High Compression Questions



  • I have spotted a RS125/Rotax 122 engine for sale, with the listing stating that the engine has been rebuilt with a 140cc Italkit and the crank has been rebuilt too. There's most likely a new con-rod in there as well, which would justify the cost of the crank rebuilding. However, the seller states that the engine has a high compression of over 140psi. He has a video of the test and also one of the engine running, so I'm curious to find out the following;

    1. Is 140psi normal?
    2. Is that psi acceptable or not?
    3. What would cause the compression to be too high?

    https://www.youtube.com/watch?v=drz8WtfLYCM



  • Okay, well I'd be wary about buying stuff from people who don't really know what they're on about.

    There is having high compression, then there is having a high compression ratio.

    These are two different things.

    High compression is just telling you that the engine is healthy. No blowby past the rings, the head gasket is sealing and it's all working as intended.

    High Compression Ratio, is the volume swept over the squish. The higher this value, the more power you're likely to make, but the higher the risk of detonation.

    It's the latter you really want to understand.



  • @Calum Thanks for the advice, I asked around on the FB RS groups and got replies of 120-160psi is the average range. With 150psi being pretty much the optimal pressure. So it appears that your comments of the current owner not knowing what he's on about, as being on the mark.

    Oh decisions - decisions!!! πŸ˜•



  • @NINJA My advice would be to just build your own engine to the spec you want.

    The last Rotax engine I bought had "Recently" been rebuilt.

    When I stripped it I found that the little bearing was missing a few needles, and that it was generally pretty worn. Barrel scored etc.

    It just comes back to bite you.

    You really want that compression ratio dialed in to how you want to ride the bike.



  • Yep gotta agree with Calum if you wants something doing right do it yourself don’t trust strangers to tell the truth



  • Thanks for the advice and although I have to agree with you both, I decided to go ahead and bought it. I got it for a decent price and my thinking is that the parts alone are worth the value, as I will have a spare engine to work on, whilst I have a running bike to enjoy. The engine has been rebuilt but not yet run-in, so the parts should be virtually 'new'? Only an internal inspection will confirm that, but even if I just end up with spare parts I will have a full set at my disposal.

    Anyways I've paid for it now so we'll see what she's like when it arrives, but my first job is to give her a damn good clean!!!

    0_1543242633288_Rotax 122 Engine-15.jpg



  • Well she cleaned up nicely and now looks much better now and the internals look really good too;

    IMG_2556.JPG

    IMG_2557.JPG

    IMG_2560.JPG

    IMG_2553.JPG

    IMG_2558.JPG

    The piston looks good and is unmarked;

    IMG_2563.JPG

    The barrel is very clean with no signs of any scours;

    IMG_2568.JPG

    Rebuilt crank and what looks like new bearings too;

    IMG_2566.JPG

    Sadly I did not do enough research before buying this motor, as I will need a lot more parts from the RS, (Loom, switches etc) and also the flywheel is a lot heavier for the electric start to crank her up. This will reduce overall performance slightly, due to it being heavier rotational mass and added weight to the bike overall due to all of the electric start components. Which all means a major expense all just to get the electric start to work. Plus I have also discovered that my MX frame will have to be altered in-order for this lump to fit in. Mainly due to the starter motor's location and slightly different engine mounts. So it's a lesson learned the slightly hard way on this one, but all is not lost as I am now going to respray her and sell her on - TBC..............................................



  • @NINJA Flywheel acts as an energy store, you won't gain/lose performance from added weight, since it's directly related to the running of the engine. What you'll find is that it'll accelerate harder, but the inverse is true. It's also not great for the bearings to run super light flywheels. Not that the MX flywheel is super light.

    Performance wise, I don't think they'll be much between the two.

    Why wouldn't you just blank the Starter off and go with a kicker?

    To get the electric starter working wouldn't require a lot at all. The starter is wired directly up to the battery and the Omron relay. The relay is wired directly to the battery also. The other side is just a 12 volt low current micro switch, could get a universal one and poll from the battery also. Shouldn't cost more than Β£10 to sort out.

    The picks up are identical, I'm running an MX pick up on my RS, since my RS was one shagged.

    What head was fitted to that big bore kit? if you got the engine cheap enough, the cylinder and head would be worth it alone. Starterm otors are cheap fortunately, and the flywheel and running gear can also be had cheap. Crank cases are well worth keeping though



  • @Calum It has the 140cc Italkit, with a single ring piston. This engine was rebuilt in 2013, fitted to an RS, started and ticked over but never ridden or ran-in, as that owner never finished off the project. So it's pretty much a new engine really. Sadly I can't return it so I need to sell it on to recover my expenditure. If I left it as a spare engine, or just donor parts it would really just be an expensive luxury collecting dust. It's better off with a new owner and someone who will enjoy it for-sure. πŸ˜‰



  • Update; I'll take some better pictures tomorrow, but here's how she looks today;

    IMG_2633.JPG

    IMG_2635.JPG

    IMG_2636.JPG

    She's all ready for sale now. πŸ˜‰



  • @NINJA Hey, that's looking very tidy!