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Honda XR 125 - bogs over half revs

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Jefr0
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Joined: 29 Jul 2007
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PostPosted: 17:51 - 10 Jun 2018    Post subject: Reply with quote

So today had another go at it. Problem still not found. Here’s what I tried:

- swapping coil and ht lead over
- swapping over spark plug
- swapping fuel tanks
- swapping over CDI

The carb is going to have a full clean again although when I swapped over carbs it still had the same problem so can’t be that.

Could it be a bad earth somewhere maybe? If so is there many earths to check?
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Rides: Honda XR 125 ('03) Honda CBR 600F ('91), Ducati 916 Biposto ('95)
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moonzoomer
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PostPosted: 18:54 - 10 Jun 2018    Post subject: Reply with quote

Possibly the choke is stuck in the closed position
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Teflon-Mike
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Joined: 01 Jun 2010
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PostPosted: 19:00 - 10 Jun 2018    Post subject: Reply with quote

You are like a drunk that has dropped his car keys veinly hunting under the street-lamp, 'cos thats the ground he can see, not looking under the car where he was trying to undo the door when he dropped them.

Its an old engine... coils and CDi units tend to be some of the most reliable bits on them; carbs, not faffed to hell and gone, likewise; old age and wear and tear starts inside the engine... thats where the drunk has lost his keys... get a compression tester on it.. or just rebarel the thing and be done, and know that you have checked under the car, so if the keys aren't under the light... go check the bar!

Bad earths? Would tend to cause a harfd fail that stops the bike running at all, or a pretty random intermittent problem, not one you see every time you try opening the throttle....

Rings and guides MAY not be the root cause, but they are a very likely contributor; process of swapping a barel, eliminates the rings and bore wear; having head off and lapping valves, eliminates a lot of chance its them; whilst act of rebuilding and following book settings, doing proper tappet and timing check, sets everything as the book says it should be INSIDE the engine..... £30 and a lazy sunday with spanners in the doing, while you could wast half a life time chasing ancillaries like coild or carbs or nieve hopes of finding rusted earths, neither fixing the problem, or leasrning anything that gets you closer to finding and fixing..... all for the sake of not wanting to see oily bits?

Bite the billet, tear the thing down and rebuild, to the book, and you will odds on 'fix' the problem in the endevour, as well as put life back in the bike and gain oomph for it.
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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?'
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Jefr0
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Joined: 29 Jul 2007
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PostPosted: 15:07 - 13 Jul 2018    Post subject: Reply with quote

Well this still hasn't been solved. Here's a list of what I've tried:

- Replaced the spark plug and air filter
- Cleaned carb
- Tried another carb
- Bipassed ignition
- Checked fuel tap
- Tried different fuel tank & tap
- Swapping coil and ht lead over
- Swapping over spark plug
- Swapping fuel tanks
- Swapping over CDI

I know Tef mentions it above too but could it be the piston rings had it?

The time it changed I was doing about 50mph full throttle accelerating. Had enough oil.

Not sure what else to try apart from a strip down?
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Rides: Honda XR 125 ('03) Honda CBR 600F ('91), Ducati 916 Biposto ('95)
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Jefr0
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PostPosted: 14:11 - 23 Jul 2018    Post subject: Reply with quote

To give a bit more info:

The bike idles fine.

The first couple of gears can be ridden to redline. After that or running for a little while it'll do low revs only.

Could the bike not be running right due to not enough compression?

If so, are there any compression test kits that you'd recommend for my bike?

Would either of these be suitable? Not sure on fittings?

https://www.ebay.co.uk/itm/Quality-Petrol-Engine-Compression-Tester-Test-Tool-Gauge-Car-Valve-Motorcycle/401419172030?epid=2218853112&hash=item5d767278be:g:w3UAAOSwg7VZpn7W

https://www.ebay.co.uk/itm/Silverline-Petrol-Engine-Compression-Tester-piston-ring-cylinder-car-motorcycle/361369182536?epid=17019448419&hash=item5423482148:g:2dQAAOxydB1SlynZ

Thanks

Jeff
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Rides: Honda XR 125 ('03) Honda CBR 600F ('91), Ducati 916 Biposto ('95)
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Tedwood
Two Stroke Sniffer



Joined: 08 May 2014
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PostPosted: 14:56 - 23 Jul 2018    Post subject: Reply with quote

I'm not familiar with your bike/ignition system.

However, for what it's worth, I had a similar problem with my CB125RS.

What I found was my ignition timing was way out as the genny rotor had spun-up so that the tdc marking I was using to set the timing was wrong.

I'm not suggesting you have the same problem, just that I would check the ignion timing.
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Jefr0
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PostPosted: 13:48 - 11 Aug 2018    Post subject: Reply with quote

I’ve bypassed the ignition too by the keys to eliminate that but haven’t tested ignition timing?

I’ve just done a compression test on both my working XR and the XR in question. I’m getting the results so can’t be engine related.

I’m thinking this could be electrical? Not sure what else to test now. Bad earth?
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Rides: Honda XR 125 ('03) Honda CBR 600F ('91), Ducati 916 Biposto ('95)
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Tedwood
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PostPosted: 15:08 - 11 Aug 2018    Post subject: Reply with quote

Check the ignition timing is correct before anything else.
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Jefr0
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PostPosted: 12:02 - 12 Aug 2018    Post subject: Reply with quote

So yesterday I thought I’d try swapping over the coils again.

I took it for a run and it ran fine?! Absolutely perfect.

So I thought I’d try the ‘duff’ coil on the other bike. Took it for a run and it worked fine also?!

So swapped them back. Ran them both again and they both running fine now.

I’ve no idea what’s changed but it seems to have fixed itself.

Any ideas? Just got a bad feeling it’s going to happen again.
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