1970 25 hp, weird exhaust sound....

[BF]

Hi all,

I had a funny thing happen to my 25 hp yesterday. While in cruise at full throttle, the exhaust note of the engine changed (got loud). Sounded like an exhaust leak into the cowling. There were no noticeable change in RPMs or power (kept plugging along). When I throttled back to idle, it went back to normal, and and then stayed normal up to at WOT, until it again suddenly started being noisy again. I popped the cover off, nothing to see there and no obvious exhaust leak. It kept doing that off and on, and sometimes would persist even when I throttled back. A couple times when it was doing this at lower rpms, when I opened the throttle, the rpm's would increase, but no acceleration. The exhaust leak thing seemed to only start to show up after a few minutes of WOT running.

My thoughts are, that maybe there is an exhaust gasket under the power head, above the leg that is just on the verge being totally bad. When it gets hot, it starts to leak (?). As for the rpm jump, I'm thinking it might just be a coincidence that the prop has spun at the same time (?) It is a drive pin kind. It seems a bit odd to me that it can push the boat fine 99% of the time, but only slip a couple times. It only "slipped" when the engine noise was loud, not when running normally. How could they be related (?).

Compression test is great (120+ on each), it was pumping water the whole time, and no, there were no weeds on the prop (I checked).

Any thoughts? Thanks in advance.

 

[tashasdaddy]

Re: 1970 25 hp, weird exhaust sound....

did you check the shear pin, they will some times break, and then the center part will slip out enough to grab on side of the prop.

 

[F_R]

Re: 1970 25 hp, weird exhaust sound....

BF, your guess is as good as mine, however I seriosly doubt it is the powerhead gasket. Also, I sort of doubt it is under the hood, because a significant leak under there would make it run bad due to ingesting it's own exhaust.

I am leaning toward the prop question. Could there have been anything creating a disturbance forward of the prop, causing cavitation? I have had motors do that from just a blade of grass hooked around the gearcase and it falls off as soon as you stop to look. 'Course we have a lot of water weeds here in FL and are used to it.

 

[BF]

Re: 1970 25 hp, weird exhaust sound....

yes, good thoughts. I'll check the pin today, yesterday was crazy busy, so I didn't do anything but bring it home. It is possible that something was whacked a bit (but prop is still pristine)... there was the odd bit of wood floating around the lake. I'm pretty sure weeds were not the culprit... this was different.

I know exactly what you mean about 1/2 a shear pin being left there... it was that way when I bought the motor. I'll let you know what I find.

Thx

 

[CalicoKid]

Re: 1970 25 hp, weird exhaust sound....

It sounds to me like the motor is just high on the transom and that when you're going WOT the exhaust is out of the water and the prop is ventilating. How hight is the anti-vent plate in relation to the bottom of the boat?
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[BF]

Re: 1970 25 hp, weird exhaust sound....

Hi, nope I don't think that's the problem... it's a 16" transom and short shaft motor. Anti cav plate is about 2-3" below the boat. That's where it should be with this old style motor.... it has 2 water pickups, one in the leading edge of the fin behind the prop, and the other a port just above the anti-cavitation plate.

and when it was rev'ing with little go, I wasn't on plane.

thanks...

 

[BF]

Re: 1970 25 hp, weird exhaust sound....

well, drive pin is intact, albeit slightly bent. With prop in hand, I can tell one of the blades has a slight bend at the last 1/2" or so at the tip.... so I'm thinkin' I whacked something hard enough to bend the shear pin and probably loosen the hub in the prop. I'm not sure if I'll have time to do the magic marker thingy on the prop hub and water test. There isn't really obvious scarring in the hub showing that it was spinning, but it looks like the end of the drive pin may have made a scratch in the outer aluminum like it would if it had spun. Maybe I'll get another prop before I go out next time.. worst case is that I have a good spare. Seeing how good my prop looks, and suspecting that it has a bad hub makes me kinda leary about buying anything used (e.g. ebay).... maybe I'll try and support Iboats by ordering a new one from here (depends on shipping).

Last time I dropped one of my Dad's prop off at the local "prop guy" he told me that he doesn't re-hub the drive pin props because too often the hub spins again anyway (?). Anyone else heard this??