Help with two-stroke tuning

[Skunk]

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]

Not enough backpressure.

[Mr. Peanut]

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

[Ixion]

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]

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]

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]

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]

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

[Skunk]

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]

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

[Skunk]

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]

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]

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.

[k14]

It could be running very very rich. Sounds like the symptons of very rich jetting. Could be hundreds of other things though.

[Ixion]

Whats the ignition system? Maybe there's actually a problem.

No, scrub that, it wouldn't start idel and rev in that case.

[Skunk]

It could be running very very rich. Sounds like the symptons of very rich jetting. Could be hundreds of other things though.
I'll stick with the carb at present. Rich you think. I'll drop the needle first and see if there's any improvement. If there is I'll get some smaller main jets. I don't think this carb has any choice in needle jet size but at the throttle % I'm using it shouldn't be on the main jet.

[Ixion]

Not something simple and silly, like the needle being loose and not rasing when the slide lifts. But I'm also wondering about that carb, cos under no load even revving hard it's probably only got the slide open a bit. So maybe main jet is too large once the needles well up.

[Skunk]

Not something simple and silly, like the needle being loose and not rasing when the slide lifts. But I'm also wondering about that carb, cos under no load even revving hard it's probably only got the slide open a bit. So maybe main jet is too large once the needles well up.
My understanding is that the main jet is the jetting above 75% throttle. The needle and needle jet do 20% to 75% throttle. That would mean you could be right... I need a smaller main jet, as it only dies under load (wider open throttle).

I've checked the needle. It won't move. (Strange clamp system - haven't seen one like it in my limited experience)

[Skunk]

The needle is a 5DH2 and the slide cutout is a 2.5, for those who have Mikuni charts.
The slide cutout should probably be smaller (ideally) but I have no option.

[Mr. Peanut]

Are you sure its not a back pressure/porting problem? RGVs/TZRs have powervalves, which even when open restrict flow.

[Ixion]

I suspect it is , partly. Prolly one of those things where there's no one single thing actually WRONG, just a bunch of things that don't get along. Insufficient back pressure will mean a wider throttle opening will be needed to generate enough power to get under way. If the carb is too rich , that means even more throttle needed and so on.

[Skunk]

It's an early TZR chamber (1kt '86-'88 model). I didn't think they had the powervalve.

I'll be checking the richness tomorrow.

[Mr. Peanut]

Yeah they got a powervalve.

[Skunk]

Expert advise is that it's lean. Confirmed by covering half the carb to increase the air velocity and it goes better. Theory is that the 100cc is not sucking as much air as the "125" the carb would normally be on, so it's not pulling the fuel into the air correctly.
I'm going to drill the main jet and see what happens.

Edit: Seems better. Need open space to run it now.

[Ivan]

hope it works

[Sparky]

You can raise the needle by moving the circlip that holds it in place. you have to remove the throttlecable and spring, under that is a "thingy" with 2 scewrs, remove that to take the needle out. there are 3 grooves in the needle so take your pick.
Maybe worth trying?? got and exploded view picture of carb if you need it


PS you can always pull the choke up to increase the fuel and see if that makes it better or worse

[Skunk]

You can raise the needle by moving the circlip that holds it in place. you have to remove the throttlecable and spring, under that is a "thingy" with 2 scewrs, remove that to take the needle out. there are 3 grooves in the needle so take your pick.
Maybe worth trying?? got and exploded view picture of carb if you need it


PS you can always pull the choke up to increase the fuel and see if that makes it better or worse
Choke helps at an idle but as the slide lifts its effects are negated. I've drilled the main jet and raised the needle to full rich. Still looking for a test road.

[k14]

Expert advise is that it's lean.
Hmm, well glad you didn't listen to me then :doh:

Hope you get it sorted.

[Skunk]

Mate, it doesn't mean they're right! I'll find out when it's going properly. I did listen, and I tried.

I'm betting that if anyone was here they'd know straight away. Reading someone describe it is never easy.

[k14]

Yeah glad nothing bad happened on listening to my advice. On reading it again lean does make sense. Revs easy but makes no power is the sign of lean jetting. Found a "closed road" for testing yet?

[Skunk]

I've got one in mind... :shifty:

[Skunk]

After a quick run (3rd gear, 60-70kph, 50% throttle, 1km long) this is the plug. Last photo shows colour best.

It wasn't running smoothly.

[Ixion]

Well, that's definately not lean!

I'd say quite rich, and rather oily. Probably dirty enough to cause misfiring which would not help.

[k14]

Yeah thats definately rich. Is that plug fairly new though? It could be a bit of a miss read if its an old plug that has been used whilst trying to diagnose the problem. Need to give it a good thrashing all through the revs to get a really good read.

Did you end up drilling out the main? If you give the throttle a quick rip from 0 to 100% whilst its idling does it rev up cleanly or does it bog? Does it fully rev out?

[Skunk]

Slow or fast twist with no load it revs clean
Slow with load it dies
Fast with load it starts to die then picks up.

I think it's rich with the wrong needle.

Ferk it; I think I'll just have to pay someone...

[k14]

Yep I reckon rich on the main.

[Skunk]

Hmmm, I'll go with than. It was a 137.5 that I drilled. It wasn't any good with the 137.5 either, so what would you suggest?

[k14]

Maybe you went from one extreme to the other? How much bigger did you drill it out by?

[Skunk]

It was a 1.4mm hole (Guessing thats 1.375mm). I drilled it with a 1.6mm

[k14]

Hmm, that seems like quite a big increase. Only going on rs experience here but going from say a 180 to a 172 in some cases can be from very very rich to dead on. Maybe want to solder it up and try 1.5mm?

[Skunk]

OK. (Goes to fine a 1.5mm drill bit...)

Thanks K14 for all your advise.

[Ivan]

have you had any luck with it? my standered carb is too rich as well went full noise down the road for about 2-3 Ks and killed it took plug out and it was black and oily and im running it at 30:1 Want to try it now tho now that I opened the reed stops up as it revs quite hard now:gob:


Need a way of transfering the video from my phone to my computer

[FilthyLuka]

blutoof is your friend... hehe. Go to dicksmith or an it store and get a blue tooth dongle ($15), that should sort out the phone to computer thing...

DENIS!

[geoffm]

Any reason you can't use "real" jets - it makes the job a lot easier. The problem with drilling them is the hole isn't smooth or consistant, nor is the hole necessarily in line, hence the flow won't necessarily as per the hole size. To mak elife more intersting, one of the Jap carb makers rated their jets by flow, the other by diameter. All very confusing. 137 ->160 is a big jump.
Don't forget number drills of you want a bigger range of sizes.
Don't forget to mark the jet with it's real size if you have drilled them, otherwise you will be very confused later on...
Geoff

[Skunk]

Cheers. I'm still looking for a 1.5mm drill (had to work today and it was too busy to get out).