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91 sp compression?

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stephensimmons

stephensimmons

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What should my compression be? I checked it and im only showing 70psi on both cylinders. Is that about right? Seems kinda low to me. It cranks and runs and sounds good out the water but have not put it in the water yet.
 
It should be in the 140-150 range, I think you have a faulty gauge, it shouldn't run at all, even out of the water at 70psi. Take the compression on a cold engine, with both spark plugs out, spark plug boots grounded and full throttle.

Lou
 
Yep... I doubt it will start in the water if it's truly at 70 PSI.

This is why we always say to give it a ride before buying. A crap engine can start without a load on it.

But before we jump to conclusions... are we sure the gauge is OK?
 
Yea thats what i thought. I had both plugs out and the engine was cold. Didnt hold it at full throttle though. Will try it again. The gauge i have is new........well it was only used once about a month ago when i bought it new.
 
Opening the throttle will only give you around 10 psi. So... you would still be in the "Dead" range.
 
Dr Honda said:
Yep... I doubt it will start in the water if it's truly at 70 PSI.

This is why we always say to give it a ride before buying. A crap engine can start without a load on it.

But before we jump to conclusions... are we sure the gauge is OK?
Click to expand...

I only paid 150 for it as a project and thought it didnt run but it does. Just checking it over before attempting to put it in the water.
 
Dr Honda said:
Opening the throttle will only give you around 10 psi. So... you would still be in the "Dead" range.
Click to expand...

Well ill retry it when i get home again. I used an automotive compression tester. Would that work on it or is there a different kind for the two strokes? It will start right up if i start it right after shutting it down but if i wait like five or more minutes i have to choke it to get it to start. Is that normal?
 
stephensimmons said:
Bad crank bearings could cause low comression?
Click to expand...

Nope... but a new set of rings would cause the crank to explode if it's rotten too.

stephensimmons said:
And if this is the problem how much would it cost to rebuild a crank? Or is the gearings something i could replace myself?
Click to expand...

If you think the bottom half is done... and you are considering a rebuild... just get an engine from SBT. You can't do it yourself any cheaper, and they will give you a one year warranty.
 
Dr Honda said:
Nope... but a new set of rings would cause the crank to explode if it's rotten too.

Very true. Who sells a crank or parts for one?


If you think the bottom half is done... and you are considering a rebuild... just get an engine from SBT. You can't do it yourself any cheaper, and they will give you a one year warranty.
Click to expand...

SBT? Its not so much of trying to be cheaper, i just like to build my toys. But if its something that l cant do because of a special tool being needed then i send that part out. I dont know about you but i feel a whole lot better and have more fun when i build them myself. So just out of curiosity, who could rebuild my crank if needed?
 
Last edited by a moderator:
I fully understand about doing it yourself. But, since you don't save any money... and you really won't have a warranty when you are done... you may as well just get a rebuilt.

But... if you want to do the labor yourself... you can get a reman crank from SBT for about $250.
 
Dr Honda said:
I fully understand about doing it yourself. But, since you don't save any money... and you really won't have a warranty when you are done... you may as well just get a rebuilt.

But... if you want to do the labor yourself... you can get a reman crank from SBT for about $250.
Click to expand...
Who is SBT?
 
LouDoo said:
www.shopsbt.com

Lou
Click to expand...

I want to thank the two of y'all for all the info you have given. Going home today and recheck the compression and will go from there. After i think about it i checked the compression when i first brought it home before i got it started. Now that its started ill recheck it to see what it is now. Dont know how long it was sitting so its possible the rings could have been stuck?
 
Sure... the rings could have been stuck, and a short run will free them. Also... it's not good to add oil to the cyl's, because it will give you a false high reading... but you want the internals coated with a "Normal" running amount of oil.

But... with that said... I've never seen a dry cyl jump from 70 to 150 just because it ran for a minute or two.
 
Dr Honda said:
Sure... the rings could have been stuck, and a short run will free them. Also... it's not good to add oil to the cyl's, because it will give you a false high reading... but you want the internals coated with a "Normal" running amount of oil.

But... with that said... I've never seen a dry cyl jump from 70 to 150 just because it ran for a minute or two.
Click to expand...

Me either. Ima get another testet and try it again. I never add oil to the cylinder when testing. I know the engine was dry so we'll see what happens.
 
stephensimmons said:
Me either. Ima get another testet and try it again. I never add oil to the cylinder when testing. I know the engine was dry so we'll see what happens.
Click to expand...

Well I retested it today and I got 90psi on bith cylinders. Then I tested my tiller and got only 40psi. So I figured let me test something with a fresh topend so I pulled out my goped and pocket bike, both have new topends with less than 24hrs on them and one tested 60psi and the other 70psi. And my nissan sentra only tested 105psi. So I guess my brand new compression tester that was only used once is JUNK!
 
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