Help with two-stroke tuning

**Skunk** · 2nd February 2007, 20:26

Need a bit of help here. This my first attempt at tuning.

The setup is an AX100 engine that has been ported and a chamber fitted. The engine (when first built) was suited to an early RGV250 chamber but currently has a TZR250 one.

The carb is a KR250 flatslide semi-downdraft 28mm Mikuni. It has no air bleed screw I can find.

I've tried following the guide in Graham Bell's book but I'm not clear as to the cause of my issue.

Now we get to it. It idles fine. It'll rev fine, slowly and with a fastish snap. What it won't do is run under load unless it's reved up to 6000rpm or more and the clutch feathered out. It still tries the stop. I say stop cause it's not a stall. More like it has no power.

What do you think?

**Ixion** · 2nd February 2007, 20:28

Not enough backpressure.

**Mr. Peanut** · 2nd February 2007, 20:30

Not enough back pressure? Sounds like my NSR with the powervalves stuck open.

**Ixion** · 2nd February 2007, 20:33

Did it ever run differently? What you describe (no power under 6000) doesn't necessarily sound wrong for a two stroke racer? Assuming it goes well over 6000.

**Skunk** · 2nd February 2007, 20:50

The old unported engine with the 18mm std carb stopped reving around 8000rpm.

Backpressure eh. What's the cure? The chamber I guess, but how?

**Ixion** · 2nd February 2007, 20:55

There's a bunch of formulas you have to work out. But I can't remeber them. I think Mr Motu posted them once. Try a search for two stroke chambers

**Ivan** · 2nd February 2007, 20:58

Check the packing in the muffler if it dont have goodpacking it wont have good bakc pressure something I know haha hope it helps

**Ixion** · 2nd February 2007, 21:01

If its got a chamber it wont have packing. Or even a muffler.

**Skunk** · 2nd February 2007, 21:01

Originally Posted by Ixion

There's a bunch of formulas you have to work out. But I can't remeber them. I think Mr Motu posted them once. Try a search for two stroke chambers

Yeah, done all that. The chamber should be quite close to what I need.

Ivan: There's no packing, just a baffle which means it's noisey.

My feeling is it's either the cut-out on the slide or the needle.

**Ixion** · 2nd February 2007, 21:04

The carb sounds quite big for a 100cc too. Was there a rationale to the components? Are you sure its not overported?

**Skunk** · 2nd February 2007, 21:10

Originally Posted by Ixion

The carb sounds quite big for a 100cc too. Was there a rationale to the components? Are you sure its not overported?

It was a competitive engine a while ago but I never could find out what carb it had. The inlet has been ported to 28mm and the rubber carb mount matches and takes a 28mm carb.

In theory the only unknown is the carb. The chamber should be ok, the timing is set, the porting is known to work (with a certain suck and blow setup).

Graham Bell recommends a 32-34mm carb for road-race setups and I think Speedpro runs a 32mm on his MB100. So carb size should work...

**Ixion** · 2nd February 2007, 21:16

What are the plugs readings like? And, once you get it over 6000 does it develop decent power - ie is it just gutless all the way , or is it a wicked narrow power band?

**Skunk** · 2nd February 2007, 21:19

Can't get a meaningful plug reading but it appears OK to rich at present.

Still seems to lack real power over 6k rpm but it doesn't just die. When I say die it's like the kill switch has been pressed almost. It just drops off.