No Fuel in 3rd Cylinder

[ppidgursky]

I need your help again. My trustie lil 50 HP 1989 Yamaha has give me troubles again.
I took it to the shop to do the sync and link. The mechanic called me and said the bottom cylinder was not receiving and burning fuel. Got 125 psi in all cylinders, good spark, clean carbs, and the reeds were checked and found to be ok. I have had this motor rebuilt with new gaskets pistons and rings. I have 3 mechanics stumped on this one. This last mechanic mentioned a seal on the crankshaft in between the pistons could be bad. He's talking as if it is a ring on the shaft that seals the cylinder. Problem is when I had the block open I don't remember seeing anything like that. I saw the half moon I think they call it the labyrinth seal. Which leads me to another question. That labyrinth seal, my manual shows it in the block, the half moon I remember seeing was on the shaft, is that the same and is it supposed to be there on the shaft.
Any input would be great.
Thanks in advance.

 

[Ray Neudecker]

Re: No Fuel in 3rd Cylinder

Have they checked fire? Defective CDI's have been a problem on that model series.

 

[ppidgursky]

Re: No Fuel in 3rd Cylinder

Thank You for the response.....
They said they checked for and got good spark on all three. He said he did a simple test by I believe he said by covering the #3 carb with his hand to choke it, then immediatly pulled the plug. The plug was completely dry.
You can see fuel come in the carb, but it isn't going in to burn. He supposedly checked and swapped around the reeds, they all looked ok and the same problem ocured even with the reed swapping.

Any vadility to the labyrinth Seal theory? I hate to have to crack the block again...but I will.
Any ideas?

Peter

 

[rodbolt]

Re: No Fuel in 3rd Cylinder

if its not getting fuel then its also not getting lubricated. find a real tech before you wipe a cylinder.
by covering the carb throat you force the mixture very rich. I am with ray, a careful check of the ign system and fuel system is in order. the labyrinth seals rarely give problems,however crank seals can. but its rare.

 

[ppidgursky]

Re: No Fuel in 3rd Cylinder

I am going nuts here. I got the boat back from the shop. Before I crack this thing open again, is there an easier way to see what the problem is. I pulled the plugs and they are a little wet, even the 3rd cylinder looked wet a couple times. I tried swaping carbs around and there was no difference in the wetness of the plugs or the way the motor sounded.
When I pulled the carbs, you can in down into the reeds, and they were all wet. When you accelerate the motor you can see fuel droplets coming into the carbs.
Any Ideas?
Any good mechanics out there near Georgia? I am at my wits end.