2 stroke repair

Discussion in 'Mechanic and Repair' started by rjxj, Mar 10, 2008.

  1. rjxj

    rjxj LawnSite Member
    Posts: 80

    A lot of times people seem to run into a dead end when working on their 2 strokes. They do the carb rebuild thing, clean the exhaust port and still no luck. The engine has to be in good shape basically to start with. It should have enough compression to blow your thumb off the hole. That’s just a rough gauge. The engine I’m showing here would do 120 psi with 2 pulls, no carb or use wide open throttle. If it wont do that look in the ex. and intake ports for scoring or scuffed piston. I put together a few photos (sorry about blurry)and tips to maybe lessen the pain and frustration. 2 strokes kick butt. They are simple, light weight, and even fun when riding a toy that is powered by one.
    This first link shows the basics. (hope I'm not ticking anyone off by linking)


    Its showing a reed valve engine. Reeds are just a check valve. A one way valve. They keep the mixture from being pushed back into the carb. Many engines are also of the piston port design. If its not a reed valve or rotary valve design its a piston port. The piston comes down to cover the carb port during the fuel mixture transfer so its not pushed back into the carb.
    People understand that the air fuel mixture is created by the air rushing past the venturi and going into the engine crankcase as the piston moves up. When the piston gets to the top of its stroke it has compressed the mixture, fires and pushes the piston down. . At this point they often don't picture what is happening next. The air/fuel mixture that has filled the crankcase now has to be pushed through the transfer ports and into the cylinder and push the exhaust gases out. As the piston moves down it closes the carb port, opens the exhaust port and compresses the mixture in the crankcase, moving it through the transfer ports and into the cylinder pushing the exhaust gas out. Now the cylinder is full of mixture and the piston starts to move up again compressing the mixture. As the mixture in the cylinder is compressed the crankcase is drawing in more air/fuel mixture.
    NOW THIS is the important part...............IF there are any leaks in the crankcase, this process is affected. All the carb kits, spark plugs, clean exhaust ports, etc in world will not make it run properly if the crankcase is leaking. Sorry I sound like a smart ass, but this is very important. Places that leak are the crankshaft seals, cylinder base gasket, crankcase halves, plastic intake manifolds. Lets say the piston is moving up and the crankcase is filling with mixture. If there is a leak it will be pulling in air that has no fuel/oil in it. (lean) and when the piston is coming down it will want to push some of the mixture out of the leak instead of transferring it into the cylinder (low fuel/air fill=low power). The carb is also affected because the pulse signal to the diaphragm is reduced. It cant flutter the diaphragm from the piston movement if its leaking. It’s the path of least resistance theory. If it can shoot out the leak easier than it can flutter the diaphragm, it will.
    Some of the symptoms would be hard to start, poor idle, no power, erratic or ineffective carb adjustments, and would probably run its best at full throttle. It will just feel doggy and unresponsive. If you went and picked up a new machine of the same model it would seem crisp, sharp and responsive.
    There is an easy way to find these problems. Its called a crankcase pressure test. WELL, I know,. Easy for guys with the stuff to do it, right. No it is easy. OK here we go. First you need to properly torque the spark plug, then make a rubber gasket to block off the carb port, Put it between the carb and the plastic manifold or cylinder, then an adapter plate to block off the exhaust. If your carb uses an external pulse line you can connect the pressure pump to it. If you have an internal pulse line to the carb you need to add a fitting to the exhaust port as I did in the photos. You don't want high pressure. A few pounds will work

    A coring tine from an aerator makes a good gasket punch. The exhaust side needs a hole in the center for the pump fitting feed. The carb gasket is solid. If you have an external pulse line to the carb both of your gaskets will be solid.


    Carb side blocked off


    Exhaust side plate with fitting. This engine had an internal pulse hole to the carb so its plumbed this way. If you have the small pulse hose from the crankcase to the carb you can use it to pressurize.


    I made the plate out of 3/16. I still need to make some spacers instead of all the nuts that I used on the long studs. Buy a barbed fitting and rubber gasket material at the hardware store. The pressure tester is available on line. You can get by cheaper by making one from a boat fuel line squeeze bulb. You will just need a small shut off valve in the line so you can shut it to hold the pressure in as you pump it up.

    The first place I found leaking was the starter side crank seal. I used soapy water. In the second photo I submerged the whole cavity to show how bad it leaked.


    Pull flywheel. DONT beat on a two stroke. They don't take a beating as well as a four stroke.

    The PTO/clutch side has very small bubbles but its not leaking. It will be replaced also. Its old.


    Some guys dig the seal out. If you do, be careful not to gouge the shaft or housing. I used a small screw. Don't go in far or you may damage the bearing.


    Tip the engine so the shaft is down and clean the shaft and bore. Lube the shafts and replace the seals. Be careful to not cave the side in while installing.


    What’s surprising about this machine is how good it started. 2 pulls in the cold. It ran ok but it didn't idle as good as it should. With new seals it will crackle like new. Its over 25 years old. I just repaired a Stihl bg72 hand held blower. It would hesitate, didn't't idle good. Pressure tested it and found the cylinder base gasket was wasted. Repaired and what a difference! It snaps when accelerated and idles nice.
    So a four stroke has four strokes........suck, squeeze, bang, blow. The two stroke actually has four things going on also..... 1. compressing the cylinder mixture while 2. drawing mixture into crankcase 3. blows (ign) the piston down while 4.transferring/savaging the mixture. Then it starts all over. Its doing everything a four stroke does, just does it using two strokes or cycles. When anything to do with the crankcase of the four cycle is leaking its very obvious because oil is all over the place. When the crankcase on the two cycle is leaking it will cause performance problems and eventually destroy the engine. With a tight crankcase it will snap, crackle and kick butt! Tight is good :):drinkup:
  2. khouse

    khouse LawnSite Bronze Member
    Posts: 1,465

    Crank seals are probably the most overlooked problem. Nice photos and good job. Some people replace the carb, new plug, fuel etc. then junk the engine not knowing that crank seals are important on 2 strokes.
  3. ed2hess

    ed2hess LawnSite Fanatic
    Posts: 13,206

    That is certainly true on Echo....just finished tearing down two blowers and both had a least one bad seal replaced both. And one had a bearing that had rust in it so it got replaced.
  4. Two Seasons

    Two Seasons LawnSite Senior Member
    Posts: 791

    Excellent thread! Thanks.
  5. Breezmeister

    Breezmeister LawnSite Bronze Member
    Male, from South Jersey
    Posts: 1,547

    Welcome to Lawnsite rjxj, great first post

    Wow, reeds, that last thing I worked on that had reeds was an old Jake commercial push mower and I won't tell you how long ago that was :rolleyes:

    I went out and got me a set of seal pullers from SnapOn a few years back
    (SGSR104) If any one is interested that make short work on those seals that are about as wide as a penny or nickle. They where worth every dollar I spent on them :laugh:

    ROTFLMAO I don't know why my ex came to mind when I read that :p

    Yes it is :drinkup: Welome and injoy :cool:
  6. panther man

    panther man LawnSite Member
    Posts: 4

    hi some fantastic advice and really helpful.i just wanted to ask can you leave the piston in the bore aslong as it is at top dead when you are doing the test?or does it need to be removed?thanks
  7. Phil G

    Phil G LawnSite Senior Member
    Posts: 843

    Excellent post. Thanks for sharing that with us, very informative'
    atb Phil :)

  8. kayeproperties

    kayeproperties LawnSite Member
    Posts: 141

    Excellent and informative to say the least!!! Thats what this site is all about sharing our knowledge and experience. THANKS!
  9. mowerknower

    mowerknower LawnSite Senior Member
    Posts: 766

    Good post, dont forget to check intake gaskets. Also presure test fuel line, at the same time you will be able to tell if you carb holds .8 bar. On chains saws dont forget to check intake boot and impulse line. If everything test out ok its time to replace or rebuild your carb. Also, put a small amount of grease on impulse port an pull the engine over it should blow the grease out of the port, other wise you have a plugged impulse

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