Honda XR100

**SHELRACING** · 9th February 2011, 18:35

Originally Posted by TZ350

.

The big question is, why is Shellracing asking? ........

I was thinking of repowering my ZX6 and turning it into a bucket

**SHELRACING** · 9th February 2011, 18:39

Originally Posted by TZ350

Scraped from the net..........

RE:1971 SL125 -150cc big bore-help
(Date Posted:01/05/2008 14:10:32)

The exhaust information is basically what I sent to XLint many years ago. My link on XLint is no longer valid because I took my web site down a few years back when I had to change web carriers and haven't ever bothered to put it back up. For a 125cc race engine, you can run a 1 1/4" primary pipe. For a 145cc engine, I would recommend at least a 1 3/8" promary and you can use a 1 1/2" for the bigger 175cc engines. Leave a step at the exhaust port, do not blend the port into the exhaust pipe diameter. You CAN open up the port diameter some, but make sure that you have a step or else build an Anti-Reversion cone that will mount at the port/pipe junction. The AR cone works best, but a simple step is easier to do. The primary length in the formula is actually for the entire exhaust length, so that includes the port length which is about 2-2 1/2" long. Measure it and subtract it from the pipe length that the formula gives you.
For the reverse cone, you can just make it a simple 45 degree angle and depending on the maximum diameter of the meg, the length of the reverse cone will vary. Depending on whether you need to run a muffler (advised because the megs are really loud, I used to run them in the '70s) the outlet of the meg should be within an 1/8" of the inlet diameter. The muffler core should be larger than the meg outlet and be as long as you can fit it on the bike. The muffler outlet will help determine how quiet the bike is and the smaller it is the less noise it willl make. Also the smaller it is the more restrictive it will be and cut back some on the power. Tough choices, but you have to decide what you can get away with and how loud you want it to be.
For carbs and racing engines, there are some nice new replacement carbs on Ebay that are 28mm (fit XR200s and 185s) and even have small accelerator pumps on them. A 145cc race engine can run a 28mm carb without a problem. I ran a 28mm on my 135cc engine back in the '70s.
I have a 2-piece head I bought from Motoracer838, which he says came off a '77 XL125. I can't swear that it's the original head, etc. (perhaps Joe can chime in), but it definitely has larger valves and combustion chamber than my '71 SL125 head.
I'll check the castings on the bottom of the head and let you know.
*edit* - I just had a look - the head has 383 E-3 cast in it. Hope that helps.
Stock CB750 piston is 60mm, compared to stock SL100 bore of 50.5 mm and stock SL/XL125 bore of 56 mm.

Stock 100 & 125 stroke is 49.5 mm.

Old articles describe stroking the crank to provide 59mm stroke.

Stock bore, stroke = 122cc
750 bore, stock stroke = 140cc
stock bore, long stroke = 145cc
750 bore, long stroke = 167cc

15,400 is right at 5000fps piston speed and that is high for an older design air-cooled engine. Powroll ran their flattrack race bike (longer stroke) at 13.6k (documented)
I picked up a 24mm carb from ebay, from a TL125. This is bigger than the stock SL's 22mm, but I'm also watching for a 26mm from an XL175. We'll see.

Well, I'm using the 383 two-piece head from a '77 XL125, as I mentioned above.

From what I remember of my diggings, the early one-piece XL125 heads came with bigger valves than the one-piece SL125 heads. And the later two-piece XL125 heads came with bigger valves than that.

All the 100's (SL, CB, CL, XL) had the smaller valves like the SL125.
XL100 1000001 = 74
XL100 1100001 = 75
XL100 1200001 = 76
The SL100 had a crank with a 14mm wrist pin, the125 has a 15mm pin. You will be limited to 120cc in the 100 class. Powroll offers a bore in kit for the 100 crank that boosts the displacement to 118cc. They offer a high compression piston recommended for racing and a low compression for pump gas but you won't gain much in the way of power with the low compression piston. These bikes hold thier own with the 2 strokes because of the smooth torque and flat power band.
i have been told all kinds of things. about the CB trans ratios. i know the CB100 is the closest one, of all the little 100 and 125s. but is the CB100 the same as the CB125????? hummmmmm
According to the early Honda Factory shop manual, A:: of the early 100s had the same trans ratios and the CB125 also had the same ratios. The CD125 had only a 4 speed and the SL125s had a wider ratio box with a lower first gear and a higher 5th gear. Don't know about the CD125, but the SL125 and the 100 gears would swap if you swapped the pairs, such as 1st gear set and 3rd gear set etc...I ran the 100 1st gear set in my 125 because of the big ratio gap from 1st to 2nd gear in the SL125 box. 2nd gear was the same ratio in all the boxes.

I don't know how it's going to work but I swapped out the cb100 into a sl125/150cc kitted ahrma bike. Any pointers on this? Thanks
This will give you a fairly close ratio box without the big drop in ratios that the 125 trannies have, and a lower top gear in 5th. Should work fine for MX work. Adjust the overall gearing with the countershaft and rear sprockets and go for it.

The transmission with the ratios you want are in the CB100, CB125S, CL100, and supposedly in early SL100s. Latter SL100s were (in my experience) the same as the SL125s. I learned this the hard way. I used a transmission out of a CL100 for my Powroll SL145 stroked motor. I also used a 26mm Keihin from a Honda Hawk, with a Powroll Cam and valve springs.
For a CB100, CL100, or CB125s the number of teeth and ratios are as follows:
Mainshaft Countershaft Ratio
First 14 35 2.5
Second 18 31 1.72
Third 21 28 1.33
Fourth 24 26 1.08
Fifth 26 24 0.92

All motors have 4.055 primanry ratios 18/73
I hope this helps. If you would like stock SL125 transmissions, I have a couple of them that I would be willing to part with.
Also I was told the CB100 had closer ratio's between 1st and 2nd. You can check wth someone that has the factual ratio numbers. Also the extra power allowed me to change the sprockets sizes. In the front I went to a 16t and the rear I dropped from the 49t stock to a 40t with good results.
the SL125 has wider spread gear ratios. over the closer cb125 gears.

Wot !!!

**F5 Dave** · 9th February 2011, 19:56

Paint it charcoal grey & sit on the fence.

watching.

**gav** · 12th February 2011, 20:16

Originally Posted by kel

CHAPTER 24
24 ROAD RACING - MINIATURE
24-2-4 Engines must be derived from non-competition motorcycles. Motocross, Road Racing, Enduro and Go Kart motors and transmission parts are not permitted.

Rule states non-competiton not road legal. Id agree with Henk that the XR was a trail bike not a competition motorcycle, it was built for kids to hack around the farm on.

Well, tbh I dont think Mr Honda had that in mind when the XR100 was built.
It was more than likely built for the US market originally as the XR75, and yes was the main stay for US mini mx for many years.
Is it legal, technically, probably not, but if Global Motorsports can run their CBR150's in 150SS then you should be fine!

**NSR143** · 6th March 2011, 18:35

The XR100 IS a trail bike, if you raced it in MX all the suspension would be wasted after about 3 races. If you excluded it you would have to exclude all TS TF engined bikes. BRING IT ON... more F5 bikes would be good.

**NSR143** · 6th March 2011, 18:38

Originally Posted by SHELRACING

I was thinking of repowering my ZX6 and turning it into a bucket

I think the real move would be a FJ1200 frame with an XR100 engine!

**SHELRACING** · 7th March 2011, 12:15

Yep it's a done deal now, the bike is half built. I had to flog a few bits back off wobblers bike that I lent her. She's not too impressed at the mo. Anyway my need was greater than hers. Ha !

**Buckets4Me** · 7th March 2011, 17:22

Originally Posted by SHELRACING

Yep it's a done deal now, the bike is half built. I had to flog a few bits back off wobblers bike that I lent her. She's not too impressed at the mo. Anyway my need was greater than hers. Ha !

so who is cooking ?

will we see you sometime soon

**Fooman** · 10th March 2011, 11:12

Originally Posted by jasonu

Not so sure. In the past XR's were considered 'competition orientated' and not allowed. I would do some thorough investigations before starting an XR project.

Was following a Greater Wellington Regional Council ute on Tuesday. It had a XR100 in the back, with a rifle sling mounted on the handlebars. Obviously makes the XR a working vehicle, but what is the maximum calibre allowed in F5?

Cheers,
FM

**F5 Dave** · 10th March 2011, 15:26

ahh, mr Fooman makes his annual visit to the hallowed pages of KB. You selling house? Still have suitable wheels, there's racing Kaitoke on the 20th.

**SHELRACING** · 11th March 2011, 11:37

Originally Posted by Buckets4Me

so who is cooking ?

will we see you sometime soon

Yep Sometime, maybe soon. Maybe not so soon.

**Fooman** · 12th March 2011, 19:14

Originally Posted by F5 Dave

ahh, mr Fooman makes his annual visit to the hallowed pages of KB. You selling house? Still have suitable wheels, there's racing Kaitoke on the 20th.

No that is the neighbours somewhere up the right-of-way. Wheels are still in bits, waiting for motivation, opportunity, time and money to stick em together.

Cheers,
FM

**gav** · 12th March 2011, 20:06

Originally Posted by NSR143

The XR100 IS a trail bike, if you raced it in MX all the suspension would be wasted after about 3 races. If you excluded it you would have to exclude all TS TF engined bikes. BRING IT ON... more F5 bikes would be good.

But the Honda equivalent to a TS/TF is the XL, the Suzuki equivalent would be a DR.
Would you consider a 150cc XR200 legal to race?