Topics

1

Technical / Carb Size

« on: May 01, 2010, 05:16:40 AM »

I'm thinking of updating my old Mikuni VM carb to a newer Keihin PWK carb. I know they're easier to tune and are likely smoother across the rpm range. What I'm wondering is, should I stick with the stock 38mm size or go to the 36mm.

The bike uses the std 68x68 bore/stroke but has been bored to 70x68. The reed cage may be one of the largest ever used in a production jap mx bike, it's a huge 8 pedal. With it being so huge, the bike really hits strong mid to top. I can't afford to get it ported right now and am hoping to shift some of it's power to the low end (hence my consideration for the 36mm).

Would going to the 36mm carb actually improve the low end?

2

Technical / Odd thought on Power Valve

« on: April 16, 2010, 10:15:43 AM »

If I were to sand down and paint my power valve with a hi temp clear coat, would it lessen or eliminate carbon build up?

I know it's an odd question, what's your thoughts?

3

General Two Stroke Talk / yamaswap

« on: March 28, 2010, 12:55:13 PM »

What is yamaswap? MXA used the term in the April 2010 issue regarding the '85 YZ490.

4

Technical / Toning down a YZ250

« on: March 27, 2010, 10:51:50 PM »

After reading some other threads, I see there are some very knowledgeable people here. So it seems a reasonable crowd to toss a question to.

I have an '83 YZ250, have had it for years and love it. What I'd really like to do is tone it down and make it a better trail bike as that's 95% of what I do with it nowadays anyway. The suspension still needs some work, but I've got a handle on that. I can't find where flywheel weights are available or different spring rates for the power valve (doesn't use springs anyway). I guess the timing could be messed with, but that's an area that I know very little about. When I got the bike it was geared taller than stock, it was at 14/48 (stock 13/48) and I re geared it lower to 13/50. I recently rebuilt the top end on it and found it to be .080 over but ring gap is at the minimum spec so it's tight. In the process, I changed the stock reeds for TDR reeds and I've jetted the stock carb. It does have a DG pipe and silencer and I've added a cobra spark arrestor. I've noticed the '84 has other more modern pipes available, but this one the pipe runs off the left side instead of the right so a bit of modding would have to be done to change to a different pipe.

It runs really good and has a really strong punch in the mid range. I'm mostly curious what, if anything, can be done to broaden and perhaps mellow out the power a little more for a better trail/woods rider on this thing.