The Bucket Foundry

[Frits Overmars]

My experience with an engine water dyno is that the dyno load curve had a lower slope than the engine torque curve leading to exactly this type of instability - it either runs away or stalls.

Same here Neels: https://www.kiwibiker.co.nz/forums/s...post1130883533

[Flettner]

Cheers, Daryl.

Typical 125 carb, 38?
Amount needed to adjust cylinder from closed to full open 13mm , approx three to one advantage
I am ultimately going to get the ECU to do the job via a small servo or a ballscrew.
No combustion pressure is exerted on the servo/ballscrew apart from ring/piston friction and that seems to be in the downward direction.
But yes it may not be as easy as I think

[Frits Overmars]

No combustion pressure is exerted on the servo/ballscrew apart from ring/piston friction and that seems to be in the downward direction.

It is, so you won't even need a 'throttle-closing' spring. But an ignition cut-out provision would be nice, just in case....
If you use one of those Honda 'pull open/pull shut' throttles, you could use the shut-cable for this cut-out, so you would not need to take your hand off the bar
(always awkward when a throttle sticks WOT).

I once used one of those pull-pull throttles to operate a valve lifter on an ice speedway bike. These have no brakes at all, but when lifting the exhaust valve 1 mm,
this bike would really throw an anchor, compared to the competition. It would have given the rider a big advantage, but he deemed it too dangerous, with all those
other guys with huge spikes and no brakes right behind him. Oh well, it probably would have been forbidden anyway...

[husaberg]

It is, so you won't even need a 'throttle-closing' spring. But an ignition cut-out provision would be nice, just in case....
If you use one of those Honda 'pull open/pull shut' throttles, you could use the shut-cable for this cut-out, so you would not need to take your hand off the bar
(always awkward when a throttle sticks WOT).

I once used one of those pull-pull throttles to operate a valve lifter on an ice speedway bike. These have no brakes at all, but when lifting the exhaust valve 1 mm,
this bike would really throw an anchor, compared to the competition. It would have given the rider a big advantage, but he deemed it too dangerous, with all those
other guys with huge spikes and no brakes right behind him. Oh well, it probably would have been forbidden anyway...

Decomp valves were comonly used in US flat track and trails bikes in the 60's.
The push pull throttle is a very intersting twist on it though.
It would be nice though if that bleed of energy could be used effectively.

[Frits Overmars]

Decomp valves were comonly used in US flat track and trails bikes in the 60's.

Are they still being used? And if not, why not?

[Pursang]

Are they still being used? And if not, why not?

Here is AMA Pro Flat Track racer Joe Kopp, winning the Bultaco Astro Cup in 2015, a "Point of View" indication of the value of the decompressor, at least in that style of racing.



Joe Kopp was AMA Grand Champion in 2000. In the Final he passes and beats 7x Grand National Champion Chris Carr.

Wonderful that such top level riders enjoy racing Old 2T Astros.

Cheers, Daryl.

[Pursang]

Are they still being used? And if not, why not?

In the early 70's a very influential guide for dirt racers was Gary Bailey's "How to Win Motocross".

A whole section, derided the decompressor, because motocross requires maximum Acceleration or maximum Braking & nothing in between.

Flat track, on the other hand, requires maintaining maximum Speed, throughout the corner, without loosing rear wheel traction by excessive acceleration or braking.

As my Old Mate would say...You don't need brakes...they just slow you down.

Cheers, Daryl.

[Michael Moore]

I had a compression release on my 1971 TS185 Suzuki. It made a wonderful flatulence sound, but the brakes did a better job of slowing it down.

[Grumph]

Are they still being used? And if not, why not?

They were used in the era when real brakes were banned. The game changed when brakes were allowed - from memory, at the same time as the 750 OHV engines came in.
I have sympathy for your ice racing fellow, I've seen an attempt to run true speedway bikes - brakeless - with flattrack bikes which are braked.
The red flag went out after one lap as it was immediately apparent it was far too dangerous.

[F5 Dave]

I had a compression release on my 1971 TS185 Suzuki. It made a wonderful flatulence sound, but the brakes did a better job of slowing it down.

. . . barely

[Ocean1]

I use the decomp on my 525 EXC all the time, it's good at controlling the back end going into corners on loose stuff, where the back brake is more or less useless.

Also on extreme descents, helps manage speed and helps keep the back from getting too far out of line.

[F5 Dave]

I'd like a robust thumb brake for when legs are flailing (part of my dirt riding style) and as a poor man's traction control in the mud.

[peewee]

I was thinking the xr series as well, they were pretty much unchanged for a big number of years, by if 5 speed is his thing a CRF450 might be a little more current they are based n the CR250/500 gearbox.
if he was using the horozontal split i would go for a VT250 gearbox beefy gears and also used on MVX and CBR250's obnly thing is they have a planetry sector mechanism
i can't imagine any of these motors would wear out a gearbox before totally killing an engine

the cr500 had probly the finest gearbox of any dirt bike imo. it shifted very well and near perfect spaced gaps in the late 80's-early 90's. after that they increased the 4th-5th gaps which i didn't really like but mechanically it still functioned well. the problem is, that bike hasn't been made for 15yrs and the spare parts have started to dry up. if you could make one gearbox last a lifetime then you would be fine but if something needed replaced you might be in trouble. if you could find a used 6spd gear box from the newer ktm dirtbike it might be worth a shot. atleast youll have extra parts for a good long time

[Flettner]

the cr500 had probly the finest gearbox of any dirt bike imo. it shifted very well and near perfect spaced gaps in the late 80's-early 90's. after that they increased the 4th-5th gaps which i didn't really like but mechanically it still functioned well. the problem is, that bike hasn't been made for 15yrs and the spare parts have started to dry up. if you could make one gearbox last a lifetime then you would be fine but if something needed replaced you might be in trouble. if you could find a used 6spd gear box from the newer ktm dirtbike it might be worth a shot. atleast youll have extra parts for a good long time

I guess having to make the odd gear from time to time as a spare part is not the end of the world.
The YZ 250 twostroke gearbox is an excellent gearbox, also good in that you can buy new YZ X gear clusters, wide ratio.
But YZ twostroke gearboxes are hard to find. YZF gearboxes are different, I've already cast a few sets of cases to suit this 250 twostroke gearbox and I'm not about to bin them because of this. I have investigated gearboxes out of USA but by the time you add freight they cost almost a new one from Blackwood Yamaha.
No problem I'll just keep an eye open for another gearbox, I've found one at the Chatham Islands of all places just have to get it sent out in a box with some crayfish.
The new 125 RV cases have a common Kawasaki trail bike gearbox, robust enough with a nice wide ratio just what I want with this engines power spread.
I couldn't use a KTM gearbox if you paid me.

[husaberg]

the cr500 had probly the finest gearbox of any dirt bike imo. it shifted very well and near perfect spaced gaps in the late 80's-early 90's. after that they increased the 4th-5th gaps which i didn't really like but mechanically it still functioned well. the problem is, that bike hasn't been made for 15yrs and the spare parts have started to dry up. if you could make one gearbox last a lifetime then you would be fine but if something needed replaced you might be in trouble. if you could find a used 6spd gear box from the newer ktm dirtbike it might be worth a shot. atleast youll have extra parts for a good long time

The CR box is hamstrung with its 5 speeds, ESP in the tight going, if it is geared for first to be useful in tight woods sections it loses too much top speed on the open. (I hate getting blown away by smaller four strokes with 6 speeds)
6 speed imo is far more verstile
PS i have a Cr500 and a HRC supplied wide ratio transmision (Honda of Holland came up with it i think, for the ISDE) i have never bothered to fit it, as it is escentually the same as the later model gearbox. (Which i have anyway)
The fax from HRC that came with it, is rather cool though.

Here is AMA Pro Flat Track racer Joe Kopp, winning the Bultaco Astro Cup in 2015, a "Point of View" indication of the value of the decompressor, at least in that style of racing.

Joe Kopp was AMA Grand Champion in 2000. In the Final he passes and beats 7x Grand National Champion Chris Carr.

Wonderful that such top level riders enjoy racing Old 2T Astros.

Cheers, Daryl.

My Husaberg has a manual decomp on the left ktm and husky used the same lever control, I must say i have never used it.........
Either this a thumb throttle off an atv mounted upside down on the left side would be a neater install than the pro/am AMA set up.

Are they still being used? And if not, why not?

I know not.
To tell you the truth I am not even sure if they still use the 2 strokes for the Juniors/Am's
They were used back in the Bultacco/OSSA/Montessa days
pretty sure Bultacco did a production flat track racer that was the model to have. (Looks like pursang has posted it above) Astros
Plenty of old villiers trials bikes had the decompression valve fitted for downhill sections.