DT125R ARCHIVE

dread123 · Moderator

RE: bike revving by itself after rebuild

AndyYam · Guru

Sounds like its the wrong lower throttle cable as i said at the start lol. If it had a non standard carb on then the cable could be any length.

mr karnage · Guru

Putting it in the wrong way leaves it open about half way haha

I know there's some right bodges on this bike and its taken the piss to sort them. It ran like this on the other carb aswell before I stripped it.

I'm going to have to have a proper look, I'll keep you posted

carl13671 · DTR Senior Member

Have you had the air screw right out?

Has it got the rubber o-rings on it?

mr karnage · Guru

Haven't checked but I know the slide screw does so I'm guessing the mixture screw does aswell

MDK · DTR Legend

mr karnage wrote:
Putting it in the wrong way leaves it open about half way haha

I know there's some right bodges on this bike and its taken the piss to sort them. It ran like this on the other carb aswell before I stripped it.

I'm going to have to have a proper look, I'll keep you posted

if t did the same with 2 diff carbs then I think we can say its not the carbs .

when you changed carbs did you change all the carb or keep the same top and slider / needle with same cable on each time ?

maybe remove cable from carb altogether and test it that way use a bit of string if need be to lift slider a little if need be it will eliminate cable for problems .

Victor · Guru

what happens if you throttle it?

sometimes mine is a little sticky on start up and revs high till i blip throttle

mr karnage · Guru

Yh I'll mess about with it, see what happens

It just dies if I throttle it, which lead me to believe it's not a carb problem only

AndyYam · Guru

That shouldnt lead you to think that. If you throttle a bike thats cold on choke it will die. You're giving the engine the complete wrong mixture that it needs to combust. Thats all caused by the carb.

MDK · DTR Legend

I still say its air leak maybe at pv or cable problem .

wheres that other topic just like this we went on for weeks saying all the same things with the owner saying he did this and did that then in the end it all came down to the cable trapped/pinched or routed wrong .. just by passing the rev cable around the frame wrong or squished in tight behind lights or wires can cause this..

done it myself and on one bike it would rev high if I turned left was fun on corners lol

1DOWN5UP · DTR Respected Member

Was that slob's MDK it had a video of it too

MDK · DTR Legend

1DOWN5UP wrote:
Was that slob's MDK it had a video of it too

thats the one yeah chears helped me find it ..

his was reving high but a smaller kink or bend can have the same outcome

http://dt125r.activeboard.com/t54411488/bike-revving-up-full-throttle-on-its-own-after-rebuild-video/

slob · Guru

Those videos bring back horrible memories of suffering. Was my own fault in the end, was a 1 min fix rather than a 4 month one.

Definatley check it properly.I was in denial for so long because the slide was snapping shut.... but it wasnt

mr karnage · Guru

slob wrote:
Those videos bring back horrible memories of suffering. Was my own fault in the end, was a 1 min fix rather than a 4 month one.

Definatley check it properly.I was in denial for so long because the slide was snapping shut.... but it wasnt

did you see how open your throttle slide was before you fixed it?

Also I've no experience with 2 stroke engines. 4 strokes don't really bog at cold and my bikes have never needed to run with choke.

Bit of a newfag when it comes to carbs on two strokes

AndyYam · Guru

You should start it with choke if needed then turn off asap. Then let it warm up before revving it much.

The slide should be completely closed. It's really not difficult to take the carb off. remove the slide from the cable and drop it in. You'll see how far it should go down and see that it closes fully.

You've also told us that the bore has scores in and the old piston has gone back in. Without starting to remove the possible causes of the problem im not sure how you're expecting to fix the problem. Theory is all well and good but put the ideas that we are suggesting into practice. I know its frustrating to go back over the same things you say you have checked but we all make mistakes. you may well find something that you hadnt noticed the previous times.

My suggestions are

compression test- if low, rebore and piston kit
if good compression...

...remove carb and check the slide can return fully.
If it doesn't then youre throttle cable is probably wrong.

slob · Guru

when I had just rebuilt. I got it to idle with carb slide right down with a screwdriver. So I would say it should be practically right at the bottom.

mr karnage · Guru

AndyYam wrote:
You should start it with choke if needed then turn off asap. Then let it warm up before revving it much.

The slide should be completely closed. It's really not difficult to take the carb off. remove the slide from the cable and drop it in. You'll see how far it should go down and see that it closes fully.

You've also told us that the bore has scores in and the old piston has gone back in. Without starting to remove the possible causes of the problem im not sure how you're expecting to fix the problem. Theory is all well and good but put the ideas that we are suggesting into practice. I know its frustrating to go back over the same things you say you have checked but we all make mistakes. you may well find something that you hadnt noticed the previous times.

My suggestions are

compression test- if low, rebore and piston kit
if good compression...

...remove carb and check the slide can return fully.
If it doesn't then youre throttle cable is probably wrong.

Yh get what your saying, the bikes in someone else's garage and I've only the weekend to work on it as I don't have the keys . Hence why I haven't got anywhere or tried anything.

I watched slobs video and thats pretty much how it sounds except I didn't let mine go past 5k, I bet it is throttle cable.

dread123 · Moderator

which is what a lot of us have said earlier in thread!

hope it is as its an easy fix for you then. :p

mr karnage · Guru

Mate, if it does turn out to be that I'm gonna feel like a right dickhead Haha

MDK · DTR Legend

mr karnage wrote:
Mate, if it does turn out to be that I'm gonna feel like a right dickhead Haha

and we will all be sure to remind you once a week with a big I Yold you so

but realy I hope it is something simple like that be a nice ending to this thread ........

mr karnage · Guru

Just checked compression and I'm getting 90psi

MDK · DTR Legend

mr karnage wrote:
Just checked compression and I'm getting 90psi

that sounds ok ? around 100psi is the nrom I think ..but pls feel free to correct me

did you check the cable then ? remove cable from bike and check .

mr karnage · Guru

I'm doing it now and after some messing around I've got it to close fully

I'm just putting the cable back on to the bike right now

I've read less than 100 is bad but I'm sure someone will clarify

mr karnage · Guru

So this is what happened, choke being on unless stated.

Kicked it over a few times, wouldn't start

Turned idle in a bit screw, started after a few kicks and died

Kicked it again and it started then revved to about 4k and turned the choke to settle it about 3k. Let the bike warm up and gradually turned the choke down. Got to about 1.8k and with the smallest amount of choke, every so often it would sound like it was going to die so I turned the choke up a bit and back down to let it settle again. I had it running like this until it got quite hot at least running temp but it still wouldn't want to idle. Then it died with no word of warning

Started it again with choke fully on and it would still rev up like mad, I let it get to 7k before backing off and it slowly settled like previous.

Started it again and it wouldn't want to settle and now it doesn't want to start.

-- Edited by mr karnage on Saturday 25th of January 2014 05:19:52 PM

AndyYam · Guru

90 psi sounds low. I thought it should be about 110-120. the haynes manual should tell you. I think you're rings/bore are not sealing well enough.

mr karnage · Guru

That's what I've been reading, that it should be at least 120

I'll check the Haynes manual tomorrow

Just gives me a reason to get the head machined to take o rings and a tighter squish clearance

AndyYam · Guru

Whaaaa? dude. get the bike running as it should and ride it a bit before you start machining the head for or rings. You've probably spent more time on here that it would take to whip the barrel the off and check the clearances and condition of the bore and piston/rings. You dont need a tighter squish clearance or o ring head gaskets. you need a good seal between the rings and the bore and a genuine/athena head gasket.

Calum122 · Guru

Lol.

Tighter squish. Yeah brilliant. It's great in theory in practice. Mate not being funny but you wouldn't last long.

If you machine the head down to OEM specifications you have to be so meticulous with the build.

There is a reason why Yamaha don't machine it down properly. Because they allow for human/machine error. Let's say that the squish clearance is bang on. If the connecting rod stretches any more than it's suppose to you will hit the head. Let's assume that doesn't happen. But you put a new piston in. Tell me honestly. Have you ever measured the dimensions of a piston to make sure that its height is exactly the right size. If it's any bigger you will again hit the head. Even if that's not the case. Let's say again the squish clearance is perfect. But you over torque the head nuts. Again you risk hitting the head.

The clearance is purposely bad to allow errors during the build to take place. The wrong mismatch between too much stretching of a connecting rod and too tall a piston still allows the bike to be within tolerances.

O-rings are great for race engines BECAUSE THEY WILL CHECK ALL THESE MEASUREMENTS. And if you intend to do so then fair play. That ought to pay off for you. But if like me you check the basics and bash it together then just leave it as it is.

mr karnage · Guru

Yes I knew that and after checking the squish clearance I don't need to have it reduced and as far as getting it machined to take o rings the original head gasket is measured then the head is machined and the squish is set last so your running the same clearance as before but with o rings. standard squash ranges at about 1mm to 1.2mm reducing it to .7 Isn't exactly race tolerances

Do you really think it's a bad idea?

On the piston and bore, the piston had signs of blow by at the sides and particularly at the second ring end gap. The cylinder definatley needs a rebore

AndyYam · Guru

As far as the o ring goes i dont understand why you want to? You know your compression isnt running too high as the head gaskets arent failing. You also know your compression is in fact low. You know the barrel needs rebore and the piston has blow by marks. Why are you trying to find another cause for your issues when you havnt address the most obivous one that you already knew about when you built the engine. round and round and round in circles we go...