Mercury500 Starting mystery

[TwinMerc500]

I have a 50hp Mercury500's that I have been working on.
The motor doesn't want to fire.
The compression seems alright all cyl are ~115-120.
I have spark.
I have rebuilt the carbs and they are getting fuel.

It seems that there is no suction from the head. No gas is getting to the cylinders. I have removed a carb to check for any draw when I start the motor, and there doesn't seem to be any.

Any suggestions before I pull the block and check the reeds , since this model the reeds are in crankshaft and I really don't want to pull the head if I don't need to.

Oh ya, and the last piece of the puzzle..
I suspect that this motor overheated on its last run.

 

[Laddies]

Re: Mercury500 Starting mystery

If the powerhead don't have vacume there must be a reed problem but the engine should still start on the other 2 cyls so that would not explain your problem.

 

[bullbat]

Re: Mercury500 Starting mystery

sounds like a reed problem to me but like laddies said it should try and start on 2 cylinders i dont see u fryin all the reeds al one i wouldnt think sounds like a buger to me

 

[TwinMerc500]

Re: Mercury500 Starting mystery

Thanks for the replies...
Hopin for a magic bullet...
Im off to pull the power head of my motor...

 

[CharlieB]

Re: Mercury500 Starting mystery

You can take the reeds completely out of the engine and it should still start.

It would immediately flood from the overly rich mixture of a reedless engine, but it would start.

Even tho you still have acceptable compression you are going to find scored cylinders from the overheat.

The lack of ring seal near the bottom of the cylinders is causing your lack of intake vacuum.

Pull the exhaust cover and inspect the cylinders thru the exhaust ports to verify.

Total tear down, if you are lucky you may get by with a hone and new rings, don't count on it.

 

[Laddies]

Re: Mercury500 Starting mystery

Have you ever seen a engine with a bad reed? Apparently not because there is no intake vacuum for it to induct fuel air mixture.