2003 Derbi Senda SM50, Yamaha YZ85 Engine Swap?

[ThatDude]

Hello everyone,

Thanks for all the great resources! I have a 2003 Derbi Senda SM50 that is pretty fun around the neighborhood, and yet I wish to tinker with it...

I know that big bore kits are frowned upon for the most part, and as a 2T fan I get why. This bike may be gutless but it is very reliable in factory trim! That’s why I want to leave well enough alone and swap out it’s heart for a Yamaha YZ85 engine and gearbox (keep the Derbi engine for re-sale). I’m 6’+ and 240lbs, very experienced and YZ85 parts are readily availible where I live. Besides, I believe that ‘03 was the last year these were Spanish made so I do not want to do any permanent mods in case I want to put it back to stock and sell (decent re-sale value in the big cities near here).

FYI, I have bigger bikes (GasGas XC300 enduro, and a Yamaha R6 which is my dedicated track bike) and appreciate the fun you can have on smaller bikes on the street!

Licensing and insurance for one of these are not issues where I live or where I am moving next year, as long as I stay under 100cc or so. Same rules apply as my R6 except the Derbi isn’t allowed on Highways since it’s only good for about 50MPH max. To be fair, I don’t care about increasing the top speed, just how quickly I get there

The big question is how to fab a bolt on frame cradle for a YZ85 heart? I see many videos of people hooning these with an 85 or 125 MX engines, but no how to’s on mounting to the frame?!? I want to do an 85 since a factory 85 will be easier on the chasis than a 125, but fun enough to keep me entertained! (Please don’t tell me to buy a bigger bike, plated 2-strokes are a rare find here and worth a few bucks to tune;). Plus a factory YZ85 engine is easier to find parts for and maintain around here than the factory Derbi 50...

By the way, I want to swap YZ85 suspension onto my wife's trail bike, so I already have reason to buy a YZ85 donor bike, and a few $$$ aren’t an issue in the grand scheme of things. Any resources on mounting an 85 in my SM50 frame would be appreciated!

Thanks!

[stevo as b4]

The Derbi engine is supposed to be a strong unit and with alot of performance in std trim. Can you not find a used engine of the same type to use to build a big bore stroker crank motor in the 85-95cc bracket. If you have some cash to spend, buy a brand new race spec uprated stroker crank, and do. Careful re-build with lots of attention given to measuring clearances, and getting your compression ratio and squish band optimised for the fuel your going to run.

I can't see why you can't with time and care build a 16-18bhp motor that'll be explosive in such a small light bike, a bit like the TPR86 kits are on the AM6 engines.

[ThatDude]

That’s not a bad idea. I struck out on parts at all the local scooter shops, but I bet they’re more common in one of the bigger cities. Yes, 15+ hp would be very fun on this little bike. I’ll do some digging to see what I can find. I know a Yamaha engine would be easier for me to find parts for, but if I could build a Derbi engine up to handle the extra oomph, that would save a lot of fabrication and alignment headaches!