How To Increase Two-Stroke Torque?

[ClareQuilty]

Question about my 120cc two stroke Tiara - if I want to have the mechanic tune it for more low-end torque and immediate throttle response, what needs to be done?

The bike feels like there's almost a flat spot from low engine speeds, and then just comes on almost too strong when you rev it up. I'd like it to be a little more balanced and not so nothing-to-too-much on the power band.

[karlos]

A big bore kit would be you easiest option, then replace the carb or re-jet to suit.

If you don't want to go that far, you could play around with reeds, maybe dual stage reeds, but if you're going to do this, a full exhaust system and a bigger carb would help no ends.

Either way to yield noticeable gains you will need to be spending 6-12k on the bike and ensure the works being done by a garage/mechanic who has a proven record, don't just take his word for it and assume because he works on bikes he knows what he's doing

It might be cheaper and easier for most thai mechanincs to fit a larger two stroke engine from a similar period, but factor in suspension and brakes too

**(sorry i read the title and didn't read that you just want to rid the bike of it's flat spot)**

Edited January 23, 2012 by karlos

[karlos]

If there's a flat spot down low, and that's the only issue with the bike's power delivery then it's most prob to do with the bikes carb. A quick strip down and clean should clear any flat spots and/or highlight any faults the carb or inlet-tract might have.

Couple of hundred baht for most shops to do this for you, make sure the carb rubber is given a good look over too, make sure there are no cracks and that the rubber it's self isn't rock hard.

Also it might be worth changing the spark plug while you're at the shop but make sure it's the correct one for your bike with the correct gap, i'm sure details can be found using google as to what gap and plug you'll need

[KRS1]

Like Karlos said, its probably a bigger than standard carb. The original should be 24-26mm, these bikes usually get 28mm carbs installed and sometimes bored out to 30mm. Bigger the carb more mid-top end you'll have and less bottom end, especially in the case of a two stroke. Where after it winds up it'll take off like a scorched cat.

I use to run offset crankpins 10 years ago when i raced, if you look at the head gasket, if there is an aluminium spacer about 2-3mm thick, more than likely theres an offset crankpin installed. The spacer needs to be there so the piston doesnt smack the head. Alternatively the spacer can also be placed at the bottom of the cylinder instead.

The pulser sensor may have also been filed to advance the ignition, this is a cheap fix at around 150 baht installed, its located by the magnetic flywheel.

The judder springs are also commonly removed for a better bite off the line, if the bike wobbles and lurches on take off, they've been taken out.

If you want a quick fix go down one front tooth, the bottom end will be stronger and blend better into midrange. If your having wheelie problems (is that really a problem ?) this may not be the best solution and may want to go up one tooth instead of down.

[Semper]

If your having wheelie problems

He is driving a 120 cc Tiara.

[KRS1]

You've obviously never ridden a souped up underbone two stroke.

Almost no frontal weight on these bikes, all you have to do is goose the throttle and it'll lift.

[Semper]

You've obviously never ridden a souped up underbone two stroke.

Can't see what the "underbone" (AKA step-through) thing have anything to do with wheelies. We are talking 120 cc here.

[KRS1]

A very light 120cc two stroke, with no gas tank on top, but at the back and lower, under the rider. Lack of front fairings also, throwing the bikes balance to the rear. Add to that a VERY short wheelbase.

First time i rode a race prepped underbone i dam_n near shit my pants i was laughing so hard at this 'thing'...

[karlos]

Talking of short wheelbases, i used to have one of these

I tuned it's water cooled two stroke 50cc Polini engine myself to around 13hp, that's what the top guns with similar mods had out of their bikes and mine kept up with them at Hullvington race track.

Still on the subject of small displacement and short wheelbases...yep i know it's one extreme to the other, but this little thing would point skywards out of every corner if you wern't over the front of it.

[ttakata]

Talking of short wheelbases, i used to have one of these

I tuned it's water cooled two stroke 50cc Polini engine myself to around 13hp, that's what the top guns with similar mods had out of their bikes and mine kept up with them at Hullvington race track.

Still on the subject of small displacement and short wheelbases...yep i know it's one extreme to the other, but this little thing would point skywards out of every corner if you wern't over the front of it.

Nice stuff Karlos.

I have this back in LA and debate if I should bring it here.

It's a 10HP Polini powered DM Midi, but I added Hope 6 piston brakes.

Are 2 strokes even known to provide torque?

All I have experience is with pocketbikes, cheap Chinese and the Polini ones.

All of them were slow down low, but in the upper revs they would just fly.

My DM didn't make its power until 12-15,000 rpm.

[pieandmash]

For anyone doubting the wheelie capabilities of a small capacity highly tuned 2 stroke:

[ClareQuilty]

Thanks guys. I should have clarified I don't want to spend more than a thousand baht or so on the fix.. maybe 1500 if it meant getting an all new carb or something like that.

The bike is really fun, but yeah, it just feels dangerous to me with that all-or-nothing power, and its so different from my other Tiara which is a better all-rounder.

I think I sort of shot myself in the foot because I kept taking it back to my mechanic complaining of the bikes lack of power (before it just didn't seem to have much power at any engine speed), and he switched the big sprocket from a 35 to I think he said a 37 tooth without even asking me (I have to admit the bike felt higher-geared than my other two strokes), and previously he did something to it which made it run a bit differently at start-up and idle as well as during driving. What I want is as much low-end power as possible in a two stroke, for a solid, pleasant driving experience. I have no use for the high-end power wheelie craziness (and yes, the 120cc does feel scary on this rather small bike).

[Lickey]

Get your mechanic to fit extra cylinder base gaskets, it lowers the compression ratio and flattens out the power curve, it might need re-jetting but try it first and check spark plug colour,

[VocalNeal]

The bike feels like there's almost a flat spot from low engine speeds, and then just comes on almost too strong when you rev it up.

CQ,

You will not get the same characteristics out of a 120 2-T that you will out of a 110 4 stroke.

What you are describing here is typical two-stroke characteristics. The exhaust is a crucial part of a two stroke engines power curve. The exhaust is designed along with the porting to give a certain power curve and the curves are usually quite narrow. Nothing much down low and then when the rpm matches the characteristics of the exhaust, yipee and away you go.

What sort of speeds do you ride at? What gears do you use. Try riding around at one gear lower everywhere you go. The extra RPM won't hurt the engine and you will get better power to speed ratio. The larger sprocket is OK but there is a limit to that progression as first gear will end up only being for pulling out stumps. If you ride at more or less the same speed everywhere in a certain gear then you could match the sprocket size to your ridding habits. Sort of like gearing for a particular race track.

You can get better lower down power by lowering the port heights slightly but as you are not doing this yourself or if your mechanic is not a closet two-stroke tuner the effort may not be worth the result. Next time it goes in for some engine work you could ask to have an extra gasket put on the bottom of the cylinder, which will effectively lower the port heights. Then you can gauge the difference. If this is going in the right direction then you can machine some of the cylinder head to restore the compression ratio. Shortening the front part of the exhaust but 1/8 - 1/4 inch will also lower give better bottom end.

For a beater 120 2-T commuter best just to shift down a gear and keep riding.

I'll dig in my drawer and have a look and see what is available as a big bore for your Tiara. maybe 140cc+

Another tack would be to befriend any 16 year-old 2 stroke soi racers in your are and see where they get there parts etc.

Edited January 24, 2012 by VocalNeal

[Kwasaki]

Talking of short wheelbases, i used to have one of these

I tuned it's water cooled two stroke 50cc Polini engine myself to around 13hp, that's what the top guns with similar mods had out of their bikes and mine kept up with them at Hullvington race track.

Still on the subject of small displacement and short wheelbases...yep i know it's one extreme to the other, but this little thing would point skywards out of every corner if you wern't over the front of it.

Last season for my son we had to tune the Poliini but we were cheating, it was the only way I could help him keep up with the little guys.

Edited January 24, 2012 by Kwasaki

[MaiChai]

What petrol are you using? Non 91 bensin will give you less power and thus you probably need a larger main jet on the carb? As others noted, 2 strokes are very sensitive to fuel, exhaust and carb. One of these will be your issue. I would put my money on a copy carb being installed or the jetting has been messed about on the carb? Assuming the exhaust is standard and doesn't have any leaks... You might find the exhaust has a leak underneath where it has hit the ground? In this case some filling with an arc welder will sort this out. Or it might be clogged up? Process of elimination; start looking at the bike and see whats different to the good one?

[ClareQuilty]

The bike feels like there's almost a flat spot from low engine speeds, and then just comes on almost too strong when you rev it up.

CQ,

You will not get the same characteristics out of a 120 2-T that you will out of a 110 4 stroke.

Yeah, I know, nor would I want to. The 110-112cc four strokes I have are not very powerful. What I'm getting at is that the bike runs relatively poorly at the low end compared to other two-strokes I've had. Its a drive-ability issue about throttle response. So, maybe somethings slightly wrong, or maybe the bike is 'tuned' more towards the higher end.

What sort of speeds do you ride at? What gears do you use.

I mostly drive around at 30-80 in 3rd through 6th gear, but the difficulty with the bike is mostly from take-off to about 30kph - it just doesn't have a smooth, steady power delivery (torque?) - again comparing it to other two strokes that I've had. Its kind of weak and a very slightly stumbley, and then just 'switches on' once it hits a higher rpm and goes a bit wild. I know this is a characteristic of two-strokes, but like I say I've had several (Tenas, Tiaras) in which this characteristic was considerably less pronounced.

Could this be due to the 'mix'? Rich vs. lean? Or is it the 'jets'? As in not enough fuel going in at the lower engine speeds and too much at the higher rpms? My understanding is very rudimentary, but I think there is a different 'jet' for the lower end and the higher end? (or perhaps the lower end one has a different name than a jet -- the 'needle' or 'needle clip'? Maybe also something called the 'pilot'?). I hate to have the mechanic tear down the engine again - he just rebuilt it a month ago, and it seems solid.

Edited January 24, 2012 by ClareQuilty

[VocalNeal]

CQ,

Sounds like the idle/low speed so either the air screw adjustment is wrong or the pilot jet is partially plugged or both. I guess it could also be the reed valves.

I'd start with a carburetor cleaning and readjustment.

DIY?

Buy a small bottle of fuel system cleaner from Tesco or the gas/petrol station... bung it in the tank as per instructions and run as normal and see what happens, it can't do any harm.

[KRS1]

Adjust the clutch so the engine winds up while your releasing it. In other words tighten the clutch cable.

Power is being transferred to fast. The clutch sounds like it is currently set for drag racing.

Check how far you have to release the clutch before the bike will move, if its almost instantly thats it. Setting it to 1/2-3/4 release will allow the engine to have more wind up torque before starting off. This is just a work around though.

Edited January 25, 2012 by KRS1

[Semper]

CQ,

Sounds like the idle/low speed so either the air screw adjustment is wrong or the pilot jet is partially plugged or both. I guess it could also be the reed valves.

I'd start with a carburetor cleaning and readjustment.

DIY?

Buy a small bottle of fuel system cleaner from Tesco or the gas/petrol station... bung it in the tank as per instructions and run as normal and see what happens, it can't do any harm.

Yes, sound like the mixture is too lean. Try turning the air screw clockwise 1/4 or 1/2 turn and see if it helps.

[oldsailor35]

For anyone doubting the wheelie capabilities of a small capacity highly tuned 2 stroke:

Bloody good job he had his safety thongs on .

[VocalNeal]

Try turning the air screw clockwise 1/4 or 1/2 turn and see if it helps.

I am sure the OP has a screwdriver but to get at the mixture screw first you probably have to remove the plastic cowling, then find the carburetor, then find the air bleed screw and the idle speed adjustment etc. but that maybe either too much or not of interest. I think knowing that we concur and that next time he goes for some work he knows it is a fix on the outside of the engine and not inside and doesn't require any new parts. Maybe!

[William Osborne]

I use to run offset crankpins 10 years ago when i raced, if you look at the head gasket, if there is an aluminium spacer about 2-3mm thick, more than likely theres an offset crankpin installed. The spacer needs to be there so the piston doesnt smack the head. Alternatively the spacer can also be placed at the bottom of the cylinder instead.

The pulser sensor may have also been filed to advance the ignition, this is a cheap fix at around 150 baht installed, its located by the magnetic flywheel.

The judder springs are also commonly removed for a better bite off the line, if the bike wobbles and lurches on take off, they've been taken out.

Mate... what are you talking about .... the op is talking about a 120cc scooter

What series did you race in ?

[KRS1]

I use to run offset crankpins 10 years ago when i raced, if you look at the head gasket, if there is an aluminium spacer about 2-3mm thick, more than likely theres an offset crankpin installed. The spacer needs to be there so the piston doesnt smack the head. Alternatively the spacer can also be placed at the bottom of the cylinder instead.

The pulser sensor may have also been filed to advance the ignition, this is a cheap fix at around 150 baht installed, its located by the magnetic flywheel.

The judder springs are also commonly removed for a better bite off the line, if the bike wobbles and lurches on take off, they've been taken out.

Mate... what are you talking about .... the op is talking about a 120cc scooter

What series did you race in ?

obviously something you know nothing about.

[VocalNeal]

I use to run offset crankpins 10 years ago when i raced, if you look at the head gasket, if there is an aluminium spacer about 2-3mm thick, more than likely theres an offset crankpin installed. The spacer needs to be there so the piston doesnt smack the head. Alternatively the spacer can also be placed at the bottom of the cylinder instead.

..and offset bores , cross drilled cranks, titanium con rods, custom manifolds and slipper pistons, et al but the post was about how to help a guy, not to tell us what you have done. This Is TV not Facebook...

Oh and forgot stuffed crankcases..

Edited January 25, 2012 by VocalNeal

[ClareQuilty]

CQ,

Sounds like the idle/low speed so either the air screw adjustment is wrong or the pilot jet is partially plugged or both. I guess it could also be the reed valves.

I'd start with a carburetor cleaning and readjustment.

Yes, sound like the mixture is too lean. Try turning the air screw clockwise 1/4 or 1/2 turn and see if it helps.

Thanks guys - any idea how to tell the mechanic to do this in Thai? I'm sure he's already cleaned out the carb when he rebuilt the engine.. its just a matter of telling him to adjust the mixture and check/fix the 'pilot jet'.

[KRS1]

I use to run offset crankpins 10 years ago when i raced, if you look at the head gasket, if there is an aluminium spacer about 2-3mm thick, more than likely theres an offset crankpin installed. The spacer needs to be there so the piston doesnt smack the head. Alternatively the spacer can also be placed at the bottom of the cylinder instead.

..and offset bores , cross drilled cranks, titanium con rods, custom manifolds and slipper pistons, et al but the post was about how to help a guy, not to tell us what you have done. This Is TV not Facebook...

Oh and forgot stuffed crankcases..

Whats your problem? i was explaining why there might be an aluminium spacer if it existed on his bike.

I'll just assume your having a bad day and wont take it personally.

Edited January 25, 2012 by KRS1

[Semper]

CQ,

Sounds like the idle/low speed so either the air screw adjustment is wrong or the pilot jet is partially plugged or both. I guess it could also be the reed valves.

I'd start with a carburetor cleaning and readjustment.

Yes, sound like the mixture is too lean. Try turning the air screw clockwise 1/4 or 1/2 turn and see if it helps.

Thanks guys - any idea how to tell the mechanic to do this in Thai? I'm sure he's already cleaned out the carb when he rebuilt the engine.. its just a matter of telling him to adjust the mixture and check/fix the 'pilot jet'.

This would be a DIY job. A small screwdriver, and turn the air-screw (the one without a spring) 1/4 - 1/2 turn clockwise.

[VocalNeal]

I use to run offset crankpins 10 years ago when i raced, if you look at the head gasket, if there is an aluminium spacer about 2-3mm thick, more than likely theres an offset crankpin installed. The spacer needs to be there so the piston doesnt smack the head. Alternatively the spacer can also be placed at the bottom of the cylinder instead.

..and offset bores , cross drilled cranks, titanium con rods, custom manifolds and slipper pistons, et al but the post was about how to help a guy, not to tell us what you have done. This Is TV not Facebook...

Oh and forgot stuffed crankcases..

Whats your problem? i was explaining why there might be an aluminium spacer if it existed on his bike.

I'll just assume your having a bad day and wont take it personally.

No I'm not having a bad day. An no I don't take it personally.

Why would an old Tiara have an offset ground crank pin and a spacer under the barrel. Doesn't make any sense. Had you have started your posts with "Check under the cylinder and see if you can see an aluminum spacer. Then the bike has a modified offset crank".

What your idea of why that should effect the bottom end torque could have then been stated. You started your post talking about YOU not about the problem. If some of the guys on here started explaining what they had done before to engines or bikes TV would need more server space.

Oh and introducing a personal aspect into a discussion is what high school debating members or politicians do when they are on the back foot.

Go for a ride today up Doi Sutep and relax.

[William Osborne]

I use to run offset crankpins 10 years ago when i raced,

what series did you race ?