2STROKE STUFFING Thanks man! And hopefully this is just the beginning :) Definitely beat the 2 minutes longevity record today. I'm going to put in a smaller idle jet tomorrow and see where that gets me because it seems to start running cleaner at the end with a little bit of throttle. If that doesn't help, maybe the float level is wrong. Or maybe the choke disabling plug that a previous owner of the carb put in there isn't quite working and it has full choke all the time. We'll see, can't be too hard to fix. Looking forward to a test ride!

Thank you :)

Alexander Jäderblom Thanks! Yes I was pretty excited about it ;) I was trying to signal "first pull". I'm thinking I should open it up and take a look at things before I give it full throttle and high rpms.

Hollaendaren Yeah I'm not saying you should rev it, but the engine definetly likes a little more rpm, i guess the pipe is made for it and the timing too. Would like to lear more about tuning and i own some books about it. Good idea checking it out first, would have done the same. :)

jnuxca Definitely! I hope it stays together but looking very promising so far :)

Thanks! My cameras microphone really doesn't do it justice, it has an awesome sound! I think the twin plugs may be partially responsible for that, I use 2 different plugs, and 2 seperate coils, so they are likely not firing at exactly the same time so I think that might add a bit of "depth" to the sound.

Does that run 2 coils standard ?

AuMechanic This engine has a single plug standard, completely different head. The dirtbike that the head is off seems to use a single coil with 2 outputs as far as I can tell. I ran a coil like that on my old engine (same type head) but I wanted to switch to MSD ignition and they told me the dual output coil was discontinued so I'd have to use 2 single coils instead. I've connected them in parallel to the output from the box, not sure what's better, parallel or in series, but so far this seems to be working.

That could be a problem I've tried using 2 coils on a twin optical ignition system on my racing RD350LC Yam in the 1980's Result was poor ignition current, would not rev out, and fouled on one plug at low revs. I removed one of the optical (Lumenition) systems and used only one unit connected to a twin lead coil off a non CDI 4cyl 4 stroke GS750 (induction ignition not CDI), the original RD twin lead coil was for CDI so couldn't use that. That sorted it and never fouled a plug again and one system only used less battery power and I had no alternator on it. If the MSD is an induction system you could use a 4t 4cyl twin lead coil of an older bikes with points or try to source a twin lead coil for induction spark system. As to why, the twin lead coil has a positive and negate firing plug and each plug grounds through the other, it is not 2 coils in one, (i have a video on how they work) this means that the secondary current is always present at both plugs even if one is fouled, which is desirable in both twin plug and twin cylinder. In 2t twins the twin lead coil runs as a dead spark system, but likewise current is always present at both plugs. A lack of current through one plug that is fouled (a spark must occur at some point to generate secondary current) leaves no means to clean itself.

AuMechanic Thanks for the explanation, I'll check out that video later! On the old engine I've ran 2 coils in parallel like I have now with good results too, one coil gave up on me though probably because of vibrations (it was mounted on the crankcase) but the engine still ran okay on the other coil/plug, I actually didn't immediately discover it was broken. I didn't have a spare coil of the same type laying around so then I tried a dual coil instead, which also worked fine. From what I can tell by eye these coils seem to give me a stronger spark than the previous setup, nothing scientific though, just eyeballing. I've read that using different coils are a popular upgrade for Ignitech to fix its relatively weak spark so there's probably something to it. A guy on Instagram told me he runs a 2cyl Ignitech box connected to a single MSD coil with good results too, he runs funny fuel and very high rpm compared to what this thing will be turning. So I'm hoping for the best, just need to sort out the jetting and bolt the chassis back together and then I think it's time for a proper test ride.

Benoît Genest That's why I have ear protection ;) ear plugs while riding. You should hear my 797, when riding slowly through the pit at a race people start covering their ears when they see it coming :D I think that's hoa modern 2-strokes do it, an external balancing shaft driven by a set of gears. I'm mostly into old stuff as you can probably tell. But might be worth having a look at in the future! My minibike with an old Briggs flathead actually has an internal balancing shaft inside the crankcase, but that's a 4 stroke.

The 797 is a dream machine! Hope I get to see and hear one in real life one day! Another solution for balancing would be investing in a titanium rod ;)

Benoît Genest It's my dream machine too :) has been highest up on the "coolest sleds of all time" list for a long time! I'm counting myself lucky that I had the opportunity to buy it a couple of years ago. Titanium sounds too hi-tech for me, and expensive ;)

DrBOB101 Might be time for that.. ;)

William Lunney I'm not sure about the exact sprocket numbers but for hillclimb I gear 2 teeth lower than for drag racing. I think a 21 upper for drag and 19 for hillclimb. I have a little extender piece for my chain that I put on for higher gearing, basically just 2 master links in a row because as you probably know replacing the entire chain is a bit of a hassle on these things :)

Thanks!

Vns workshop thank you! Currently I have 28 degrees at 6000 rpm, ramping down to 18 at 9000. It's going to need adjusting though.

Thanks! Nope, it has a CVT transmission :)

andli461 Thanks! I'm a happy camper today :) I know it's really low, but that's what a longer stroke, heavier rod and heavier piston did to that 20% it has stock. Which seems very low too btw. But that's what I'm hoping, if I can get low enough idle speed, and high enough engagement maybe it'll work as it is. Same problem with the stock engine, but the shaking is spread over a wider rpm range now.

Hollaendaren Hard to say what will happen really. But of course, getting something more “mainstream” like say 50-50% would probably not hurt. But then it’s about the “frame” etc etc so what is ideal is hard to say. Trail and error. IIRC, older foul stroke bikes whit a vertical cylinder often run up to 70% bf. Just got to think about it. You better double check that though.

andli461 The stock 20% has been working great in previous years, so I know that will probably work this time too. But I have an old Husqvarna 250 dirtbike that has an almost vertical cylinder and its crank is around 50%, can't remember the exact number. But we'll see, I need to take a test ride before I do any crank hacking :)

Hollaendaren Get what you are saying. This is no obvious thing to “just do the calc and follow the result”. Way more complicated than that. So absolutely, give it a go! The 20% stock bf must be there for a reason. 👍

andli461 Yeah I figure there might be a reason for it. I will be turning it around 9200-9400 rpm with the new pipe, so as long as its really smooth there I'll be happy :)

Vns workshop the piston skirt is too narrow for a bridged piston port, I would lose area by doing so. I considered boost ports for the exhaust but the studs are in the way for the boost channels so they won't work either. So I had to stay oldschool :)

Ok, all is very clear now and make sense, you have a great approach in tuning and study engines!! Great, i stay tuned on your channel !

Christopher Blair I think a single row chain should be fine for that as long as the chain is in good shape. I use a chaincase and chain off a twin Elan on this one, with duplex chain. Never broke a chain so far!

I'd say for a stock engine a 34 would be plenty, maybe a 36.

snocrushr It's 335 cc now, from 268 stock.

I've made it for this project, haven't really gotten to test it properly yet. I'll do a report on it after I've actually tested some full throttle action :) I just mentioned what it was because otherwise the question would inevatably come up anyway.

I'm interested in your adjustable jet too, some of my old Puch mopeds have a adjustable mainjet too. Pretty simple actually, some holes and a needle that adjusts the flow. I guess yours is similar, a taper bored jet and a needle that adjusts into, or what? :)

Carsten Lorenzen Exactly like that :) I just need to get the low speed jetting sorted out first and then I can test this one :)

Thanks! I'm looking forward to hearing it at full throttle and high rpm! :)

Hollaendaren yes man ThaT Rotax is Like music to my ears ! kzfaq.info/get/bejne/f9yFiJub1KjGYYU.html

15taggarn Finally :)

Sorry, it's not for sale.