75 hp Merc question

[ewirz]

Question for you mechanic types:

I have a 1998 75hp merc (2 stroke carb) on a Alumacraft Tourny pro 175. This weekend the motor started acting up. Here's the symptoms:

Sputters a little on hole shot (idles fine). Once at WOT, the rpms would periodically drop to about half or 2/3 then back to wot. This would happen in short bursts and only last for a second or 2.

I was thinking it was sucking air so I replaced the fuel line and tightened all the clamps. That didnt fix it.

Powerhead was rebuilt 2 yrs agp with about 60hrs on it. All three CDM's were replaced 3 yrs ago. Has fresh gas.

I'm thinking its still a fuel problem-dirty carbs or fuel pump, or do I have an ignition issue?

Thanks in advance for the advice.

Eric

[Heartman]

If rpm's are coming down, gas flow may be your problem. Gas line replacement was good start - now on to fresh fuel - then have fuel pump checked. Those are pretty bulletproof engines if you give them a chance.

[boatfixer]

Eric, if you have a timing light or access to one, I would hook it up to each cylinder while running it as you normally would until it misses. Watch for the light to go out (you will probably need someone with you). this will help determine if its ignitin related. The sudden drop in RPM's is indicative of an ignition problem.

[ewirz]

Bruce-

Thats what I was affraid of. I have a timing light. Is this something I can check with the muffs on in the yard (obviously not WOT) or does the motor need to be under load? Might be a bit of a circus on the water since its a tiller.

When I replaced the CDM's, one went bad (was running on only 2 cylinders.) After I replaced the bad one, the other 2 went. Once I got 3 new ones, ran fine. This is different. Its firing on all 3. Like I mentioned, it sputters just a little on hole shot. The hesitation is only @ WOT. If I back off a little, no hesitation - thats why I was thinking it was sucking air. When it does hesitate, it will go 2-3-4 minutes running fine, then hesitate for a second or 2.

Do you think its the stator or voltage regulator?

I'll pull the carbs and give them a good cleaning, recheck all the plumbing and see what I can come up with with the timing light.

Thanks

Eric

[Gordie]

I had simalar with my classic fifty but it would spudder a bit more at WOT then when you let off and gradually throttled up it was fine but once in a while it would spudder I rebulit the carbs, still did it then rebuilt the fuel pump took care of it.

[boatfixer]

Bruce-

Thats what I was affraid of. I have a timing light. Is this something I can check with the muffs on in the yard (obviously not WOT) or does the motor need to be under load? Might be a bit of a circus on the water since its a tiller.

When I replaced the CDM's, one went bad (was running on only 2 cylinders.) After I replaced the bad one, the other 2 went. Once I got 3 new ones, ran fine. This is different. Its firing on all 3. Like I mentioned, it sputters just a little on hole shot. The hesitation is only @ WOT. If I back off a little, no hesitation - thats why I was thinking it was sucking air. When it does hesitate, it will go 2-3-4 minutes running fine, then hesitate for a second or 2.

Do you think its the stator or voltage regulator?

I'll pull the carbs and give them a good cleaning, recheck all the plumbing and see what I can come up with with the timing light.

Thanks

Eric

Since you now say it wont do it if you let up, I'd lean towards fuel. The timing light thing.. You dont need to watch the light on the flywheel, just watch for ther light to flicker or go out, watch the end of the light. Actually easier to do by yourself with a tiller.

[ewirz]

Well just got done rebuilding carbs. Pretty clean. However I did find a fuel line between the carbs that was missing a clamp. Culprit? I'm headed out on the water now and will have the timing light with. Ill report how it goes.

ERW

[ewirz]

Here's my (late) report. Fixed the fuel line and cleaned the carbs. Running better but still not up to par. Hooked up the timing light and no. 3 cylinder was firing sporatically. Ran a bunch of ignition tests, turned out the trigger was bad. $60 for a new trigger, a bunch of swearing at the fly wheel and it runs like a top.

Thanks again for all the advice.

Eric

[chucker]

Eric,

What is the trigger? Is that the coil pack?

[ewirz]

Chucker-

The trigger sits inside the stator under the flywheel. Its what tells the CDMs (Capacitance Discharge. Modual=which are basically the coils) when to fire. there were a bunch of test I had to run to make sure that was the issue. If you buy an ignition part and install it and then find out you have the wrong one, you cannot return them. Pretty easy to install, only bugger is getting the flywheel off.

ERW