86' 25hp Merc Tiller Dies at half throttle

[Case450]

My 86 two stroke merc dies at half throttle. I have new fuel lines, new filters, new gas, cleaned carbs, new plugs, and new coils.

When none of this helped the problem of the motor dying at half throttle I took it to the marine mechanic who after an analysis said he didnt know what was wrong with it exactly and suggested throwing parts at it.

He said he's start with a new cdi but at 500 dollars canadian he wanted my permission to change it first. I told him not to bother and that I'll do it myself.

So the mechanic is no better off then where I am at right now so I might as well throw parts at it and save money on the labour.

My question is, how or is there even a way to test the cdi? Is there any special tools needed to test it? What exactly does a cdi do? Is the cdi on my motor interchangeable with any others?

I know it's an older girl but the compression is good in both cylinders and i've practically rebuilt her so to stop now is a huge waste of money so I might as well go all the way and do it right.

Lookin for some guidance or suggestions from a merc guru who can get me pointed in the right direction.

Thanks yall!!!

 

[jbjennings]

Re: 86' 25hp Merc Tiller Dies at half throttle

I believe CDI stands for Capacitance Discharge Ignition, but either way, it is the part that basically provides spark.
If you hadn't said you cleaned the carbs, I'd have thought your high speed jet was clogged, since the problem occurs at half throttle. You might try spraying some propane from a propane torch around the carb while the motor is running and see if you can locate a vacuum leak---it will run faster on the propane if it's sucking some in through a leak. ALso checking that your throttle plate is all the way open when the throttle is turned to wide open.
You might also try running the motor, and getting it up toward half throttle and seeing what happens when you spray a little premixed fuel into the carb throat. IF it picks up, you know you have a fuel delivery problem, if it kills it, you probably do have spark troubles.
One of the things I don't like about CDI ignition is that they are darned expensive to replace when they foul up.
Good luck,
JBJ

 

[James R]

Re: 86' 25hp Merc Tiller Dies at half throttle

JB gave you some good info. If both cylinders are firing then it is unlikely that it is a power pack. 'I' boats have power packs for your motor and a lot less than $500. You can replace it yourself.
Could be a syncronization problem if not carb/fuel pump.
Could be a leak in the fuel line allowing the pump to suck air.
Finally it could be crank seals.
Had one here a few months ago.