1971 Fastwin 18hp - Missing cylinder

[pl2442]

At WOT the motor seems to run good on both cylinders, but when i drop down to trolling speed it drops a cylinder. I also noticed afterwards there was black oil leaking out of a weep hole in the lower unit. Im assuming i am running really rich. Would running rich cause the cylinder drop or what else could it be?

 

[F_R]

Well, yes running rich on that one cylinder can do it. So can a list of other things. Some black oil drainage is normal. Excess oil drainage can be caused by that list of things. If the cylinder isn't firing, the fuel/ oil mix isn't being burned and will run out the exhaust.

Begin at the beginning. Check compression to see if you have anything worth fixing. Then check spark.

 

[RCO]

If you do have good spark and compression, check your fuel pump . When the diaphragm leaks it will flood one cylinder.

 

[F_R]

How long have you had this motor? Have you trolled with it before?

Sure, they are good motors, but not the best in the world for prolonged idle (trolling). It's all due to the changes that were made for environmental reasons. They took the excess fuel that used to be dumped into the lake and ran it to the upper cylinder for burning. That led to loading up and plug fouling on that cylinder if it wasn't run fast now and then to clean it out. But that probably isn't the reason for this present problem. Just sayin....

 

[oldboat1]

You doing this troubleshooting in the lake or in the driveway?

 

[racerone]

Thermostat needs to be checked.----They usually fail in the OPEN position.

 

[petryshyn]

pull the flywheel and check for cracked coil insulation.

 

[pl2442]

The fuel pump should be good, i replaced it a few years ago.
Im not actually trolling much with the boat its just when im at that speed, trolling/ Slow no wake, when it drops down to 1 cylinder.
I added some gas to the tank to dilute the mixture, still missing when going slow, but noticed almost no black oil leaking from the weep hole.
Compression needs to be tested still, have the tester sitting there just need to do it, i will update after.
My grandfather bought the motor new in '71 he always has complained about it fowling plugs but cannot remember it dropping a cylinder. I actually got the motor after a mechanic told him it would be better suited as a boat anchor then a motor because he couldn't get it running, turns out all it needed was a new fuel pump, lines, carb rebuild, new lower unit oil, spark plugs, impeller and shes running now.
Troubleshooting is mostly happening in the garage, but it is on the boat and ive taken it out fishing a few times this year already and thats where ive noticed it droping a cylinder at slow speeds.

How would a thermostat cause this issue? the thermostat might be bad. Admittedly i did overheat the motor when i replaced the impeller and the water tube didnt seat correctly.

 

[racerone]

Thermostats typically fail in the open position.----Motor will run ice cold !

 

[Saintnowhere]

check your float setting could be flooding the bottom cylinder at low rpm duel to high fuel level in carb, also file/clean the points if you have not replaced them. check plug wires also. and a new set of plugs couldn't hurt. and check the thermostat as stated, just put it in a pan of boiling water to see if it opens, but if its open cold its bad