Backfire '72 120 Merc.

[johnnynegaunee]

Lately I've been trying to chase down the source of a fairly frequent backfire through the carb and more frequent popping through the exhaust. The electrical looks good, I'm drawing fresh fuel from a new tank. However, I have found that the exhaust manifold leaks just above the 3rd mounting stud from the front; adjacent the Rochester 2bbl. GC carb.

Now to the carb : I can turn the port side idle adjustment down to it's seat without affecting (400 rpm) idle one wit. The starboard screw I can also seat and the engine still runs, though ragged. In fact it will idle with both screws against there seats. The base gaskets don't leak near as I can tell spraying with carb spray. R&R the carb? Replace the manifold gasket? Both? Any hints on where next to look for leaks? I sprayed top of the intake joint too without finding anything.

 

[ziggy]

Re: Backfire '72 120 Merc.

do the throttle plates close all the way?

if not. the idle mix screw will be rendered inop

 

[searay3]

Re: Backfire '72 120 Merc.

If that engine is really a 72, you have to make some decisions. That carb is a problem from reading your other posts. A rebuild is iffy at best if the castings in good enough shape to take a kit. Comp numbers are not bad, but you have to think about the age of the mans and risers. You could have an intake manifold leak, either from a gasket or again with it's age possibly rusted thru somewhere. Gonna take some bucks to put this motor in shape and then you still have a 1972 hull. You will never recover the costs involved in the rebuild. If you think the hull is worth it......

 

[johnnynegaunee]

Re: Backfire '72 120 Merc.

Thank for the feedback, I'll check your tip Zig about the throttle plates to make sure they are closed, I think they are though.