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4 stroke question - 7/20/2008 5:00:02 AM   
poison


 

Posts: 58
Joined: 2/26/2006
From: Gentry, AR, USA
Status: offline 		I just got a os 91 fs surpass from a trade on rc universe I was wondering about the compression on the motor it seems like it lose to me I have not had any 4 strokes before just 2 strokes and a gas motor my 2 strokes have a good bump in them you know good compression right. Well this motor has just a little if in bump to it I'am wondering if the motor is any good or if I got ripped on the trad I called the local hobby store and they told me 4 strokes are little different then 2 strokes and would be looser intell you put fuel in it I'am wondering if anyone knows anything about that
       Post #: 1

RE: 4 stroke question - 7/20/2008 2:49:05 PM   
Lomcevak Duck



Posts: 973
Joined: 2/22/2004
From: Warner Robins, GA, USA
Status: offline 		The .91 Surpasses have plenty of compresssion under normal circumstances. I have had them lose compression temporarily though, and there are a couple of possibilities here.

1) The obvious- make sure the head is on tight and the glow plug is properly tightened.

2) You may have a broken valve spring. Take off the valve cover and look to see. These are cheap parts and very easy to replace.

3) They have a ringed piston and the ring has a tendancy to set up if they are stored for a while. It can just 'gum up.' If it has any compression at all, try mounting it and running it. If you can get it to run the ring should relax and normal compression should return. This would be the easiest fix, but you should still take the time to look under the valve cover before you run it just to make sure you don't have a vlave hanging down.

4) If none of that works, disassemble the engine to see if your piston ring is broken. This is also a cheap part, but can be more difficult to rebuild if you've never done it before. The good news is that you don't have to have any special tools, just patience.

Perhaps the best news here is that the piston and cylinder liner on these engines are quite hardy and in my experience, not prone to wearing out. Most likely you won't have to replace much, if anything.

-Steve

< Message edited by Lomcevak Duck -- 7/20/2008 4:40:45 PM >

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RE: 4 stroke question - 7/20/2008 3:43:34 PM   
airbusdrvr


 

Posts: 865
Joined: 11/3/2005
From: New Port Richey, FL, USA
Status: online 		I would run the engine before getting all concerned. I bought a used OS 70 Surpass II that had not been run for quite a while. Same thing, it had hardly any compression. But after running it, it was fine. If it doesn't run right, then you might investigate valve clearance and such. But OS four-strokes can have less compression after sitting a while.

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       Post #: 3

RE: 4 stroke question - 7/20/2008 4:42:20 PM   
ChuckW



Posts: 1894
Joined: 6/15/2006
From: Clovis, CA, USA
Status: offline 		4-strokes don't always feel as "tight" as 2-strokes. I would verify the valve adjustment and make sure neither valves is stuck. then put a few drops of oil in the carb, flip it a few times and run it. The real proof is in how it runs.

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RE: 4 stroke question - 7/20/2008 9:57:16 PM   
bassmanh



Posts: 540
Joined: 7/29/2002
From: fond du lac, WI, USA
Status: online 		poison,

is this a ringed engine ? if its a ringed engine it will have less compression than 2 strokes. my magnum .91 feels like there is hardly any compression BUT its a fine runner.



bassman

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RE: 4 stroke question - 7/20/2008 10:08:52 PM   
Gray Beard


 

Posts: 2031
Joined: 4/21/2003
From: Hemderson, NV, USA
Status: offline 		Ring rotation, it will pick up again after running.
Your two strokes have A taper at the top and that's what you feel, it's the squish of the piston against the cylinder.
Go on over to the glow engine forum then to the OS section and ask about ring rotation or do A search, it's been covered more then once.

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RE: 4 stroke question - 7/20/2008 10:24:00 PM   
Flypaper 2



Posts: 4308
Joined: 3/29/2002
From: Kingston, ON, CANADA
Status: offline 		4 strokers will feel like it has less compression with the throttle closed compared to when it's open.

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RE: 4 stroke question - 7/21/2008 1:40:27 AM   
Lomcevak Duck



Posts: 973
Joined: 2/22/2004
From: Warner Robins, GA, USA
Status: offline 		Yes bassmanh, it is a ringed engine.

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RE: 4 stroke question - 7/21/2008 8:20:48 PM   
poison


 

Posts: 58
Joined: 2/26/2006
From: Gentry, AR, USA
Status: offline 		hey guys thanks for the info I'am getting ready to see if that OS will start i noticed last night thow that I hear a ticking sound when I turn it over I think the valves need justed as well if so I'll have to order a tool kit

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       Post #: 9

RE: 4 stroke question - 7/22/2008 2:52:48 AM   
B.L.E.


 

Posts: 1309
Joined: 8/27/2005
From: Austin, TX, USA
Status: offline 		The ticking sound is normal and it's not from the engine's valve clearance. It comes from the cam gear backlash, when the cam drive transitions from driving the cam while a valve is opening to holding the cam back when the valve closes, the gear tooth play or lash makes a little click.

The new Triumph Bonneville motorcycle's overhead cam drive gears uses spring loaded scissor gears for the sole purpose of eliminating that gear lash clicking.

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       Post #: 10

RE: 4 stroke question - 7/22/2008 4:47:31 AM   
poison


 

Posts: 58
Joined: 2/26/2006
From: Gentry, AR, USA
Status: offline 		Started it today it ran fine all of my worries were for nothing I guess once it ran it tightend up a bit it ran just fine thanks for all the info and help all appreciated

(in reply to B.L.E.)
       Post #: 11

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