Bad Switchbox? - Max RPM 4600 on Inline 6

[mmccurry]

Guy's,

I've got a 1987 115 hp Inline 6.....

I recently replaced a faulty upper switchbox (lost ground b/c of loose mounting bolt). I replaced the old switchbox with a new OEM from Mercury. While the motor now has good spark to all 6 cylinders, it will not run higher than 4600 RPM. It previously ran 6,000 RPM WOT.

I put a timing light on the wires and found that the spark pattern on ALL cylinders at WOT is a little erratic. Whereas at idle the timing light binks nice and constant at idle speed, at WOT it does not have a perfectly consistent blink. I'm not sure if this is just a limitation of my $40 Autozone Timing Light or if this is truely an indication of a high speed spark problem.

I'm thinking that my old lower switchbox was also partially damaged by failure of the upper unit and is now causing problems in the bias circuit of both units. Should I just buy a matched set of CDI switchboxes and keep the "newer" merc unit as a spare?

Everything else has been done: New bulb, rebuilt carbs, rebuild fuel pump, new spark plugs, link and sync,etc. Boat now idles and starts great

Does everyone agree erratic high speed spark pattern on all cylinders is the fault of the remaining original switchbox?

 

[Laddies]

Re: Bad Switchbox? - Max RPM 4600 on Inline 6

It is common for the bias in the other switch box to fail if one box does with OEM boxes as you are learning that is why they recommend changing both boxes. That is why CDI electronics equipment make theirs look like junk, you have the right idea and if the rest of the system tests out go with 2 new CDI boxes.

 

[mmccurry]

Re: Bad Switchbox? - Max RPM 4600 on Inline 6

Thanks Laddies,

Sounds like you agree that not replacing both swithboxes could cause the ignition to fire less than perfect at high speed. Before I spend the money I'm going to take one more look at spark plug # 6 and make sure it doesn't have water intrusion. An earlier look at the spark plug made me suspicious but I couldn't tell for sure because the spark plug wire end had rusted and the cylinder was not firing at all......I've repaired it and taken another look since then and everything looked fine. I want to do one more long hard run and then check the spark plug and make sure it isn't "clean".

After I've absolutely ruled that out as the possible cause for the RPM issue I'm buying the new switchboxes from CDI.

I appreciate your help.