Fire in carburetor

[Nbeheler]

I have a 1990 johnson gt200. I am seeing a flash of fire in my number 6 cylinder but it jumps to number 4 as well. Never fires in both at one time. Timing seems good but missed every once and a while but at idle the engine runs great. Reved to 2000 rpms is when the fire starts. It happens every 15 to 20 seconds. Pulled heads and pistons and cylinder walls look good. I'm no mechanic but I have one helping me. Someone please tell me they know what's happening. It has caught fire twice now luckily only burning choke line and fuel line once. Trying to get it fixed to sell because I've already bought another boat.

 

[Nbeheler]

Also compression is 110 on all cylinders except number 6 which is 100

 

[jakedaawg]

...

 

[Chris1956]

Sounds like backfire into carbs. It would need to get past reed valves. So check timing for too much advance and check reed valves.

 

[Faztbullet]

This a lean crankcase backfire and you will see this more running on a hose than in water with the airbox removed. You have the restrictive airbox and removing it will lean carbs. Also being a 1990 the rubber coating on reed block could be degrading causing reeds not to seal.Also remember you have the correct cylinder as looking at carbs left side feeds 2/4/6 and right side feed 1/3/5..this is cross over intake.

 

[Nbeheler]

Timing is dead on center with an occasional miss. And as for the reeds, I replace the gaskets on them 2 days before the last fire. They seemed fine, sealed off good and all.

 

[racerone]

Are orange wires going to the correct coils?------Seal rings on crankshaft checked ?

 

[Nbeheler]

All wiring checks out. Haven't checked seal rings.