength of the spark you can also get a spark gap tester, Amazon has some inexpensive ones. That would allow you to compare the strength of the #3 cylinder against the others.

Amazon.com : spark gap tester

 

[Sea Rider]

With stuck piston rigs the PSI reading will be darn low, not your case. To check spark intensity need a spark gap tester to check if all jumps the same distance. There are other electrical components under the fywheel that feeds the CDI and that in turn feeds the ignition coils which must be electrically troubleshooted. But if misbehaving badly due to internal overheating/cooling cycles under load will drive you more than nuts to find the culprit fast..

Happy Boating​

 

[Burgieman]

With stuck piston rigs the PSI reading will be darn low, not your case. To check spark intensity need a spark gap tester to check if all jumps the same distance. There are other electrical components under the fywheel that feeds the CDI and that in turn feeds the ignition coils which must be electrically troubleshooted. But if misbehaving badly due to internal overheating/cooling cycles under load will drive you more than nuts to find the culprit fast..

Happy Boating​

Thanks sea rider. I suppose your eluding to testing the stator. I will try that tomorrow. The multi meter with a DVA addition is not giving any logical readings when the + is connected to the output of the CDI to coils and the - is connected to a coil grounding terminal. So no luck getting any usable peak measurements. Can a poor stator cause a dead cylinder with spark and fuel?

 

[pvanv]

That motor does not have a stator; It has an exciter and of course trigger coils. Full info is in the Factory service manual.

 

[Burgieman]

We’ll,

update. New timing gun shows spark working on #3 coil. New CDI did not change anything. What else could it be? Components look clean. Motor is not corroded.

 

[pvanv]

Did you go through the lower carb?

 

[Burgieman]

It was the only one I took off and it looked spotless. Main jet was clear. I didn’t view the other jet. I will remove again and view reeds on that carb too. I just don’t feel I will find much. It all seems so clean. The piston is getting fuel as the plug is damp compared to #1 & #2… I tried squirting a fuel mix through the open butterfly and nothing changed. I tried squirting fuel into the piston, replacing plug and firing it up. No change either…

 

[Burgieman]

Ok so I thought an update would be helpful.

After I replaced the CDI (as per a 30 year marine techs advice) and tested all electrical systems and timing adjustments and the lower carb jet to no avail, I had a mobile marine tech come by today.

First he tested spark on all cylinders and it was good. He said it could be bad but he’ll run the compression test and we’ll know quickly.
The compression test on his gauge was great.
His test came out 118, 120, 120. He said this engine is “mint”. So that was very exciting.
He pulled the airbox cover and saw the lower carb weeping fuel mix. Took one look at the butterfly choke valves and saw that one was just a hair off from the others. The lower one. He adjusted the rod one turn, put it back on fired it up, within a few seconds the engine was purring on all three. He said just a hair off and the air flow is increased dramatically compared to the others forcing it to run rich and flood the cylinder. Incredible, I noticed that when I pulled the carb and thought, “ah that would never matter… it’s barely visible.”

tach still doesn’t work and jumps around but now it’s ready for business. Michael Pachette is the tech. Great guy out of NH covers southern Maine also. Great rates.

 

[Sea Rider]

Yep, all carbs should run with their butterflies well adjusted to factory specs for motor to run top.

Happy Boating​