Help on top-end of a 2-stroke
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    Help on top-end of a 2-stroke

    by Monkeywrench » Thu Sep 08, 2005 8:59 pm

    Hey guys,
    I need to ask the advice of seasoned 2-stroke owners who may have switched to 4's nowadays.

    I friend of mine bought a 1987 yz-125, and has been riding it for a few months now. Not a bad little ripper for its age. Even has an FMF pipe on it (looks nice, not sure what gains it gives the bike).

    He had been noticing less and less resistance when kicking the engine over, so I brang it home to my garage to do a compression test on it. I got a reading of just under 55psi, and that took about a dozen kicks.
    Also, I find it's really hard to start -- kick and kick, take the plug out and find almost no gas on it. I finally did get it started, but somewhere around 60 kicks (I not kidding).

    :?: What should the compression psi's read on a healthy top-end?

    :?: What should be replaced? I'm guessing the piston and rings, and probably any gaskets I remove to get at them?

    :?: Lastly, should the cylinder be honed before finishing the rebuild? Also, any special instructions on the exhaust valve/port? I took that puppy out of the side of the jug, and wasn't sure if the brass sleeve on the cover plate should have been "stuck" in the spot it was stuck in, or should be able to move freely. I not really up to speed on the 2-stroke engine, but have always been fascinated with their design. I figure this might be a cool thing to learn.

    Appreciate any help! Thank you in advance.

    2006 450R
  • b.lee
    Posts: 6
    Joined: Wed Jul 20, 2005 12:14 pm

    by b.lee » Sat Sep 10, 2005 6:48 am

    Well, if you plan on spending any cash or doing any work at all on this bike, or any bike for that matter, go buy a service manual. All of these questions will be answered in there.

    As for the compression, that is very low. 100-130 psi would be good, but I don't know the exact factory specs off the top of my head. It's also hard to say what would need to be replaced without seeing it, but at the minimum, I'd go for a piston and rings, wrist pin, and wrist pin bearing. Of course you'll also need a top end gasket set.

    I can't remember if the cylinders back then were plated of not, but that really doesn't matter, since your cylinder could have had a sleeve in it at some point anyway. Take the cylinder to a reputable shop and have them give you your opinion on it.

    And lastly, don't spend a bunch of money on it. The unfortunate truth is that you can spend $500-600 on a $1000 bike and it will still only be worth $1000.
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    by mustang » Sat Sep 10, 2005 8:28 am

    2 strokes need good rings and a good round cylinder wall to be able to seal , as stated in the last post have the cylinder checked for size , it should be within spec , ie. if std size is 50mm your manual will give the tolerance that this bore should fall within , also you should check for tapered cylinder walls as well as out of roundness , you shouldn't see any more than a couple of tenths (.0002)

    I have seen 2 strokes that will run like yours and the repair is just a new set of rings sometimes . although if it were my bike I would at least do rings piston wrist pin and needle bearings for wrist pin . any good piston company usually gives you everything when you buy a piston .
    cylinder work is going to cost you ,if its an old design , ie no plated bore than you can have the cyl. bored and honed to the next size up , usually they go in increments of .010 , .020 , .030 , .040 would not suggest boring over .040 .
    I would see what is available for piston and rings 1st and then decide , also previous owner could have already had the jug bored .
    the cylinder is what needs to be carefully checked out b4 you start , if its still in spec than just a simple piston and ring replacement will make the yammie run like new again . if the cylinder is junk you could very likely end up with a expensive repair bill and will need to decide if it is worth that much to repair or use the money to buy another used dirt bike .
    I'm guessing that if the engine is shot the rest of the bike has been ridden hard also ! :-({|=
    It aint rocket science but two strokes do need a good seal in the cylinder to be able to deliver fuel (suction)and also needs that same cylinder seal to produce HP under the fire stroke !

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    by oldtimer » Sat Oct 08, 2005 6:28 pm

    Monkeywrench . After new piston and rings the compression was 195 on my 88 CR250. I put about 30 hrs on that and it was down to about 150. So I just replaced the rings last time and that brought it back up to 185.
    If it was down to 55 and there are no other signs of problems it has been in there for a long long time. After that kind of use I would be most worried about the skirt breaking off and causing a pile of damage to your cases and crank.

    Speaking of cranks when you have the top end off check the crank for any play. If you hold the rod and move it side to side and up and down there should be little or no play. The manual will give you exact specs but any more that .001 and it and or the crank bearings should be done.

    Also when tearing it down if you want to save a few bucks and the top gasket is holding fine just leave it alone. These are a metal gasket that rarely go if everything else if good. Just remove the jug from the base and replace that one only. This will still allow you to inspect the inside of the jug and cap.

    If the jug is plated and in good shape leave it alone. Don’t hone it. Just use a scotch brite pad to break the glaze that will be built up.
    06 CRF230, BBR F&R Springs, BBR Triple Clamp and Link,BBR skidplate and cam cover, RSW fork brace, Pro Taper Fat bars, Ready Filters, Power up Kit, Airbox Mods and Trail Tech.
    06 CRF80
    04 CRF70
    88 CR250
  • cr125racer
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    Joined: Thu Feb 02, 2006 5:46 pm

    by cr125racer » Thu Feb 02, 2006 6:05 pm

    hey! monkeywrench,
    i have a cr125 that had the same problem, it had no compression.
    all i did is take out the piston and use a really soft sand paper to clean the imperfections.
    then i did the same with the chamber.
    then i changed the ring for a bigger one(make sure the new ring seals the chamber completely.)
    then i build it again and changed the seals.

    it now runs like the devil.

    (just replace the seals and the ring.)
    good luck
    ((((BoLiChE)))) ;)

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