Ignition system upgrade?

[pnwboat]

I have a 1988 125 HP motor that I rebuilt 3 years ago ( thanks to help from this forum) and mechanically, has been performing flawlessly. Had a few issues with the Prestolite ignition system (Blue coils and CD units). It was a bear to trouble-shoot. Wires everywhere! Plus when I look at all the wires behind the plate that the coils and CD units are mounted on, it looks like a ball of spaghetti! I've tried to straighten them out but it still doesn't look very neat. The CD units were not cheap either!

Anyways, I've always wondered if you could replace the Prestolite system with a late model Mercury Thunder Bolt CDM system that came on the 1996 and later 120 HP motors. Bottom line....yes it can be done! I gathered the necessary parts over the winter on line. Flywheel, stator, trigger, top mounting plate, CDM units with plate. Total cost was about $120 in used parts. Time will tell if it was worth the effort. The CDM units are cheaper and much easier to replace. Don't know if they are more reliable, but I don't see as many posts regarding problems with them. I haven't had a chance to take it out on the water yet, so it's still has not proven itself yet. The fact that the motor even started after the switch is promising.

One other thing I noticed was the NGK BUHX plugs for the Prestolite Ignition system have a gap of about .030, where as the NGK BUHW plugs for the Mercury ignition system have a gap of about .050. Does that mean the Mercury system has a hotter spark?

Before and after pictures.

 

[Josh P]

Re: Ignition system upgrade?

Thats interesting i might have to look into it as well mine is a 91 do you know what the differences is between 88 and 91 if any? where did you scrounge your parts from? Thanks for the info!

 

[pnwboat]

Re: Ignition system upgrade?

The flywheel was the most difficult item to get right. I scrounged parts from E-Bay, Craigslist, local paper. I wanted to take advantage of the 16 amp stator, so I assumed I needed a flywheel with the 6 separate magnets. Mine has the single strip magnet. I discovered that there are at least two versions of the flywheel with six magnets. The newest version has a threaded 1/2 inch high lip around the center hole. All of the examples of this flywheel that I came across has a center hole that is approx. 1 6/16th inches in diameter on the bottom. This hole is tapered so it's bigger on the bottom than the top. I needed one that was approx. 1 3/16th inches in diameter on the bottom. The only flywheel that I found would work that had the correct diameter hole and six separate magnets was off of a 1992 120 HP. At least that what the fellow that I got it from believed it to be. Anyways, it did not have the 1/2 inch high threaded lip around the center hole.

 

[pnwboat]

Re: Ignition system upgrade?

Josh P, here are the pictures you were requesting. This is a flywheel just like the one I used.

Do you have the Prestolite ignition system with the blue coils/CD Modules?

I've had it out 4 times since the ignition upgrade. I also have to say that in addition to the ignition system, I also put a set of later model carburetors that do not use the choke plate. They use the "fuel enrichment solenoid". WOW what a difference! It takes two tries to start when stone cold. The rest of the day, only takes one attempt. So far it has been consistent each time. I do use the neutral fast idle technique each time. It idles down to 500 RPM's in gear very smoothly. Before, anything below 700 RPM's it was a little rough. Also hardly smokes at all. Before when I took off from a dead standstill, I could see a trail of faint smokey haze hanging in the air following my path. Now, nothing.

 

[Bigprairie1]

Re: Ignition system upgrade?

PNW this is a really interesting change and upgrade. I have a (very) nice running (Chrysler/Force) '90/90hp and have a mechanically identical (Mercury/Force) '91/90hp waiting in the wings should the first one fail.
So it's good to know this change or upgrade can be done without too much difficulty. I'm often looking and comparing the two motors and systems so if I ever have to trouble shoot this...on either...you might be hearing from me.
Now, I've heard a few guys mention here the Merc system is better than the Chrysler system...and it's best to get the Merc upgrade on the years prior to Merc's supposed 'mechanical changes' in the mid-late 90's...which were 'downgrades' supposedly.
As for the spark plugs I have had wayyyyy better luck running the BUHX plugs. When I ran the BUHW plugs (prior to a big decarb, etc) they would load up pretty quick at slow speeds.
Anywhoooo, nice work!!
BP

 

[pnwboat]

Re: Ignition system upgrade?

Yes the BUHX plugs are a "boost gap" plug specifically for the Prestolite ignition system. If you put an ohm meter on them they read open. There is an internal gap inside the plug itself. I'm not an engineer or anything like that but from what I can gather, the purpose is to allow the voltage to reach a certain potential before it produces spark at the electrode end. Seems a like they're trying to insure maximum voltage potential before it actually fires. Not sure if that's a good thing or not.

Anyways, the BUHW plugs don't seem to have an internal gap as they read continuity with an ohm meter. The gap on the BUHW plugs is definitely larger which I think might help with ignition. Maybe someone with more experience in this area can chime in.

So far, Ive not had any problem with the way the BUHW plugs have been running. Very clean and they all look exactly the same. Just a small amount of carbon around the ceramic insulator portion of the the plug.

With the new flywheel and stator, I also noticed that my voltmeter gauge on my dash indicates 14.5 volts at 1000 RPM's. The old system I had to be 2000 - 3000 RPM's before I saw it get to 14.5 volts. I guess that's good?

The stator I used was the 16 AMP version. It originally had the bullet connectors. I changed them to spade lug connectors and a terminal strip. I see a lot of the stator bullet connectors on this particular stator where the clear plastic and the metal bullet connectors are discolored from what looks like too much heat. I suspect that the bullet type connectors may not be robust enough to carry the current generated by the stator.