DT125R ARCHIVE

Some of you may know I was having problems after my rebuild with over reving after my rebuilt.

At first I thought it was the carb slider not closing properly but I think I have found out the real problem today luckily.

I got the bike started and it started Reving again, I pulled the choke up and the revs went down meaning I was pritty sure it was an god dam air leak after all.

I noticed some smoke coming from behind the bit where the powervalve cables go down to,

So I stripped the powervalve out and noticed I had been a cmplete idiot and had not put the rubber O ring on the end I had put it on the Notch down from It.

Great I thought that has been the dam problem all along, put it back together and it was even fecking worse.#

I noticed the powerevalve seems like rubber has perished where it pivots and there is a bit of play, 

So my theory is that when The O ring was on wrong it was preventing the gas escaping meaning it would idle ( sometimes ) but as soon as I touched the throttle the servo would move it making it rev.

Sorry for the lng posts just trying to get what my problem across as clear as possible for you kind fellows.

Here are the pics,

 

 

 

 

The air leek is coming through the middle I am sure of it, what do you guys think, Just want the bike running nice after all the time and effort.

Does that thing seperate on a thread or something couldn't work it out.

Was looking on Aj sutton and you can buy the seal thats in there but I cant figure out how this thing seperates.

 

 

whers the other end ? also the end caps make up a good part of sealing the thing wouldnt say that this is causeing you over reving

the tzr I have atm is running ok with no powervalve fitted at all .

Got it mate but thanks for the link, this thing must come apart cause on the diagram on sutton it is in peices.

He was only running the bike for a split second i think. Not ramping it about.

only an air leak on the inlet side would cause it to rev up on its own dude. its gonna be around the reed block/carb manifold. always between the carb and engine for air leaks. n tht pv end looks a bit dodgy looks like the plastic part where the cables clip into, has seperated from that arm thing on the end of the valve in the first pics.. but a slight exh gas leak wouldnt cause the symptoms your havin m8. try blocking off the hole in the top of the manifold where the boost bottle goes could be a slight leak there if not. reed block off n check the gasket. might be worth replacing it anyway and even a very thin layer of grease on both sides of the gasket to make sure. check the surface on the cases wer it mounts is clean, flat / no pitting or gouges in the cases. check carb is all the way home in the manifold sucured tight. i even put a bit of grease around there aswell to make sure its sealed up good.

Vac Leaks:


A vac leak is unmetered air allowed in error to get by the intake. This effects carbs and injection equally bad....
You do not want a vac leak on any engine ever, but they are very common

Sometimes you might not know it if the leak is small, and other times a engine won't start at all. So symptoms run a broad range of what a vac leak can do.


Some symptoms are a sudden lean condition, which can result in loss of power instantly and a matching increase in fuel consumtion.


This might be found to be true if heat cracked a vac line to the pet****s while you were riding, or the line just lifted off


Other types of symtoms cause what is known as "Hunting" Which is idle RMP that will not stay correct. The idle goes up to a given range maybe even to 2,200 rpm and then will drop to 600 RPM and go right back up as if a demon has the grips.. Or the bike might go to 2,200 RPM and stall forcing youi to restart.


Lesser leaks might effect idle, and what was correct yesterday suddenly is high today.


Turning down the over all Throttle linkage screw (THE ONE MAIN ONE MADE FOR FINGERS)
will work to lower idle sometimes but is a mis-adjusting when you should not do that..


If the leak becomes worse the idle will do what ever the leak demands..


The leak leans out the correct mix of 14% to 17% fuel to air and makes the mix undeterminable, ALWAYS lean...


On bikes each carb can have leaks, and manifold mount for any carb can have vac leaks.


Any throttle plate shaft can leak on either end. Any vac lines can leak on either end.


And any test port can have a bad cap, and so also leak.


Most bikes don't have vac operated accesories, with the one exception of vac operated pet****s. A vac operated pet****s WILL say, Pri = prime, On/Run, AND Res = reserve.


There is NO OFF setting..... Also the pet**** will have 2 lines each... One line is for fuel and the other is a vacuum line telling the pet**** the engine is MAKING vacuum, and to turn on the pet**** diaphram to pull open the on off valve with in the pet****.


The way a internal combustion engine works creates vacuum. I have never seen any bike with a vac pump.


In my experience vac lines in general do not deal with heat and weather well. They crack, split, and become brittle, and should be replaced once a year, as well as gravity feed fuel lines.


To locate a vac leak you need a can of WD-40 which is probably the best thing you can use WD-40 for.


Also you can use WD-40 to test whether or not idle mix is right. This chemical beats ether hands down for use as a engine starter as well, and will not cause engine damage in moderate amounts.


WD-40 makes what you can't see, and probably what you can't hear findable.


You need to listen to know....


SO to tell if idle mix is right, Spritz a shot right at the intake with a running engine, and listen...


Does the idle go up? Or does the idle go down?


If things are correct the engine has all the fuel it wants and the idle will drop, as the engine wants no more...


If the idle goes up you are lean.......


If you have 4 carbs and all go down but one, then that one is lean.... Why it is lean remains a question.


Maybe the setting is wrong, and the pilot is in to far (mix screw on a car).


[ Often a book setting will say 2, or 2 1/2 turns out. That is a place where a fresh built engine should run to start, and IS NOT always the best mix for any given clyinder]


Or maybe you have a vac leak...and so adding fuel in the form of WD-40 causes the idle to jump to who knows what, and that depends on the unmetered air.


It is possible for a bike to run on 2 clys out of 4, and have the two dead clys fire up above idle speeds as the engine approaches mid range RPM.


So finding leaks becomes a bit of hit and miss, as you spritz about the carbs after a initial shot into the carbs.


Each time you spritz you must listen, so with a air cooled bike you might want a fan on the engine.


Places to spritz are the manifolds looking for loose clamps, throttle shaft ends, and any vac line ends and components vacuum operated. On injected bikes any Throttle body lines, and injector bases, also any vac operated components as you find them.


Often times vac leaks are mis diagnoised as clogged carbs, and bad plugs, wires, pick ups coils and more... Mac

slob you shouldnt be using a cable tie to secure the carb lol, your suppost to have a nice tight jubilee clip on there, get a nice stainless one on there done up tight, but not over tight :)

it is an air leak no doubt about that you have all the syptoms

you wanna fix the powervalve as we discussed in PM, as that will do nothing but cause you problems anyway..

on the manifold inlet, make sure you use a thin jubilee clip holding the carb on yes. also make sure the gasket inbetween the ENGINE CASING and the REEDBLOCK housing is fine

it DOES NOT have a gasket inbetween the reedblock and inlet manifold, there is a small square shaped o-ring, smear grease on this before installing the reedblock screws in a criss cross pattern :)


could also be the cylinder BASE gasket potentially, but more unlikely

i dont think there is anywhere else air can leak in except for crank seals or inbetween the crank cases, but as youve just rebuilt your engine i doubt theyll be the problem

you can get it to idle forever with an air leak!


dont listen to your friend


an air leak is un-predictable, everyone on the forum will agree :)



use wd40 to find the air leak

bike can idle fine for weeks mate then suddenly go up or down its called hunting ,on my dt it was the inlet manifold gasket ,it really doesnt take much to make it rise , if you can get her to start spray wd 40 on all the mating surfaces to see if she rises or drops .also if you stripped the carb make sure evrything is seated properly .the pulse i put a vid up of wouldnt start at all because of a 10mm nut not holding the inlet in properly easily corrected

Yeah I read it all, just getting completely fed up now, while fiddiling endlessly again my fuel tank fell of and smashed on the garage floor powdercoat on it is ****ed, going to have to get the tank re done if the thing ever starts that is, feels like all this work has been for nothing.

I will try and get WD40 sprayed on the inlets, just dont like a freshly built engine reving like that as you can imagine, I am pritty sure it is an air leak , wrong jet would be a bit to hopefull.

if it ran fine before, your jetting is fine, if i was you id get some genuine yamaha gaskets and just replace them all where air leaks can form, theyre cheap enough

smear some grease on the inlet manifold o-ring too.


sort out and clean your powervalve, replace them o-rings and the seal on the pv and you should be laughing, as its due them to be replaced anyway isnt it bud


dont give up!