Logo
FULL SITE MENU

Mercury 650 running rough

P

phishinphree

New member
Hey all
So I picked up a 65hp mercruy 650 pretty cheap the other day and I'm working on getting it running good. I don't have much experience with outboards or two-strokes. I mostly work on small tractor, lawn mower engines or my personal vehicle.

The motor starts right up, seems to idle and rev fine and has a strong stream out the tell tail. I changed the gear oil in it and the old oil looked new and it shifts fine so I don't suspect any problems there.

When I changed the plugs out, I noticed the plug on the bottom cylinder was cleaner than the rest which made me think it wasn't firing. I checked it with a spark tester and it is fine. However, when I pull the plug wire while its running, I don't notice a change. When I try the same on the top and middle cylinder, it does start to stumble but doesn't die. The weird thing is if I only connect the the bottom plug, it'll still start (and run bad ofcourse). I'm not sure what to make of it, maybe theres no issue at all.

Anyway, I came to the conclusion that perhaps I don't have a good sense as to what these things should sound like at an idle out of water. It seems rough to me on the video below

I also did a compression test and all 3 cylinders gave 90psi. I take that with a grain of salt as the tester is old and came from harbor freight.

Any pointers in general would be greatly appreciated. I've searched around a ton for more info on these motors and I'm not finding much helpful. Are the Seloc manuals worth it? I'm often disappointed with the chiltons I've bought in the past when working on cars.

I really appreciate any advice the community has to offer.
-Jason


 
Nice looking motor. So a lean lower cylinder could be caused by partial carb main jet blockage, or even a bad lower crankshaft seal. If it will run on just the one (lower) cylinder, then the low speed channel in the carb must be okay. Have you run it down the lake? How is the power? Compression is a bit low. I consider that accurate especially if it seems to lack power once you get it out and run it. Good to see 120 psi on that Merc.....seems like a nicely maintained machine.
 
Last edited:
The harbor freight compression gauges don't have a schrader valve in the end of the hose so those read LOW. Grab a loaner from autozone and make sure it has the valve and retest...bet you see 120. My harbor freight gauge read 30psi lower! thats the difference between a worn out motor and a great motor!
 
After watching your video, and being VERY familiar with these triples, I'd say it idles fairly normally.

You can make it idle the best it can by using an infrared temp gage on the metal plug body. Tweak the idle adjustments until you get all three plugs about the same temperature, then add a 1/4 turn OUT on both idle adjustments.

Good luck!

Jeff
 
Thanks everyone!

I plan to take it on the water later this week and see how it does. I also managed to score another 650 for cheap locally. Its in the next serial number bracket so not 100% compatible but I spot checked a few parts and it seems most everything except the block is identical. SN 4046505 vs 3453537 which is on the boat and in the video above. Apparently it was running earlier in the summer and taken off as the new owner only wanted the fiberglass hull which needed repair and was going to put on a new outboard once he was done. unfortunately it wasn't set up to run when I bought it but for $100, I'll get my money back in parts.

Thanks for the tips on adjusting idle Jeff. I suspect I'll be taking the carb apart soon to rebuild. Looking forward to wrenching on these things as they're completely different than the typical motors I work on.

I'll report back once its been in the water.
 
Back
Top