283CHECK

I have noticed that my 502 is developing a miss around 2500-4000 rpm under load. Such as getting on plane, once on plane don't notice it that much but that could be due to speed and wind noise? I replaced the cap and rotor, but didn't replace the plug wires. For some reason the boat shops around here can't get them in. I am getting some accel 8mm spiral plug wires(make your own)and plan on installing them. As far as I know the wires that are on there now are original? Any input on the make your own,good or bad. Any other thoughts as to the miss, don't notice is while ideling.. This is an efi 502..

Thanks
Chris

mcollinstn

plug wires plug wires plug wires.

Barring many other nasty problems, the most common source for this is plug wires.

You should also be down 150-250 rpm on top.


Other possibles are a dirty injector. Backfiring is usually a symptom if you have a broken valvespring or a collapsed lifter so I'm hopeful that you are on track with the wire issue.

Spiral cores are a must.

283CHECK

I have been down 200-250 rpm ever since getting the motor put back in. I guess that the shop and I got our wires(pun intended)crossed and they did not replace cap, rotor, and wires. I replaced the cap and rotor the week before I noticed the miss. So my thinking is the increased spark is breaking down one if not more wires.

Yes the new wires I have are spiral core...

Thanks

DonMan

I agree. Spark plug wires are most likely the culprit. I installed a set of Accel 8mm spiral cores on my engine. They worked great for a few months, then it started having symptoms similar to yours. I replaced them with MSD wires. Problems went away. Not sure why the Accels went bad, but now I have a sour taste in my mouth towards Accel wires.

Tinkerer

I use the TAYLOR SPIRAL PRO wires. You can get them in your choice of colors also.

FindMe

Are you putting it into "Base timing mode" when, or if the timing has been checked?

service bulletin
No. 94-2 TO: SERVICE MANAGER TECHNICIANS
PARTS MANAGER
A. THUNDERBOLT V IGNITION SYSTEM
Models
All MCM/MIE Models with Thunderbolt V Ignition System.

Description
This new ignition system uses the same distributor that is used on the Thunderbolt IV. The change is the Ignition Module mounted on the distributor. This new Ignition Module controls Idle Speed spark, Acceleration Spark, Mean-Best-Timing (MBT) Spark Advance and has a built in Rev-Limiter. A Knock Retard Spark Control Module is also mounted on the distributor to help protect the engine from detonation. A more detailed explanation of this system and troubleshooting procedures are outlined in the Service Manual Supplement “Thunderbolt V Ignition System” (90-823224941) that was mailed out to all dealers. Rev-Limiter The Rev-Limiter is set 50 RPM above the maximum engine RPM. When the engine reaches this limit, it starts to miss because the spark is turned off. When the engine drops 300 RPM, spark is turned back on. Because of this, prop selection tests should start with
higher pitches then work your way down to the correct pitch for the boat.
IMPORTANT: Do Not prop engine at Rev-Limiter RPM.

Audio Warning System
The Audio Warning System that we ship with these engines do not have a “timer” on the buzzer. When the ignition key is turned on, the buzzer will sound until the engine is started. After the engine is running, the buzzer will shut off. If it doesn’t, check the Engine Oil Pressure, Engine Coolant Temperature or MCM Models: Drive Lubricant Level, MIE Models: Transmission Lubricant Temperature. = Revised April, 1994

Timing Engine or Adjusting Carburetor

Because the Thunderbolt V Ignition Module has Idle Speed Spark Control, it has to be locked in “Base Timing Mode” whenever you: Set or Check The Timing

Adjust Engine Idle RPM
Adjust Idle Mixture

BEFORE turning ignition key on or trying to start the engine, connect a jumper wire from the Timing Lead (PUR/WHT wire, see wiring diagram) to a good ground. This locks the Module into “Base Timing Mode”. Now any of those three adjustments can be made. IMPORTANT: The jumper wire must be connected to the Timing Lead and ground BEFORE turning ignition key on, otherwise the Module will not go into the “Base Timing Mode”. Adjustments are made the same way they were done on Thunderbolt IV engines. The Idle Speed Spark Control in the Module is only active within a certain RPM range. Because of this, engine idle RPM has to be set exactly to the specified RPM shown and in NEUTRAL GEAR. The boat can be either in the water or on a trailer.

IMPORTANT: Use an accurate shop tachometer when setting engine RPM because it has to be at an
exact RPM in forward gear.

Timing at Idle RPM: 10° BTDC
Idle RPM in Neutral Gear: 650

IMPORTANT: Be sure to disconnect the jumper wire between ground and timing lead or ignition will remain in “Base Timing Mode”.

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