'89 Tohatsu 15- missing a cylinder

[Ro-Tator]

All right...

I've got a sweet tohatsu 15 that's having problems running in the mid speed range. it starts very easily and idles well. at top speed it really pushes my 14' skiff hard. the issue is the high idle to mid speed range. I was given this motor a number of years ago. I have used it and loved it but the mid range issue has gotten worse each year. It's so bad now that I don't want to run the motor anymore without hunting this problem down and repairing it. Being experienced with many kinds of engines on land and sea, I have taken precautions to be sure that it's not being starved of lubrication.

Here is what i've done so far (with no effect on the problem):

thorough carb cleaning (3 different seasons)
carb rebuild
new ignition coil
new spark plugs (every year)
replaced: cd ignition unit
replaced: upper crankshaft seal
checked: reeds (immaculate)

The upper cylinder is the one having the problem. I know this because the exhaust port for the lower one is immaculate with small portions of carbon the way a normal port should be. the port for the upper cylinder is gross with large amounts of unburned oil residue/fuel (looks like creosote from a chimney).

I will do another round of compression testing (readings were 115 and 125 last time).

Edit- I just did a compression test and my previous statement was wrong.
top cylinder got 125 psi
bottom cylinder got 124 psi both were dry. using oil did not increase these numbers.

Any opinions and suggestions are appreciated.

Thanks

Tator

 

[TOHATSU GURU]

Re: '89 Tohatsu 15- missing a cylinder

The compression difference doesn't sound like a lot, but since you have the low cylinder there and that's the one with the problem...Depending on the mechanic you talk to you will hear that a 5 to 15 difference is acceptable. I'm a 5 guy so I would say you have a mechanical problem at a 10 difference on that one. I would re-ring it at a minimum.

 

[Ro-Tator]

Re: '89 Tohatsu 15- missing a cylinder

Well- I wanted to re-check the compression since my memory of the last test was vague.

I don't think compression variation is my problem. 125 on the upper and 124 on teh bottom is pretty good imo. However, I don't have the specs. Then again, they're practically the same so I'm happy about that.

After spending some time with the motor today, I speculated that the gasket on the crankcase side of the reed plate could theoretically cause issues if it leaked between the two separate crank cases. I have sprayed wd-40 on the outside of the intake side of the engine while running it to check for vacuum leaks. It didn't make any difference. The leak i'm wondering about would be in the part of the gasket that separates the two sets of reeds.

Thoughts?

Thanks for the speedy reply, guru.

~Tator

 

[TOHATSU GURU]

Re: '89 Tohatsu 15- missing a cylinder

I think your back to basic troubleshooting. Eliminate the ignition system by testing the coils under the flywheel as well as the ones you have already replaced. Once your done with that start looking at the gaskets....Did you reuse the old gaskets when you had the engine apart when you replaced the crankshaft seal? If you did it's possible that when they clamped back down they were off from the old impression enough to leak. You can usually reuse everything but the base gasket and headgasket, but sometimes it will just not reseal with some of the other ones too.

 

[Ro-Tator]

Re: '89 Tohatsu 15- missing a cylinder

I agree with you about doing some more electronic testing. however, I can't see how a coil could cause this problem, since one coil feeds both cylinders and only one cylinder is having a problem. Then again, I won't dismiss the electrical aspect of the motor.

I don't reuse gaskets and I've gone out of my way to replace some gaskets "just in case".

What are the typical symptoms of a leaky upper seal? Isn't it a lean upper cylinder and sometimes seizing of the upper rod bearings?

thanks

Tator

 

[Ro-Tator]

Re: '89 Tohatsu 15- missing a cylinder

All righty then... Update:

I thoroughly tested the ignition. Since the motors vary, I want to say that this one has one coil which feeds both of the two cylinders. The small wires going into the coil consist of one ground wire (black) and one which runs to the CDI unit (orange). Since the CDI and the coil are brand new, I doubted that they would be the source of the problem.

Regardless of the above, I figured i'd proceed with some spark testing. Both cylinders are sparking very well at all rpm ranges. I've also double triple checked all connections both under the flywheel and on the exposed wires. The connectors are immaculate and the multitester shows great continuity.

I have a brand new upper crankshaft seal as well as carburetor seals. The problem is still with me.

Any more info, Guru?
Does anyone else with experience in dealing with this issue want to comment further?

Thanks.

Tator

 

[TOHATSU GURU]

Re: '89 Tohatsu 15- missing a cylinder

If the upper seal failed it could result in a lean cylinder. But, If that seal failed you would have unburned fuel/oil blowing out through it making a mess. I really don't think that's the problem. Ether around the block would be one way to determine if the halves are are sealed correctly. You might also try running carb tuner through the engine and cleaning out that block. Then run it for a few hours and inspect it again and see if it cakes up again on the top.

 

[Ro-Tator]

Re: '89 Tohatsu 15- missing a cylinder

That makes sense about the upper seal. Since I replaced that and sprayed all over the sealed edges of the crankcase (where the halves meet) when the engine was running, I don't think those things are an issue.

As far as running cleaner through the engine, the crankcase is immaculate and the cylinders are also clean. The exhaust passage for the upper cylinder is the only dirty spot. And yes, i'll be sure to clean it before re-assembling this sucker.

My next step is going to be cleaning everything and putting it back together with a new base gasket and new intake manifold gaskets since those were suspicious looking. We'll see where it goes from there.

Thanks

Tator