2 Stroke Powervalve Help needed

[darkangel]

Looking for help with a strange problem with my bike.

My DTR125 has the strange problem of defying physics, the powervalve in the exhaust port makes less power when fully open then it does when turned backwards on itself, i have attatched some poorly drawn images to try and show this.

Does anyone have any idea why this could be? The bike has been de-restricted, gutted exhaust, rear DEP, reed switch mod, inlet manifold modded e.c.t
I have done a rejet down from a 260 i put in there when i derestricted the bike down to the stock 240, that increased the power but the fact is still that having the valve turning backwards on itself is making the bike faster...

When alligned to the hole (which is where it's supposed to be for maximum power) the bike feels guttless and restricted, moving it to the left creates much more power, it transforms the top end from a low grumble of sucking air into the traditional two stroke ring when the revs are above 7k.

It does this both when using the powervalve servo to controll it and when pegged manually.

The engine got fully rebuilt 6k miles ago and starts every time so compression is not a problem, it has also been like this ever since i owned it 13k miles ago.

Thanks for any help anyone can offer

*Edit*

To make sence of the pictures

'Open' is the correct position the valve is supposed to be in for maximum power

'Closed' Is the position it moves to at low revs to give more bottom end

'Backwards' is the position i have to run it at for the bike to make any power, with it at the 'open' position i can barely get past 65, with it at the backwards position i can do 75-80 and the bike is more responsive and can be wheelied in the first three gears MUCH easier

[ben350]

I cant figure out what's happening as it should make full power with the valve wide open

When you say inlet mods what do you mean as I didnt think that they were restricted that way?

BTW I hope you dont work as an artist
____________________
Ducati 996 Biposto
Suzuki Goose 350
DT125R Supermoto (Project)

[darkangel]

what's wrong with my pictures.. :'(

haha its what you get from 2 mins in paint!


Its a strange problem isn't it...

The inlet manifold has a rubber section that funnels the flow to the tip of the reeds, i cut this off as instructed in the derestriction manuals, its kinda contraversial some people say do it some people say dont, it made no difference when i did it though, same performance as before.

[Kickstart]

Hi

Suspect chopping back the car rubber might have lost you a small amount of power.

Does the DT use a 1 piece power valve? The TZR we had years ago had a 2 piece valve which could be assembled out of line. Only thing I can think of that would cause it (well, other than someone doing a bad job or porting it).

All the best

Keith
____________________
Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[darkangel]

It is assembled perfectly, it is a two peice one that pulls out either side of the cylinder, i had it out cleaned and regreased it earlier in the week, at the open position it is flush with the exhaust port as shown in the pictures.

Maybe cutting the manifold lost some power im not sure, i felt no difference and the reports on forums are that it makes either a small difference or none atall.

The engine has not been ported, it has been rebored once by a yamaha workshop and this strange problem was around even when the engine was stock and the only mods were electronic speed limiter removed, snorkel removed, a half gutted exhaust (one of the restrictor rings fell out) and a rear dep pipe.

[darkangel]

Do you think it could be something to do with my exhaust system needing replacing or something.. its the only thing i can think of.

Allthough i cant really find any leaks with the exhaust, maybe a tiny one in the baffles of the end can.

What could possibly cause this? 'Normal' Dt's produce the power mine does by being alligned to the hole, why does my bike have to be weird like this!

If i could get to the bottom of this i could probably see some improvements by getting the powervalve working properly when fully open.

Either that or maybe a jetting problem?

[Kickstart]

Hi

To be honest not sure. Possibly is worth checking all the odd things (jetting, timing, etc) and that nothing is rattling round in the exhaust cuasing a problem, but to be honest not sure that is the problem.

All the best

Keith
____________________
Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[darkangel]

Thanks Keith

Im really out of ideas myself.. just hoping someone here has had a similar experience or has an idea what might be causing this mystifying problem with my bike...

[darkangel]

In the pictures i have drawn, the open and closed ones are correct for a powervalve system right?

[binge]

No I don't think so.

I'm pretty sure its a 180 degree rotation between open and closed.





Ben

[Philious]

[darkangel]

I will try swapping the cables round tonight, i thought the point of a powervalve system was to extend the duration the piston has to travel to release the exhaust gas (extending the exhaust port) :/
My bike is a 2000 model

[Kickstart]

Hi

Power valve is there to lower the effective top of the exhaust port at low revs and raise it at higher revs. Which appears to be the way round you currently have it.

However, thinking about it my TZR250 power valve resets itself to the open (high revs) position when you switch on the ignition and only turns to the closed (low revs) position when you start the engine.

All the best

Keith
____________________
Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[darkangel]

As does my bike, this brings us back to the question of why is my bike making more power with the valve set in a backwards position rather then fully open...

[Kickstart]

Hi

Will the bike pull a higher speed with the valve open if you thrash it through the gears? Ie, is it just a large flat spot that in higher gears it cannot pull through.

All the best

Keith
____________________
Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[darkangel]

The servo operates exactly like your TZR, it resets to wherever you set it to (the allignment hole, which is the fully open position) and then rotates to the right (lowering the height of the exhaust port) when the engine is started and when at low revs.

With the powervalve servo operating as it is supposed to, having it alligned correctly to the hole (fully open, flush to exhaust port position) the bike feels gutless, like a stock DT, when accelerating it sounds really low pitched and the acceleration to 60 is poor allthough the final top speed is pretty much the same in the right conditions (75-80 rev limiter) it gets there much slower and the chances of getting past 65 are slim.

With the servo operating and alligned to the left of the hole as in my 'backwards' picture once going over 5-6k rpm the bike turns much more high pitched and sounds like a traditional two stroke, it also makes much more power being able to get to 70 regardless of wind or going uphill, acceleration through the range is much better allthough top speeds are limited still to 75-80 because of the limiter.

Kickstart, when the bike is set to the hole like its supposed to, it feels like the whole rpm range is a flat spot!
it's really deep and flat when in the high rpm range, could the bike be getting too much or not enough fuel/air to properly operate with the valve in a fully open position?

Moving from a 260 main to the stock 240 helped allthough ocasionally i get a ping or a knocking sound when rolling off the throttle, allthough that also happened sometimes with the 260.
The 260 also used to flood the engine frequently and make starting the bike once warm a pain.
Some places (the haynes manual) state the stock jet for a DT is a 210 main, but all of the bikes since atleast 2000 have shipped with a 240.
Going full throttle also sometimes doesn't seem to make any difference, there is no power benefit then going half or 3/4 throttle.
When i had the 260 jet i frequently used to back off the throttle when nearing the 9k rpm range, backing it off and slowly reaplying it helped allot to accelerate to the limiter at 10.5k

It wouldn't suprise me if i had jetting issues...

[Kickstart]

Hi

Could be jetting, and on large throttle openings it would be the main jet that is to blame. But not sure I would expect THAT much difference. What does the plug look like if you do a plug chop?

Afraid the only books I have only list up the 2001 model with a 210 main jet.

All the best

Keith
____________________
Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[darkangel]

I havn't done a plug chop yet, but whenever i replace the spark plugs they are usually black/dark brown with carbon deposits.

My bike is a 2000 model that came with a 240 main...think i should try a down jet?

When i had the 260 in opening the airbox helped performance

Now i have the 240, the performance is better then the 260 with the airbox open, and i no longer have to run with the airbox open.

Performance right now is identical if i slowly roll on the throttle or if i hammer back the throttle full twist. Should it be like that?

[Kickstart]

Hi

Think I would try and confirm what the stock main jet is really meant to be. Autodata lists it as a 210 from 1991 to 2001.

With a plug in normal use it is difficult to know from the colour. That is most likely related to the last few miles of bimbling home.

That openign the airbox helped things on the larger jet suggests that was too rich.

Opening the throttle more than necessary isn't going to give you any more power and possibly less (ie, full throttle at low revs). However once the revs are high enough it should pull full throttle well.

All the best

Keith
____________________
Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[darkangel]

I doubt the carb has ever been rejetted before me, it came with a 240 stock. Couldn't they have been sending the bikes off rich though to reduce power?


I am going to go run some tests with the powervalve servo disconnected.
will peg it shut in various places and see where gives the best results.

I also have my module 2 tomorrow! can you suggest any theorey sites for roundabouts? there are a few 6+ exit ones with new layout near me that are a bit confusing.

[Kickstart]

Hi

It is a quick change to change the main jet and some people seem to think that just putting in a larger main jet will give more power. Hence it wouldn't surprise me in the slightest if it has been rejetted.

Good luck with your theory test.

All the best

Keith
____________________
Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[darkangel]

its not a theorey test! its my practical
But the route i will most likely get taken on has new layout roundabouts

[Kickstart]

Hi

. Good luck with the practical. Afraid I don't know of any sites that would help.

All the best

Keith
____________________
Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing