I just purchased a Derbi GPR50 with supposedly a 74cc kit brand new.
Questions:
1) How do I know if it is really is 74cc and not 49cc without dismantling
the engine to measurs the circumference size of the piston?
2) The top end appears to be about 102-105kmh(about 65mph). What
is the top speed for the 49cc model? What is the top speed for the
74cc model?
Any how, it is a great bike and fun to ride! Thanks in advance for any
responses.....
Jericho
im sorry to say but you've been ripped off. your bike is mostlikely a 50cc. a 74cc kit should do 80-90mph. have a good look on the cylinder and cylinder head for a 50cc mark.
Good information can be had from "Oz" at Cycle Imports, tel. # (305) 597-4440. If your bike is a factory 75cc version, i.e. without the kit added to a 50cc bike the rear sprocket will have 42 teeth as opposed to 52.This could cause low top speed if running on the 52T. sprocket.
Apparently the factory produced 75cc. bike has a diferent cylinder, I believe the stroke has been changed so if you ever want to fit a cylinder with better porting, a 75cc. kit for a GPR 50 will not fit! Have not yet found a source for a performance kit for the GPR 75.
Another way to confirm a 75cc bike is obviously to measure the bore. This can be done without stripping the engine quite easily. Take two pieces of stiff wire bend one piece at 90" so that it measures 20mm(close enough for half the bore of 39.88mm for the 50, the other piece bend so that it measures 23.5mm. half of the 47mm. bore of the 75). Leave enough length on the long side so that you can't drop it into the cylinder! Remove the plug and insert the wire into the cylinder you should not be able to feel the cylinder wall with the shorter wire if it is a 75cc. machine. The longer wire will hit the wall with the vertical portion in the center of the plug hole, just make sure you measure near the top so you miss the ports.
My cylinder has the Derbi logo cast into it and a part # AO6982 stamped on the rear, and is a factory GPR 75.This bike was not purchased in the States, but I believe they are just being imported now. Oz would be a good person to call if you have any further questions or need parts.
Hope this is of some help.
Tony.
Thanks for your input. I do have a 42 tooth rear sprocket. I have not yet checked
for the stamp on the rear of the cylinder head; I will though. I will also measure
the bore.
Thanks again.
I found a dealer in NJ that claims to have these factory GRP 75s. The guy I talked to said that the bore was the same but the stoke was lengthened. I know that this will definately effect the rev of the engine. A shorter stroke allows for a higher revs. Measuring the bore might do you know good. I haven't found any proof on the net that the factory even makes these. I have emailed both the UK distributor and the factory for info. I haven't recieved any yet.
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