Rough low rpm, Fall on face at WOT

StlChris

Recruit
Joined
Jul 1, 2010
Messages
5
This is an 87 black max xr2 150 with about 15 hours on a rebuild on an 87 charger.
The boat is new to me and when I bought it a couple weeks back and several times in the driveway it fired right up and just purred, no choke, throttle nothing as I would a fresh rebuild would run. I took it out the first time this weekend and all went well the first day. I was out several hours getting used to it how it ran, reacted, etc. It ran like a champ in all scenarios. The next day it fired right up and we went out to play for the day. Shortly into the trip I went from WOT and slowed to make a turn and got in it again. I hit WOT at 5500 and it fell on its face down to about 4000 rpm so I backed off for a sec and punched it again and it ran at 5500 for a bit and did it again. I stopped and checked things out to find nothing. It was a little hard to start which was suprising to me as every other time immediately fired and purred. This continued througout the day. Sometimes it would run several minutes before falling but it eventually would. We decided to call it a day and try again in the morning after I could give it a onceover and see if I could find anything obvious.

The next morning it was very difficult to start and keep running. I had to choke it and throttle it to keep it running and was a chore to get it in gear without dying and even then it ran bad until I could get it up to about 1500 or so and then it would just jump on it and run fine until the previous problem would take over. Once warm the starting and getting in gear was a little better but would hardly idle without help from me and the throttle. It was backfiring and popping through the carb at low idle occasionally.

I bought a new fuel line, bulb, and fittings thinking initally it was starving or getting air in the line but have yet to put it on to see if it helps. I was also reading somewhere about a timing advancer or something similarly named going bad but I cant find it anymore to read it again but the low rpm issue could fit with a timing issue I would think.

I am no boat mechanic for sure but am comfortable around motors and did get a manual but am no good with diagnosis. I would sure appreciate any help on what to do or a direction to look at. If I can provide any more info just ask and I will do my best. Also if we cant figure this out if you know of a good mechanic in the Lake of the Ozarks gravois mills or dam area would be great or in the ofallon mo area.

Thanks for taking the time to read this, I know its kinda long, and thanks for any help in advance!
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
Re: Rough low rpm, Fall on face at WOT

Pull the spark plugs and have a good look at them. Then do a compression check.... Did you find out why the engine was rebuilt in the first place? and was the reason for the failure that required the rebuild (I suspect a piston burnt up) ever fixed?

Chris.........
 

StlChris

Recruit
Joined
Jul 1, 2010
Messages
5
Re: Rough low rpm, Fall on face at WOT

It was rebuilt due to the previous owner trying to install the wrong impeller and forcing it to work, and obviously didnt, and in fact did burn a piston. I was thinking of doing that if this does become something more complex but when it is running it runs like a striped *** ape and I didnt think it would be something that serious(popped piston) but like I said I aint no boat mechanic. Ill have to see if my compression tester has the right thread if I can even find the damn thing anymore, its been a long time since I have used it.
Thanks for the reply Chris.
 

Doh

Petty Officer 2nd Class
Joined
Aug 12, 2008
Messages
193
Re: Rough low rpm, Fall on face at WOT

Most common is Varnish in the bowl of the carbs. A good carb cleaning should do wounders for you.
 

StlChris

Recruit
Joined
Jul 1, 2010
Messages
5
Re: Rough low rpm, Fall on face at WOT

OK, ill put that on the list, I just really wasnt thinking plugs, carbs, things like that as it was running so well for hours before it took a crap.
Thanks for your reply.
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
Re: Rough low rpm, Fall on face at WOT

OK, ill put that on the list, I just really wasnt thinking plugs, carbs, things like that as it was running so well for hours before it took a crap.
Thanks for your reply.

That's exactly how a popped piston would appear. Runs well until it doesn't, and then it gets worst.

Do a compression test, have a good look at the plugs. Before you go tearing into carbs. It may be them, but plugs and compression don't involve any disassembly, and are quick. I'm asking you to look at the plugs to get a read on what's happening in the combustion chambers, not to see if the plugs are faulty.

Chris............
 
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