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Old 04-13-2014, 08:40 PM
Techbiker Techbiker is offline
Motorized Bicycle Elite Member
Join Date: Oct 2009
Location: DFW, Texas
Posts: 165
Default Found a Sweet Engine, What Now?

After building HS 50cc, a HT 70 and a GT50R CAG bikes, I have decided that I am ready for something a bit more reliable, powerful, and comfortable (but still a motorized bike).

For me, these requirements mean finding an engine that includes true power generation abilities and is shift-able for comfortable mid-thirties cruising. I thought that a high performance 4-stroke would give me what I was looking for.

Anyway these previous engines had many upsides and drawbacks. The HS is quite reliable, however it has virtually no charging (for lights) or shifting options and power is not exciting. The Happy Time was reliable but so poorly built that horrible vibrations hurt long road-trip prospects. External charging options are better but still limited. Finally, the GT50R is incredible power-wise and is as smooth as silk. Unfortunately, no real external charging or shifting options exist. I also can't help destroying clutches on the GT50.

I spent a long time perusing Ebay and Alibaba for an engine without too much success. I almost settled on the CG125 engine, however I could only find a couple in the states due to overburdensome EPA regulations. I was also concerned that 125cc and around 11 hp would be too much for a bike frame. The smaller Honda clones are typically horizontal engines and may have trouble fitting almost any frame.

I called up and they gave me a ton of very helpful engine suggestions. We decided that I should consider engines from small Enduro dirt bikes. After quite a bit of searching, I realized that the XR80 might be the perfect engine for my goals. With 110 watts of power generating capabilities, it can drive a fuel injection setup, a headlight and a tail light! This engine also has 5 close-ratio gears, a light crankshaft, kick-start, and a 9.7:1 compression ratio. Topping off the engine package is the fact that it produces ~5hp and weighs no more than ~45lbs. Finally, I have heard that the XR80 is incredibly reliable.

At 10.5" wide, the XR80 engine is narrower than my HS50 with Grubee 4G.

$315 netted me a clean real Honda XR80 engine with 180 psi compression! It should include a pre-wound stator to help charge a motorcycle headlight since I got it off an Enduro bike. The seller also gave me his Big Gun exhaust for only $80. Finally, I picked up a service manual, wiring harness, and carb.

Here's the rub though. I could not find information on the height of this engine online. It turns out that the motor is between 14" and 15" tall, probably due to the OHC design.

What should I do? Should I try to find/ modify a frame to fit this engine or should I try to source another type of engine? I do not mind adding a drop loop to a frame if it isn't too much money.

I feel like the Felt Bixby Cruiser is a close fit, however I do not have any specs on the space inside the frame. The benefit of the XR80 is that the jug does not look enormous (only 10cc larger than a HT 70).

Note that the engine mounts are in the front and rear of the engine. I should not have to support the engine from the bottom.

Thanks very much. Pictures attached.
Attached Images
File Type: jpg 2014-04-12 23.51.16small.jpg (702.8 KB, 388 views)
File Type: jpg 2014-04-12 23.51.37small.jpg (705.5 KB, 115 views)
File Type: jpg 2014-04-12 23.51.43small.jpg (679.6 KB, 101 views)

Last edited by Techbiker; 04-13-2014 at 08:47 PM.
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