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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 21 Sep 2008
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Question Xt 600 Sputtering and messing about

Hi there,
I have been a regular viewer on this forum for a while but I now actually have a problem that I wonder if someone can help me with. It concerns my xt. I bought it as a basket case a number of years ago and have since built it up. I have been happy with it apart from after the first few times I tried to open it up to freeway speeds. Bottom range is great but it suddenly started sputtering when it hit about 60. I could previously wind it up to about 100 with no probs. I know that is still getting enough air as I have checked and cleaned everything and nothing else has been changed in that department.

Like a tool, the day after I noticed it spluttering, I adjusted the air fuel mix and took it for another run. This time when I gave it some gas it started really messing about and led to me pushing it for about half a mile. When I was trying to crack it up again, it blew a fuse and killed the spark. after a load of kicks it sputtered into life again and cleared itself completely (at low speeds). It then died again and I lost the spark completely. After a couple hours checking the wiring for shorts after blowing the fuse and testing the voltage going to the coil (thinking it could be CDI related) I got nowhere. After undoing, cleaning and redoing all connections in the spark circuit including the HT lead, it started showing 12v and a decent spark out of nowhere when kicking it over.

So....I can now run it at fairly low speeds and it revs freely to the red line in neutral. However if I gas it in fourth or fifth I end up with it sputtering and farting and either dying or finally clearing itself.

I have monkeyed around with the air screw but trying it in a number of positions seems to make no difference to performance. Could it be a dodgy coil cutting the spark? The CDI? a blocked jet in the secondary carb? (the only reason I haven't torn the carbs down is that after it has started messing at mid to high speed it still sputters when I slow down to low speed and also in neutral for a good while before clearing.)

It's been a bit of a bugger and after sorting the head gasket, clutch, charging system, whole electric start system as well as the cosmetic side of things, I now just want to ride the damn thing!!


Any help would be greatly appreciated!

Cheers,
Mike
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  #2  
Old 22 Sep 2008
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Hej Mike,

it would be helpful to know which XT model you own and if there are any changes to the original set-up (i.e. exhaust).

Try to find what causes the problem - carbs or electric.

For the electrics - side-stand switch, kill switch, ignition lock - common trouble makers. Ask if you don't know how to run the bike without these items, make sure how the kill-switch works as it is not the same system on all models.

For the carbs - check spark plug colour - indicator for too much or too less fuel. If it is a dark grey up to brown - go to find the fault in the electrics. If it is white or black - post the result and we'll continue the checks.

As you had the engine in parts (head-gasket), check also ignition timing.

But due to the lost spark I would go for electrics first!

Good luck

Uwe
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  #3  
Old 22 Sep 2008
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Fuel and spark

Seems you have the seldom combination of ignition AND carburettor problems.

Eevrybody is already advising you on the ignition problems, so I'll concentrate on the carb problems.
Start with the easy part - is enough fuel arriing in the float bowl? Check this by opening the drain screw with the fuel tap open. You should get about 25 liters per hour or 0,5 a liter per minute in this way. If not, check the filters in the tank, an inline filter you have installed or, the nasty one, the little filter on top of the float needle. This last one - remove it!

Good luck!
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  #4  
Old 22 Sep 2008
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Sounds to me like the diaphragm of the 2nd carb is u/s.
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  #5  
Old 22 Sep 2008
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1VJ Tenere 1987

Thanks for your relplies so far I am using them to eliminate potential issues.

Ok, the exhaust is a full stainless with micron end pipe. This was on when the bike was working perfectly.

I have changed the ignition barrel since building the bike and it has never caused problems ( i checked the wiring when I was flumuxed before and nothing has come out of place.)

I got rid of the sidestand switch in the build as knew they could cause trouble.

The kill switch is fine (checked that with the previous wiring checks)

The bike has always run rich and the plug is still a touch blackened. I could do with help as to what the factory setting for the air screw is, so I can start on an even playing field it runs in my mind that it should be about 1 1/4 turns out. Am I barking up the right tree?

I'm going to check fuel output and whip the carb off tomorrow night to see if that's the cause. Possible that it could be the fuel tap filters inside the tank as I replaced the tap recently as well as the fuel pipe as it had started to vapour lock due to the wrong bore fuel pipe.

By the diaphragm becoming u/s do you mean unstuck or unserviceable?
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  #6  
Old 23 Sep 2008
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Hej Mike,

reading your last posting I would vote for Auke's tip: the small filter inside your carbs.

Tried to work on your fuel pump with some gasket fluid? If the stuff doesn't work with fuel it will be in this small filter. Then the bike works on low revs and will not get enough petrol on higher ones.

A way to check this before ripping the carbs into pieces may be a testride and pull the choke when sputtering starts. If this extra portion of gasoline helps:

if not are still the fuelcocks to check.

Someone else has to help with the setting for a 1VJ air screw - anyway the setting is unimportant when you accelerate. The correct setting just keeps your eninge running on idle speed and when throttle is closed down, also it helps to start the bike.

Good luck and don't loose the spark again!

Uwe
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  #7  
Old 23 Sep 2008
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Fuel pump

Hi again,
Just a query. I have done a lot of work on the xt previously, but I have a question. The bike has a much later tank on it and I found from the completion of the build that it ran best without the fuel pump on. The later tank has only one petcock, which may or may not make a difference to the fuel pump operation. Am I been a dullard? Does it still require the fuel pump say for extended high speed use? I've got to admit for the type of riding i do it seemingly made no difference at all. (I still have it if I need it.) This is obviously just an aside to my current problem but I need to check I'm not cutting my nose off to spite my face or making my current situation worse than it needs to be!

Again your ideas are more than appreciated.

cheers,
Mike
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  #8  
Old 23 Sep 2008
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I had very similar problem with my 3AJ, run sweet at low revs but open the throttle suddenly and it went very fluffy. Stripped and cleaned carbs, adjusted the secondary carb opening screw ( the one that opens second carb at higher throttle speed ) but I think the main cause was that some inline fuel filters I had fitted had broken up and filled the fuel pump with the remains, small bits of filter paper. Cleaned this out and was running perfect. Worth a try.
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  #9  
Old 23 Sep 2008
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I've had a few 'unusal' running issues with my bike recently too at higher revs.

It turned out to be the spark plug lead vibrating loose - it gave a a few tickles from the coil on the bars too.
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  #10  
Old 5 Oct 2008
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Question Hmmmm still strange things a-happening!

Ok, firstly a massive thanks to the replies that got me on my merry mending way. Finally got chance and opportunity to get into the carbs this weekend.

the filter above the float needle looked like a tiny rat's nest and the diaphragm of the second carb had unseated itself. Got those sorted cleaned all the jets and stuck it back on. Also checked the timing of the opening of the secondary carb and that was fine

The post repair run -
Once the carb had cleared itself of residue and carb cleaner it was working much better but still stuttering a little when moving onto the secondary carb.

However after a few miles riding it around it died. For no reason. sounded like either the float bowl ran dry or the spark was cut.

Ground to a halt. Kicked it over a few times but nothing. Hit the starter button ( I have a weak battery at the moment so I just use the starter to ease kicking it over.) and with a kick as well it sparked back into life. half a mile later did exactly the same again.

The weirdest bit was when the engine was dying for the third time I hit the starter by mistake and it came to life again. The rest of the way home was fun, as every so often it would be dying and I would blip the starter and it would either sort itself out straight away or I would keep my thumb on the starter for a longer period until I could take my thumb off without it dying again.

Now obviously there is a little electrical jiggery-pokery going on but Before I take the tank off (again) to look into the loom can anyone tell where I need to look fist. I presume it is a bad connection somewhere or that it is losing it's earth at random intervals - any help would be hugely appreciated!!!
Cheers,
Mike
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  #11  
Old 5 Oct 2008
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Hi Mike, well out of my depth with bikes but had the same problem on my 4x4 - replaced the plug leads which had all tested fine - problem solved.

I don't know the age of your bike but my wiring is 20 year old now and is starting to degrade in places.
Just a thought
good luck
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  #12  
Old 6 Oct 2008
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Fuel Pump

My XT660 went through a phase of cutting out every few miles then you could stat it again and go a bit further. It was the fuel pump playing up and not allowing enough petrol through. After a few miles you run out of whats in the lines and carb. Then it trickles back through and runs again.

I bypassed it with a pipe and solved my problems. Might be worth a try.

The only problem with this it that you cant use the last bit of petrol in the tank without the pump as the tank is lower than the carb.
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  #13  
Old 7 Oct 2008
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So it's an electrical thing!

Thanks again for the replies.

I know it is now not fuel related as the float bowl never stops filling and the line is clear. It doesn't have the fuel pump on it and it is only the starter switch that saves it every time it dies.

I think I better start looking at the wiring diagram again!!

Again, any help to point me in the right direction would be great!!


Cheers,
Mike
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