Trials Bike Powerband for an 85cc Class Bike???

[MACE]

I'm looking for a bike to move my daughter up from an XR70. I'd like something very light with good suspension and brakes like an 85 mx but with easy power like one of the entry level diesels (TTR125, etc).

Is it possible to modify an 85cc engine to have a very linear powerband for $500 or less? What about a budget of $750?

I'm thinking welding a head pipe extension on the pipe, deck the cylinder base and modify the head to lower the ports - as part of an EG port job and maybe a smaller carb and a flywheel weight. Noise is an issue so I would adapt a large volume silencer and SA.

I'm thinking this would be a reasonable task because 85s are pretty cheap used and I could build something fun for at least this daughter and would be passed down to the next one when she is ready.

 

[David Trustrum]

In which case maybe a TT125 is in order? They are small & if 2 youngsters will get use of it perhaps it is the economic way when you consider that old 85s will require more maintenance & a rebuild at the min.

If you could stretch to a GG Pampera which does have a trials engine that would be cool. Right. . .

The 85 would be an interesting project though. I think the biggest change you would have to make is the pipe. Just adding to the header pipe won’t do enough to remove the power 'hit'. You really need to design a new pipe with far more shallow diffuser & baffle cones. Combined with the other mods it may work out ok, but the large ex port will always be a compromise.

Unless you could find an old Suzuki DS80 & graft the engine in the newer bike until she was ready for the RM? the crankcases were similar to old RMs, but that was generations ago.

 

[Patman]

MACE, that's prob going to be similar to what I'll be looking to do in about another year. The current engine configuration in my son's XR80 is AWESOME but the chassis will be lacking in about 12 months or less. I suspect we'll either go with a CR85 and try to "trialize" the power or swap the engine for a XR/CRF based power plant similar to the 95cc setup Rich & Eric built for his current ride but in the 125cc range.

 

[MACE]

You know, after spending some time on the mini-racer section of DRN, I think what I really need (I mean what my daughter really needs...) is a KX100. Put on a flywheel and an FMF Q and we should be good to go for several years.

 

[cujet]

of course you could always sleeve the cylinder and use very mild port timing. This would get you exactly the mild power you seem to be looking for. I would guess the sleeve would cost far less than $500. A throttle stop might be in order if ports are really small.

Chris

 

[dbrace]

How about a rag in the airbox? ;)

 

[snb73]

Chris,

My son started on an XR-100 when he was 9. At 12 years old he was ready for a bigger/more advanced bike. He would only get one summer out of an 80/85, and a 125 would be too big. I read lots of reveiws and settled on the KX-100. I was looking for something that would grow with him. The bikes power and suspension are great.

The power is smooth, with good low end for the beginning 2-stroke rider. As your daughters skill increases, she just needs to twist the throttle for more power. The little bike can scream.

The suspension is great too. My son can take every jump I do on my RM-250.

Depending on how big she gets (height) the bike could last her for years to come. I've read that most women in the 5'2"-5'8" range really like the bike for it's ergonomics, light weight and smooth power delivery. My son's 2003 only weighs 150 pounds.

I think you (your daughter) would be happy with the KX-100.

Hope this helps, Steve.

 

[fishhead]

Mace,

The best 85 for your use is probably the kx85. The motor makes the best power down low of the 85's. I have a similar challege but I am going the cr80 route with a 100cc kit and low end porting with a flywheel. My son is whacking the suspension of the 70 real good since we put the 82 cc piston in it. You can turn a kx 85 into a 100 with a few parts from the parts bin. I'm lining up a cr80 expert set of wheels and swing arm so he won't outgrow ithe cr. I'll keep you posted how it goes. Ideally a ttr125 motor in the cr80/85 frame would be hot setup but I don't want to buy two bikes to make one.

I really considered the xr's and ttr125 but the way he rides I'm sure he will find the suspension limits in short order.