over sized pistons

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Charger25

Charger25

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OK Fellas, got another one for ya. I got my mitts on another 25 rude cheep! Compression is 75 on top and 78 on bottom cylinders, looks to be an early nineties model. It idles and runs but has an obvious lack of power. This motor was on a work boat used in the Rappahannock River. (brackish water ) Haven't pulled the head yet but shining a bore light in spark plug holes ,the cylinder walls appear to be smooth with no score marks. Kicking around the idea of doing a total rebuild on this one and going with a 35 hp carb and intake. My question is , IF I bore this out 20 ths. SHOULD I go with over sized pistons or COULD I get away with just the over sized rings?
any and all opinions are welcome
 
What year 25 do you have? Anything over .002 should be bored to .030 over and oversized pistons installed..... that is assuming we are talking about the 77' and later 3" bore 31.8 ci blocks

My guess is when you get in there you may find the taper out of spec or the roundness of the walls may be out also. I would bore it out myself if I were thinking on doing a complete rebuild.
 
No, you cannot get away with just using oversized rings.............period!
If .020 over pistons and rings are available use them. If not you may have to go .030 over.
I would check that compression with another guage as well.
First sign of lack of compression in a two stroke is that the idle quality goes away. You stated the engine idles.
Lots of things can cause low power in a well worn engine. Would start with checking throttle blades and wot timing, etc.
 
Pappy said:
No, you cannot get away with just using oversized rings.............period!
If .020 over pistons and rings are available use them. If not you may have to go .030 over.
I would check that compression with another guage as well.
First sign of lack of compression in a two stroke is that the idle quality goes away. You stated the engine idles.
Lots of things can cause low power in a well worn engine. Would start with checking throttle blades and wot timing, etc.
Click to expand...

OK , If I bore it out I'll go with the over sized pistons.
Yep it idles pretty good, spits a little though.
A bud of mine has a compression guage also,might borrow it to compare.
She's got alot of worn parts, throttle cable and linkage is sloppy. gotta tighten that up.
Not sure how to check timing on WOT though.



Johny25 said:
What year 25 do you have? Anything over .002 should be bored to .030 over and oversized pistons installed..... that is assuming we are talking about the 77' and later 3" bore 31.8 ci blocks

My guess is when you get in there you may find the taper out of spec or the roundness of the walls may be out also. I would bore it out myself if I were thinking on doing a complete rebuild.
Click to expand...


From the paint and electronics it appears to be an early 90's model.
The ID plate is long gone.
What I might do first is decarb it ,then check the compression again.
While typing this ,I was thinking.............how much more hp would I get if its bored out and the head shaved? Just thinking out loud so to speak.
 
for an early 90's 75 and 78 lbs sounds pretty low for your compression, i have an 80 25hp johnny and both are between 100 and 110 lbs...
 
Its been confirmed thats its a 92. Rechecked the compression with a buds gauge got 82 & 85 lbs. Guess mines just old. This project is going on hold for now, just got word that we are being shut down. So I'll be looking for work.
 
Back when I was young dumb and without money, I restored compression to my Yamaha two stroke motorcycles by just re-ringing them. When the pistons were collaped - that is, when the taper was gone - I knew a guy who would shot peen the inside of the skirts. A quick shot would blow the skirts out a few thou, enough to restore the taper. That part was important because the pistons must hold the rings square to the bore. Rocking pistons interfere with ring seal.
 
Charger25 said:
Its been confirmed thats its a 92. Rechecked the compression with a buds gauge got 82 & 85 lbs. Guess mines just old. This project is going on hold for now, just got word that we are being shut down. So I'll be looking for work.
Click to expand...

glad you mentioned yours is old(i assume you mean it has had a lot of hours of run time)....my 1980 up until 2000 ran 95% of the time on no wake lakes and half of that time the minn kota did the work, was not cracking on your motor at all its just that mine only has a few years worth of actual wear and tear since it was used so little and maintained to a T b4 my grampa gave it to me.....
 
Your new compression numbers should run that engine okay. Glad the second guage did the trick. Incidentally, if your gauge has a long hose that can make a difference in compression numbers.
Run OMC Engine Tuner through the engine and let it sit overnight. Let the engine come up to thermostat temp and raise the RPM a little. Inject the tuner down the carb throat until the engine is close to stalling. Use 1/3 to 1/2 can on your engine. You will be surprised what comes out of the engine the next day. After the tuner treatment check the compression again and you will probably find a little higher number. May take more than one treatment. This is a dedicated product and works incredibly well. Somebody is bound to jump on the SeaFoam bandwagon now but thats a general do all product vs Engine tuner.
 
Pappy said:
Your new compression numbers should run that engine okay. Glad the second guage did the trick. Incidentally, if your gauge has a long hose that can make a difference in compression numbers.
Run OMC Engine Tuner through the engine and let it sit overnight. Let the engine come up to thermostat temp and raise the RPM a little. Inject the tuner down the carb throat until the engine is close to stalling. Use 1/3 to 1/2 can on your engine. You will be surprised what comes out of the engine the next day. After the tuner treatment check the compression again and you will probably find a little higher number. May take more than one treatment. This is a dedicated product and works incredibly well. Somebody is bound to jump on the SeaFoam bandwagon now but thats a general do all product vs Engine tuner.
Click to expand...

ive always ran sea foam through mine each spring and always wondered if there was a different product to try and see if one was better than the other, im gonna try the OMC next spring. thanks for the post pappy
 

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