CDI to CDM force 90 Sport Jet

[Redbarron%%]

OK I finally got my 90 HP three cylinder sport jet running properly after converting from CDI to CDM ignition. It has been quite a journey.
While the CDI trigger coil will happily fire the CDM modules there are issues to be taken care of.
Back in the 90's Mercury sold a kit to upgrade from the CDI to the CDM 95XR specifications, It included all of the wiring harness and mounting plate, trigger coil and a new flywheel with the different magnet setup as the old system has push-pull coils and the new one has single ended.
The new 95XR flywheel will not fit the early crankshaft as the hub is larger so swapping that out is a non-starter.
So after fighting the timing due to the bias circuit being in the old CDI boxes and the CDM bias circuit is in the trigger coil I decided to buy a used new style trigger coil and stripped the potting out to determine the circuitry and test the system.
What happens with out the bias circuit or with a bad bias circuit if you set the WOT timing at 30 degrees the idle will be way too low, perhaps less than 0 degrees. This is due to the advance in the voltage rise as the speed increases which would normally be offset by the negative bias voltage generated and rectified from the trigger coils.
So now I have the circuitry external to the trigger coil, between the trigger coil and the CDM trigger input.
Now if I set the WOT timing at 30 degrees the idle will be at 9 degrees as specified and the idle set screw is at 1/8" also as indicated in the service manual.
I had a chance to test the system today in my Hobie Jet Fisherman and I can report that it works great with smooth acceleration and smooth running at WOT and idle and points between.
The system starts rapidly perhaps as I also included a circuit to increase the coming in speed of the ignition.
The 12 volt start signal that pulls in the starter solenoid also pulls in a relay that bypasses the bias circuit to fire the CDMs at a lower RPM and since this is just for start and at cranking RPM it should nor be an issue.
So can you convert to the CDM modules?
Yes you can.
Buy a set of CDM and a wiring harness off ebay and make arrangements to mount them on the base plate and then rig up a bias circuit. While I have figured it out I would like to recover some of the cost of figuring this out and the cost of the engine overhaul due to a burned piston while testing earlier.
My thoughts were that it is easier and cheaper to replace one CDM rather than troubleshoot the CDI, coils, etc.
I can keep some spare CDM modules in the emergency tool box and perhaps get home if one fails.

 

[jerryjerry05]

A success story!!! I like it!!

 

[Redbarron%%]

I have been well pleased at last with the results of the experiments. Good power, smooth acceleration, runs good at WOT and cruise. The old Hobie Jet runs about 25 mph at 4200 with two up, two large batteries, trolling motor on the bow and gear.
With the new trigger bias setup it starts almost instantly and idles well trolling. A lot of work, but a pleasure so far (after a lot of headache) .
I figured out a few things
1. If the bias is incorrect the timing is very difficult to set for 30 degrees WOT and 9 degrees at idle.
2. The throttle was difficult as well and I finally drilled a new hole in the cam to get the travel correct.
3. This leads me to believe that the bias circuit has a great effect on the travel of the timing tower and and the throttle throw.
4. If the bias gain is incorrect the spread between the WOT and 30 degrees means that the 9 degrees is difficult to get especially with the idle screw set as indicated in the workshop manual with 1/8" from the head of the idle screw to the stop nut.
5 As the negative bias is rectified from the trigger pulses the condition of the capacitor and the resistors would make some differences as they age.
6. With the modified system using the CDI trigger coil and copied CDM bias circuit with the late model CDMs the bias is was not exactly correct, but was close enough and I was able to get it to work with a little fiddling with the cam modifications
If you are having problems with the timing at WOT and idle and getting the carb opening right check the bias circuit. In the CDI system the bias circuit is in the CDI module and the Black/White wire should build a negative voltage as the rpm increases. The CDM trigger common (Black) is grounded as the bias circuit is within the trigger coil Assembly.

 

[Wavewarrior]

I have a 1994 sea rayder 90 sport jet , flywheel lost magnets and ate the stater , replaced stater (red same as it was ) and bought a new flywheel ( $580 at Cowley) , still no spark!! The original post above is slightly above my pay scale lol , any pointers on where to go next with testing???

 

[jbcurt00]

The original post above is slightly above my pay scale lol , any pointers on where to go next with testing???

Lets start w not hijacking someone else's topic and start a topic of your own about your motor

 

[Wavewarrior]

Ok then , thanks , I'll try another forum

 

[Redbarron%%]

Just make another post and I will be glad to help as much as I can.

 

[Mike Golding]

Sent you a PM I'm interested in your bias system

 

[Mike Golding]

sent you another pm

 

[Redbarron%%]

I had a little set back when the engine kicked back and bent the shaft on the cheap Chinese starter with the start bypass relay in the circuit. I think the higher speed in the partial turn to the trigger point advanced the timing much further than I had assumed. I had assumed that the initial cranking speed would not build up the positive voltage and advance, but I seem to be wrong and I am removing the start relay and the tie in to the starter solenoid.
Initially I had a dragging stater and the bypass did produce a flash on the timing light a very low speeds where without it there was no initial firing of the timing light, spark plug and no start.
I have installed the new board and removed the bypass wire the the start and now I need to get on the water to test the second board I have built before I send one to someone else for testing with the CDM setup.

 

[Mike Golding]

Any updates on the new setup

 

[Redbarron%%]

I have been pretty busy with things other than my boat.
I have a board I can send you that I have run in my boat and I believe it will work for you. It currently has the bypass relay for starting that I am going to remove as I think that this is what caused the kickback that bent the shaft on my cheap Chinese starter.
PM me and send me your address etc.

 

[Redbarron%%]

I have the board I built and ran on my Sport Jet with cover on it and can send it to you. I removed the relay and the jumper to the starter to eliminate the possibility of the kickback when hooked up direct. This board worked well n my 90 Jet and should work for you. PM me if you want it.

 

[pnwboat]

I had a 1988 125hp motor that had a crack in the block. I took all the guts out of the 1988 motor and put it into a later model big bore block. I had to buy larger diameter pistons to fit the block but it worked just fine. I have no idea what year the replacement block was, but it had a setup for oil injection. It had a mounting boss for an oil injection pump.

I also converted the 1988 Prestolite ignition system to the later model CDM modules. I used a flywheel from what I believe was a 1994ish 120HP motor. It has the same magnet set up (trigger and stator magnets), at least the dimentions were the same as the later model CDM motors, but the crankshaft mounting hole fit the older 1988 crankshaft. This flywheel allowed me to use all of the CDM components...... coils, trigger, trigger mounting ring, stator, etc etc. It's been running flawlessly for the past 6 or 7 years and no more ball of spaghetti wires behind the coil mounting plate to deal with. I don't know if this flywheel is the correct one to use as far as the elctronics, I'm not an electronics expert, but all I can say is that it works great! Attached is a picture of the flywheel that I used.

 

[Redbarron%%]

The trigger will fire the CDM modules, but the advance due to the increasing RPM causes the split between the idle timing and the WOT to be higher than normal and the timing is less stable. This is due to the lack of the trigger bias circuit in the early trigger coil. It is possible that the late model trigger with the solid black ground wire will work with the early smaller hub flywheel, I have not tried it as the magnets are different in the hub. The early trigger has a white with black tracer that goes the the old CDI box and it's bias circuit it in it.
I would be interested if the new trigger worked with the old flywheel as that would take care of the trigger issue.
What happens is that the voltage preceeding the timing point increases as it nears that point and it only has to get to .6 volts to fire the CDM. The bias circuit rectifies the AC that is generated by the trigger coils in the negative direction and stores it in a capacitor and basically the trigger pulse has to be high enough to overcome that negative voltage (about -20 volts) to get to that +.6 level at the proper point.
I hope I have made what I understand is happening clear. The CDMs can work with the early engines, but the timing will not be correct and will not be stable. I ran without it for a long time, but I had issues with starting etc and finally burned a piston, perhaps not directly due to the timing.
A bad bias circuit means that to get the WOT timing correct the idle is much too low the spread being more than the 9* to 30* (call it ~20%) it will be closer to 30* without bias.
Since this engine initially accelerates initially more on timing than carb plates opening it makes the mid-range more problematic.

 

[pnwboat]

The flywheel that I'm using seems to be working just fine for me....well at least as far as I can tell with my limited knowledge of electronics. WOT timing is set to 30° advance. At idle, I'm seeing about 6° ro 7° advance. Motor starts right up and runs great. Excellent throttle response, runs great all the way up to WOT without any miss or hesitation. Haven't had any issues with pistons or compression in the past 6 or 7 years. I usually run 89 octane gas. Probably one of the best modifications that I've performed on the motor to increase the ignition system reliability and ease of trouble-shooting. The motor is on a 1988 18ft Bayliner Cobra w/17pitch prop. The family uses it for skiing hence the 17 pitch prop, however, you have to really watch the RPM's as occasionally it will go up to 6000 if you don't pay attention.

 

[Redbarron%%]

That sounds great. If it works for you go for it. The idle timing should be 8 - 9* but close enough if it idles and accelerated well.
I agree about the greater reliability as that is the reason I did it myself. That and with parts off eBay it is less expensive than replacing the switchbox CDI. Also you can carry a spare CDM with you and replace one on the water if need be.

 

[Mike Golding]

sent you another pm yes i want your bias i wonder if pnwboat might have had the conversion flywheel on the motor he bought there are some out there

 

[GA_Boater]

This has turned into a sales venture.

Closed.