efi F650gs engine in a KTM640e/a chassis? Has anyone done this?

Like the title says, has anyone here put an efi rotax 650 from a BMW F650GS into a KTM 640e or a chassis? I have this idea tickling my brain, and wondering if it's been done. I can't seem to search anything on this forum and get anything I'm looking for.

My silly brain is thinking that the super reliable and smooth rotax single in the much lighter and more dirtable KTM chassis would be sweeter than the buzzy grenade LC4 (which IS a sweet motor, but they don't seem to last) I love that Rotax designed engine, but Xchallenges are hard to find and expensive, the the new TR650 is way to heavy and poorly suspended. And the F650gs is WAY to heavy and poorly suspended.

Is it possible, has it been done? The final drives are on the same side, but I havn't been able to look at both bikes together to see just how it might work.

never heard of this happening but all the best if you do will have to get some measurements to see if it'll fit okay, there are a ton of mods for the rotax engine especially in Germany to get some more poke out of it.

Maybe I'm missing something but I really don't see why anyone would do this, that's why you may not have seen this done before. My reason being that changing a large thumper motor for another large thumper wouldn't really be any different than the orginal.

I personally would find a better use for the rotax. Maybe use it in a smaller (lighter) chassis, something that goes beyond what BMW had in mind. Like building an F650 that didn't break apart every time the front wheel hit something bigger than a cigarette butt.

do you have the engine and chassis?
see lukas , maybe he can shoot some info your way ,he has worked on other possibilities but my have the info you need.

jammed into a 450 frame would be a good prospect

Having jammed my fair share of engines into donor frames I can say that fitting the engine is the easy part. It's everything else that has you running for the booze. Airbox, exhaust, fuel all require much more consideration than just remounting an engine.

I agree Lukas is probably the most well equipped to answer this question as I believe he has extensive experience with both the bikes in question.

To build a better 1 lung dual sport travel bike. I love the 640A chassis, it's light enough for what I want, has a large fuel capacity, and is well suspended. But the engine, while it has plenty of punch, is buzzy at speed, gets 40 mpg, and I have not seen very many high mileage examples. They seem to grenade around 30k miles or so. Some have gone further, but many don't even make it that far.

The efi Rotax 650 single, on the other hand, has similar power output, but better bottom end, is very smooth (for a single), gets upwards of 70 mpg, and they run and run and run. I've seen more than one of those engines with over 100k miles on them. But the chassis they put them in is 100lbs heavier then a KLR, and the suspension is just as crappy. They make an excellent commuter and highway bike, but are not so good in the dirt.

Anyway, it's just an idea I've been kicking around. Take the best features of both bikes and put them in one package. I don't have any of the parts yet, but was wondering if it would be a worth while endeavor. I have no experience welding chrome molly though, so that might be interesting.

This question should never be asked in this forum (well this section at least) because if you have to ask there's a good chance you won't understand.

No, it won't work without huge mods to the 640 frame. The FI rotax engines have a downdraft inlet port, eg the port points almost straight up out of the of the head. The throttle body would need to be right in the middle of the 640's frame backbone would be.

An earlier non-FI rotax engine might fit, they have a more conventional port arrangement (straight out the back of the head).

BMW went from a backbone type frame to a twin spar frame in the F650's when they went to the new engine with FI. The new Husky also has the downdraft port, it's better for flow being a straighter path into the combustion chamber.

Just go crazy on an F650 like I did (this thread needed a pic);

There were 2 rotax ktm models 504 mx and gs 1983-84 .
Regards,Ed

Maybe I don't get this one, then again someone turned a sportster into an ADV bike and I didn't get that one either. Although I totally respect what he did and ended up with. And considering that it's something that you could never buy it made it that much more impressive.

In this case replacing a large single thumper with a large single thumper seems ... well... like a dog chasing his tail. But hey, I've suscribed so let me just sit back and see what happens, I may end up learning something!

Concur. Strongly.

Also agreed. I almost lost my shit on another forum with that kind of talk. Plus, seems like the OP listed plenty of good reasons to me. Longevity is a big deal. No sense taking out your favorite bike if you don't trust her to get you home anymore.

Turn the spine into an airbox?

ala twin spline, nice thing about that is you can then build it around the eccentials,will have to go with a different tank(small considering the rest of the project), just fab a jig to hold the frame geometry in place BEFORE you cut .maybe use all the fuel system for that engine and build around it using the beemer factory frame dimensions(tube dia-thickness etc) as a guide.

Looks sweet, but how much does it weigh? How's the handling at high speed in the desert AND at low speed in the rocks? I've thought about doing this too.

[QUOTE Turn the spine into an airbox?[/QUOTE]


Hmmm, interesting idea.

It weighs 184 kg with a full tank of fuel (20 l) so about 170 dry, a good few kg more than a 640.

High speed is fantastic, it feels planted at ~150km/h on the dirt even without a steering dampener.

It turns like a bath tub is the tight stuff...

I just rebuilt the engine (2nd gear selector crapped it's self) and put in an aftermarket 11.5:1 piston meant for a DS650 quad, it should hopefully have a few more hp now.

Something I did on my KLR650 / Ninja 650 merge.




I'm not sure if it's the best thing to do engineering/performance-wise, but it worked out for me.

It isn't as if the KTM plant and Rotax plant are equal motors. Show me a KTM 640 motor with 80,000 miles on it that hasn't had serious work done to it and still runs as strong as it did day one? Hell, show me one with 50,000 miles on it, period. You're also talking a skirtless piston in a high strung motor built for performance vs. a Rotax which is built for reliability. The KTM motor has more jam, in my experience-- which is admittedly one afternoon-- but is extremely vibey at highway speeds.




You'll have just as difficult a time finding a 650 Rotax that left someone stranded as you will finding an 80,000 mile 640 motor unmolested.

In one year I rode F650 Dakar from Windsor, Ontario to Death Valley and back, solo, and to Newfoundland and back, solo, and to North Carolina and back, solo, without ever once wondering if would start. The EFI is rudimentary and dead simple.

In nearly 40,000 miles the only mechanical I had was a dead radiator fan.



A YZ450 front end, sprung for the weight of the bike, and a well sorted Ohlins rear shock makes a hell of a difference. Still, the bike weighs a lot, the standing position is 'challenging' and the whole thing is too damn wide at the saddle.

I'd love to see that Rotax motor in a nicely set up KTM frame. Hell, I'll take my XR400 motor in a really nice modern aluminum frame with a great suspension.

^ Nice bike.

You smell what I'm steppin' in. A super smooth, reliable single in a bike that can travel, but still handle off road.

BMW had the right idea once, then promptly dropped the bike from it's lineup before it even had a chance to catch on.

KTM built an excellent chassis with the 640a, and put an high performance engine in, but unfortunately high performance=high maintenance.