1997 Force 120 spark issue

[Harperdude67]

Motor is running but seems to start to lose power at higher RPMs. Back off the throttle and it seems to come back to life. Runs all day at half throttle.

While running and flushing the motor, I reached over and unplugged the #1 spark lead, there was no change in the motor running, plugged it back in and did the same on #2, no change. Plugged it back in and unplugged #3, engine dies. Engine dies when #4 is unplugged as well. Only unplugged one at a time.

Does anyone have a troubleshooting guide for the CDM modules? I know these things are tied together by grounding in strange ways, but is there a guide for this?

Any help is greatly appreciated!

 

[Jiggz]

First you need to check compression on both #1 and #2 cylinder. Then check make sure fuel is getting into each cylinder (inspect plugs for fuel wetting) if not check the carb. If compression is good and fuel is actually getting into the cylinders, then it could be electrical. I am presuming you have the Prestolite ignition system.


#1 and #2 cylinders are fed from the same CDM module. There are two separate circuits in each module. Seldom they failed at the same time but it can happen. There are also two separate ignition coil for each cylinder, and yes both of them failing at the same time is unique but not impossible.

Starting with the easy ones through process of elimination:
1. Switch #3 ignition coil with #1. And see if the problem follows, i.e. #3 cylinder loses spark and #1 regains spark. If it does, ignition coil on #1 is bad.

2. Do the same steps with #2 cylinder.

3. If problem does not follow, i.e., #1 and #2 cylinder continues to have no spark, switch #1 CDM (feeds #1 and #2 cylinders) with #2 CDM. This involves more wires do make sure you take a pic and draw each wire connection first before disconnecting.

4. If problem follows, then you have a bad #1 CDM . If problem DOES NOT follow, check resistance reading on both the stator and the trigger.

 

[Harperdude67]

Thanks Jiggz for the great response! Tonight I will start with compression test and taking a look at the plugs, carbs, etc. I just replaced the fuel tank in the boat and have a new water/fuel separator, lines and fuel filter so my very first thought was a lack of fuel issue. Then when running it Sat to flush it, I took the engine cover off and unplugged that first spark plug wire and was amazed that it didn't change the way it was running. Same with #2 and then with everything plugged in and running unplugged #3 or #4 and it dies.

This may sound like a stupid question but is there an easy way to determine if there is spark on a cylinder? Other then the thing lighting you up when you grab it....just kidding. The spark plugs don't have a traditional piece on the end where you can observe the spark so is there something you can plug in there to see it?


Thanks again for the procedure write up.

 

[Jiggz]

No Title

The best way to check for spark is to use an inline spark tester (~$5) which is available from auto shop or even at HF. Just need to do the checking at night or under a shade. It's really very hard to see the spark during day time or with bright light. Hers' a pic.

 

[jerryjerry05]

If you have the gapless plugs,get a plug that has the electrode on it.
​Then hook the "new plug" to the wire.
Hold it next to the block on a screw/bolt head.
​Have someone turn the key or use a jump box.
​The plug should spark.
​Oh yea use something that won't telegraph the spark to you.

​I bought 2 of those spark testers in the pic.
​Junk. No matter what I did they wouldn't work right.
​They are real hard to see the spark too.

 

[Harperdude67]

Didn't get a chance to mess with it last night but will this evening if the rains give me a chance. I have an older timing gun with an inductive pickup that should show a spark as well. Of course it doesn't mean the spark plug is working but at least it will show the coil pack is working.

Thanks again guys for the info. This is exactly the reason these forums exist.

 

[Jiggz]

That is correct. An inductive type timing light will serve the purpose.