Up to 3000 rpm you are using the primary carb and in part the idle jet. Hence we can say with reasonable certainty the issue isnt in the primary carb. Also this carb is cd-slide. i.e. forced operation
The secondary carb comes into operation at 3000rpm. However it is a venturii and will only come into full operation if the vacuum draw is sufficient. It was designed to assist mid range bog down of the primary. For this carb to come into full operation it really needs to have the throttle open wide consistently. i.e. flat out or when accelerating. Above 5000rpm matches this model hence my earlier supposition tht it is a secondary carb issue.

Have you ensured the emulsion tubes are clear?

Faulty coils that are broken have the reverse symptoms. As the engine gets hot they break down, and they operate better then above 4krpm than below it during these conditions.

However carbon tracking in HT leads will tend to arc out to earth before reaching the electrode (plug) too hence why I didnt discount it.

I would not worry about the slide on the RH carb wearing out. We have a tenere with the same carb that still goes strong after 160.000 km.
I am puzzled by the issue, but 2 more tests you could do:
- Do a run with the air tube of the coasting enrichment blocked off, so that nothing could enter the barrel of the RH carb from the left and vice versa.
- Take carb off (again, sorry for that), and suck with a vacuum cleaner at the outlet side with the butterfly valve open. The slide should open smoothly and you should be able to control the opening of the slide with the butterfly valve.

BTW, the theory of the blocked emulsion tubes / holes does have some credibility as the emulsification has a larger influence with colder engine, evaporation is better when the engine is warmed up. So, that would be worth a check also.

Did you also raise the RH carb needle with a significant amount? Effect?

Auke

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Auke
http://aukemarieke.blogspot.com

Thanks for the responses guys. I'll double check the emulsification tubes, but I'm pretty sure I got them properly clean.
I only raised the needle on the left hand carb. Just did it to check if it made any difference and to rule it out.

Could an improperly adjusted inlet valve on the right side reduce the air flow enough to prevent the secondary slide from opening effectively? That might explain the heat related aspect of it also ?

I'll try the thing you guys have suggested. Bruken, it is definitely running lean from your symptoms description.

Thanks for hanging in there with me.
Cheers, Julian

Shite.... Sorry, mistook the troubled bike for a never one. Thx, your data seems more right. Thx.

Can you explain carbon tracking? Is it within the HT lead itself?
Thanks, Julian

Not necessarily. It can also develop along the porcelain outer of the sparkplug. Basically if the resistance of the plug is too high or the HT lead is a bit iffy or plug cap for that matter, then the spark will arc across the point of least resistance, not necessarily the plug or only the plug. Each time it does this it leaves a carbon track following the path of the arc. Do this enough times and the carbon track develops further. This can lead to a point where the carbon track, carbon conducts electricity, offers lower resistance enough at least to rob the plug of a good spark across the rev range. Remember plug resistance changes with heat, pressure, and fuel mixture.
Carbon tracks on the porcelain of the plug are very visible and most commonly develop when moisture gets under the suppressor cap.... washing, rain etc.
Silicon based suppressor caps are prone to this with age. So too HT leads as they lie flush against the frame or close to, which supplies the earth.
Carbon tracks in the HT leads and cap can be identified by what looks like black pin pricks...... which in the XTs case is nigh on impossible to see as the HT and cap are generally black too.

I would not say this is your first port of call. Just advise you to change them in trouble shooting as best practice. I also reiterate I have not experienced this myself on a XT but have had the exact same symptoms on large scale model aircraft using 100cc plus engines and TCI ignition. There it is a common and recognised issue.
I still maintain you are most likely running lean. BTW I take it for granted you have wet checked your float levels?

To answer some of your questions:
Yes I did wet check the float levels of the carb. I assume by this you mean putting a clear tube on the drain to see the fuel level after idling. Also did it when the carb was apart and measured the 26mm height to bottom of float.
All the emulsion tubes etc looked fine and clear. Nothing looks amiss anywhere. I'll try your suggestions in the next couple of days Aukeboss.

My gut feeling is the second slide is not opening properly, but what could cause this. Every airway has been cleaned, I'm pretty darn sure there are no airleaks on the carb rubbers.

Maybe I should try raising the fuel level in the float bowl? Just to rule it out. Seems a bit of a tricky thing to measure and set accurately.
Thanks again for sticking in there with this.
Cheers, Julian

The thing that is really bugging me is the absence of misfires. You seem to have checked everything in the carb. I am beginning to wonder if your valve timing may be out. Especially if it has advanced. It may be worth checking if the cam timing marks all line up. It's not uncommon for the cam chain to jump a tooth during maintenance on the head or cam chain tensioner if the cam was under load during a tightening sequence. Ie suppressing valves.

Hi.
Posted before about a similar prob I had some years back.
Have you checked the valve timing, i.e, I had a customers bike showing similar problems and traced it to one tooth out on one of the camshaft sprockets.
At the very least it would tick it off as I went down the carb route before checking for it.
Hope it helps mate.
Dave.

I can check that. I'll have to do a bit of reading in the manual first. Something I should check anyway.
I had kind of discounted a timing issue because of the fact that after 10 to 15 mins of riding it would run well. If the timing was out, I didn't think there would be a change in performance. IE it would consistently run poorly and not improve. But, I'm open to anything. I'll get back to you on that.
If it is a carb problem, it's got me stumped, or it's an air leak/fuel blockage that is so small it's difficult to isolate.
Thanks for being fellow head scratchers. I owe you all a .
Cheers, Julian

Do you need to remove the cylinder head cover in order to check the valve timing, or can you do it through the timing holes.
I've got a Clymer manual that I'm really starting to get annoyed with. The explanations are crap at best. CHeers, Julian

Yes, unfortunately. Quite an operation, I'd suggest to first check your other options. In more than 300.000 km of riding XT6's I never had a timing chain jump a tooth ...

If you decide to take the cover off, do not forget the nastily hidden bolt between the upper engine mount. I guess that you do not have to drop the engine, just remove the carburettors and 'rotate' the cover off backwards. This is how it is on the older XT6's.

A

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Auke
http://aukemarieke.blogspot.com

Probably because you assembled correctly .... .. but there is no accounting for it's history.

"In more than 300.000 km of riding XT6's I never had a timing chain jump a tooth ..."

Indeed!
However, this wasn't suggested, jumping cam chains that is.
The bike was bought with this problem and who knows what has been (or not) done?
I have seen it before, as in my earlier post, but quite right bruken, you just never know do you?
Dave.

You'll be suprised what some people can do, and I'm no exception. I had a good mate who fully assembled his camshaft tensioner before putting it into the bike. How he torqued it down is still a mystery to me. Within 20 odd miles the cam chain had broken. He also complained bitterly about how much oil he was leaking until he found he had 6 litre in instead of 2.6 because he was overly fastidious. I have a early Tiger Steamer that you HAVE to take into concideration that when you torque down the cam caps it will almost certainly jump a tooth. Granted the XT doesnt have cam caps but the load principle still holds true. These engines are so easy to work on that it also has the unfortunate consequence that less than profficient peeps are willing to and do climb in happily...wouldnt dare do the same with their cage or gixxer tho. I cut my teeth on the XT550, and I'm not even going to begin telling you the cockups I made while learning, now I restore classic bikes as a hobby and even now I still have "fun" moments.