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48 spl rough and erratic idle

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greasemonkeyjmj

Contributing Member
I have a 87 48SPL model E48ESLCUS iI bought llast Week on a pontoon boat.It has been sitting since the first of the year, anything before that I'm unsure of. Engine has 125 pounds compression on bottom cylinder and 120 on top, spark on both will jump across 7/16 gap with new QL78YC plugs.I'm running 50:1 mix, has the non adjustable carbs, it starts fine, but tries to die if you let it sit at idle,and when you put it in gear, you have to go ahead and throttle up a little to keep it running but after you come up from idle it throttles to WOT smoothly with no stuttering or hesitation, but at idle the rpm will run up and down, it doesnt have a tach, but i would say idle is around 650 or so, but it climbs as high as 3,000 rpm's, high enough that i quickly shut it down to keep from slinging it apart. I can pull the plug wifes one at a time while running, and the top cylinder actually runs smoother and at a higher RPM than the bottom cylinder, I know that a linc and sync may would help the dying when coming off of idle but I don't see where that would have an effect on the erratic idle speeds I tore into the carbs and they looked clean, but I soaked them overnight with no effect on the engine. I can hold my hand over the carb throat while running, and the top carb has more suction against my hand than the bottom carb.

thanks
 
think the climbing rpm's maybe a suction leak causing carb to lean out just before engine dies from no fuel
did you have carb rebuild kits, hope so old gaskets really don't help, dis-assemble, remove jets carefully, if can't remove use welders tip cleaning tool to clean out jets and passages, remove core plugs and get those little holes too
yes, the link and sync adjustments are important, you'll know when you get them right, and the throttle shaft tool helps lots.
you say your spark is good, mine way bad and did't know it until charge coil failed, and when replaced, it was like a new engine idling, turned idle screw in 1/2 turn, even better
and the one weird thing about my setup, have two 6 gal tanks, on one tank engine idles forever, other one dies in about 2-3 min.
have removed fuel connection at cowling, trying to eliminate possible air leaks, think of changing tank connections to mercury ones
enough rambling, hope one or two gold level guys chime in to help too
 
oh yah, tach important, so as not to over rev at wot, or get prop-ed right to reach 5000 to 5500 rpm, regular tach connections fairly easy, and as i've said before check out Tiny Tach's online, really like mine
 
Thanks for the reply, but after pulling the powerhead to get a good look at the botton crank seal to make sure it wasnt pulling air, it looked good but after looking into the exhaust port, i think i found my problem, the piston and cylinder is scored, i cant see the top piston, but i can see the scoring in the cylinder wall, and scoring on the botom cylinder
 
yeah the sad thing is I just bought the rig last monday, so congratulations to me. I am going to price having it bored out, i have already priced pistons, the other things I need. I figure if i build it, at least I know what shape the internals are in, I can buy a used one but may be in the same shapeas now,, the engine has been being ran on 100:1 i myself always run them on 50:1 and thats what i have ran this one one for the last week. I will keep you updated on the procedure.
 
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