Both plugs spark rear cylinder won't fire

[genesass]

Just finished a rebuild on a 90 SP Rotax 587 yellow engine and it fires right up but in the water it would only reach "parade" speed. Took it out and found both plugs are sparking but rear cylinder won't fire. I tinkered with the rotary valve position and I'm confident it's right. Compression before rebuild was good. Any ideas before I leave it behind tomorrow when we leave for camping?

 

[seadoolover]

fuel issue?

Are you getting fuel to the rear cylinder bore?

 

[genesass]

Looks good

fuel line looks good. rear Piston head is coated with unburned mixed while front head is clean.

 

[kustomkarl]

Are you running new spark plugs? Double check the compression, just for giggles...

Karl

 

[genesass]

You know I got it out on the water and it runs fine most of the time althought it does occasionally missfire. I'm starting to think the rear cylinder compression is borderline. I'll probably end up taking the heads off and checking the seals and gaskets.

 

[kustomkarl]

Just do a simple compression check.You'll need a compression tester. Go to Auto Zone. I think they sell for like $25.00. The compression gage, will screw in the cylinder head in place of the spark plugs. To test compression, remove both spark plugs. Place spark plug caps on the plug cap studs near the cylinder head to ground the empty caps. This completes the circuit of the ignition electrical system and prevents any electrical problems from the caps being un grounded. Using the correct adapter for the threaded end of the tester,( same length of the spark plug threads length)Screw in the tester in one plug hole. Hold the throttle wide open. Push the start button. Watch the compression gauge, when it peaks out at the most compression, let go the start button. Read the psi number. I would do it 3 times to be sure it is accurate. Check both cylinders the same way. The ideal compression is 150 psi per cylinder. If it is less, it's not a problem as long as they are close to being the same. If the psi is less than 90 it might need be time for a tear down and a rebuild. If the psi in 1 cylinder is say 140 psi and the other is 80 psi you need to tear down and repair. This difference is a lot and there is a problem. I hope this helps you.

Karl