Making a 250 Behave Like a 200

[trapezeartist]

There's a general consensus around these parts that the Beta 200 is the ideal bike for novices and other riders of limited ability. The trouble is, in the secondhand market, 200s are as rare as virgins in Essex. So I got to wondering what it would take to tame a 250 down to 200 level.

You can get low compression heads at modest cost. How much difference do they really make?

What about retarding the ignition? For sure it would reduce peak power to a manageable level, but would it then hurt the responsiveness or the ability to trickle around on next-to-no revs? Is it even possible with the CDI ignition?

[heffergm]

Slow throttle. Call it good. 

[nhuskys]

Tryals Shop just did a Beta 250 Evo for a customer's novice wife.  He used a slow throttle and welded a washer in the head pipe to restrict it to about 50%.  The customer is very pleased with the performance. 

Mike Komer at the Tryals Shop said he didn't adjust the jetting, when he restricted it. 

Edited December 19, 2018 by nhuskys

[guy53]

The slow throttle and a little bigger rear sprocket are in my opinion the only mod you can make without playing with carburation

Guy 

[lineaway]

 The 250  is a low compression bike in the first place, look at the Factory specs. I run 9 and 44 sprockets. Not for a low first, but for a great 2nd gear. My 3rd gear is exactly the same as a stock second, and 4th is just barely taller than the stock third. It gives you such a broad range having 4 workable gears in a section.

Edited December 19, 2018 by lineaway

[trapezeartist]

I've played with quick and slow throttles on another bike. When I was an even rawer novice than I am now, I thought the slow was good. But then I found that occasionally I wanted a good handful of throttle without needing to move my wrist too far. So I'm not a big fan, but it's a cheap mod and I would almost certainly give it a try.

Washer in the head pipe, nhuskys? Do you mean the exhaust? I would be a bit concerned about the back pressure and the effect it might have on cylinder temperatures. And I would certainly expect it to affect carburation. I had thought about a restrictor in the inlet, but that's really no different to adding a throttle stop. And a throttle stop is not much different to a slow throttle where it becomes impractical to wind it all the way round.

These are the reasons I was thinking about low compression or retarded ignition. They would hopefully tame the engine all the way through the rev range.

[nhuskys]

Email the Tryals Shop and Mike Komer will explain.  https://www.tryalsshop.com/

He said a few years of Montesa 315R models came with this washer in the head pipe (along with a restricted air box) and he used one of the Montesa washers he saved all these years in the Beta. I know my 2000 Husqvarna WR250 Enduro bike came with a conical washer in the head pipe to restrict it.  I think this was a common way back then to restrict street homologated 250 2T off road bikes to low HP for EU Learner bike HP standard at the time, which I think was 33HP max?

[turbofurball]

Slow throttle alone works a treat on my Cota, haven't had any issues giving it a good gob of extra throttle when needed

[al_orange]

Putting a thicker base gasket certainty softened my 270, as does running it a bit richer. I did use a slow throttle to start with but have now moved up to the normal throttle as it makes steps and big logs much easier. 

[trapezeartist]

3 hours ago, al_orange said:

Putting a thicker base gasket certainty softened my 270, as does running it a bit richer. I did use a slow throttle to start with but have now moved up to the normal throttle as it makes steps and big logs much easier. 

An extra base gasket is obviously similar to a low compression head. As well as affecting compression ratio, it will also increase the squish clearance which will also be good for softening power. It will raise the port heights too. I'm struggling to get my head around what that will do. The higher exhaust may reduce power as the last bit of pressure will be lost. Higher transfers might reduce low speed power and restore a bit of top end. Am I right?

Richening it up is an interesting idea. As an ex-karter, I'm naturally disposed to running as lean as possible so I'm going to have to rethink this a bit. How did you do it? Main jet or needle position?

[section swept]

4 hours ago, trapezeartist said:

I've played with quick and slow throttles on another bike. When I was an even rawer novice than I am now, I thought the slow was good. But then I found that occasionally I wanted a good handful of throttle without needing to move my wrist too far. So I'm not a big fan, but it's a cheap mod and I would almost certainly give it a try.

Washer in the head pipe, nhuskys? Do you mean the exhaust? I would be a bit concerned about the back pressure and the effect it might have on cylinder temperatures. And I would certainly expect it to affect carburation. I had thought about a restrictor in the inlet, but that's really no different to adding a throttle stop. And a throttle stop is not much different to a slow throttle where it becomes impractical to wind it all the way round.

These are the reasons I was thinking about low compression or retarded ignition. They would hopefully tame the engine all the way through the rev range.

If you ever bought an Aprilia Mille ( road bike) these were restricted when new, they had washers in the exhuast front pipes and an ecu power restriction circuit. Bloody quick in restricted mode but dealers were supposed to remove the washers and snip the ecu wire to release full power while performing the first service. A great many owners never knew about the restrictions and a great any dealers never actually removed the restiction. So a washer in the front pipe of a two stroke would certainly cause coke build up but I doubt it would cause any increase in heat around that area. Retarding the ignition is not a good idea, timing to close to TDC can cause the engine to run in reverse ..not good for a timid rider! Advancing the ignition will introduce starting issues and idling problems...again not good for the timid rider. Restricting how far the throttle slide can travel is a relatively easy thing to achieve and will prevent unwanted wheelies or tree hugging if too big a handful of revs is grabbed. There are two ways to do this restriction, one is to insert a plastic ring cut from a tube of appropriate diameter and fitted on top of the slide so that it hits the underside of the carb top, the plastic spacer must be free to move and not jam. The second way is to merely work out how much of a restiction to throttle slide movement is needed and then to drill? a small hole and screw in a self tapper point removed. Easily returned to normal by plugging the hole when the screw is removed using suitable filler. Sounds bodgey but it worked for a ‘ try a trials bike’ event that I ran for a charity do, we had no idea who was going to ride or their skill levels so had to account for the lumbering great fairground workers that were next to my display area and the fairer sex that took great delight in riding around and over obstacles. ?

[mcman56]

I spent quite a bit of time experimenting with a 250 Rev3.  (It is surprising what some people will do for entertainment.)

- Instead of welding a washer in the inlet of the exhaust pipe, I made a spacer with smaller ID that fit between cylinder and pipe so it was removable.  The pipe still cleared the fender but did not slide into the muffler quite so far.  It softened the power quite a bit but also made it bog proof.  You could use large amounts of throttle at low RPM and it would just chug along.  If you are interested I may still have this spacer.  I don't remember if I changed jetting.  

- In the US, Betas come without flywheel weights so adding one would soften power.

- I made a carb spacer with small boost bottle.  If you run the Mikuni manifold with a Keihin carb, you pick up an extra 1/2" to insert a spacer with port for boost bottle.  IIRC this produced the same bog proof low end chug power but did not soften the overall power much. 

- A 26 mm OKO carb ran the same as a 28 mm Keihin but the response was just a tiny bit softer.

- The expensive option is a 200 top end.  The Beta 200 is a small bore 250 instead of a big bore 125 like most other brands.

- Thicker base gaskets raise the ports to push the power band up in the rpm range.  This would soften the low rpm power where most trials riding is done.  It also drops the compression so would be easier to start.  The 300 super Smooth runs low compression and is surprisingly easy to kick for a 300.

 

 

[trapezeartist]

Good point about the engine running backwards, SectionSwept. My Fantic does that occasionally at the moment.

The improved chuggability (is that even a word?) of the exhaust restrictor is attractive and I would guess it would also balance out any top-end boost that might come from the higher ports. Provided there isn't a heat issue (and you guys are reducing my concerns on that score) it sounds promising.

I've just been doodling some calculations and it looks like a 1mm extra base gasket lowers the compression ratio from 9.0 to 8.0. That sounds about right. Easier starting would be a bonus too.

Costs of the two compression ratio options are closer than expected. Assuming the head has to come off to change the base gasket, and the engine will end up with two base gaskets, that's going to cost about £70. A low compression head and head gasket will cost about £85.

Whatever ends up being the solution, I fancy buying a 250 will be the way to go. Unless a good 200 lands in my lap.

[2stroke4stroke]

The somewhat obvious best way to do this would be to remove the additional flywheel weight and fit a 200 top end.  Perhaps not as expensive as you might think - I know at least one dealer keeps refurbished barrels in stock to exchange when needed.  If you can locate such a barrel for a 200 then you're made.  Or try a breaker.

[sirdabalot]

I've had 200 and 250 Evo's and found the 250 with the additional flywheel weight and semi slow throttle (QA tube dremelled to a SA tubes diameter over the 1st half of movement) more forgiving than the 200. Standard the 200 span up to quick on muddy rocks, I tried fitting the 250s additional weight but it was awful, too sleepy, like trying to accelerate through dry sand.