DT125R ARCHIVE

If the compressions low it might rev like that and die.

But i'm sure that if you've rebuilt the engine twice you must have reasoned that the piston and rings were fine.

Another thing it could be would be an air leak somewhere maybe where you've skimped out on gaskets and reused some. Or where maybe you did not clean the surface properly before reassembly.

What sort of head gasket did you use.

I'd go round with a can of wd40 and spray round the seals to see if the idle drops.

Or it could be (quite common) you've caught the throttle cable on something which is causing the high idle.

Did it run okay prior to the rebuild?

Sounds like air leak or carb problems as Cal says, Spray WD40 around the areas that it could be whilst the engine is running. ie, inlet manifold, l/h crank seal.

if all of that seems ok then I would suggest carb. Strip it totally down and clean everything including jets. Check the float height. Make sure the carb is sealing into the inlet rubber.

It is also worth checking the slide is completely close as that could cause it to rev high. If youve changed from the roudnslide to a flatslide then the lower throttle cable from the junction box to the carb is slightly different length and the flatslide carb needs a longer cable.

I'm also assuming, like Cal, that you've double checking ring gaps, piston condition, bore condition and clearances etc. Personally I would always rebore and use a new piston, especially if you've totally rebuilt the bottom end.

Keep us updated.

Test the compression as that would show if there was a problem with the rings and head. If theres scoring it seems strange that you wouldnt have got it rebored or at very least honed?

With it revving up high its an air leak 99% of the time. I know you say you've replaced all the seal but have you tested them? The wd40 method is the best.

If you test the compression try it normally at first then put a little 2 stroke oil in the sprak plug hole and re test. If the compression comes up noticably then chances are the rings/bore are damaged as the oil will fill the gaps and offr greater compression.

Again to reemphasise Andy's point I don't quite get your logic behind not servicing a scored barrel. Seems a bit silly to have the engine in bits and to not spend £35 having it sorted. I took my bore to my local Yammy dealer to have it serviced. Some slight score marks on the barrel and they've just honed it put and said it is well within tolerances. As said for the price of £35 for a hone and measure seems a bit silly to pass up.

Granted £40 for the piston kit as well takes it up but if you're trying to diagnose a problem and you evade key steps. Chances are there is your answer.

Another reason I didn't have it done is because I've got a near brand new barrel that I'm giving to mick abbey then getting rebored because it's got some scoring, so thought I'd skip the rings for now and save money

I was going to get a mitaka if it did turn out to be the rings

It's got a new prox con rod that came with small end bearing

Is the athena gasket actually better than a genuine?

There's actually a yam dealer right opposite where the bike is haha

Does it need choke to start it up?

Athena gaskets are equal to the quality of Genuine. Most people use a pattern gasket set and chuck the head gasket and use athena or genuine. ( athena is cheaper). Other pattern head gaskets always fail.

I was thinking air leak if not in the carb rubbers maybe at the magneto crank case bearing seal

Cables fine, I've checked and rechecked

The inlet manifold hasn't got any cracks etc. It seems to be in good condition and its been tightened twice over now. There can't possibly be any leakage from there.

I haven't checked the pv but I'll change the o rings just to be sure

I did turn the mixture screw all the way and then out 1.5 turns and it wouldn't start. Rich mixture and air leak, you'd think it would have made the condition worse. That's why I'm stumped

where did you get the right hand crank case seal from??? was it in akit from yambits or genuine. theres some retailers supplying the incorrect seal. mine was doing the same. it was the righthand seal, it should be a twin lipped/sprung seal that only goes in (1) way round altho may look normal to the untrained eye

mr karnage wrote:

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Standard jets and I know it is that's why I set it at 1.5 turns out

All gaskets and seals are genuine norfe

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 You've said you "did" set it to these settings and it wouldnt start, suggesting that you're not running it on this setting now. It's not advisable to run it with the air screw settings wrong. If you dont know what the problem is you could be adjusting other things and then have it set wrong on the air screw, then put the air screw to the correct setting and it will stop running right. I'm only trying to help here remember.

mr karnage wrote:

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Sorry mate, your right I've set it to what it was before now

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 No worries. I know how frustrating it can be. It sounds like what my YZ was doing. It would start and run fine for a limited time but as soon as i started reving it, it would start bogging down and then die. turns out the bore was damaged. It wasnt revving up high on its own though.

I still think the problem is probably your bore. try puting some oil down the plug and then starting it and see what happens. If it still revs high it has to be an air leak really. one other thing i dont thinks been mentioned yet is the boost bottle above the inlet/ reed block. Is that sealing?

This problem sounds a bit familiar to me? I know the pain all to well.

I was a kinked throttle cable. After hours of heartache and ££££,s and destruction.

Just to clarify is the throttle slide meant to be totally closed ie no gap whatsoever or just slightly open?

It doesn't run off choke and when I open the throttle it dies straight away so it couldn't be the throttle cable I'm sure 

-- Edited by mr karnage on Sunday 19th of January 2014 10:20:47 PM