72cc Kit for GPR Question [Archive] - Bay Area Two Stroke

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r6elmo
12-20-2006, 10:40 AM
Hello There,

I just picked up a 2003 GPR50. Modifications it has:

Arrow Pipe
24mm Metrakit Carb with Filter
Carbon Reeds
1/4 Throttle no Oil Pump 36:1 Premix
Metrakit Race Clutch
54T rear sprocket

My question is if i get the Metrakit 72cc Kit, which i believe comes only with the cylinder, piston, ring, gasket, and head, would i need to change any of the above parts? Would i have to rejet?

Thanks!
Danny

eddiemcrider
12-20-2006, 11:37 AM
welcome to BATS Danny

bad news: rejet? bigger motor requires more fuel so you'll need a larger jet.

good news: MRD carb = OKO = PWK Keihin MX carb knockoff so you can order Keihin jets from a local dealer.

even better news: your bike is gonna rip!

do a search - I have a simliar setup - sans the arrow pipe (I have a thrower which is more of a midrange pipe)

Dangerous Dave
12-20-2006, 04:35 PM
More bad news; I don't think you're going to be happy with the Arrow pipe with that kit, since the Arrow is made for a 50 only. Second, you're going to need to do something with the bottom end. A stock 50 crank and bearings simply won't survive the additional revs or stress of the 72 kit. If you don't believe it, I'll take pics tomorrow of the stock 50 crank that came out of my 72cc MK motor. I know there USED to be a bearing of some sort between the rod and crankpin, there's nothing resembling one there anymore. That happened after about 100 miles and the top end was fine, but the bottom was trash. You may also wish to change final drive gearing before all is said and done. Every modification takes at least 3 more to make it work right. The good news is, the bike will be surprisingly fast when finished...if your wallet can get you there. :D

r6elmo
12-20-2006, 05:21 PM
thanks for the replies guys...

I'm not really looking to put a lot of money into this, if it's a couple of hundred then sure.

Maybe a better question to ask is that after the mods that are on there now, what's a good mod for 2-300 to add more power? is there a 50cc kit out there that would add more power?

Pierce
12-20-2006, 05:41 PM
Actually the stock crank will work for a while but will have issues the longer you run it at higher rpms. I ran a SP kit for a year on the stock bottom end at the track. It lasted until the bearings let go.

Anyway you can run a HM 72cc kit with out real issue to a stock crank. The arrow pipe will work with it also as it is a lower rpm cylinder.

patrickmotox
12-23-2006, 09:02 PM
News to us. we Bought the 72cc kit from Dan with the sp pipe carb, etc.. ran it at least 1000 street miles and then another 440 race miles, then we put the pro race top end on, Shane did two practice days and two race days on that, all with the stock crank. It is still running so strong, with no vibration etc... we are putting in the race crank and bearings, but only cause he wants to scream it to 15k.... THese things are rock solid.

eddiemcrider
12-24-2006, 01:46 PM
News to us. we Bought the 72cc kit from Dan with the sp pipe carb, etc.. ran it at least 1000 street miles and then another 440 race miles, then we put the pro race top end on, Shane did two practice days and two race days on that, all with the stock crank. It is still running so strong, with no vibration etc... we are putting in the race crank and bearings, but only cause he wants to scream it to 15k.... THese things are rock solid.


I thought you had the stroker crank to bump up the motor to 80cc.

Manimal
12-24-2006, 06:22 PM
Wow! sounds like a Pro-Crank is a must for a 72cc kit, for the long haul.......ouch! I can already feel the pain in my wallet!

What is the cost of a Pro Crank and what is the cost in labor to install one, along with new main bearings?

looks like my Yerbi will remain a 50cc for a while :(

Pierce
12-24-2006, 06:56 PM
Na Manimal you will be fine for a long time with just running on the street. You won't be pushing that motor to really tear it up. My buddy has raced two years on the stock crank and he will be racing it next year on the same crank. You should be fine.

Manimal
12-25-2006, 12:05 PM
Well then, I'm confused :confused:

then, what's up with the "stock" main bearings failure comment towards running a 72cc? Or was the comments directed towards to Pro Race 72cc version?

Please explain......

Pierce
12-26-2006, 06:32 PM
The stock crank will last for a while. The bearings will begin to fail after a while of running. You will feel a weird vibration from the bike when they start to go.