Opinions needed on rebuilt engine misfiring
 

I had my twin 454 mags freshened up with new roller cams, roller rocker arms, timing chains etc from RM builder. Real confident in my parts purchase. One engine runs and screams like a dream! However, the starboard engine ingested something and bent a valve shortly after initial start up. Not knowing that something was ingested and the engine builder stating the problem of not idling had to be an accessory on the outside of the newly built engine, I took the boat to a certified merc dealership. Dealer verified all systems(ignition, etc)were good. Merc mechanic said need to pull valve covers and check engine builders valves since backfiring thru carb. Merc dealer found milky oil around valves and pulled the intake to find a leak in the gasket/reused intakes. At this point I paid the merc dealer and said I thought the engine builder ought to be involved. Got the engine builder to inspect the engine and found two slivers of metal and a bent valve. Builder pulled the head,replaced the valve and installed new edelbrock rpm air gap intakes. I paid him for his time and heard the engine idle and rev on the hose at his business yesterday. Couldn't wait for weekend and went to drop the boat in the water and idle and cruise around today. Turn the key, sounded a little dry, pump the throttle while cranking and varooooooom boom huge missfire. I think it was thru the exhaust because it lurched the boat forward. Cranked again and same thing. Then nothing I tried would crank it. Got engine builder on phone at 5 o'clock, he said problem has got to be something minor. Told me to try changing plugs, they are probably loaded up and if that didn't get it he see me Monday. Said he could hear one loaded up when hose yesterday and it should have burned off. Went and got another set of rapid-fire's. Boy, I'm frustrated and seems like this engine is snake bit. Am I wasting my time changing the plugs or do you think it's something internal? Any suggestions appreciated.

Did you check timing?

Have not checked the timing. Builder set the timing on both engines base 10 degree. I literally witnessed the envine run, rev, idle 10 mins yesterday at his shop. Put boat in shed and today it went the varooooom boom on the trailer and wouldnt crank after that. I can deff check the timing if I could get it cranked. Keep the opinions coming. Thanks.

Swap carbs from starboard to port to see if a carb problem?

If the builder used the wrong keyway on a multiple keyway crank timing gear this would happen,

This morning pulled the distributor cap and checked the timing with timing mark on the crank. It was dead on. Found one nut loose on a carb stud. Tightened that corner of carb down and checked the other studs. Pulled number one spark plug and it is glazed over black and wet with fuel. And I get no spark while cranking. Proceeding to change all the plugs. Any do's or dont's to keep from fouling the new rapid fire plugs again?

And I get no spark while cranking
Don't bother cranking it over with new plugs unless you get to the bottom of this as obviously it will not spark. Modern ignition is strong enough that even wet plugs will spark, might not run right but they will still spark with no load out in the open. Hopefully a minor connection problem somewhere, Smitty

What year is it, what distributor are you running ?

Guys got it running and appreciate the tips and what to look for. These are 454 gen vi with thunderbolt V ignition.

This is what all I think caused the fouled plugs: plugs weren't changed after bent valve was replaced, carb wasn't bolted down on one corner, #2 spark plug was loosened by hand! After new plugs it started immediately and idled at about 2,000 rpm. I guess the Builder set idle high to compensate for all the other problems.

Again thanks for tips, Brian

Good to hear ya got her going. Sometimes it's the small things that will drive ya nuts. New plugs and some re-tightening...... good to go.



Darrell.