I’m thinking about getting an OK-S engine to play with. They just look like a blast. But I spoke to one of the main European engine builders. They said the bottom end needs rebuilt every 5-6 hours, for recreational use! That’s about 2 days at the track for me. Does anyone have experience with these? I know they’re a top of the line competition engine, but even KZ’s go 15+ hours without a bottom end. The KA100 is supposed to go 20 hours and also revs to 16k+ rpm. Can anyone confirm they need a bottom end rebuild every 5-6 hours to avoid failure?
My goal is to have a fast single speed engine, 40hp+ so the braking zones on my local track (very open and flowing) actually require hard braking and teach me to use the brakes. With my KA100 you just brush the brakes as you turn into most turns. I’ve also been recommended to look into a 144cc conversion for the Rotax Max?
6 hours on the bottom end is a slight exageration, altough they are high maintance platforms due to the high compression, high RPM and direct drive characterestics.
The 6hours quoted are real in racing conditions, running the engine at 3% oil and lean. The problem with the platform is that it doesn’t like running rich and with a higher percentage of oil due to the exhaust valve requiring lean conditions to opperate as intended. The valve also happens to be the difficult part of the entire thing, which further complicates carburation.
All in all, i wouldn’t necesarily suggest it as a recreational choice (altough they are incredible to drive). Consider the OK-N as an alternative, which lasts more than twice as long, procures the same driving sensations and is more forgiving.
For recreation, the Rotax is hard to beat. More power than a KA and 30-50 hour service intervals!!!
Unfortunately Rotax ruined the market with endless design revisions or they would still have a strong presence in the sport.
This surprises me given how prevalent powervalves are on motorcycles (race and road). Is this due to the use of a pumper carb you think? Most (Basically all) powervalve engines use a slide float bowl carb so your statement got me thinking. I have an older KF2 knocking around and while the powervavke itself was a bit of a pain, I didn’t find it anymore picky on carb. That said, I was using it fairly recreationally.
So anyway, im curious to hear more about carbs and powervalves with OK
The OK valve is much simpler than the KF era, as most of the dimensions are specified by the technical and homologation regulations (In KF, there were some insane designs, dual stage blades and many many many iterations of geometries)
The incidence in OK is limited to the valve membrane and the spring used, along with how much tension its under by the adjuster screw. The exhaust also plays a very very big part, as it governs when the valve opens more or less.
They are fineky by design, but even more so at international level where maximum optimisation is searched. It’s a platform that fundamentally is very very hard to operate in racing conditions by “amateurs” (non factory racing, bassicaly and very crudely).
The carbs are also very very sensitive (a 2 min change on the needle makes a world of difference).
Couple this with the fact that you really change piston every 1h in racing conditions, as they like to detonate, it becomes very dificult to deal with.
On the other hand (and paradoxicaly), the OKJ is bassicaly indestructible, running 30h on the bottom end and 7h on a piston without much issue. The OK-N is very close to this in terms of usage interval.
Is it possible to convert an OK-S to an OK-J with a different cylinder/exhaust?
I ended up just ordering an SRP TM OK-S this monring for fun. Feeling a little worried after this post about getting the jetting/oil mixture/everything else right so it runs decent.
I’m going to run a KA100/x30 mostly and just put this on for fun occasionally. I have a TM R2 also but it’s a major pain to swap from a KA100 to the R2 (reconnecting front brakes, setting up the shifter, different exhaust mounts, seat position, hubs, gear/chain, etc, etc). It seems like it shouldn’t be too bad, maybe a 20-30 min job to swap the other motors though.
KZ with rear brakes only will help you get the braking practice you’re seeking
I don’t have any data to support this… but I suspect the OK-S bottom end will hold up better than stated. Probably worth cleaning the powervalve once in a while and maybe check for rod play on the crank end periodically.
Haha, I thought about it, but it’s still a huge pain to switch, compared to the OK.
I’m hoping so. I’ll probably try to do the top end myself every 3-4 hours. Then send it out around 8 hours to have the bottom end checked out the first time. Hopefully after that I can gauge the rebuild intervals better. I’d be happy with 8-10 hours of use before needing to send it out for a bottom end.
We have run our Rotax over 100 hrs. with no issues, repeatedly. No better engine for karting, so far as I can tell, but I prefer the simplicity of KA100.
Never seen a Rotax engine pass 50 hours of bottom end service. They always broke earlier than that. That’s why our local Rotax dealer stopped saying they can last 50 hours some years ago.
Although he started divulging that again because he wants to sell them to another club.
The Micro Max engines are the only ones I saw that can last a long time
