dt125r keeps missing at high revs

[madcookie38]

Can anyone please help as iam pulling my hair out as ive ran out of ideas.

My dt125r has got a missfire at full throttle in every gear,

Ive tryed cleaning the carb, No joy

Tryed a new plug, No joy

Checked the power valve all seams ok, ie open and closes

looked at the air filter a little dirty but ok

Tryed running it without air filter still misses,

I should add that this bike starts up seconed kick nearly all the time and will tick over nice once warmed up. it revs clean up to about 7 rpm

it just wont rev past 7 to 8 rpm without missing even when not moving, You can hear the revs sort of coming in and out,

AM I MISSING SOMETHING ?????

ILL BE GREATFULL OF ANY HELP YOU CAN GIVE ME,

[Fizzer Thou]

Check that the reed valves are not split around the edges and that they are seating correctly.They are cheap and easy to replace.

Check also that the plug cap is of the correct resistance.It should be between 5-10kohms.Some internal resistors inside the plug cap are removeable and able to be cleaned of any corrosion.
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Just talk bikes.What else is there?

Always have a 'Plan B'

[DMCpro]

Spray wd 40 around your manifolds and listen carefully with the engine running for any drops in revs even the slightest noise
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Honda CB-1, TS125X
VW Golf Estate

[madcookie38]

Right had another go at the Dt today

Had the reed block off and all seams ok ?
I tryed another coil, lead, and cap still missing no change at all ???

so still pulling my hair out

This bike is going to see a big hammer soon.???

Thanks for the help so far guys.

[Teflon-Mike]

1/ Has any-one tried de-restricting it?
2/ Does the Speedo Work Properly?
3/ Any signs that the speedo has been tampered with / Broken?

If I recall correctly; DTR has a reed-switch speed governer behind the speedo, that is activated by a magnet on the needle, at about 55/60mph, which tells the ignition CDi to use a retarded timing.

Aparently DIY bodges to by-pass the switch result in malefariouse problems, with the retarded ignition map constantly triggered, or bouncing in and out. Damage can cause similar effects.

So that's a likely candidate.

Next up; its a two stroke; oft de-restricted, often not done well; and with the little niggle that attempts at de-restricting are more likely than basic mantenence....

DT lumps a pretty hardy example of the species; but bores still wear, rings still gum, small end bearings still go, and mains dont last for ever; especially if owners aren't too clued up, and run them without checking two-stroke oil reservoir, or decide to follow some old bit of 'lore' like using chip fat instead of two-stroke oil.

It could be that the thing is in need of a top end rebuild & rebore.

Next on the list; exhaust. Miscalaniouse bad running, often down to the exhaust, and two chekcs I would make is first, has the exhaust got a gasket behind it..... I have actually been handed a copper crush washer by an eager owner telling me that it was the 'Restriction Washer' and that having taken it out thier bike was now 'De-Restricted' Sort of explained why it ran like it was fueled by last nights Vindaloo vapours! Meanwhile, the convoluted routing of the high-level expansion chamber puts a neat 'U-Bend' just beyond the exhaust port; this, like the U-bend under a sink, can catch condensation quite well, and is often where the pipes rot out. So close to the port, even a small pin-hole there can do some very strange things to teh carburation. Check for pin-holes; check for gloopy crap smeared over potential pin holes or even complete sieve like remnants of pipe!

And lastly, if all else has turned up a blank; main seals. Ends of the crank, one sealing crank case from atmosphere int the alternator cover, other the primary drive and gear oil. Pull mag cover, sniff for petrol. Check if gearbox oil milky. Does the plug foul quickly?

The seals dont always go catastophically, they go intermittently, sealing less and less well, and often acting like a one way valve, letting pressure out of the crank case at full throttle.

Do you have the Work-Shop manual though?

There is section on 'Fault-Finding', and list gioves the full fault tree of stuff to look at and eliminate, and following down eliminating what its not, you will eventually get to what it is..... just takes time patience & perceverance.
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My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'

[Vincent]

Loose flywheel?
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Space Is Deep

[madcookie38]

Thanks again for taking time to try and help me sort this Dt out.

Well firstly I should say that this dt125r has had the following done to it in the last 3 months

New piston and rebore to a 1mm over size

New crank seals

A skimmed head and new gasket

The exhaust is in good nick with the washer removed from the front pipe

I have removed the snorkle from under the seat to help it breathe
better

I haven't jetted the carb as i dont think it needs it as its running a 240 jet which i belive to be standard,

This isn't the first Dt125r ive had but so far the worse, My last 1 was a 1996 bike which done me proud and never let me down in nearly 20.000 miles and would do 70mph all day.