Home Forums 2-Cycle Racing KT100 Ideal compression to be competitive.

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    • #34977
      FREDDY SANDOVAL
      Participant

      Can someone please tell me what a good compression reading should be for a super Sr pipe or can KT100 motor?

      I keep hearing 125, 130 and 150 psi.
      What would be an accurate compression number to be a decent/competitive motor?

      Thank you all in advanced. Fast Freddy.

    • #34980
      Jim Silverheels
      Participant

      At least 160 psi. Ours were 160-165 psi using two different gauges.

    • #34981
      FREDDY SANDOVAL
      Participant

      Jim
      Thank you! That’s what I’m talking about
      We’ve been getting our fresh engines back with about 95 psi to 120 psi, at the most.

      It’s no wonder we can’t finis on the podium in every race, we’re getting told it’s our chassis tuning, so we’ve tried every different set up we can gain any fraction and still lack of power.

      Till we decided to check our compression.
      Thank you for your help. Fast Freddy.

    • #34985
      Jim Silverheels
      Participant

      Freddy, I should have said 160 plus cut to WKA limits.. .025 squish and a tad over 11 cc’s cylinder volume at TDC. Try your gauge reading against another gauge to verify it’s not a bad gauge you have.     chi wa wa  my friend

    • #34987
      FREDDY SANDOVAL
      Participant

      Jim
      It’s all good, I understand
      We’ve tried 2 different ones, including a new one from MAC tools
      And it’s about the same readings

      I’m only getting 90’s on mine (just broken in, at a very slow pace and gentle )
      And 120 psi on my Son’s, with only 2 races on it, and carefully broken in also.

      Something’s telling me I’m dumping money where it doesn’t belong?
      I would bet money that the guys running upfront have strong motors, producing at least 130 to 150 PSI?

    • #34994
      Ted Hamilton
      Participant

      QUALITY guage neccesary, and a fresh battery on starter when turning it over to test… or so I’m told. The figures I was hearing on Bob’s were around 200, with ring job and/or rebore when it was below 140…

      http://karting.4cycle.com/showthread.php?11090-kt100-port-timing — post #6

      and depending on your application, this is interesting reading too:

      http://eknclassic.com/viewtopic.php?t=98674&postdays=0&postorder=asc&highlight=kt100+open&start=0

      www.facebook.com/hamiltonhelmets/
      2014 Praga Dragon / IAME KA-100

    • #35032
      Jim Silverheels
      Participant

      Ted I think the 200 figure is a compression volume of 9 cc’s.

    • #35040
      Ted Hamilton
      Participant

      Perhaps, but I think that thread was talking about to the bottom of the plug hole, not top (~2cc difference).

      www.facebook.com/hamiltonhelmets/
      2014 Praga Dragon / IAME KA-100

    • #35079
      FREDDY SANDOVAL
      Participant

      Thanks guys
      I should have specified earlier
      This is for a legal motor, and national level, not a hot rod motor

      What is the normal compression PSI I should be expecting from a reputable engine builder?

    • #35082
      Greg Lindahl
      Participant

      Be certain you’re using a small engine compression gauge.

    • #35092
      FREDDY SANDOVAL
      Participant

      Cool
      Where can I buy one of those from?

    • #35222
      brian downing
      Participant

      Google is your friend, check Amazon too

    • #35283
      Paul Lyda
      Participant

      I know that you should test with fresh battery in starter to get consistent/comparable numbers.

      Also, Always been told to do it with Wide Open Throttle.

      Question:  Should you have fuel line hooked up or should carb be dry?  Just dawned on me that may be some of differences that I see in my numbers.

       

    • #35289
      FREDDY SANDOVAL
      Participant

      Fuel line off, pulse line off and blocked off/sealed
      I don’t know about wide open throttle, but I gess I could try that for learning
      We’ve tried several motors from our other karts, that are set up the same way, with fairly fresh motors, and surely there is a difference, no matter how we put it.

      The point is that I don’t think you can race 2 stroke motors with low compression, especially when they only read 95 psi. That’s why I asked how many psi you guys are running on your motors? And this is for a spec pipe/can class at a local level. ( not open class )

    • #35447
      Walt Gifford
      Participant

      Some builders don’t hone the bore right. I’ve seen guys that just scratch it with a big spring loaded brake hone with no torque plates, don’t even measure and call it done. Loose as a goose because, as long as it doesn’t stick the piston they did a perfect job.

      That being said, let us know what reading you get when you check it with open throttle.

      Gif

      FAA certified jet engine and aircraft technician,
      Nicholson Speedway class champion 2001,
      Yamaha KT100 Service Center,
      41 years karting experience

    • #35461
      FREDDY SANDOVAL
      Participant

      Thanks, Walt

      Well, I finally got engine back, and was said that there was nothing wrong with it, no leaks at all, and it all looked good.

      I followed your advice to test it out of curiosity at wide open throttle, and with the pulse fitting plugged.

      I got 12o PSI, I also tested again with the carburetor closed, and same readings

      I don’t get it

      I dropped off an engine with confirmed and double checked compression reading at 93 to 95 psi

      And I get it back with 120 psi?

       

      Anyhow, Walt. There is your answer to your wandering for the new updated readings at WOT

      What do you thing? Any thoughts and pointers are always appreciated. Thank you.

    • #35486
      Walt Gifford
      Participant

      Sounds like they put a new ring in it?

      FAA certified jet engine and aircraft technician,
      Nicholson Speedway class champion 2001,
      Yamaha KT100 Service Center,
      41 years karting experience

    • #35497
      Jim Silverheels
      Participant

      Fast Freddy, pull the head off and raise the cylinder off the piston. Take the ring off the piston and place it in the cylinder above the exhaust port. Hold the cylinder up to the light and see if you can see light around any part of the ring. This is called the light test. I bet the cylinder was honed out of round. Know anyone who can check the cylinder with a inside mic for roundness for you? The ring is free on the piston land also?

    • #35508
      FREDDY SANDOVAL
      Participant

      Walt and Jim

      I think both of you guys are right, I was just wondering about that, and now it all makes cense. There is absolutely no way that this engine miraculous gained 20 more PSI, unless like Walt said “may be a new ring”? Slap it on and thank you very much?

      Jim, I think you may be right also with the trueness of the cylinder walls as well, and the light test. It’s just like when you do a straight edge on a cylinder head on a car, shine a flash light and you can crawl underneath.

      We do run a spec class that switches from pipe to 4 can every other race for the old farts, like me. With 390 lbs. Pipe. 410 lbs 4 hole can, and a 3 disk L&T Wet clutch

      And a spec class for the young guns Sr Super Sportsman at 360 lbs pipe and a 3 hole can 2 disk dry clutch.

      That being said, for a spec class (club level) KT100. What PSI is most common?

       

      Thank you all. Fast Freddy.

       

    • #35513
      Jim Silverheels
      Participant

      160-165 PSI    Freddy, quit screwing around and send the cylinder out to be checked for roundness and you’ll prob have to re hone/install new piston and ring .Yer positive the comp gauge is correct? Do the light test first. Easy fix.

    • #35517
      FREDDY SANDOVAL
      Participant

      Ok, I won’t mess around, no more. I am taking in consideration what you’re saying, Jim. I do have my back up engine package to rely on, before I do my own inspection on this engine, and before I send the cylinder out.

      Where can I order a good, accurate 2 cycle/small engine compression gauge kit?

      Any suggestions or part numbers of the one I should get, and where to order it from?

    • #35521
      Jim Silverheels
      Participant

      Fast Freddy, any auto parts stores have them. I had very very bad luck with the Harbor Freight ones. Go to a NAPA, Auto Zone, or Advance store and get one. They are all China made I believe and use a simple bourdon tube mechanism. If you run the engine and pull the exhaust off and see discolorization the the skirt ya got blowby with poor ring seal. Take the cylinder off and do the light test to verify it’s an out of round cylinder. Head gasket seals properly? If you had an air comp you may use a leak down tester to check ring seal also. Spend time reading on the net and go to You Tube for great videos. In the future you will laugh how easy it is to check this stuff.  Grab the bull by the horn and learn to do this stuff and not send everything out for others to do. Just my take on it….chi wa wa

    • #35523
      FREDDY SANDOVAL
      Participant

      We own an automotive repair facility, and I am a certified technician

      We do have a fairly new compression gauge kit from Mac tools, and another one from Snap-on tools.

      I just thought I needed a small engine compression gauge for these KT100 motors.

       

      But yeah, I think from now on if I want something done right, I’ll do it myself, I just thought there was more science to keep those engines freshened up and legal.

      What should be my ring gap, with a feeler gauge at the ends?

      Thank you for all your advice

    • #35524
      Ted Hamilton
      Participant

      Clue me in to why a “small engine” gauge is needed — pressure is pressure, right?

      www.facebook.com/hamiltonhelmets/
      2014 Praga Dragon / IAME KA-100

    • #35525
      FREDDY SANDOVAL
      Participant

      Ted

      I only brought that up, since it was mentioned by Greg, so that’s why I wondered if I really needed one or not, I thought pressure is pressure too, I just had to go in with an open mind, as I’m learning 2 cycles too.

       

      Not trying to re invent the wheel, I just thought that there may be something different here.

      But in all honestly, if you don’t think I need one, then I gess i’m fine.

      Please check the previous comments, I’m just trying to get advice here, not trying to prove a point.

    • #35802
      brian downing
      Participant

      What should be my ring gap, with a feeler gauge at the ends? 

      Install the ring in the cylinder and push it with the piston towards the upper end. The “gold ring” end gap clearance is in the .008 to .014 range. Yes, you check with feeler gauges, but don’t use shit tools from Sears, buy yourself a decent set of Starrett feeler gauges. They also make great 12″ finger feeler gauges that are good for checking piston to cylinder clearance, taper, etc.

      Also make sure you install the ring with the “T” for top on the top side :)

    • #35834
      FREDDY SANDOVAL
      Participant

      Thank you, Brian
      That is good info, and yes I only buy nothing but the best in tools
      All Snap-On, Mac, and the best brands for precision tools, You’re right about that.

      Now, I’m new at this 2 stroke engine rebuilding. How do I order the correct ring? And do people usually trim or grind the ends to fit the feeler gauge? Or Do you have to get the ring that fits the feeler gauge?

      Thank you, so much.

      I think from now on I’m just gonna send my cylinders to get trued correctly and I think I’ll do the rest, it can’t get any worse than the quality work I’ve been getting by having a builder do it all. Lol

    • #35847
      brian downing
      Participant

      Order a ring from one of the respectable online dealers, call if you need to talk with them to make sure you’re getting the correct ring.

      Usually one can get a ring .1 mm larger ring than the piston, so if for example your piston was oversized 0.5 to 52.50 mm, then you’d get a 52.60 mm ring and file equally from the ring ends until it fits to the gap spec ( .1 mm equals almost .004 of an inch).

       

       

    • #35850
      FREDDY SANDOVAL
      Participant

      That’s awesome! Brian
      Thank you.
      When a decent full rebuild is done, Should I expect to see some 150 PSI? Or something like that? Right now I’m only getting 120 psi. It’s no wonder I can’t stay with the pack. Lol.

    • #35864
      brian downing
      Participant

      I can’t say for sure, but one of the tricks I learned was to test my compression once, then again after squirting some plain old 30 wt motor oil in through the plug hole and testing again.

      If the compression jumps up, then the oil is helping the ring seal and yes, you’re not getting a good seal between piston and ring.

      Aside from that, there are too many unknown variables.
      I’d suggest buying a second motor anyway as they vary quite a bit from one casting to another.
      You never know when you’re going to need a “B” motor, just in case ;)

       

    • #35913
      FREDDY SANDOVAL
      Participant

      Thanks, Brian
      And yes, I do agree with second motor ( back up motors )
      All of our karts at the shop have 2 motors for each kart
      I currently have 2 newly rebuilt engines, sitting in my office, and that’s how I started testing all my engines, when I suspected I had no power,

      And there you go, my engine had low compression, so I started comparing everything we’ve been getting built.

      Thank you for all your help and advice, You’re awesome!

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