mercrusier 470 miss upto 1500 rmp exhaust leak?

[darot]

my 470 has a miss from idle up to 1500 rmp and then runs smoothly. it is not the plugs wires rotor cap or petronix ignition. it is not the fuel pump or filters. the carb idle adjustment screws will not make it go away. Help! is it an exhaust leak? how do i check? any common area where they crack?

 

[Bt Doctur]

Re: mercrusier 470 miss upto 1500 rmp exhaust leak?

Inspect #4 spark plug to see if it`s whiter than the rest,a sign of water from a leaking exaust riser.

 

[stonyloam]

Re: mercrusier 470 miss upto 1500 rmp exhaust leak?

Had a similar problem, turned out to be ahead gasket /exhaust valve problem on #4. A compression test will tell you a lot, a low #3 or 4 could indicate a head gasket problem (known problem area). Autozone will lend you the tester for free. In the meantime you can isolate the bad cylinder by (carefully) pulling the the wires from the distributor one at a time. When you pull the wire from the bad cylinder you should see no (smaller) rpm loss. Good luck.

"Inspect #4 spark plug to see if it`s whiter than the rest,a sign of water from a leaking exaust riser."
Mine was snow white

 

[darot]

Re: mercrusier 470 miss upto 1500 rmp exhaust leak?

thanks for the posts guys. I just pulled the #4 again and compared it to the #3 and #2 plugs. They look exactly the same, very clean. The number 4 may be the faintest shade whiter but that is stretching it.

I did notice yesterday that I thought there was more water in the boat than usual (from sking and tubing). My antifreeze level is the same (have to be the seawater side)and while the engine was running i tried to see if the exhaust manifold was leaking and could not see anything very well.

I did do a compression test and there is no difference between 2,3 & 4 cylinders.