Testing the CDI unit

[immiketoo]

I'm new to the forum, but have searched and not found my answer.

1994 Yamaha C75LTRS with intermittent miss and bogging down (won't come up on plane). I've struggled with this all summer. It may do it when cold or hot, with the cover on or off.

The fuel system is COMPLETELY new - new tank, new hoses, new water separator, new fuel pump, carbs clean as new. Ethanol free fuel used exclusively in this motor since I've owned it (10 years).

I have purchased a DVA to test the CDI unit, but I cannot find any specifications to tell me what voltages to be looking for, or which terminals to test. The best I have found is a YouTube video which is "probably" correct, but I would like to get the correct specifications.

Can anyone point me to this information? Thanks in advance,
Mike

[99yam40]

A Yamaha service manual for your motor will have the specs and how to test.
You really need to get one if working on your motor yourself.

with that said try reading this to see if it will help some\
http://www.cdielectronics.com/wp-con...side-Pages.pdf

[rodbolt17]

pulser1w/r-w/y.
pulser 2 w/b-w/g.
resistance is 330 ohms.
cranking loaded 2.5v
at 1500 7v.
cdi out 95v cranking loaded.
130v at 1500 b/w-b.

[immiketoo]

Thank you, guys. This info helps.

Any recommendations on a good manual? I do have a Clymer manual and a manual on CD. Both have remove/replace instructions, but neither had the specifications on resistance or voltage necessary to test coils/CDI unit.

I did test all three coils and found them all to have the same resistance on low and high sides, so I assume they are functional. I did find minor corrosion on the ground connections where the coil wire is grounded to the engine, also corrosion in the spark plug connections. I have sanded/polished all connections and reassembled with silicone grease, but have not had the opportunity to check it out yet.

I will continue with the CDI unit tests using Rodbolt's' info until I can do on-water testing with the CDI electronics website instructions.

Thanks again!

[immiketoo]

Found the issue - not what I thought

I have been working all summer to sort out my c75. It will sometimes run just fine, sometime miss badly and bog down under load. The fuel system has been completely replaced (leaking tank) and the carburetors have all been cleaned and synchronized.

With help from this forum I tested the ignition coils, CDI unit and the stator. Aside from a little corrosion, all tested well. Somehow I just KNEW it was an ignition problem, but just couldn't find it.

Last time out, it ran like a new motor for 45 minutes, then started the same problem. This time I found slight evidence of water on the bottom spark plug. I pulled the head and found a an almost invisible crack. Evidently, when the engine warms up, the crack opens enough to cause the problem. Compression checked good on a cold engine, never tested compression at operating temperature - maybe it would have showed up.

My experience has always been that thorough troubleshooting eventually gets to the solution, even when it is not what you expected.

Thanks for the help you gave me on testing the ignition system and avoiding replacing expensive parts.

[boscoe99]

Thanks for the report. Glad you found the problem.

[pstephens46]

Can this crack be repaired?

[immiketoo]

Brand new head - $167. Even if it could be repaired, it probably would cost more to fix than replace.

I opted to replace everything - head, head cover, thermostat & associated grommets, parts, gaskets, etc. Total bill just under $320 and I won't have to worry about warped head or cover.

[pstephens46]

Not too bad

[rodbolt17]

and that is where experience whoops.
that issue should have been spotted the first time the plugs came out.
water burn salt or fresh, leaves distinctive marks on the plug porcelain and electrodes.
anti-freeze even leaves a message on the plug.
it does take a few years to be able to accurately read a plug.

but it is one of the first things I look at.
it is a window to the combustion cycle.

[immiketoo]

I hear you, Rodbolt, but I checked plugs over and over. This is the first time I ever saw evidence of water. Whether the crack got worse or whether I just missed it, I can't say. So, I will accept the Whoops. - and add to my experience.

[rodbolt17]

burning freshwater leaves a distinctive brownish to reddish crust on the side electrode and the area at the end of the threads.
burning saltwater tends to leave tiny green flecks on the center electrode.

burning anti-freeze typically leaves a heavy crust, kinda brownish and you can smell it in the exhaust.

[pstephens46]

Some pics on the topic. Not specific to marine but interesting.
Anybody know of any more specific pics relating to Rodbolt's descriptions?

https://www.ngksparkplugs.com/about-...d-a-spark-plug

[cpostis]

Here's some spark plug info from our "shrimp on the barbi" friends.
Spark Plug Analysis - Technical Information - NGK Spark Plugs Australia

[rodbolt17]

while those pics work well in the automotive world they don't mean much in the marine world as far as water burn.
you can also use silver iodide but my eyes do about as well.

did anyone notice on the normal plug, center electrode.
see the ring around the electrode about .03-.04" from the end?

that is an indication of a correct heat range and normal combustion.