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Honda XR 125 - bogs over half revs

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Jefr0
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PostPosted: 14:40 - 19 May 2018    Post subject: Honda XR 125 - bogs over half revs Reply with quote

Hi all

Nothing seems to be going right lately!

Was riding home the other day roughly 50mph then it bogged.

Bike runs on half throttle but anything hover that it just bogs.

I’ve replaced the spark plug and air filter which were due but still the same.

Could it be a blocked main jet?

Thanks

Jeff
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P.
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PostPosted: 15:50 - 19 May 2018    Post subject: Reply with quote

Firs thing with carbs is strip and clean, I'd do just that, shouldn't take too long and making sure you put it back correctly and clean, I bet it fixes it. Smile
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Jefr0
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PostPosted: 11:21 - 20 May 2018    Post subject: Reply with quote

Well the main jet was blocked, blew it out. Back together. Still happening.

Stupidly didn’t blow out the pilot jet. Hopefully that sorts it otherwise I’m lost on what to check.
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pepperami
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PostPosted: 11:33 - 20 May 2018    Post subject: Reply with quote

How old is the plug?
Does that need to be replaced?

I’d go for carb stuff first .
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Jefr0
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PostPosted: 11:08 - 21 May 2018    Post subject: Reply with quote

pepperami wrote:
How old is the plug?
Does that need to be replaced?

I’d go for carb stuff first .


I replaced the plug, was pretty old.

Yeah have done the main jet which was blocked. Just need to do the pilot jet now.

Just going to be a bit stumped if that doesn't cure the problem.
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jaffa90
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PostPosted: 11:31 - 21 May 2018    Post subject: Reply with quote

If possible fit an inline filter inbetween the tap and carb .
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andym
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PostPosted: 12:54 - 21 May 2018    Post subject: Reply with quote

I'm keeping an eye on this thread as I have the same problem with my deathtrap project..... I've stripped and cleaned the carb numerous times (neither of the jets were blocked after the first clean), I've replaced the carb manifold, checked compression, exhaust header, air filter on and off, adjusted the mixture (which is a pain as I have to remove the exhaust and starter each adjustment), and still can't get above half throttle either.
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pepperami
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PostPosted: 13:50 - 21 May 2018    Post subject: Reply with quote

Points or electronic ignition?
My old GT 250’s would bog down as the revs rose if the points were not set up just right as would my old CZ125.

However they were both two-smokes.?
I don’t know if that would of had an influence on the matter?
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Rogerborg
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PostPosted: 14:07 - 21 May 2018    Post subject: Reply with quote

Slide carb, right? Is the needle secure in its clip? Is the clip under the spring? I've had all sorts of carb shenanigans when the needle or clip went wandering.
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Octapode
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PostPosted: 16:02 - 21 May 2018    Post subject: Reply with quote

Can you film her when you try and give her some throttle? A damaged ignition system can run on low throttle but not have the power to fire the spark when you open her up, but you should hear her pop out the exhaust from the unburned mix if that's the case.
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WmY
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PostPosted: 19:10 - 21 May 2018    Post subject: Reply with quote

Jefr0 wrote:

Yeah have done the main jet which was blocked. Just need to do the pilot jet now.

Just going to be a bit stumped if that doesn't cure the problem.


I had a very strange problem with an old bike.... there was a bit of muck in the hollow nut just under the main jet. At high speed it got sucked up into the main jet.... when the fuel flow backed off it fell out again. Rinse & repeat....
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Jefr0
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PostPosted: 13:24 - 29 May 2018    Post subject: Reply with quote

Thanks for all your replies.

So yesterday I tackled this again. I had been riding my Dad's XR when that suddenly stopped ticking over. Turns out the mixture screw had fallen out!

I replaced it and the bike ran fine.

So I decided to try this carb on my XR. Still the same problem of anything over half revs just bogs (Bike is fine in 1st/2nd though in whole rev range...)

So that eliminates the carb. Checked the fuel tap by disconnecting it and seeing the fuel run, which was fine.

I also bypassed the ignition to see if it was that, still no change.

At a bit of a loss at the moment. Can only think it's electrical related but not sure where to start.

Pepperami, not sure on the electrical front with your questions - never been my strong point!

Thanks Karma
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P.
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PostPosted: 14:00 - 29 May 2018    Post subject: Reply with quote

Erm, is it specific RPM or just whacking throttle open?

My old Cityfly had issues with plug cap being shite, I replaced with another NGK one and it was fine.
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Jefr0
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PostPosted: 15:49 - 29 May 2018    Post subject: Reply with quote

Well that's the thing, I can full throttle to top revs in 1st and 2nd gear, then attempting that in 3rd gear it doesn't happen.

Hmm I could swap over the plug caps from the other XR and give that a go then!
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rpsmith79
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PostPosted: 15:50 - 29 May 2018    Post subject: Reply with quote

Coil or Plug Lead breaking down under load?

If you have a 2nd bike to play with, i't just start swapping parts over till you find what works/what doesn't
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P.
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PostPosted: 15:58 - 29 May 2018    Post subject: Reply with quote

I'd take the coil + HT from working bike, see if that works. Could be that. Also, steal the spark plug... just because.
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RhynoCZ
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PostPosted: 17:20 - 29 May 2018    Post subject: Reply with quote

jaffa90 wrote:
If possible fit an inline filter inbetween the tap and carb .

Not always the answer. Some engines require unrestricted fuel flow.
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Teflon-Mike
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PostPosted: 18:33 - 29 May 2018    Post subject: Reply with quote

Silly question, but when you have been taking carbs off to swap or clean, you have been using or making new gaskets, not just slapping the old ones back in, or leaving them stuck to the flange face, or using some sort of gloop like RTV instead or as well as... haven't you?
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salem1987
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PostPosted: 19:06 - 29 May 2018    Post subject: Reply with quote

sounds more like a fuel flow issue if its ok in 1st and 2nd and the carbs clean.
I'd check the tap and replace/clean the filter if there is one.
Or kinks in the fuel hoses.
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moonzoomer
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PostPosted: 20:15 - 29 May 2018    Post subject: Reply with quote

Check for blocked fuel tank breather.
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Jefr0
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PostPosted: 14:33 - 30 May 2018    Post subject: Reply with quote

Thanks for your replies all, I'll try all the above at the weekend.

I'm under the impression it's the fuel flow that's why I checked the fuel tap and it was flowing fine.

Good shout on the HT lead and coil, that'll be worth a try.

I think tank off and have a look at any filters/breathers there.

Tef, I haven't been using any sealant. The o rings are in good condition still. My Dad's XR is working fine now with the air mixture screw replaced which was also tried on my XR so can't put it down to a gasket?

Thanks
Jeff
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pepperami
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PostPosted: 15:43 - 30 May 2018    Post subject: Reply with quote

Let us know what the problem was when you find out Thumbs Up
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2Hondas
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PostPosted: 17:14 - 30 May 2018    Post subject: Reply with quote

Blocked Air Intake maybe?

I had a similar problem on my Hornet many years ago (https://www.bikechatforums.com/viewtopic.php?t=182151&highlight=)

And it turned out that I'd pulled back a wiring loom cover and left it directly in-front of the air intake. At low revs it didn't cause a problem, but at high revs it was getting sucked in and blocking the thing enough to choke it.
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Teflon-Mike
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PostPosted: 23:53 - 31 May 2018    Post subject: Reply with quote

If its the 15 year old '03 model reffered to in sig line....
I will take a bet that old age is to blame, and that after lots of muggering about with what are, usually the most reliable periferals of an engine, in the carbs and coils.... it will be a clusterfuck of age related issues, of which, the main culpret will be bore and rings and valves.

The ignition, yup a duff spark-plug can do strange things when it gets hot, or a lot of charge is squashed in the pot, but coil and CDi are pretty robust, and if motor 'bogs' as in refuses to accelerate under load, then picks up slowly after lag.... the CDi is sending signal to coil and coil isn't breaking down under heat....

Carb... fuel gets sucked up jets... probided the carb's sealed to manifold... and you suggest it is... I have to admit I am sangiune of the suggestion... them rubber O'Rings in Kehin PD's dont last for ever, and bike's old.... may look good, but they need to be soft to seal, and not dinged in the rebate.... but you say the jets clear, and the pet-cock flows.... they are rather prone to sticky float needles as they wear, which may explain things, but they tend to be more intermittent....

Brings me round.... if vacuum aint being lost through gaps in gaskets... do you have decent vacuum.....

When you accelerate, you open the taps, lift the slide in carb, gives unobstructed passage for air into the inlet tract; usual cause of bog or hesitation is that that sudden opening sees an immediate drop in air-speed through the wide-open door, low air-speed means low vacuum over jets... fuel dont get sucked up jets... without engine speed riding, or riding as quick as throttle opened, air-speed stays low reletive to engine speed, and vaccum created over the jet stays small, and mix leans out until the engine catches up..... this is why on low emmission engines they have an accelerator pump to pump a bit of neat fuel down the maw, to ritchen things up when mix weakens off....

BUT... if you have knackered bore and or rings.... piston dropping wont create the suck it should to start with.. may be 'OK' at low throttle openings or steady state running, when the carb is effectively choked by the throttle slide, so there's a pressure difference from aor-box to port.... but open the tap, that difference gets very small, and if the rings aren't sealing so well, easier for it to suck air from the crank-case than through the carb..

If the valves aren't sealing... tappet clearances miles out, valves coked or pitted, seats coked or pitted... suck starts before exhaust closes.... the 'over-lap', now suck stroke will drag exhaust back into the pot through the not quote closed exhaust valve... whilst charge will be being pushed out the gaps when piston turns round and starts to put it back on compression.

Long explanations... B-U-T... this sort of anomolouse poor running, flat-spots, bogging, running out of puff early, are all symptoms of a tired old motor... and you can waste an AWFUL lot of time and effort and money chasing down each symptom trying to effect a cure, never really bottoming it, getting frustrated, re-doing things you think you have done, that should have fixed it or seemed to have fixed it....

UNTIL... you bite the bullet and get hands covered in oil battling with the black-bits, doing a recon; reboring, fitting new piston and rings, or chinky barel kit; lapping or grinding valves and seats or buying new chinky valves and head... paying attension to the cam and timing on rebuild, checking over the push-rods and rockers, making sure tappets aren't mushroomed, and doing detail tappet tickle etc....

THEN with that niggle eliminated... carb set up is often just a question of dialing in the book settings; the ignition sorted with a new plug.. and no more than clean and tweek on the HT lead when you put it back together....

And 'low'.... symptoms all gone away.....

BLUMMIN HECK! £29!!! First hit on e-blay! For a Chinky barel and piston kit, with rings! Comes with gaskets too by the looks!

At that kind of money!!!!!!! And given how 'simple' CG motors are and not having to faff with stringing cam-chains!!! JUST get one and fit one!!

Base line the problem, and eliminate possibilities before you drive yourself mad chasing anything and everything to try not look at oily bits!!!!
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P.
Red Rocket



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PostPosted: 08:15 - 01 Jun 2018    Post subject: Reply with quote

It isn't going to be bore or rings, ignore that.
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