Yamaha 8 bad lower crank seal?

[ChuckB1954]

1994 Yamaha 8HP two stroke. No sign of abuse and looks almost new condition. When I first bought it, I had to remove the carb, and clean the bowl. Nothing serious. Just some gel looking matter. Checked the jets ect also. It cleaned up good. Took it out locally and it ran great, starting on the first pull. Took it up north and had a hard time starting it, but after numerous pulls, running great again. Now what happens is I can get it to start, but it runs about 30 seconds and dies. Then it won't even fire unless it sits for a while, but then only for a short time before it dies again. I spent hours trying to get it to run normally, removing the carb numerous times and I have no doubt that everything in this carb is in order. Clean fuel/tank and filter, plenty of compression, reed valves are in order. Not running hot.

The reason I suspect the seal is because:

1: When I remove the carb, the bowl is full of fuel, it's just not making it to the plugs. Verified by removing them. Plenty of spark.
2: I notice a lot of oil residue in the water, whether idling it in the garbage can on the shore, or when it is mounted on the boat in the water.
3: Only when it sits a few minutes, I usually can get it to run the 30 seconds again, until it dies. My theory is that the oil in the engine is draining down to the seal and temporary resolving the problem.

I haven't taken it apart yet, but does this sound like seal failure?

[TownsendsFJR1300]

Have you inspected the reed valves, that their closing fully?

I tend to agree about the seals possibly being bad.

I'd also check the compression, especially for carbon build up causing the rings not to seal fully.
I had a Poulan blower, same issue. Compression was somewhat high but not near what another exact unit had.
Rings were carboned up and after a cleaning, the machine fired right up and continues to do so..

[ChuckB1954]

TownsendsFJR1300 Your reply to my post is not visible to me. Can you please re-post it again? When I clicked on the notification, it said something like I am not authorized to view it.

[TownsendsFJR1300]

TownsendsFJR1300 Your reply to my post is not visible to me. Can you please re-post it again? When I clicked on the notification, it said something like I am not authorized to view it.

It show's in BLUE to me and I had a message it had to be approved by admin for some reason, but here's what I posted earlier:


Have you inspected the reed valves, that their closing fully?

I tend to agree about the seals possibly being bad.

[robert graham]

Has the fuel pump ever been checked or replaced?....a crack in the diaphragm can cause a lot of issues.....On my 1999 C90 I usually just put a new one on about every 5 years or so....

[ChuckB1954]

Yes, I removed the reed valve cage and everything is in order. The fuel pump looks good. It is attached to the carb bowl.

I bought an excellent OEM carb on ebay for a 6 HP Yamaha. I did so because although the other carb looked good, one of the bowl screws was stripped but re-tapped for a larger screw. It always bothered me.

I took the 6HP carb and converted it to an 8HP carb by exchanging the Jet Needle (item 15 on the parts page) that was in my 8HP carb. That is the only difference between the 6 and 8HP carbs. I totally disassembled the new one and soaked it for 24 hrs. Looks brand new now. I reinstalled it along with a used front face plate. The right side of mine was broken off and missing. The right side on the new one had lettering by the fuel pipe joint that read 100:1. I have been running it at 50:1. Maybe that is the reason for all the oil residue(?) Maybe the lower seal is good and it was the carb all along(?).

I plan on looking at the impeller for the water pump, so I maybe will take a peek at the lower seal anyway while it is apart.

[mitch184]

Make sure the exhaust in the lower unit isn't clogged. Verify there is plenty of exhaust coming out of the prop.