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Yamaha Tech Originally the Yamaha XT600 Tech Forum, due to demand it now includes all Yamaha's technical / mechanical / repair / preparation questions.
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  #1  
Old 19 Aug 2012
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Carburettor problems XT600E - 3tb.

As the search function at the site seems to be down i will write the question directly:
My XT600E -98 (3tb) runs well at idle, but when i throttle up to around 2000rpm it starts to backfire and doesnt rev any further up.

1st question:
Is the problem related to primary or secondary carb? Secondary throttle function seems to open well, maybe missing gas?

2nd question:
Pilot jet adjusts air mixture? At idle speed?? Setting? Tried 4 lapturns out.

Something to start with...

/ Henrik
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  #2  
Old 19 Aug 2012
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Quote:
Originally Posted by carlhenrik_80 View Post
As the search function at the site seems to be down i will write the question directly:
My XT600E -98 (3tb) runs well at idle, but when i throttle up to around 2000rpm it starts to backfire and doesnt rev any further up.

1st question:
Is the problem related to primary or secondary carb? Secondary throttle function seems to open well, maybe missing gas?

2nd question:
Pilot jet adjusts air mixture? At idle speed?? Setting? Tried 4 lapturns out.

Something to start with...

/ Henrik
I would check air filter, maybe remove it and check what happens. If engine works better without air filter, than either you have too rich mixture, or you air filter is too dirty. Try cleaning or replacing air filter first. If that does not help take carbs down and clean them. Check gas leven in carb.

If engine works even worse without air filter that would indicate that you have too lean mixture. Check two rubber parts that connect engine head and carbs, even little cracks could cause something like this. To properly check inlet rubbers you should take them down.
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  #3  
Old 19 Aug 2012
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Start with Cleaning the carb and airfilter, and give it a new sparkplug.

And tell os more about the bike, has it sat for a while, did it just suddently start backfiring, or has it been faulty for youre entire ownership? Pretty important information.

F.x It doenst help to adjust the air/fuelscrew, if the carb is filthy and clogged up, or if theres no airflow trough the airfilter ect.
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  #4  
Old 19 Aug 2012
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The bike has been backfiring my whole ownership. But not this much! It was usable when i bought it..

I did change the intake rubbers to new ones and the rubber sealings as well.
Clean carb check! New air filter mounted.

It has a CRD slip-on mounted silencer, so i changed the jets from original 130/120 to 140/130 instead.

I even did regrind the valves i the top, because they where burned, and rubber valve-sealings leaked.
Also checked the valve clearence, seems to me its the first time thats ever been done, the bike has only gone 13000km.

Lot of work has been done, so far undone..

/ Henrik
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  #5  
Old 19 Aug 2012
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Youre jetting is rich, you dont need to rejet mainjets for an exhaust-swap. If anything, go back to stock, and go up one number on the idlejet.
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Old 26 Aug 2012
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Now I have tried three different carbs, with the same results as before.
It seems to me the problem isnt the carb at all.
Could be ignition problems. Already saw there is a weak spark..
Someone who had problem with the CDI-unit at a certain rpm??
More ideas where to look?

/ Henrik
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  #7  
Old 26 Aug 2012
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a broken piston ring... do a compression test
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  #8  
Old 26 Aug 2012
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If you have ruled out the carb, and the engine compression is good, it sounds like it might be the coil breaking down. It's quite common for coils to work OK at low revs but to break down when asked to do more work, or when hot. Cheap and easy fix if this is what the problem is.
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Old 26 Aug 2012
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Previously that was a question I made...

I've just remembered that I had a weird problem once slightly similar to yours. In my case I had a relay (one that attaches to the outside of the airbox right next to the battery with a rubber band. It came loose, broke the plastic casing and when driving it would make ground and the bike would backfire, even die...

Can't remember much else besides what you and the guys have already said.
Faulty wiring maybe? side stand switch going wrong with the vibration? The two cables (one orange and the other I can't remember) that connect to the sparkplug ignition coil slightly loose?


Good Luck!
Vando
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  #10  
Old 16 Sep 2012
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Buckxt

Quote:
Originally Posted by carlhenrik_80 View Post
Now I have tried three different carbs, with the same results as before.
It seems to me the problem isnt the carb at all.
Could be ignition problems. Already saw there is a weak spark..
Someone who had problem with the CDI-unit at a certain rpm??
More ideas where to look?

/ Henrik
My xt550 had a simlar problem at 120km the spark would brake up. I replaced the CDI then the coil and still had a problem. what i found was a loose connection on the kill switch.
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  #11  
Old 16 Sep 2012
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When it back fires? Does it back fire through the carbs or out the exhaust?
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Last edited by otr002; 16 Sep 2012 at 08:33.
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  #12  
Old 16 Sep 2012
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mine back fired through the exhaust after hitting 120km
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  #13  
Old 25 Sep 2012
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The problem is solved!
I switched back down to original main jet settings 130 & 125 and did also switch the idlejet one step up from 46 to 48, this because the bike has an aftermarket Slipon silencer, brand CRD -absolute power II.. whatever that name means in reality?!

I also discovered that ive had a broken airmixture screw. After replacing it with a new one, setting it 3 lapturns out the bike seems to run ok again.

But before I found the problem in the carb.. ofcourse did I test the ignition coil at my local yamaha workshop. And It worked as good as a new one.
Then I losened up every connection in the electricity wiring system and checked if there were any corrosion. It was also found to be ok.

And my bike did backfire on both exhaust and carb -side. The main carb was beginning to melt inwards..

Lucky for the bike i found the problem, now it works as good as new!

Thank you all for the help!

/ Henrik
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  #14  
Old 25 Sep 2012
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Excellent news, well done.
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