How to make YZ80 more rideable

[corksil]

Hey I'm still tinkering with an old 95 yz80 two stroker supermoto mini.

After lots of dickering with the jetting and carb work, I can't seem to make the bike more rideable. It is either ON or OFF.

All attempts at part throttle cruising result in the bike eight-stroking or four-stroking and the power delivery is notchy and choppy. The bike bucks and lurches at part throttle.

I put in a new piston and crank, everything in the gear box is good, and I sourced the original carb and set it up exactly how the yamaha manual describes. It has aftermarket pipe, and one size bigger main jet to account for the aftermarket pipe.

With that said -- it's very hard to ride. I feel as if I'm playing a finicky instrument with my right hand. With very precise throttle modulation I can get it moving from a stop and up into higher gears but the bike is not content to cruise at part throttle and maintain a certain speed.

Also worth noting -- I have read that these engines are entirely designed for a "race" application (duh) and have little to no midrange power. Even in stock form, the power delivery is either all or nothing.

If it were a motocross bike on a track, the proper method would be to feather the clutch to get the RPM up and cause the rear tire to spin. Then the rider would sit forward on the tank and scream the engine through the gears at WOT while keeping the front wheel pointed and the back wheel churning. With sticky street tires, that's not an option because the bike shoots right out from underneath me. I cannot get the rear wheel spinning on tarmac.

Even if I firmly plant my scrotum on the gas cap and get my chest and head over the handle bars, the bike still lifts the front wheel all the way through third gear. It's a lot of fun, but not very practical.

Just to be clear -- this is a motocross bike with street tires being operated on a closed-course kart race track. I need to tone down the power delivery and make the power more usable.

I have heard that a flywheel weight would help but I don't know what that is (aside from what it obviously sounds like.)

If anyone can help...

[MLR]

Reed valves?

Were they sealing?

Can you flip them?

[jrshooter]

a different exhaust.

[jrshooter]

note im not a supermoto guy, just a gut grew up motocross racing.
have you tried comming into the the turn a gear higher, and slipping the clutch
on the exit , when you can drive out the turn ?

[Triple Jim]

I wouldn't take the flywheel weight idea too seriously. Has it been ported? If so it would be a good idea to measure your ports and see just where you stand with port timing.

4-stroking almost certainly means rich. Can you drop the needle and get any improvement in the midrange? If it's very rich in the midrange, that will certainly tend to make it behave as you're describing. A friend has a CR80 engine in a Kawasaki G3, and although the low end and midrange are weak, it has a big, wide powerband that's controllable.

My DT100 is the opposite... it started out tuned so mildly that it didn't have any 2-stroke "hit". I'm working up on the port timing and still have a little to go.

[Motofool]

What is your spark plug telling you about the mix?

I would add counter-pressure to the exhaust first, test, then reduce the fuel proportion in the mix, test again.

If that bad, and you feel more hesitation at full open throttle and low rpm's, I would verify that the supply of sparks is continuos and strong.

Has the low edge of the slide in the carb be cut and modified?
Any thing upstream the jets that could disturb the flow of air over those?

If the ports have been modified in the wrong direction, there is no much that you can do now, it is too late for that cylinder.

[jkv45]

It sounds like the jetting is off (like TJ said).

When I got my Derbi GPR (75cc 2-stroke) it was almost un-ridable. In the midrange it would stumble so bad it almost quit, then wake-up and pull strong to redline.

I ended-up leaning the midrange a ton - new leaner Needle Jet and leaner Jet Needle as well. Slowly backed down on the Main Jet also. Leaner Pilot Jet too. Now it idles well and pulls smoothly all the way.

I would make a change to the needle and see if you can clear it up. I might go to the leanest adjustment on the needle and see if the issues get better or worse.

I've ridden a CR85 and a KX100 (not mine) that ran pretty much the same as yours. Stumbled through the midrange only to clear its throat an loft the front wheel. Strictly point-and-shoot. Not very easy to manage in anything except a straight line.

It's possible there may be some cylinder/porting mods that are required to get it to run smoothly at moderate RPMs. I have a book that may have more info, and will check back if I can find anything.

[Ralgha]

Do you have a stock exhaust you can try? 2-strokes are very sensitive to the exhaust pipe.

[Triple Jim]

Sometimes the low end of a 2-stroke race bike is set up to be very rich on purpose. If it's premixing oil and gasoline, the engine relies on the fuel mix for lubrication. On a track, if you're at WOT and close the throttle suddenly for a turn, you're also shutting off the oil supply. It's not uncommon for 2-stroke race bikes to be so rich at the bottom that they won't idle at all. But even so, you should be able to get the midrange running properly.

Is this set up for premix, or does it have an oil pump?

[Panda]

If this is an actual YZ80 like I remember as a kid the easiest way to make it more rideable is to shrink yourself by like a foot and a half and lose 100bs so you are the same size as a 12 year old.

[subxero]

Sounds like something is off, probably the jetting.

I would not expect that bike to be able to just roll on with out riding the clutch with any decent load on it especially say going up a hill. But with that being said if you are cruising on a fairly flat stretch you should have no problem rolling on the throttle and the bike reving up slowly but smoothly until you hit the power. I think that is your test. Can you get going on a nice flat section of road and while in a good gear roll on the throttle without the clutch and issue?

Does that bike have a power valve? If not then the low mid range will be very lacking and more throttle and riding the clutch might be needed a lot more than you think.

I would not suggest a flywheel weight, it may make it worse, it will make it even harder for the engine to spin up.

Getting the jug ported for more mid low end is one thing to look into, i believe Boyesen Rad valve reed cage tends to move power more into the low mid range vs stock or say a vforce set up. I doubt they make a "torquey pipe" . for an 80. but you could look into it, an expansion chamber more designed for low end could help as well but takes a lot off the top at the same time. FMF gnarly pipe is an example for bigger bore bikes. Takes off the top and puts more in the mid, vs say their Fatty pipe.

If it has a powervalve sometimes there are ways to adjust when it opens, have it open later in the revs for more low end and smoother transition.

that's about it.

I know after riding larger bore 2 strokes for a long time i am always a bit surprised how much clutch is needed in the low mid range of small to mid bore 2 strokes. Rode an older 125 few weeks ago and it ran similar to how you describe.

[JohnnyBravo]

Weld a bit of extra frame and shove a bigger motor in it

[corksil]

You guys nailed it.

I dropped the needle four clicks and changed the needle jet and now the bike is running perfectly.

What's the easiest way to get a little more torque out of this motor? I am aware that it is not designed for this purpose and I don't want to waste a bunch of time trying to put lipstick on a pig... but if I could get a titch more torque it would be a lot quicker off the line.

Sometime in the next month when I have a moment, I'll check the gearing. Pretty sure it's not stock but I don't know what's under there until I have a look.

Thank you all for contributing.

[jkv45]

Originally Posted by corksil

You guys nailed it.

I dropped the needle four clicks and changed the needle jet and now the bike is running perfectly.

What's the easiest way to get a little more torque out of this motor? I am aware that it is not designed for this purpose and I don't want to waste a bunch of time trying to put lipstick on a pig... but if I could get a titch more torque it would be a lot quicker off the line.

Sometime in the next month when I have a moment, I'll check the gearing. Pretty sure it's not stock but I don't know what's under there until I have a look.

Thank you all for contributing.

Good to hear it's running better.

A different pipe, designed to boost the midrange would help, but that may not be an option.

If you can gear it down and still have enough top-end, that may be the easiest way to get more snap off the line.

[corksil]

Numerous independent riders have confirmed my suspicions. This bike has a lot of top end but very little usable low-end power. Given, it's a small displacement two-stroke motor and it was not designed for torque production.

With that said, it's difficult to get the bike moving from a stop without feathering the clutch to keep the RPMs up.

It's also really freakin loud which is not ideal. The pipe is wrapped with a mixture of butyl rubber and kevlar but that doesn't seem to make it much quieter.

If I could find a way to make this machine produce more torque and also have a quieter exhaust note, I would be very eager to do the work and get the job done.

If all else fails, I guess I could put a CR500 engine in it and call it a day. Then again, it would need a longer swing arm to keep the operator from ending up on the ground with the bike on top of him and that would thoroughly disturb the suspension geometry so it's not practical. And certainly not within the budget.

[jkv45]

What pipe is on it? Does it have a riveted silencer? If so, you can repack the muffler and quite it down.

I did that on my Derbi, and it made a big difference.

[Motofool]

The engine should exhaust thru the original resonating pipe in order to perform per specs.
The downstream canister is just an absorbing material silencer, which needs to be repacked from time to time.

https://en.m.wikipedia.org/wiki/Expansion_chamber

I would remove any wrapping, because that changes the dynamic inside the resonating pipe, due to reduced cooling of the expanding gases.

[jkv45]

So I found some info in the book - Mini Motocross and Pit Bike Performance Handbook by Eric Gorr. (https://www.amazon.com/Motocross-Per.../dp/076032896X)

Chapter 15 - Yamaha YZ Tuning Tips, page 143 - 147.

He states that turning down the cylinder base and head 0.75mm raises the compression and retards the port timing for better mid-range. He sites the radical port timing and high gearing as the cause for poor low and mid-range performance (p 146.) There are also some photos covering port mods.

I also have another similar book by Gorr about engine tuning that probably has some worthwhile tips as well.

[corksil]

Thanks Jay I am looking at the same info.

[corksil]

Originally Posted by Motofool

I would remove any wrapping, because that changes the dynamic inside the resonating pipe, due to reduced cooling of the expanding gases.

I hadn't considered this until now. I'm going to remove the coating and see how things are affected.

The muffler/stinger/downstream-canister has been repacked and it helped a lot but it's still freaking loud.

I believe it's an FMF pipe, not sure which one though. I will look into this.

Thank you all!

[jkv45]

Originally Posted by corksil

Thanks Jay I am looking at the same info.

That looks like it. Sounds like a good place to start.