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Back Firing 198990 58 Liter 351 Winsor Block Mercruiser EngineHelp

J

Jonathan Harle

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" I was wondering if anyone mi

" I was wondering if anyone might have any ideas on how to correct my back firing engine?

The boat is a 1989 or 90 Reinell with the 5.8 liter, MerCruiser Engine / Stern Drive. The boat is my father-in-laws, which is why I am not sure of the exact year.... The block is a 351 Windsor block with a Holly Carburetor (not sure of the CFM)

The engine backfires under load (70-100% full throttle). I can make it backfire, by turning the boat hard to starboard or port and "flooring" it. It usually happens around 2000-2500 RPM.

The boat was tuned at sea level in Northern, CA and is now being used in Lake Tahoe, CA at an elevation of 6200'. It has never done this before at this elevation, until this last tune up. I feel that this is the problem, and that the engine is running too rich, but am not familiar enough with carburetors. I was also thinking of checking the spark plug gap.

Would anyone have any ideas on what the problem might be? What I could check? Proper spark plug gap settings for this engine? Am I in the ball park, or could it be something else?....

Thanks, in advance!!

Jonathan "grew up in the age of Mass Airflow Sensors" Harle :) "
 
I am kinda just checking this

I am kinda just checking this site for info on a couple old Chris-craft engines I found but think I have an answer for you. On AUTOMOTIVE windsors plugwire routing is critical. Under a load the wires tend to induce a voltage into adjacent plug wires if they are not routed properly thus firing on more then one cylinder at a time. I suspect this is happening to your engine when it has an exhaust valver open. And since it didn't start happening until AFTER a tuneup the mechanic that tuned it up may have inadvertently misrouted plug wires. My usual fix for this is to simply make sure each wire CROSSES the other 3 on that side of the engine at least once. Hope this helps.
 
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