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.