Hi, and trouble shooting question.
 

Hi All, Brand new to this forum. I have just bought an 86 xt 600 which I now have running. This is the third XT I've owned and I love these bikes. I rode around Australia on an '84 model back in 1994. However, I've got a tricky troubleshooting question for anyone that is interested.
The bike accelerates well up until about 3000rpm. After that, the power is very flat and won't really pull above 5000rpm. However, there is no misfire. It feels asphyxiated , short on either air or fuel. After about 15 minutes of riding the bike started performing as I expected it to. Pulled hard throughout the rev range. The previous owner reported similar symptoms.
My questions are this:
Could a weakened battery cause this type of symptom? (IE a weakened spark that reduces power until it gets charged enough to give a healthy enough spark under load. (I'm putting in a new battery tomorrow)

It feels like it's either short of fuel and air. It doesn't have the filter on the float valve but does have an inline filter. Any other culprits? The air filter looks clean and is oiled. I have a basic understanding of how the twin carbs work, but can someone point me to a link that explains how they operate. I looked on here but couldn't find a thread.

Is there a link to download the manual for this model. Again I looked. I have the Clymer manual but it is not very good at explaining the operation of the systems.
Thanks in advance for any help you can give.

Hi Loony,

I ask this info some time back which did come in handy. Maybe it can help you.

http://www.horizonsunlimited.com/hub...2-months-41047

or

http://www.horizonsunlimited.com/hub...ride-rev-42555

good luck.

ta-all-the-way

Xt
 

Hi.
I have had a few similar problems over the years with inline fuel filters!
It seemed as if the filter could't cope with letting enough fuel through at higher sustained revs/throttle ect.

Just try it without the filter.
Hope this helps.
Dave.

It certainly sounds like a carb issue, especially if the second carb is not getting enough fuel and the bike is running lean. The engine will run leaner more readily when hot. However one cannot discount the ignition and I mention this because of the lack of misfires. Plug resistance changes with both heat and pressure. If you have a poor ignition system (weak spark or carbon tracking) this MAY lead to this kind of symtom - although I have never experienced it on a XT, only on 100cc 2 stroke RC aircraft engines I use in that hobby- and the early XT's are known to have issues with the suppressor cap and HT leads. I would guess it's the secondary venturi carb that is giving you issues but it's always worthwhile changing the HT leads suppressor cap and plug to.

Thanks.
 

I'm going to put the new battery in today and see if that helps. Got to do it anyway for the blinkers to work. Then I'll try those options one at a time. Thanks for the input and I'll keep you posted.

Update.
 

SO, still have the same condition on my bike. Starts fine and idles somewhat okay, slightly erratic but pretty good. When starting out and riding, it feels underpowered and after about 3000rpm it is particularly sluggish and wont rev past 5000pm. Also has a bit of backfire. This condition lasts for about 10-15 minutes of riding. Then, the bike starts idling high like it is running out of fuel. After this the bike runs fine. Full power through out the rev range and runs great. Idles almost perfectly after the 15 minutes and the high idling incident. The only thing consistent about this is that it seems to follow this pattern every time.
So, since I last posted, I have completely taken the carb apart, cleaned all the orifices/airways with compressed air and carb cleaner. The coasting enricher diaphragm looked in good shape. Everything overall looked good and I found no signs of any blockages or wear on components. I checked the float bowl fuel level and it looked okay, although this is the first time I've done this. It was 6mm below the float bowl lip which is in specification. I have also changed the spark plug and put in a new battery. Took out the inline filter and there is no filter on the float needle. Also cleaned the air filter. The rubber inlet boots look in good shape. I squirted some WD 40 around the boots and didn't hear any difference in engine rpm. Also put in fresh fuel. Choke seems to work as it should and if I put it on while running the bike it makes it run worse.
SO, what is it that after this 10 to 15 minutes that changes and makes the bike run fine. Is it temperature related and what would cause that? (CDI, spark plug cap, ignition timing)
It still acts like a carb issue, either fuel starvation or asphyxiation, but that would not change with temp would it?
What other things could I do with the carb.
Ho HUM. Any help would be appreciated.

Regards, and still scratching my head,
Julian:confused1:

Further update.
 

Checked the plug out last night and it was whitish, indicating too lean. Seems to run hot as well. The previous owner had it at altitude and changed to a smaller jet. I returned it to the jet it had before and the plug was still white. So, I'm wondering if the previous owner also adjusted the needle height. How does one do this. It looks like you shim it or de-shim(?) it to raise or lower the needle respectively. How much should you do at a time?
AN incorrect needle height would fit the symptoms I have. After 10-15 minutes when the engine gets really hot and wants to run leaner, the current settings start to function properly hence I get the performance improvement. Does that sound correct?
Other options could be an air leak but this doesn't fit with it's consistency of running poorly and then improving after getting hot.
Thanks for any help and I hope this troubleshooting process might aid someone too. (That is if we figure it out!)
Cheers, Julian

Which jet did you change? Tha mainjet does nothing for standard riding, its only for wide open throttle.

My plug seams a tad lean also, eventhough it isnt.

The needles have different grooves, and which a little lockwascher sits. The "washer" can be moved to the other grooves. Stock it sits in the middel (3rd fromt he top) on both needles. Move the clip down, to make the needel sit higher, thus raising the needle.

Have you checked for airleaks on both sides of the carb, where the rubber mounts? Spray some dieselstart aorund them when the bike is idling. If the rpms change, you have an airleak.

Does the fuel fun freely from the fuel tap if you take the pipe off the carb?
If not, try removing the petrol tap from the tank and making sure that the filter stacks are clean and that the tap works as it should.
Check for vacuum leaks in the rubber manifold that the carb/s slot into, these crack sometimes allowing air in.
Have a look at the the gasket/seal between the head and the rubber manifold.
I'm sure you've done this already but an extensive air blowing of the carbs and all it's jets and air bleed drillings is a must!
Not sure if an incorrect needle height would cause these symptoms but stranger things have happened.
I see you have just bought the bike. Did the previous owner have this prob or have they done any work on the bike?
I had a similar problem on a customers bike some years ago, the cam chain was one tooth out on one of the camshaft sprockets as a result of a DIY cam chain change.
Just a thought.
Hope you make progress mate.
Stick with it, it's a great feeling when you solve it!
Dave.

Do you have the standard airbox, snorkel and airfilter on. Has the exhaust been modded. Changes here can also effect jetting. Again, I can't speak for the xt but as a general rule going for a K&N filter increases inflow requiring larger jets. The snorkel also plays a important damping effect.

I can't see how something carb related would change after 15 minutes. Usually things get worse with temperature, espeially when electrical related - cracks in coils opening up etc.

Does it have these symptoms even when started hot, or is it only from cold?

Are you using the choke to start it?

Do you turn the fuel tap off, or leave it open?

I'm assuming there are no temp sensors on a 1986 engine, and possibly a vacuum advance? (My FJ1100 has vacuum advance on the ignition unit)

definitely carb
 

The specifications for the carburettor of a German '86 XT are as follows:
main jets L/R: 130/125
needle / clip position left: 5C37/4th notch from top
needle / clip right: 4A71/4th notch from top
idle jet: /46
fuel level / float height: 6-8/25-27
idle setting: Basis 2,5 indiv. setting
synchronisation: 5 mm

I'd recommend to check the needle positions, as these have the most influence on the mixture at part load. This version of the XT seems a bit out of the ordinary as the needles are not standard in the middle. The previous owner might have changed the positions also, which would have been better for altitude adjustment than changing the jets. Also, there are some tiny washers and rings that should be under the clip, which obviously also works on the needle position relative to the slide psoition.
Then check the synchonisation, although the 5 mm indicated above was later changed to 6 by service bulleting from Yamaha. Does the vacuum operated slide in the RH carb work as it should?
Finally, although I do not think that that is the issue, adjust the idle mix setting to your bike, for any older XT the 2,5 turns out is way too lean.

A

Answering some of your comments/questions.
 

Yes, fuel flows freely from tank. I've also ridden with the fuel tank cap loose so as to eliminate the cap vent as a problem.
No signs of air leaks. Sprayed carb cleaner liberally around the manifolds with no rpm change.
Yes, I use the choke to start but once warm it does not need it. Bike starts easily.
The previous owner did have this issue.
The bike does has the standard airbox and filter.
The bike has an aftermarket exhaust. A Produro.
The jetting is standard now.
Usually leave the fuel tap in the "on" position when I stop.

How can I check accurately that the second slide is working properly. I took off the rubber boot from the air box to watch it with the engine running, but thought that the airflow change might not make it work correctly. What should I observe when watching this? What I saw was the slide jerking up and down when I revved it, but it did so very erratically. It didn't open right up either. Should it have been a smooth up and down action. Again, I thought this may have been because the air boot was off and changing the air flow. Thoughts?

My needles don't have the 5 slot positions. You raise or lower them I assume by shimming with washers. I tried this with the plastic washer but it is a very thin washer. How far apart are the notchs on regular needles. I can try with thicker washers too raise the needle, just not sure by what increments. (Ie. 0.5mm or 1mm at time)

Should I be raising the needles on both sides of the carb? Not sure how to do that on the RH carb as again there are no notchs.

My gut feeling is it is either the secondary slide not opening properly or the needles need to be raised. WHy after 10 mins of riding would these change though?

Thanks for all your help. I feel like I'm getting closer if only because I'm ruling out more stuff.

Maybe a beer would help me think clearer.beer
Cheers, Julian

Raising the needle.
 

I raised the needle up a lot and still no difference in the performance. I think if it is carb related, it is the second slide not opening up properly.
Sooo, why would this slide not open up?
Nothing looks bent or worn. It appears to move with no resistance. It is the metal diaphragm type. Can these wear out? Could a damaged coasting enricher diaphragm cause a problem. It looks fine by the way.
How do I determine if the slide is working properly?
Again though, why does it run fine after 10 minutes of riding.

Hoping to get there soon. I'm very quick now at taking out a carb and stripping it down by the way. Can get the thing out in about 15 minutes!!!!!
If there were a sport called Carb Removal Racing, I'm starting to think XT 600 owners would be at the front of the pack. We'd even have time for some head scratching and a beer while the other competitors finished up!

Cheers, Julian

SYMPTOMS: ENGINE TOO RICH.

a. PERFORMANCE IS FLAT, NOT CRISP.

b. CONDITION IS WORSE WHEN ENGINE IS HOT.

c. CONDITION IS WORSE WHEN THE CHOKE/
STARTER IS USED.

d. DARK,FOULED SPARK PLUG.

e. REMOVAL OF AIR CLEANER HELPS.

f. EXHAUST POPPING OR BACKFIRE.

g. ENGINE SOUND IS DULL.

SYMPTOMS: ENGINE IS TOO LEAN.

a. ACCELERATION POOR, SURGING.

b. PERFORMANCE IMPROVES SLIGHTLY AS
ENGINE WARMS.

c. BETTER PERFORMANCE WHEN CHOKE/
STARTER IS USED.

d. NO COLOR, OVERHEATED SPARK PLUG
ELECTRODE.

f. POPPING BACK THOUGH CARBURATOR.

g. ENGINE PINS OR KNOCKS, EXCESSIVE
ENGINE HEAT.

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.

the mystery continues ...
 

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

Thanks for the help
 

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.

Carbon tracking?
 

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.

Probs
 

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.

Valve timing
 

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 beer.
Cheers, Julian

Checking valve timing
 

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

valve cover off ...
 

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

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

Cam type stuff.
 

"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.doh 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.:oops2:

ai
 

Gentlemen, I stand corrected, in the sense that indeed everything is possible.

How do you match the described symptoms with the jumping camchain?

A

If the cam timing is advanced you generally can expect good and improved torque and power in the low range but lacking in the upper rpm range. The reverse would be true for retarded. It does not explain why the symptoms go away after 15 minutes, but based on Loony's assurances it is starting to sound like it may not entirely be a carb issue after all.

do a google search... everybody is basically reporting the same thing ... but why it clears after 15 min ....:(... it could be that there is more than one issue at fault. lets find something that is obviously wrong and work forward.

Sluggish 85 Shadow after rebuild - Motorcycle Forum

GSXR Cam Chain Timing Off - Sportbikes.net

www.vtr1000.org • View topic - Familiar symptoms to anyone?

timing belt off a tooth? - Honda-Tech

Hi All
 

Just to let you know I haven't given up on the problem, I've just got to spend less time on it. I'll get back on it in a few days. Thanks for all your help.
Cheers, Julian