No pulse from one cylinder but good compression

[tfloyd1213]

Yamaha 760. Rebuilt carbs. Will run but only with gas in the throat of the carb. Put a squeeze bulb on it and can keep it running that way.

[rodbolt17]

fix the fuel pump or pump intake supply.
yep its that simple.
unless you lost a reed then no matter what it wont run on both.
two strokes have primary and secondary compressions.
if you lose primary its on the crank side of the pistion.
if you lose secondary its the piston,head.

[tfloyd1213]

No fuel pump. It has diaphragms in the carbs. Just rebuilt the carbs. Took the reeds off and they look good, no cracks or missing pieces. Front cylinder has pulse but the rear has none. they both have good compression.

[99yam40]

Just what is a Yamaha 760?
I know it aint an outboard motor.

crankshaft seals maybe
what ever will let air in without it going through the carb on what ever this is

[boscoe99]

I was wondering myself what it was.

Thanks to goggle:

Yamaha 760: Personal Watercraft Parts | eBay

[boscoe99]

Diaphragm is part of the integral fuel pump. If it is not getting pressure pulses then the fuel pump can't do its job.

Have you actually measured the reed valve clearances?

Have you tried swapping the pressure input lines to see if the problem moves to the other carburetor?

Have you inspected the pulse lines from the one carburetor that is not running back to its attach point on the block?

[tfloyd1213]

I have not measured the clearances but they look like they are sealing. I was gonna move one reed assembly to the other side and see if the problem moves.

I did not swap the lines, they are too short but I removed the carbs and both hoses are attache to the block, When I turn the motor over with out the carbs, the front hose has pressure and the rear has none. Thats when I removed the reed valve housing to inspect the reeds. I blew through the hose and there is no blockage. If there was a crank seal leaking then there would be no pressure out of either hose, correct?

[99yam40]

there should be seals between the cylinders on the crank.
Cannot have a common crankcase and still draw fuel charge into a cylinder if the other is tied in

[TownsendsFJR1300]

+1 ^^^.

Each cylinder has a separate crankcase/separate seals...

If you have pressure on one cylinder, obviously both of those crank seals are good. Seems the outer seal on the crank on the bad cylinder is leaking.

Can you access that and look for seepage?



If you post a year, model # etc, a parts fisch will show you where its at.


Should be close; http://www.boats.net/parts/search/Ya...TON/parts.html

[zenoahphobic]

Pwc

Oh so hard working on jetskiis.
My son has the one with the 3 cyl 1800 - all intake carb fuel etc underneath. Perform surgery (adjustments /maintainance) with cameras, mirrors etc - easy on the boat ramp- NOT

[TownsendsFJR1300]

Looking at the parts fisch I posted, it appears once the engine is out, you just have to pull the lower half of the crankcase. Cylinders/heads can stay.

[tfloyd1213]

Looks like I will be pulling the motor this weekend. Pretty sure its the rear main seal.

[tfloyd1213]

Ok Motor is out and Crank is out of motor. Now, how do I get the coupler off to get the new seal on?

[TownsendsFJR1300]

Just looking at the parts fisch, it looks like its threaded on. Maybe reverse threaded.

What does the shop manual say?