The XR250R Motor FAQ Last update: 12/06/98

The XR250R motor is a four valve, overhead cam four stroke with plenty of torque. Many riders find the bike underpowered however, for serious enduro racing or agressive riding. Here's what can be done to give the XR a little more horsepower......

Q: What can be done with the stock motor to make it more suitable for agressive riding or racing?
A: The general consensus is that the best bang for the buck to get increased horsepower from the XR250 motor is to go to at least a 3mm overbore, which coupled with a 77mm 10.5:1 piston will get you 277cc. This alone, without an aftermarket cam or any other modifications, will give a significant HP increase throughout the entire powerband. In addition to the overbore, a mild porting job and a cam can really bring this little motor alive.

Q: How can I get to 280cc or above with this motor?

A: Many companies such as Thumper Racing or XR's only offer a 280cc kit or separate components to increase the displacement. Most kits come with a piston, a wrist pin, clips, piston rings, and top and bottom gaskets. You then have to have your cylinder bored to match the piston in the kit. If you only go to 280cc, you can avoid re-sleeving the cylinder, which allows for an unexpected meltdown. The biggest kit available for the XR is a 320cc kit from XR's only, which requires re-sleeving the cylinder. This large of a displacement however, may require connecting rod replacement or bottom-end modifications for a safe motor. Following is a listing of the known available kits for displacement boosts on the XR250R:
Re-sleeving required?
Kit/Parts from
270cc 76mm Piston No XR's Only
277cc 77mm Piston No XR's Only
280cc 77mm Piston No Thumper Racing
284cc 78mm Piston Yes XR's Only
300cc 80mm Piston Yes Thumper Racing

Q: How can I go about getting a cam for the motor and what is available?

A: XR's Only offer cams   for the XR250. If you get in touch with them and tell them what kind of riding you do and what type of modifications have been made to your motor, intake, and exhaust systems, they can reccomend a cam to suit.

Q: What about porting the cylinder head?

A: Both Thumper Racing and XR's only offer porting for the XR head, as do many motorcycle engine services, or you can do some porting yourself if qualified. Simply going in and smoothing the casting marks and transitions can make a noticable difference in performance. More radical porting jobs however, may not be the most cost effective solutions for increasing hosepower, as the biggest increases will come from added displacement.

Q: What else can be done to the motor to offer better performance?

A: After doing a displacement increase, a hotter cam, and maybe some porting work or other motor modifications, you may find the XR starts to run significantly hotter during long races. To combat overheating, XR's Only offers a modification to the cylinder to weld on larger fins, which should help with the dissipation of heat and allow your motor to run cooler. This is a pricey modification, and may only be necessary for very hot climates and prolonged agressive riding or long races like hare scrambles.

Q: Are there any issues with the XR valves?

A: The XR valves are really large enough, with modifications generally limited to better valve springs, and possibly the addition of racing valve guides.  A three-angle grind can be performed on the valves by XR's only if desired as well.

Summary: If you want some more horsepower but don't want to sacrifice reliability for the "ultimate race motor", then a 280cc kit with some mild smoothing of the casting marks in the ports while the head is apart should generate the best bang for the buck. The sky is the limit here though, so a 320cc kit with a full port job, hotter cam, and some rod and bottom end work will turn your little XR into a real beast. But is that what you really bought the XR for anyways?

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