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Correct ignition coil

R

ras

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Mercury 175hp black max s/n 45

Mercury 175hp black max s/n 4588041.This ob has two diffrent brands of ignition coils on it.There are 4 orange coils merc #399 5288 and two blue sierra coils #18-5186.I have a lack of power under load problem and I changed another merc coil with a new sierra coil.My problem seemed to get worse.I tested all the sierra coils and my reading was under the 8-11 range.The merc coils tested in the 8-11 range.Could this be my problem?If so do they offer a aftermarket coil that would work?The sierra #18-5186 says to fit mercury #339-7370 which does not match my orange merc coils #339 5288
 
"Randy, the orange coils were

"Randy, the orange coils were the early coils used on the Thunderbolt 4 ignitions.

They are still available from Merc (part number 339-5288T2 is the most current version) for a little over 70 bucks a piece.

You could however, replace them with the more modern coil (339-832757A4 which replaced 339-7370A13, the blue coils, which your Sierra coil cross ref's to) and the motor "should" run fine.

The newer coil is capable of a litter higher output, so would not "reduce" power for sure. In fact, you can use the new coils on very small outboards (6/8/9.9/15 horse models) which have lower power (black) coils from the factory and considerably improve idle/performance - so don't think your coils are the issue.

What else have you done/tested?

Is your compression good?
Confirmed spark at all cylinders?

How old are the plugs and how do they look? burning fine?

What exactly do you mean by "lack of power"?

eg. it used to get up on plane quickly, now it struggles... it used to top out at "x" mph and now it's only half that."
 
"Graham,I purchased this boat

"Graham,I purchased this boat which has been sitting for 5 years.The old owner was having power problems.He said if he worked the throttle back and forth he could get it to plane.That is what it did for me also.It would plane after working the throttle and seemed great.It seemed to get better the longer it was used.

After I changed bad coil I could not get it to plane anymore.The compresion and spark seems ok.I pulled and cleaned carbs and gas tanks.Spark plugs seemed ok but I changed them.Changed fuel lines and fuel pump. I also spliced in sections of wiring.Stator and trigger tested ok.The blue coils were the only thing that tested out of spec.I puchased a manual on cd.I went thru the timing and sync instructions.This is how I ended last season.
Thank you for your help!"
 
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