Random engine surging, then overheat!

[porkpiehat]

Hi All:

Heading into my third summer of RIB ownership and I am learning a lot (and spending not a little). 13' 1998 AB with seat and right-hand console.

I am befuddled by a very intermittent problem I've had since I launched my boat: Here's what's happened probably four-five times total:

Around 2/3 throttle the engine will surge and max out RPMs. Not sure if I throttle back or it drops down on it's own. It continues to spike and drop, the HOT light comes on, at which point the engine limits itself to 2k rpm. Smoke is coming from the engine cover, but there is a tell-tale.

I turn it off to let it cool. 10-15 mins later with some effort it starts up and doesn't happen again. (Once it did happen a second time in the same trip, but that was after running it for more than two hours).

It shakes and shimmies at lower rpms/idle...it is a 2-stroke and all but it seemed worse after the incidents.

factors worth noting:

-I got snookered (I think) into replacing the the lower unit (I found a used one) two years ago. The surging happened before and after the replacement.

-Compression was fine last time it was checked.

-I bought it from a mechanic (non-boat) who bought it at auction from the Maine game wardens. The engine is a 35hp 3cyl 2 stroke (97/98) that was originally self-mixing the oil and gas. Either he or the game wardens removed the oil injection because it was problematic and told me to mix at 50:1. I can't confirm that this was done correctly.

-Someone has done crude fiberglass work on the bottom and adhered a long steel plate down the center. I'm going to see if I can attach at pic.

-It eats batteries like crazy and apparently had a loose neutral safety switch.

-When I took possession it had a 16 pitch four-blade prop and I couldn't make more than 4100 rpms. I replaced it with a 13 pitch 3 blade and it gets 5800 or so. This phenomenon happened with both props.

After talking to people and researching my guess is that the roughness under the hull might be causing some cavitation/ventilation OR the engine is mounted too high/low...???

I am also suspecting the oil injector might have had a special role given the three cylinders. I think it would adjust the mix given the engine speed. Maybe it was incorrectly removed?

The impeller was just replaced. Haven't gotten it back on the water since, but the slipping happened twice during my last trip last season before storing it.

I could be on the wrong path altogether. Perhaps it needs a viking funeral. I am just worried this could become catastrophic if the timing were right, but I can't replicate it in a garage for a mechanic or even at will in the open water.

Thoughts/experience would be greatly appreciated....thanks!

[colcreate]

Sounds like a fuel problem, running too weak and overheating. Few suggestions:
Might be due to gummed up carbs especially if the premix gas been sat in them for a time between seasons.
If the oil injection (not sure what type you had) has not been blocked off or removed correctly you might be sucking in air somewhere weakening the fuel/air mixture.
Could be a restriction on fuel supply, as the engine demands more fuel at higher revs the fuel levels in the carbs drop weakening the mixture causing the engine to race and overheat.

Eats batteries? Get the charging system checked out. The rectifier/voltage regulator might have failed and is not handling the AC from the alternator properly.

If it were mine, I would have the carbs pulled off, stripped and cleaned just for peace of mind and have the oil injection pipes checked to see if they had been closed off properly.

The standard of hull repair might be an indication of the quality of engine maintenance so check everything methodically, especially the fuel supply from tank to carbs. Check that there are no restrictions and that you are getting a healthy clean supply at the carbs.

[Festinghouse]

+1 sounds like its running lean to me. ask anyone who uses a stihl saw what it does when its running out of fuel, and they will tell you that the revs increase on their own.

[Festinghouse]

on another note, when did your boat change its name col?!!!

[porkpiehat]

Thanks fellas. These all seem like fixable, relatively inexpensive problems. I was fearing catastrophic engine failure, gear case slipping, or costly fiberglass work!

One other interesting fact: it has never happened in the open ocean/harbor...just flat fresh water rivers and lakes. Or maybe another red herring!

[porkpiehat]

Took it out today after the impeller replaced and she ran completely fine! Can't say it won't happen next time but who knows. If it does I'll have someone look at the carb! Thanks for all the ideas!