4-stroke VMX conversion, ideas/suggestions

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1/1/2020 2:01 PM

Hi!

I'm able to get a very cheap RM125C rolling chassis, so I thought about shoehorning in a 4 stroke engine of that era.

The only rules are that it has to be pre-85 and air cooled. I'd prefer a wetsump.

Which model would you choose?

XT500 has a large aftermarket following which seems nice. Though Hondas XL500 could work too I guess. I've ruled out Husqvarnas due to price and availability.

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1/1/2020 2:32 PM

There is a guy racing AHRMA with an XR200 motor in a RM125C chassis. He hauls ass on that thing...

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“Men, we are surrounded by the enemy. We have the greatest opportunity ever presented an army. We can attack in any direction.”
― General Anthony McAuliffe, 101st Airborne, while addressing troops at Bastogne during the Battle
of the Bulge.

1/1/2020 6:48 PM

Look at YouTube "Alan Steele xr200"
79 / 80 model, almost the same.

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1/1/2020 7:09 PM

I have been 1/2 way through such a conversion for the last couple of years, two steps forward and one step back every step of the way!

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1/1/2020 7:55 PM

Is a 250 f allowed to race 125s in vintage?

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1/2/2020 12:23 AM

To have some garagefun and challenge myself a bit.

I think the XR200 will be out of the question, as they weren't really a thing in Europe.

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1/3/2020 4:53 PM

Do not give the XR 200 the boot just yet.
I have one in a 1980 RM 125 frame. Hi comp piston, mid range cam and oversized exhaust and it is more fun than most of my bikes.
The 200 engine is easy to find over here and since you should be building back up from the cases upward, start with a good bottom end and then source the rest. Easier to ship over and then you would also know the history of the engine.
I am 6’4” and 225 and the bike never feels underpowered.
Oh, and it is much lighter than my HL 500.

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1/3/2020 6:26 PM

RF145 wrote:

Do not give the XR 200 the boot just yet.
I have one in a 1980 RM 125 frame. Hi comp piston, mid range cam and oversized ...more

How difficult was the fab work to mount the engine ?

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1/4/2020 3:50 AM

RF145 wrote:

Do not give the XR 200 the boot just yet.
I have one in a 1980 RM 125 frame. Hi comp piston, mid range cam and oversized ...more

ATKpilot99 wrote:

How difficult was the fab work to mount the engine ?

I was very lucky, we had a fellow racer who was a welder/fab guy by trade who loved to build XR 200 builds. He felt the RM was not that bad. He also built Husky/Ossa/Montesa pre-75 bikes for others so we could race the Pre-75 class at ACR events (www.acrmx.com).
He also built a 1980 XR200.
I will post a link to a build of another 1980 XR200 made to
look like a mini HL 500 by a regular joe who’s only advanced skill is basic welding. It is a step by step instruction thread if you want to build your own.
I should be able to post it by tonight.

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1/5/2020 6:55 AM

Hi again!

I'm on board with putting in something similar to the XR200. I tried fitting in an XT500, but it was just way too big for the frame.

Over here, XL185 engines seem plentiful compared to the XR200. Can I make the 185 run like a XR200.

I'd need to overhaul the engine anyway, so boring out a cylinder and changing cams is not going to matter that much.

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1/5/2020 7:12 AM

PhotoPhoto

Here's a couple of pictures of what I'm starting with.
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1/5/2020 6:16 PM

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1/6/2020 2:13 AM
Edited Date/Time: 1/6/2020 2:21 AM

Lasse wrote: PhotoPhoto

Here's a couple of pictures of what I'm starting with.

If you go 185 / 200 the first thing is to replace the lower cradle, having said that, a better option would be to replace the lower cradle but also extend them most of the way up and connect to the single spar so your newly fabricated exhaust can exit between the two cradles.
From there the engine will bolt in with fabricated and welded mounts.
The kick start action will be the biggest head ache.

As I said two steps forward and one step back!

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1/6/2020 5:16 AM

inthebadboycorner wrote:

If you go 185 / 200 the first thing is to replace the lower cradle, having said that, a better option would be to replace the ...more

The XL engines have a slight sideport, so if I make two pipes as a cradle all the way up, I will have clearance issues.

But yes, making a new cradle will be a neccesity.

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1/6/2020 10:02 AM

If you position the engine level in the frame, the slight side port exhaust will hit the single down tube frame, the down tube must be relieved by 1/2 diameter

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1/6/2020 10:05 AM

inthebadboycorner wrote:

If you position the engine level in the frame, the slight side port exhaust will hit the single down tube frame, the down tube ...more

Do you have pictures or anything of a similar conversion? I'd like to see what I'm getting myself in to.

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1/6/2020 10:18 AM
Edited Date/Time: 1/6/2020 10:20 AM

If you position the engine level and as back in the frame as to have least posible countershaft sprocket distance the slight side port exhaust will hit the single down tube frame, the down tube must be relieved by almost 1/2 it's diameter,

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1/6/2020 10:20 AM

Lasse wrote:

Do you have pictures or anything of a similar conversion? I'd like to see what I'm getting myself in to.

Send me an email rf145 at aol dot com and I will send you pics of my 1980 RM with a XR 200 engine.

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1/6/2020 10:22 AM

inthebadboycorner wrote:

If you position the engine level in the frame, the slight side port exhaust will hit the single down tube frame, the down tube ...more

Lasse wrote:

Do you have pictures or anything of a similar conversion? I'd like to see what I'm getting myself in to.

Tonight I will get some pictures.
Did you look at Alan Steele YouTube site?

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1/6/2020 10:29 AM
Edited Date/Time: 1/6/2020 10:51 AM

inthebadboycorner wrote:

Tonight I will get some pictures.
Did you look at Alan Steele YouTube site?

I missed that suggestion, but I'll make a note to do it tonight.

Do you know the height of the engine? I'd consider an SP370, if they aren't too far off.

EDIT:

Checked Alan's YouTube, great stuff, perfect for what I want!

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1/6/2020 11:38 PM

Lasse wrote:

Do you have pictures or anything of a similar conversion? I'd like to see what I'm getting myself in to.

inthebadboycorner wrote:

Tonight I will get some pictures.
Did you look at Alan Steele YouTube site?

Lasse wrote:

I missed that suggestion, but I'll make a note to do it tonight.

Do you know the height of the engine? I'd consider an SP370, ...more

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1/6/2020 11:41 PM
Edited Date/Time: 1/6/2020 11:52 PM

Photo

An xl185, hi comp, big bore, stroker, head work, big cam, big exhaust, pumper carb with a light flywheel should have enough go for a 59 year old squid.
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1/8/2020 12:49 AM

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Here's an absobloodylutely lovely 79 RM125 chassis with 99 XR200 engine - I think I snaffled these pictures from this site.

A big advantage of an XR200 engine over the XL185 is the 6 speed gearbox.

I ran a much modified XL engine for quite a few years, until I got a 6 spd XR. That extra gear, made it So Much Better.

I'm currently just keeping an eye out for an 'as good as I can get 230' engine, for the E Start. Just because my leg doesn't even bend to 90 degrees, nowadays. I also had to run an MVX 250 (Honda 250 2stroke 'V' Triple - look it up!) Kick Start lever, which was hard to deal with, on the 89 CR Chassis I last used it on, and, which necessitated my making an ATK style rearward facing rear brake lever and master cylinder set up.

I'll transfer my top end to the ES'd cases, with it's Big Fins I put on, Oil cooler set up (with high capacity TLR250 Oil Pump - the body and pump gears are about 2 / 3mm wider than the early 200 engines - later models may be near that, as I've seen a few different Oil Pump body and gear part numbers in later model parts fiches) , and TLR 250 barrel. All those fins make the unknowing think it's a much bigger capacity engine whistling With a stroker crank, machined heavily for lightness, plus the flywheel machined right down, the bigger inlet valve, porting, 32mm carb and Megacycle 'Curtis Sparks Flat Track' spec cam, it revved to the moon (despite the 66+mm(?) long stroke), and dyno'd out to a whopping 28/29HP. w00t I tried to run it as a 249cc, but it became prone to seizures with such near paper thin cylinder walls, so sleeved the TLR barrel back to the 200 sized piston.

That was enough to consistently beat blokes on all sorts of XRS Only 280s and 630s, and the (inevitable) wankers on 4t Huskies. Though, I had to ride so, so bloody hard to win, or place, and I'm very far from that level of riding / risk taking now.

It was a fantastic little bike, in both chassis I ran it in, a real 'sleeper', but, with that massive (I'm being facetious, boys and girls ) power output (well I think a std 200 had around 13 / 14 HP) , it started snapping the main shaft, and counter shaft, plus, exploding the 3/4 (I think that was it) gear combination. Long ago I had the gears and both shafts made by a specialist manufacturer, but it's all sitting in the cupboard / shelf that this computer is on, with the 83 RM125D chassis, and the 89 CR125 chassis I mentioned previously, both hanging from the workshop wall. All waiting for me to put either combo together. There's a chance of that, what with EVO racing and VMEnduro being pretty big now , but I've got so much else to do in my semi retirement dizzy .

A Key thing with engine placement, is the Countershaft position. Replicate the position of the std engine / countershaft!

Not doing so Will stuff up handling / the suspension. I know this, as I first buggered it up with other conversions in my youth. By having it too high, because I left a std cradle in place. And, even that beautiful bike I put pictures up of, above, looks to have that serious mistake - look how high the CS center is, with relation to the Swing Arm pivot, and the rear axle.

Even something as mildy powered as a 'worked' 200 engine, with the countershaft too high, will have so much pull down effect on the rear end ( magnified by the high revs you'll be constantly using because it's so under powered) that it will not work as well as it could do, if you positioned the engine correctly.
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1/8/2020 1:42 AM

Wow, that is great information!

I bought a damage engine, so I will aim for getting a 6 speed gearbox and a CB100 ignition.

Are there any sensible engine mods, which will still retain some reliability. I don't want to go all out , neither do I want 13/14 hp.

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1/8/2020 3:22 AM

How do you plan to make the CB100 ignition work?

I looked into it although I eventually gave up and got an xr185 flywheel which looks to be maybe 20% lighter than stock.

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1/8/2020 5:31 AM

inthebadboycorner wrote:

How do you plan to make the CB100 ignition work?

I looked into it although I eventually gave up and got an xr185 flywheel ...more

No clue, the sites I've read says its a direct fit.

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1/8/2020 8:32 AM

I think the 185/200/230 engines started as a CB100 type engine in the late 60s / early 70s.

There is so much cross over of parts it would boggle your mind.

Along with the much copied horizontal 'Cub' engine ( which, in 2017 reached a benchmark of 100 Million made - the highest production of any vehicles series, ever), in all it's sizes, and the 70cc through to 110 / 150(?) engines, and this basic engine, I reckon those few engine modules have made for a significant part of Hondas profits, and their 'bank balance' to do so much of their development of the rest of their bikes, cars, and their racing.

Even with the change over to the 'new' 230 EFI engine in CR-fs, which I think is made mostly in Hondas vast Brazilian factory, but probably also in a bundle of their other plants around the World, the older 100/125/150/185 / 200/ 230 engine is still used in many, many thousands of bikes Honda produce. Quite amazing, really. It's a heck of an ROI.

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1/8/2020 12:05 PM
Edited Date/Time: 1/9/2020 8:11 AM

I came close to buying a '78 TT500 this fall for 800. The consensus is to use a late seventies Husky frame to do a proper HL500 clone or race bike. That said it'll still weigh in at over 250 all tricked out. Recently a deal came up from a local junkyard on a roached out alu. tanked M250C so I got that instead and will be way ahead in ease of build. Anyhow I now have so many frame up projects to do and am choosing the ones with least resistance to completion. That rules out 4T for me other than my '99 YZ400.

Good luck on your 4 stroke project though, they must be a blast to ride !

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78 RM250C
79 YZ250F
81 YZ465H
80 MX6 125
81 MX6B 250



1/8/2020 1:09 PM

Bearuno wrote: Photo
Photo
Photo

Here's an absobloodylutely lovely 79 RM125 chassis with 99 XR200 engine - I think I snaffled these pictures from this ...more

That is Alan's build. It rips----and so does he. Great guy and lifetime mx'er. 💪👈

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“Men, we are surrounded by the enemy. We have the greatest opportunity ever presented an army. We can attack in any direction.”
― General Anthony McAuliffe, 101st Airborne, while addressing troops at Bastogne during the Battle
of the Bulge.

1/8/2020 2:22 PM

inthebadboycorner wrote:

How do you plan to make the CB100 ignition work?

I looked into it although I eventually gave up and got an xr185 flywheel ...more

Lasse wrote:

No clue, the sites I've read says its a direct fit.

If you ever figure it out please post it, yes the rotor bolts on, from there is not so easy.
Lots of talk but have yet to come across an actual completed ignition.

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