TDR125 4GW

  • You speak of ignition upgrades as a first port of call, are you talking about changing the cdi? If so what are my options.? Aftermarket or OEM.

    With regards to the engine your absolutely bang on about building a separate one from the ground up, like you say it will take a lot of money and time, but the theory behind it makes more sense. Especially now I have a spanking new flat top mitaka piston for the 4FU barrel I mas aswell run it as is for now.

  • @tdr125greg Yeah, I don't know what your options are. But I used Zeeltronic Ignition System. I will be using them again for my new build.

    By far the single one component that I noticed.

    Yeah, I personally, would have ran a domed piston, higher compression. Usually, Higher Compression = Higher Peak Power.


    It's not a strict rule, and it does add wear, but if you tune it right, you can generally make bigger gains upping the C/R

  • So you are telling me I could have ran a dome top piston in my 4FU nikasil plated barrel.

    The only reason I ask is because I was told I could only run a 56mm flat top in my cylinder by pjme?.

  • @tdr125greg PJME know what they are doing.

    You can run whatever you want.

    But you would want to have the combustion chamber reworked.

    If you have a 4FU head then you should notice a restrictive combustion ring around the chamber.

    For £30 odd quid Mick Abbey will grind that out and put a nice squish band there. 80% of the power of a two stroke comes from the head. So getting this part right is crucial.

    Telling your tuner that you are going to run a domed piston, will allow them to correct the squish band.

    My advice would again be do this with another, specifically designed, engine.

    The Rotax 122 engine that I am building comes stock with a flat top piston. But I intead to rework the head and run a domed.

    Running at higher compression puts you at risk from detonation. This is an engine killer. To avoid this you need to retard the ignition, or up the fueling.

    So it's not always clear what the best path is.

    I have had a few 4FU's but never saw a build through to the finish.

  • Thankyou calum I will take everything into consideration and make a path for myself. I solidly agree on a second engine but for now I'd rather get the current engine running smoothly and swap the front end.

    When I have a nice set up I will give it the beating heart it deserves.

  • You cant run a Domed piston in a 4FU? Wouldn't it hit the head? I've got a 4FU head, and they never came with combustion rings, only 2RK/3BN's?

    A Flat top piston doesn't interfere with your transfer ports as much as a Domed piston, so you can take advantage of scavenging with correct porting and a proper exhaust,

    The 4FU seems to have larger volume crankcase transfers than a 3MB?

    I'm running .6mm squish and a 134cc heavy Top end porting on a 3MB barrel. Looking at 25bhp maximum on 97octane. Most power will be made by the cylinder head itself, the barrel is aiding it along.

    No doubt moving torque, and not that much more of it to very high RPM, will only make you come to be very efficient at going up and down the gearbox quicker to keep the thing in the right rev range.

    Depends what you want from it, i'd say get the ports cleaned up, but don't change the Port timings at all, and just run her standard. These engines don't make power at all like a Rotax!

    I've been bulding a race setup for mine for a while now, yet to know proper results tho

  • @Darty Here was my 4FU

    alt text

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    As said I don't know natively if it supports domed, but certainly if you removed that ring and reworked the head

  • This post is deleted!


    Mine might have been modified, the crown seems very different. No signs of a heavy skim. it's a closer match to a P' stamped head but with a squish height less than .5, A domed piston would crash into it at that point.

    A base gasket would help, but then you begin to change port timings, not good. I gave up on a 4FU, Nicasil is expensive if you lunch the bore up.

  • Sorry my bad.

    I have had a few 4FU's and they have all been this way.

    Which is why I said it...

  • Another purchase sees the trail tech vapor and indicator dash added to PROJECT TDR.
    I purchased the kit on eBay from Moore speed racing for £150 all in which I thought was a good deal. 🔥🔥

    Any of you guys run the vapor on a tdr/Dt before?

    0_1488293323206_DSC_0017_1.JPG 😀

  • Global Moderator

    I always thought replacing my TDR clocks with something like this would look a bit out of place as the stock one moulds nicely into the plastics. Bet theres a big hole there now! 😂

  • @Mightyman I have thought about the same problem but how about 3d printing a new speedo surround then fibre glassing bondo, filler paint to finish ?

  • Global Moderator

    @tdr125greg That is a very nice idea actually! Would be even better if you could save the fussing with fibre glass and just get a snug fit with something 3D printed.

  • Running a vapour on my DT. Homemade mounting bracket

  • @finnerz89 looks tidy mate, how responsive is the Rev meter and do you have the radiator temp sensor or the sensor that goes on the spark plug?

  • Temp sensor that fits inside the radiator fin. I might get one for the plug and wire them through a switch

  • @tdr125greg vapor on my dt and I love it. Makes me feel like I got a spaceship at night with the shift lights flashing in my goggles. Haven't ridden the bike for years but back when I was a kid 🙂

  • @Calum I'm really looking forward to seeing those shift lights in action

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