2003 motor swap project [Archive] - Bay Area Two Stroke


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06-24-2009, 09:38 AM
I have 2003 derbi gpr50 and I'm looking to swap to a larger motor. I was planning to install either a 125 or maybe a 250 2 stroke, for what ever makes it easier. My derbi will be street driven everyday. Does anyone out there that has done a project and maybe give some info on how to do this?

Dangerous Dave
06-25-2009, 07:34 PM
I don't think there's any such thing as a quick easy swap of a bigger motor into a GPR. I have a friend who put a 350 ATC motor in his, but it was a long term project with lots of cutting, welding, and fabrication involved. Frankly, these things are a blast with a decent big-bore kit (including crank for reliability), pipe, and carb. My GPR50 with SP72 kit is really more fun to ride than my GPR125. Unfortunately, it has had some reliability issues that I haven't been able to resolve, but I think mine is an isolated case and not typical of big-bore 50s. If you've gotta have a big motor, I suggest buying something else. Otherwise, get a COMPLETE big-bore kit (anything less is being penny-wise and pound-foolish) and ride the tits off of it.

06-25-2009, 10:34 PM
A men brotha. My 75 kit I had was so reliable I put 7,000 miles on that kit. That was after I worked out all the little set up issues.

06-25-2009, 11:46 PM
Well here's my problem. I'm only 5'2" tall with a short inseam and I really don't want buy a ysr, which there's nothing wrong. I just have to avoid the rubber necking tourist who don't know I'm on the same road with them. The derbi is the perfect size for me. I don't have to lower the bike at all and if the bike did fall I have confidense to picking up the bike off the ground. I sat on my friends 1999 aprilia rs250 and I can't even have one of my toes touch the ground. Already my bike has a new malossi big block heads but I don't actually know what size jug. There is a casting # on the side but I've looked online to find out what's in there. One thing I know speeding down the highway I can travel 75mph, under gps. I'm running a 17 tooth front and stock 35 tooth on the back. A 24mm mukini flat slide carb and a mk sp pro pipe. But I still want to ride faster.

If anyone out there has any ideas or anyone have tried please pm thank you, Tim

Dangerous Dave
06-27-2009, 07:13 PM
Well, I feel ya. But I don't think there's any easy direct swap. I know there's a Yerbi kit to put your Derbi motor into a YSR chassis, but I don't know of any kits to put anything else in a Derbi. And I do understand the "invisibility" factor of riding a YSR on the street...I did it for awhile, so I know where you're coming from. I also think my GPR50 fits me like a glove. If I could work out its issues, I'd ride it everyday and be damn happy about it. My 125 has taken alot of work to get it to where I'm pretty happy with it, but I'll be alot happier after I get a decent (lowered) shock for it. You can put whatever you want in your GPR chassis, but you might as well invest in a MIG welder now and start practicing.