Tuning a CR80 (2000)

[reedycsu]

I just bought my wife a CR80. She is so pumped to start riding with me. The bike has plenty of power for her 5'3 125, but the only problem is the power band hits way to hard up top and is almost non-existent on the low end. I live in colorodo and we primarily run trails. Yes I know the the CR80 is probly the worst bike you can hit the trails with, but its a learner bike. If I could afford to buy her a KTM 400EXC I would, but hell I tear the trails up on my kx250 So I know its doable. Anyways, any advice on how to smooth out the power band and bring some power to the low end would be much appreciated.

 

[adam728]

I tear the trails up on my kx250 So I know its doable.

Huge difference between power spreads between the 250 and the 80 (but you know that). Just because they are both motocrossers doesn't mean they are comparible beyond that point.

There isn't too much that can be done to make the little 80 have low end. You can improve it some, but it will still be weak.

A FMF SST pipe or Pro Circuit pipe are both designed to help a little in the low to mid range power. You could also add a torque ring to effectively lengthen the headpipe, which would shift the power band down a few hundred rpm in the rev range. It might come on 200 rpm sooner, but it will run out of juice 200 rpm sooner as well.

Since it's a trail bike you need a spark arrestor, so throw a FMF Turbine Core II or Pro Circuit Type 296 silencer on there as well. I don't know if either will really help low end power, but they keep you legal, which matters.

FMF also makes torque reeds that should help with low end a little bit. A reed spacer helps some bikes with low-mid power by adding volume to the crankcase. I don't know if anyone makes one for that bike, or if it would even help.

Lets not forget about a flywheel weight. Stealthy makes a 10 oz for that bike. It won't add any power, but it will smooth out what's there, plus make the bike harder to stall out. A flywheel weight gives the "feel" of more low end power.

After that it's engine work. Cutting the base of the cylinder down will lower the ports and shift the power band down in the rpm range. Like the longer head pipe you'll gain down low and lose up top. Cutting the cylinder will require the head be cut/shaped to keep the squish band and compression in check. Since you are in Colorado and at a fairly high altitude (I assume) you can actually run higher compression without detonation. This is a good thing, because higher compression means more low end power potential.


Even with all the above mods the 80 won't have any real "low" end to speak of. But the power will be shifted into a much more usable range.

 

[4unner4life]

Even if you do all Adam728 said you will still be lacking some low end, so teach her how to feather that clutch. I had an unmodded cr80 and got though some tough trails with some clutch work.

 

[Rcannon]

You can buy or build a spacer for the exhaust pipe. Something 5mm thick will help a lot. I just copied the design from these. http://www.mxbonz.com/powrplate.htm

The mod is worthwhile and a lot cheaper than a pipe. Milling the head helped some as well.

Still, the bike is an 85. Before I bought a pipe I would look at a 90-100cc big bore kit.

 

[reedycsu]

Thanks guys, I bought it used and it already has the FMF pipe and many other mods to it. I think that teaching her to control the clucth and throttle are the best bet. She has got like 2.5 hours on a dirt bike total in her life and she was going up some insane climbs. In fact I had to stop her before she killed herself. My main concern was just improving the heavy bog you get when trying to take off in the middle of a hill. Seat time is the best modification. Any pointers on sproket size? Will the 10oz fww really work, I keep hearing about people putting them on bikes like my kx250 to tame the low end. So would it improve low end on her cr80?

 

[adam728]

It tames the hit, at any rpm. It doesn't add or take away power. It makes a bike "feel" like it has more lowend because you have more mass spinning. This means that if you give it a little too much clutch too soon there is more stored energy in the spinning flywheel to get the rear wheel moving, where as without it the engine will be slowed down at a greater rate.

If it has an FMF Fatty pipe it probably has less low end than stock.

Proper jetting will make a big difference as well, and should help cure any bog that it has.

 

[husabutt]

The only thing I could add is to try a one tooth smaller countershaft sprocket and even in conjunction with one tooth off the back. That is what we are running on our YZ85 for off road use.