Bill 3

Twin 515 cu. in. NA right at 600hp. 850 Holleys with Dart intake, iron heads. MSD distributor with 6M MSD boxes. My issue is with fouling plugs, and they come out looking clean. Plugs are NGK R5674-7. They do not gradually foul at idle, but no idle is the result after a run.

I run 5 - 10 miles down the lake, 3500-4000 rpm come to a stop and both engines die. Very hard to restart, cough, spudder and barely run given some throttle. No way to put it in gear with out dying. Check over everything and all seems right. Carbs not flooding or dripping. Floats are set properly. Put in new plugs and all is well again.

I did install O2 sensors over the winter and have found that I need to fatten up the mid range, I am at 13 at 3500, and need to get that to 12 before I hurt something.

Is it possible to lean foul plugs? Strangest thing is that it is both engines at the same time. I don't run the boat the same every time out, but is seems about every 3rd time out this happens. 3 sets of plugs already this year.

My plan is to fatten the mid range and go with colder plugs.

Any suggestions or experience in solving this problem?

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14 apache

iTrader: (1)

Tighten the gap up some I don't think its the spark plugs. They look clean enough to run. I would be leaning towards ignition. Do the plugs come out wet?

kvogt

fatten up your carbs. Are cooking fuel off your pistons when you slow down to a idle after a run? After the pistons cool some it runs ok again.

kvogt

How much timing are you running and what compression are the motors?

Bill 3

At first my gaps were at 0.030, I changed to 0.035 on the last two sets. Shouldn't that be tight enough? I was actually thinking of opening it up a little more.

Some of the plugs come out slightly wet, my assumption is from not firing and me hard on the throttle (acc pump) trying to get it going again.

I won't say that it isn't ignition, but after changing out this last set of plugs in the middle of the lake, I went on another 5 miles, had lunch and then back home (15 miles) stopping once on the way. No problems whatsoever. Will not leave the dock again without a spare set of plugs on board though.

Thank you for the reply!

Bill 3

34 degrees 9:1 compression.

14 apache

iTrader: (1)

Post some close up pictures of the plugs

SB

iTrader: (1)

Chinese Spark Plug Factory by abarndweller, on Flickr

Chinese Spark Plug Factory by abarndweller, on Flickr

Bill 3

These are from the port engine. The pic in first post is the stbd.

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14 apache

iTrader: (1)

My money is that its not the plugs they look ok. Put a set of the old ones back in after it stalls again see if it starts.