Low Compression After Rebuild

Want to post a reply to this topic?
Login or register for an acount to join our online community today!

Low Compression After Rebuild

  • I just did a top end on my 800 Storm due to low compression (60psi all 3 cylinders). I honed out all the cylinders and installed new pistons, rings, pins, and gaskets. The cylinders had no scoring and cleaned up nicely with the hone. I started the engine and took the sled for a ride (about 10miles) and it ran great!  Today when I went out to start it, it began backfiring through the exhaust, was extremely boggy, and would not go over about 5mph. I took a compression reading after doin some other simple checks with spark and fuel and I have 70psi on all three cylinders.  This was takin with the throttle pinned wide open and engine warm. I know my gauge is good, as I use it at work frequently.  My piston to cylinder clearence was on the high side (.008").  I used a SPI T-Moly piston kit. I pulled one cylinder off and expected to see a broken ring or something but everything looked good.  I would like to hear any suggestions or opinions anyone may have because at this point I'm thinkin I wasted my money, and now I'm going to have to buy new pistons and have my cylinders bored.

    Replies to this thread are ordered from "oldest to newest".   To reverse this order, click here.
    To learn about more about sorting options, visit our FAQ page.
  • rings stuck? and theres to much piston slap.... plus arnt those nickisil cylinders.... or do you have liners?

    stupid doesnt fix stupid
  • The rings were not stuck and their tension is still good. My cylinders have the iron sleeves in them. They are also ported using a race logic template which gets me thinking that my piston to wall clearance should be even more important since there is less support on the piston. I ordered a set of +1mm pistons today with the money I dont have lol so I guess I'll have to see what happens. I'm going to be seriously PO'd if this doesnt correct the problem!

  • your piston clearence should be 5 thousands

    stupid doesnt fix stupid
  • Rccars18

    The rings were not stuck and their tension is still good. My cylinders have the iron sleeves in them. They are also ported using a race logic template which gets me thinking that my piston to wall clearance should be even more important since there is less support on the piston. I ordered a set of +1mm pistons today with the money I dont have lol so I guess I'll have to see what happens. I'm going to be seriously PO'd if this doesnt correct the problem!

    Who ported the cylinders using the template?  [Exh. port to high / transfers open to long / incorrect blow down timing ??] [ Have you mapped out your port timing? ]

    Are the pistons installed facing the proper way?

     

     

  • not trying to be a smart a--, use another compression guage yours might be bad  also try a leakdown test also check flywheel key might have sheared this would not effect compression. good luck

  • I ported the cylinders myself using the template. They are pretty much idiot proof and came with very clear directions. I was very impressed with them and extremely precise when doing the port job. The templates for my application didnt change the timing on the exhaust but rather opened the port a bit wider towards the top to increase flow. These motors already have huge exhaust ports. The majority of the porting was on the intake side. If i recall the top of the intake port did not change but the port was opened up on both sides significantly and slightly on the bottom (maybe 1mm?). The transfer ports were just cleaned up equally on all cylinders  as there was a ton of casting flash there. The port edges were then radiused to prevent the rings from snagging which was my first thought of what may have happened. I'm ASE Master Certified in auto and heavy trucks as well as Advanced Diagnostics certified but I'll be the first to admit I have basically no experience with 2-stroke port timing and have been trying to learn more about it, which is why I went with the templates in the first place. I was told its a little more than "trail ported" but not "race ported". I have not mapped the timing... Does this require computer software?? And yes the pistons are in correctly

  • Where did you find that piston clearance spec?? The service manual I have does not include my engine, though I've found the majority of the polaris engines spec are between .004" and .005" and the max service limit at .008" for all of them.  It would make me feel a whole lot better knowing the exact spec to tell the machine shop to bore it to since I'm putting factory pistons back in it.

  • you want a minimum of 5000 and no more than 6000 other wise your pistons will chatter inside the cylinders... did you re use any of your gaskets??

    stupid doesnt fix stupid
  • ? whats the end gap on the rings ...    is it possible  your crank seals between the cyl are bad ?  Are you doing the comp test with all 3 plugs out or just one.    If its not piston clearance or ring end gap then just about has to be case seals or a bad gauge..

  • I installed all new gaskets and crank seals when I did the pistons. Compression test was done with all three plugs out. The ring end gap was huge... something like .026" which I figured was because the SPI rings are junk. I've used cheap rings in a few rebuilds before and have had similar results. I'm hoping I will need to file fit the factory rings. I completely forgot how big that gap was... Thank you all for your suggestions. Hopefully the new +1mm pistons will be here soon and I'll let you all know if it takes care of the problem. I pulled the other two cylinders off today and theres a strange wear mark by the intake port of the mag cyl. I'm thinkin its got to be a combination of too much wall clearance and the wide ring gap