Vintage Dirt Bike Q & A

Is there a way to smooth out the power hit during acceleration. I would prefer a flatter powerband curve like modern bikes but don't know if this would be possible without a power valve. I don't want to reduce peak horsepower, I just would like more throttle control. Would adding fly weight help? playing with ignition timing?

Adding flywheel weight will definitely smooth out the power delivery. I have done this on many of my "built" late model CR250s to smooth out the hit a little, especially
on slick or hard pack tracks. I also found my arms don't pump up as much late in the moto. While the abrupt power delivery from a stout two stroke may be fun for play
riding and useful for supercross type obstacles, it is not the best way to go fast around a race track for 30 minutes plus two laps.

There is a company in Oregon that makes flywheel weights for a bunch of different applications. Here is the link: http://www.steahlyoffroad.com/ I don't know if they
can help you with your '79, but I bet they can tell you who can.

dogger

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1973 CR250M
1979 CR250R
1993 CR250R
2008 CR500R
2009 CR250R

The 1979 CR250R has an unusual "inside-out" rotor setup that will mean clearance for a flywheel weight is limited. If you want to smooth out the power get a "torque" pipe, porting and/or go down a couple of teeth on the rear sprocket. Or better yet ride a more modern 450 4-stroke and then go back to the CR....

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Michael Stiles
1979 Honda CR250R Elsinore | 2006 Husqvarna SM510R | 2007 Service Honda CR500R-AF

Hi,
I had a 1980 CR250R Factory Enduro bike that I bought directly from Honda at the end of the season. It had the stock rotor with a large steel ring pressed, tack-welded and balanced. It was a pretty trick scooter with a wide ratio gearset, C&J swingarm, Ohlins shocks and lots of titanium fasteners. The only thing that I have left from that bike is the titanium swingarm pivot bolt.
Anyway, back to the rotor. It provided plenty of torque for goat trail rides through the forest and slower, more technical trail riding. The revs were slower to come up, but I learned to live with that. I defeats the design of the motocross bike however, being that throttle response is much slower. Great for a woods bike! You could always change rotors for different riding conditions. Wish I still had it.

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1978 CR250R
1979 CR250R
1980 CR250R
1980 ME360
1986 CR250R

I didn't have that model back then, but in the mid 80's there were some low end tricks going around.
1) The reed stuffer was a little hollow block that you'd place within the reed cage to fill up the dead space in the corners. I think FMF made them. When I put one in my Kawasaki, I was really suprised with the low end boost I got out of it.
2) The reed spacer was a 1/4" plate that you'd bolt between the reed cage and the carb flange. The idea behind it was to make the intake track longer to do lord knows what to increase low end power. I went on and off with that thing and can't tell you if did much or not. It's super easy to make one of these out of scrap aluminum and give it a try though.
3) Cutting the exhaust head pipe section and welding in 1/2" or so of extra length to the pipe. This changes the length to the reversing cone to better time exhaust pulses to the lower rpm range. Depending on the tuning of your particular bike the ideal length may be more or less than 1/2" and may or may not cause a significant change in high rmp power. The opposite change was to cut 1/2" to shorten the length of the header to better match the exhaust pulses to high rpm's. Where's your sweet spot? You like experimenting?
4) Dual stage Boyessen reeds are a given. Put them in and the lower end power responds better.
5) Boost bottles were suppossed to help low end weren't they? I have a couple of these to try on my yz's, but I haven't done it yet. Does anybody know if they actually did anything other than cost money and look neat?
That's my bag of cheap add on tricks.
I do have a 79 250 Elsinore in the garage, and will get to it in time. I'd love to hear what you try and what works. I'm old too, and could use sane power delivery.

You can get a reed spacer at http://www.mossbargerracing.com/ and they also sell a reed

valve kit that might help.

Allthough being new to VDR and not wanting to spend much money on the sport untill I know I like it, the pipe and porting option is an interesting subject. If I got an aftermarket pipe for the bike; which one would you get and would or should you do some exhaust porting? I would think if I was going for a flatter torque curve, I would want to add material back into the port. If this is correct, then adding material to the cylinder head port would be difficult if not impossible. How about spot welding a step smaller tube in the ex port flange thingamagig to create a smaller ID or would the custom pipe compensate for this? Do the '78 or '80 heads have different exhaust port designs? BTW, my 79 Elsinore is all stock at the moment.

1978 and 1979 porting is very similar - the bridging of the rear transfer-intake port being the obvious difference . 1980 porting is fundamentally different as Honda went for more mid-range and in doing so lost some top-end. Pro-Form did a torque pipe that was longer than stock and came with a longer than stock silencer. Skimming a little off the head will help with the bottom-end, but do too much and you'll need run race gas. Boysesen reeds are more responsive than stock. Moose has a reed block spacer.

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Michael Stiles
1979 Honda CR250R Elsinore | 2006 Husqvarna SM510R | 2007 Service Honda CR500R-AF

The '79 CR250R can also be tamed-down a little with use of the 1980 rotor, stator and CDI set. It's not difficult to fabricate mounts for the smaller CDI module without modification to the module, or frame mount.

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1978 CR250R
1979 CR250R
1980 CR250R
1980 ME360
1986 CR250R