351 engine question

[348tripower]

I have a problem with a 1999 Kayot deck boat. The 351 gets a miss at 2200 to 2800 and starts to backfire thru the carb. They have had the carb apart and rebuilt it twice now and the miss comes back. They are blaming it on the ethanol in the fuel. If you can get past the 2800 rpm you seem to hide the miss problem but it is a bit scary trying to do that with the backfire going on under the deck. The short ride I took didn't turn anything obvious up. Any suggestions on where I should look?
Thanks,
Don

[wayne petty]

think at that point i would really look closely at the ignition system...


examine the ignition coil... for signs of high voltage leakage..

check the various spark plug wires for signs of leakage.. and then ohm them....   digital ohm meter to 200K Ohms scale..   the wires should show less than their length in inches on the display...

a 24 inch wire should read less than 24... probably 12 to 15..

examine the spark plug wire routing...

where the wires leave the distributer cap and run over the left valve cover.. the wires need to be in the separator in this order...

7, 5, 6, 8      this separates the 7 and 8 cylinder wire as 351s just like small block chevys have those two cylinders 90 degrees of crank rotation apart...   the length of the wires running together can induce a false spark in the number 7 wire and fire that cylinder off 90 degrees early causing it to detonate and run lean... this melts the piston and scores the cylinder walls...   and it REALLY important ...

how is the cap and rotor???

what does the inside of the distributer look like???  nice and clean.. or rusty..    

if the reluctor is rusty... it may not be properly working..  but it probably has a hall effect or optical distributer in it...

[348tripower]

I am bringing this back to the top to tell about the results. Wayne was spot on. It was an ignition problem. I originally suspected ignition, but I was looking for a bit of reinforcement in my diagnosis. The owner of this boat returned it to the dealer only to have them call OMC and get replacements for the whole ignition system.  Showing me that they don't know how to find a problem. They are just part changers. I was willing to sort this out, they were in a hurry. I didn't ask the cost.
Don

[jaybee]

Wasn't cheap to replace the entire ignition, that's for sure.

[kb426]

I love this story. Not understanding how things work is a real detriment to repairing anything.