Logo
FULL SITE MENU

1987 48 SPL on 24 foot pontoon not turning up enough rpm

G

greasemonkeyjmj

Contributing Member
i have a 1987 48 spl with 3 hours on it after a engine rebuild, compression is 145 on both cylinders, rebuilt carbs, fresh gas, carbs are opening fully and timing is advancing fully. the engine actually has a 74 model big gear case lower unit on it, an is ran on a 24 foot pontoon. when i bought it, they were running a aluminum 3 blade 13 3/4 x 15 prop. and no tach. This could of had something to do with the burnt pistons that i replace. i added a tach and it shows 650 at idle in the water in gear , but only jumps up to 2800 at WOT in gear. i decided to change pitch so i bought a 13 7/8 x 9 hydrus pontoon and workboat prop and installed it, the boat still only turns up to 3500 rpm. i also installed a brand new tach and rectifier just in case either was faulty. I am running out of ideas on how to get it where it needs to be, does anyone else have any ?
 
thanks for the reply, plugs are a medium brown color, not sure on the gears, was told they should be a 2.42, couldnt get the jets to break loose out of bowls, so i had to soak carbs with them in, and made sure they were clear, but when i built the engine I had a miss at idle, so i bought used carbs that were supposedly from a 86 model 50 HP, any idea on what size jets they should be ?maybe i should swap bowls and jets from my original carbs ? Engine is in Arkansas, so not a high altitude engine. I definitely dont like lugging the rebuilt engine.
 
the idle air jets are both# 36, i went ahead and swapped the high speed jets to the #50. what effect would the wide open timing have on the RPM ?
 
Additional info, i was looking at pictures of before i pulled the engine for the rebuild, when i reinstalled the engine, i mounted it as low as it would go, it was originally in the nex to the bottom hole, which means it is sitting 2 holes lower

 
Is the tach you installed set correctly if you look at the back there is a button labeled poles that needs to be in the right spot
 
i used an online instruction of how to set the wide open timing with the engine spinning over and advanced to wide open but not running, it calls to set it 4 degrees below the actual setting,so i was going for 15 degrees it says when running it will make up the 4 degrees and be where it needs to be at 19 degrees. when i initially checked it by this method it was about 18 degrees which would put it at 22 when running at wide open. the engine had melted pistons when i rebuilt it so i thought maybe this was part of the reason why. the idle jets in the carbs i bought were # 36 jets, also i removed a trolling motor a electric anchor and a pair of waterlogged couches and armrests with marine plywood bases and went back with plastic base furniture, so it is sitting higher up in the front than it was when i bought it last fall. i am 365 pounds so tomorrow i will set in the front while my wife drives and see what effect it has on it
 
The old 1974 unit was a 2.66 or .375 ratio and the original one was a 2.42 or .41 ratio,This is a little smaller gearing and the bigger case has increased drag. The 8 pitch prop will put you in the 12-15 mph range at 5500 rpm. You may be better off installing a factory original lower....
 
Last edited:
yes on the tach, i set it where the manufacturer instructed me to set it at, and all research i have done confirms it to be the correct setting
 
Run the motor on a dynomometer or with a test prop.---------Need to determine if the motor is developing its rated POWER.-------------If there is something wrong then no adjusting will help you get the performance you want.
 
in my humble opinion a 48 hp will not effectively get a 24 foot pontoon up on a plane and run it efficiently without undue stress on the motor regardless of any modifications you may make..its a good rig just to Cadillac around in and enjoy the scenery..a 24 foot john boat would be under powered with a 48 hp..
 
well guys thanks for the help on this, But I think i am at the point to where it is best to throw in the towel and cut my losses. raising the engine and swapping the jets made no change at all. On top of that, now the engine is running rough at sounds like a totally different engine than it did yesterday. i checked the compression its at 148 on top and 135 on bottom. looks to me like a internal problem brewing.
 
I just wanted to give a update on this post , I bought a used 70 Horsepower Evinrude and put it on in place of the 48 SPL and solved my problems, but I was lying in bed and thinking about the 48 last night and a thought popped in my head. So today I pulled the upper carb off of my engine and rounded up the original carb from my engine. I measure the throat at the rear of the carbs, my original is 1.5 inches across, or 38MM, the carbs i was running measures 1.2 inches or 31mm across,.HMM sounds like I could be starving for air trying to get it to turn up like it should
 
Back
Top