1989 Yamaha PRO 50LF water intrusion Cylinder 3

[Powdershredder]

Looking for a few ideas as to why I may be having water intrusion on cylinder 3 on my rebuilt Yamaha PRO 50LF. Compression test reads 120 at all 3 cylinders and all 3 cylinders have good strong spark.

The motor will start and run fine until it kicks into low idle at which point it begins to mis and sputter. The motor is consuming a large amount of fuel and there is a large amount of fuel coming through the exhaust, which I assume is being expelled from cylinder 3 un burnt due to no ignition as a result of what I think is a water intrusion issue.

The plugs have have been pulled and examined cylinder 3 is wet and clean as is the piston itself, as if they are being washed clean. Cylinders 1 and 2 are firing with no issues and have a nice even noticeably darker color from cylinder 3.

Any ideas?

 

[rejesterd]

It might be just the unburned fuel that is cleaning the #3 cylinder. If water is getting in there, you should be able to confirm that by draining the #3 carb.

The first thing I would do is make sure oil is actually being pumped to the #3 cylinder. There's a check valve in each line, so that's a potential point of failure.

If oil is flowing at the same rate as the other 2 cylinders, then test the #3 plug wire to confirm that it's producing good spark. If these plugs were replaced recently, I would assume they're ok.. but if you get to this point and find that it's producing good spark, I would replace that #3 plug just to be sure.

 

[Powdershredder]

Thanks for the reply. The oil injection system has been disconnected on this model and fuel is being premixed. I have also installed new plugs properly gapped and confirmed that a hot spark is present. I suspect water in the cylinder simply due to the cleanliness of both the plug and piston. Is it possible do you think that the plug is firing at the bottom of the exhaust stroke after all of the unburnt fuel has been expelled through the exhaust? Bad CDI? I guess I’m not sure here.

 

[racerone]

I doubt that the plug is firing at " the bottom of the exhaust stroke " on this motor.-----Check the fuel pump diaphragm , it may be ruptured.----Guessing does not work.----Good trouble shooting needs to be done..

 

[scoflaw]

Pinhole from water jacket to head ? They can corrode thru

That head should be easy enough to pull

 

[rejesterd]

Thanks for the reply. The oil injection system has been disconnected on this model and fuel is being premixed. I have also installed new plugs properly gapped and confirmed that a hot spark is present. I suspect water in the cylinder simply due to the cleanliness of both the plug and piston. Is it possible do you think that the plug is firing at the bottom of the exhaust stroke after all of the unburnt fuel has been expelled through the exhaust? Bad CDI? I guess I’m not sure here.

If the fuel is premixed and you have spark, then I would assume it's not the CDI or timing. I would confirm that fuel flow to that carb is the same as the others. If it's not the same, look at the fuel pump diaphragm. If fuel flow the same, then I would check for incorrect linkage/sync. If it's not that, then possibly corrosion somewhere.

 

[Powdershredder]

Ok everybody I just wanted to update this post. First off thanks for all of the help and suggestions, it’s always appreciated.

I suspected water intrusion into the #3 cylinder, simply because the plug was extremely wet and was not firing even though spark was present. However I seemed to have been sorely mistaken about my problem.

This engine was a complete rebuild for me, actually my first engine rebuild ever. In the process I decided to remove the oil injection from the engine an premix my own fuel. I mistakenly left the large worm drive gear inside the crank case which drives the oil pump. I got to thinking about how I disconnected the system and decided to investigate. I removed the pump and removed the worm drive that sits in the crank case and them reattached the pump, capping off and sealing all hose connection ports of course. I crossed my fingers and fired her back up and she purred like a kitten. I even disconnected #1 and#2 plugs to confirm that combustion was taking place on #3 and to my delight she ran on #3 all by itself. So I can only assume that the worm drive gear was causing #3 to flood and not fire. Either way she is running smooth now. Thanks again everybody.

 

[Powdershredder]

Update... bad diaphragm on fuel pump as well. Fuel dumping into crank case through vacuum port causing flooding on cylinder 3.