69 125 L6 Tower of Power runs rough until cowling is off.

cjleigh

Cadet
Joined
May 12, 2010
Messages
11
Back again with a question about the ol' tower of power attached to my boat. I took her out on the lake yesturday and we cruised out of the little inlet where the ramp is located. When I gave her some throttle she started to run really rough and then died. She started right up and we idled along for a while, I gave her some gas and again... rough and then died.

I figured it was a fuel starvation problem, so I checked the bulb (it was good). I took the cowling off to make sure things were okay inside. I was hoping for something really obvious like a mouse stuck in a carb or something, but no such luck. I left the cowl off and started her up just to check things out. I had my kid give her some gas while I was watching the insides for something obvious, but the motor ran fine. Took her up to full throttle, and she ran smooth as a top. So we stopped and put the cowling back on, figuring it as one of those worked itself out miracles. Unfortunatly as soon as we started off again, the problem re-emerged.

Once again I removed the cowling and the motor ran like a top, so we left it off for the rest of the trip and had no problems, other than it was a little louder than we were used to.

The only thing I can think of is air starvation inside the cowling, like the carbs are not getting enough air to mix with the fuel while the boat is running at full tilt. I looked all around for some kind of air vents that might be plugged but I couldn't find any? Any suggestions? Should I just drill some holes in the cowling to provide more air, or is there some kind of adjustment I need to make?

Thanks.

C.J.
 

76canaventure

Seaman Apprentice
Joined
Mar 27, 2009
Messages
32
Re: 69 125 L6 Tower of Power runs rough until cowling is off.

Hi, It sounds to me like your exhaust plate gasket is leaking. That will produce the symptoms that your describing. I'm pretty sure you will have to pull the powerhead to replace it. That may open a different can of worms but at least the gasket is cheap, like $15 bucks. Someone who knows for sure will come along and chime in here I'm sure.
Regards,
 

emckelvy

Commander
Joined
Jan 16, 2004
Messages
2,506
Re: 69 125 L6 Tower of Power runs rough until cowling is off.

I've never seen an Inline Six spew exhaust gasses into the powerhead. If it were enough to affect operation, you'd surely hear it and see the fumes.

I'm thinking you either have a leaky spark plug wire, or bad spark plug boot. When you reinstall the cover, it's arcing thru the defect to the cover, killing your power. Seen this quite a few times on the baby Mercs where you scrub on the plug boot almost every time you pull the cover off & on.

One thing you could do is firmly clamp an insulated wire to engine ground somewhere (under a bolthead, etc), then rub the wire along all the sparking-related surfaces. If you get an arc to the wire, you've found your culprit. Another good way is to lite 'er off when it's dark outside and check for arcs & sparks.

Other than that, check to make sure the cover is not crimping or otherwise impinging on any wire or hose when it's installed. Anything shorting to ground would cause issues, as would a crimped hose.

BTW the motor breathes thru the front bottom of the motor pan, amongst other places which aren't hermetically sealed.

Anyway, a couple more things to think about.

HTH & let us know what you find.............ed
 
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