Vintage Dirt Bike Q & A

Was just looking at a couple of "bikes of the month," in particular a 1976 MR250 and a 1976 CR250M. Looks to me like they have removed the boat anchor/muffler on both of the bikes. Am I wrong?? Beside the obvious noise problem, will this cause any other problems?? I hate the giant 10-pound muffler on my '76, but it just seems too darned loud without it.

IMHO: I don't think you can make any exhaust change in any 2 stroke and not have some jetting issues.

If you did not need to change anything, it would be more luck and good fortune then an inherently flexible design.

2 strokes depend on the rush of gasses out the exhaust to pull the next charge into the cylinder. The theory of operation for a tune exhaust is it not only pulls fuel/air mixture into the cylinder but actually into the exhaust itself. The sound wave/tuning pushes that extra back to into the cylinder for a kind of turbo charging effect.

I have run some two strokes briefly w/o a muffler and while they did run, they were down on HP.

Kind of Kismet that you would post this today. I'm pretty sure I damaged an engine just yesterday running it to lean. I set the high speed jet for max RPM with a fixed load water type dyno set up. At the time I thought it was awfully lean. I got into a postion where I had to run the engine WOT for an extended period and it went to lunch.

Thanks for the reply. I've tried running mine without exhaust one afternoon and it was just toooo flippin loud. I have to say, though, the '76 CR and MRs look pretty bada__ without that ugly muffler hanging on the left side.

Mitchie,

I've run a Supertrapp (with the disks) on my MR since 77! Dennis Kirk still sells them I believe, as well as other brands.

Supertrapps, Old Skyways, etc, pop up on ebay from time to time. Just clamp one over the stock pipe, and you can always clamp the stock one back.

Keep it, as it's worth a few bucks to those who want to convert to original. the same anchor muffler came stocl oon the CR models, I think most racers just tossed it as soon as they got home.

I agree with ricortes, removing the muffler will hurt your hp, and more than likely hurt your powerband (top end performance). But besides that, the noise is so excessive, we all have to be aware of the amount of noise were making out riding to help keep our areas open.