Miss Fire at 4500-4800 RPM

[Glipat]

Good day,

So I finally got my boat in the water, I have a 1990 RInker Captiva 185, the engine is a 4.3L Mercruiser, with an alpha 1 gen 1 drive. Prop is a 14.5x19 3 blade quick silver prop.
It has new plugs, plug wires, rotor, distributor cap, ignition sensor under the distributor cap and rotor. The only things that are not new on the ignition system are the coil and the distributor. When I had it on the water,it is sputering and misfiring at 4500-4800 RPM. WOT on this boat should be between 4400-4800RPM.
The top speed was 40.3MPH according to GPS.
Would this issue be a timing issue, or a spark issue.

 

[Bondo]

Would this issue be a timing issue, or a spark issue.

Ayuh,..... Welcome Aboard,....... I'd be lookin' at fuel delivery,.....

 

[alldodge]

Would this issue be a timing issue, or a spark issue.

If this was a TB5 I would think its hitting the rev limiter, but being it should be a TB4 then maybe your ignition module or pickup is breaking down

 

[achris]

Weber carb or Rochester?

 

[Glipat]

So it’s a 2 barrel Weber carb, the ignition pick up is brand new

 

[Grub54891]

Did ya set the timing?

 

[Glipat]

It runs fantastic up until that point... I’ll check the timing today!

 

[achris]

So it’s a 2 barrel Weber carb, the ignition pick up is brand new

No such beast... It's either a 4 barrel Weber, the 9600. A 4 barrel Rochester, the quadrajet. Or a 2 barrel Mercarb (which is a 2 barrel Rochester copy)...

 

[Glipat]

I really meant to say 2 barrel rochester, brain was thinking rochester but I typed weber

 

[QBhoy]

I’m going to take a pretty certain guess that with a 19” and that set up...your tach is a little off and you are actually hitting the rev limiter at just over 4800 rpm.

 

[Glipat]

I had a 21" prop solas brand and the RPM was maxing out at 4300RPM

 

[Glipat]

It may be the rev limiter, just seems weird, How would I verify the accuracy of the tach?

 

[Grub54891]

It may be the rev limiter, just seems weird, How would I verify the accuracy of the tach?

You take a known good tach, hook it up @ the coil and make a run with it at various rpms.

 

[alldodge]

It may be the rev limiter, just seems weird, How would I verify the accuracy of the tach?

Can verify with a hand held tachometer, but you should have a TB4 module and it does not have a rev limiter

Remove the distributor cap and see if there is any side play in the shaft.
Connect a voltmeter on the + side of the coil and see if the voltage drops.

As Bondo mentioned could be your running a bit lean if your getting a backfire

 

[nola mike]

Had a recent issue trying to diagnose a bad coil, misfire > 3k rpm. Got some advice here to try to use a timing light. I was able to see pretty easily with a timing light connected that it was ignition and not fuel related