Formula 5 question?

[nudemetalz]

Heya,

Yes I do still look at the buckets even though I don't do it anymore...

Got a question...has anyone done a competitive four-stroke F5 bike?
I assume a good base would be an XL-100S (as the limit is 104cc) and ...ahem,..XR-100 parts are fairly easy to find (I would say they're identical internally).

Just,...ummm....curious...

[richban]

Heya,

Yes I do still look at the buckets even though I don't do it anymore...

Got a question...has anyone done a competitive four-stroke F5 bike?
I assume a good base would be an XL-100S (as the limit is 104cc) and ...ahem,..XR-100 parts are fairly easy to find (I would say they're identical internally).

Just,...ummm....curious...

Yeah I have been thinking about that one. CBR125 sleeved down or stroked one or the other. 20mm throttle body. Reving to 15k. That would do the trick.

[SHELRACING]

Yep, I have an XR100 based bucket, and it's for sale.

It's not quite finished and needs work, but most of work is done. I'll post up a pic soon if anyone's interested PM me

BTW : Avalon Biddles bike was an XR 100 I think in an RG frame, and it is/was a great little bike

Steve

[nudemetalz]

How competitive is an XR100 engine next to some of those hot 50's ?
I've heard of 16hp out of the RG's and and XR100 stock is around 9-10hp.

[F5 Dave]

. . . .
I've heard of 16hp out of the RG's . . .

[nudemetalz]

oiii..Dave, I said I only heard.....

We know yours is only 6hp.....

[Grumph]

How competitive is an XR100 engine next to some of those hot 50's ?
I've heard of 16hp out of the RG's and and XR100 stock is around 9-10hp.

Fit pedals.....

[Ivan]

Easy to work up the parts all interchange from 100 to 200 and if you use the XL/XR 100 bottom end and cylinder put on a XR200 head you have bigger valves and ports use the XR200 carb even bigger carb build a better exhaust system my mate used to build lots of these motors

[gav]

F5 four stroke is limited to a 20mm carb though ...

F5 4 stroke engines over 53cc are restricted to carburation
equivalent to a single 20mm carburettor.

Might want to check, think a XR100 might run a 22mm carb though?

[husaberg]

Easy to work up the parts all interchange from 100 to 200 and if you use the XL/XR 100 bottom end and cylinder put on a XR200 head you have bigger valves and ports use the XR200 carb even bigger carb build a better exhaust system my mate used to build lots of these motors

Sorry Ivan the Xl100s and the XR100 are a different beast from the normal cb100 xl100 125s and 185 200 etc.


The motor is based on the cb50 XR75 80 model engines.

I think gav may be right I think some were 22 mm carbs and I think some were 20mm Maybe the XL was.

the Xl100S is a points ignition and has (I am lead to believe) a different crank taper from the XR100 though the XR100 which is CDI.
Most of the rest of the engine is similar. The motors are lazy sparkers like a pitbike. They are more of a revver than the old style 100 motors. They do have hop up potential and are lighter than the cb100 Xl100 etc.
the head has a cam carrier bolt on inside the head cover and there is a lot of Cams etc and parts available from the states.

They are a 53mm x 45mm Motor with a 5 speed.
I was always wanted to see if the MB50 6 speed fits (I bet it does) as they in std form share the exact same ratio as a 5 speed MB5 MB100. Which is to large a coincidence.

Stuff that's not legal but bling bling.

http://www.mx-performance.com/id53.html

[nudemetalz]

Fascinating info, Husaberg. Thanks for sharing.

Back in 1985 I used to ride an XL100S (1980 model), with a nice booming Supertrap on it. Used to rev nicely to 11,000rpm (schoolboys have no mercy of course ). Always thought one day I'll get another and see what I can do with it.

[nudemetalz]

Been doing a bit more research lately.

The XL100S engine is 53mm x 45mm (as Husaberg points out)
If you can get an XL75 engine (same as an XR I believe) the bore/stroke is 48mm x 41.4mm. If it can be resleeved to 56.5mm (most 125 4T SOHC Honda's are a 56.5mm piston) you get a capacity of 104cc.

You can get some serious revs without excessive piston speed - ie 14,000rpm still under 20m/s.
Wondering though, even with a hot cam, piston compression etc if the 20mm carb restriction would negate any advantage?

[F5 Dave]

Main bit of cost would be the cam. you won't find a cam profile suited to doing mondo revs for that engine & developing one will be a little trial & error & expensive.

[nudemetalz]

Interesting, thanks Dave.

I guess not having a DOHC head like a CBR150 would make it not quite as easily tunable.

[F5 Dave]

Well just if you are building an engine to do stella more revs than std you need cam timings that will match.


Hey Nudz, why not just find an RG50? I hear they go alright & are light & cheap.