Vintage Dirt Bike Q & A

Finally got my cr250 -82 running, after replacing all the shiftforks and all gears.
It runs really well, but dont seems to be much midrange power. I would like the powerband to be a little smoother, if possible.
Is the difference on switching to boyesen reeds noticeable? Or do you have any other suggestions?
The engine is all stock with all new bearings.
/Mikael

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KX250F -09, CR250R -82

the boyesen dual stage reeds will give you a bit in the midrange, but don´t expect too much.
the reeds, a reedspacer and a longer bigger muffler will help you a bit but won´t switch it into an endurobike.
don´t know if there´s a pipe avaliable that´s desingned to give more power in the mid.

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80 CR250Ra
80 CR250Ra
78 CR250R
80 CR80
82 Bultaco Sherpa 340
78 Bultaco Pursang 250 MK12
76 Bultaco Frontera 370 MK10
76 Bultaco Alpina 250
73 Bultaco Lobito 125 MK6
76 Suzuki RM125A
90 Kawasaki KX250

Thanks b-joe.
No I am not expecting that much, but any improvements would be nice.
What thickness should the carb spacer be, something like 5-10mm?

The feeling of the engine when riding, is that it has higher compression than a modern 2-stroke.
Dont know if thats the case tho.

Not looking for enduro power, still need the topend power on the mx track.

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KX250F -09, CR250R -82

I don't know if this will help or not, but a few weeks ago Jeff at Moto Tassinari hooked me up with a vForce3 reed block for my '82 RM250 (V3110-87). They sell it as a replacement for an LTR250 quad motor, but it fits my cylinder perfectly. I haven't installed it yet, but double the reed area - and half the reed travel - probably won't make anything worse.

I only mention this because they may also have something that fits your bike, even though it's not shown in the application guide.

I tried Boyesens on the RM, and I actually preferred the low end response of the more flexible stock reeds. I wasn't looking for more power, only a improvement in the sluggish-then-OMG! stock powerband.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Thanks rayivers.
Probably start with a 3mm reedspacer and maybe the boyesen reeds.
Have you rode it with the vForce3 reed block?
Me neither is looking for more power really, it just feels a bit harsh on bottom then not much i the middle, but pulls quite good on top.
Would like to get bottom to mid little smoother if possible.
I do realize it`s an old bike, but always fun to try improve.
/Mikael

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KX250F -09, CR250R -82

Mikael,

No, I haven't ridden the RM with the new reeds - so many other projects to do first.

I also want the bottom-to-mid to be more smooth, and more bottom power too.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

some newer bike riders told me they have to change the V-force reedblades every 1-2 years cause they are worn.

_________________
80 CR250Ra
80 CR250Ra
78 CR250R
80 CR80
82 Bultaco Sherpa 340
78 Bultaco Pursang 250 MK12
76 Bultaco Frontera 370 MK10
76 Bultaco Alpina 250
73 Bultaco Lobito 125 MK6
76 Suzuki RM125A
90 Kawasaki KX250

Thanks for the info! I'll get an extra set to have on hand.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

All good stuff so far, except I'd start with a 3/8", 10mm, intake spacer since I think they were all made out of 3/8" plate back when they were popular. I don't recall every seeing any thicker nor thinner.

You can do some similar tuning with the exhaust. The length of the pipe clear through to the reversing cone is determined by how long you want the exhaust pulse to take to return the pulse wave back to the engine. The returning pulse wave reverses the exhaust gas flow and stuffs unburned air/fuel back into the combustion chamber. If you want more power higher in the rev range, you can shorten the exhaust header tube. The shorter exhaust allows the exhaust pulse to return more quickly which is better matched to the faster engine speed. When you want more low end power, you can add 3/8" length to the header pipe to allow the exhaust pulse to take a longer time going out and coming back. This better matches the pulse wave speed to the lower engine speed, thereby maximizing power at the lower RPM range.

What is right for one particular engine is something I don't know. But the factory length isn't always optimal for everyone. On some motors this little change might cost tons of top end. On other motors, you will get the mid range you seek with no perceptible loss on top end. Somebody out there may know precisely, but you can experiment by fabricating a longer mounting spigot or just cut and run.

Enjoy,
Jon

Thanks mrmikkelsen.
If the reedspacer and boysen don`t make me satisfied, I might try the exhaust mods.
The season for that bike is nearly over here now, so I will do some changes during the winter.
What would the most benefit from a reedspacer be, longer intake or lower crankcase sompression? or maybe both.
I believe that lower crankcase compression would resault in lower cylinder compression pressure, Is that right?
/Mikael

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KX250F -09, CR250R -82

I don't really know about the crankcase pressure question. I can't quite figure how lower crankcase pressure would increase lower end unless there was a tuning defect to begin with, like an overly high compression ratio or something. I am nowhere near the tuning master, so maybe somebody else can shed some light here.

Before you alter your pipe permanently, you might consider grabbing another pipe manifold and just extend it to experiment. Easy and reversible.

With a good modern pipe design and porting, I'd say the reverse is true; a larger crankcase (i.e.; lower primary compression) = more fuel/air mixture available for the ports and pipe to pull up into the cylinder = possibly better scavenging / cylinder filling = more cylinder compression, and potentially more power. I think nowadays the primary (crankcase) compression mainly affects the initial puff of mixture when the transfers open, then the pipe and porting do the 'heavy lifting' and pull the remainder into the cylinder as best they can.

With vintage and early EVO porting and pipe designs, I think the crankcase pressure is more important, as scavenging and filling are more dependant on the pumping-in effect of the crankcase.

In the course of several hundred simulations on my MR175 motor using the engmod2t program, the one variable I really couldn't change - crankcase volume / primary compression - seemed to be the most effective at increasing power potential. The MR175 is basically a 250 piston on a 125 bottom end, so the crankcase is tiny, the primary compression way high (1.6:1), and power potential is limited because of it, regardless of how good the porting and pipe may be.

I always thought reed block spacers were intended to tune the intake-tract resonance lower by increasing its length, but the spacer would also increase crankcase volume which would surely have an effect as well.

Intake tuning can be very effective at increasing power in specific rpm ranges. For what it's worth, the MR175's unusual intake tract has a very noticeable effect on the very low end (from idle to maybe 3,000 rpm) especially the air boot from the carb to the airbox (!). After seeing the simulator results I said "that can't possibly be right!" and pulled off the air boot; goodbye bottom end.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Thanks for good answers.
rayivers, I believe your right, did not think in that direction. Will be interresting to try it out.
/Mikael

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KX250F -09, CR250R -82