1970 mercury thunderbolt want start

[joro]

I have a 1970 65hp thunderbolt that want start in the water. Carbs have been rebuilt and cleaned. It runs great in a barrell or on a hose. I have had it in two shops and both places claimed it was ready. But as soon as it hits water it want start. A third mechanic said it might have the exhaust ports stopped up. Is there a way to clean them from without taking the motor apart? It has a distributor and I don't want to mess with it.

 

[Faztbullet]

Whats the compression as only possible cause would be backpressure(motor submerged deeper) is higher when actuallly launched...

 

[rvaughn704]

I agree. Don't spend another dime on it until we know what the compression is.

 

[joro]

Compression is 120 on all 4 cylinders. Mechanic said leak down on #1 was around 30, which meant a tapered bore. Both mechanics claimed it should be running but wouldn't idle right anymore. The second mechanic said it might be the exhaust ports stopped up.