Xt600 Misfires, then cuts out... Help Needed
 

Hi Everybody!


I recently bought myself a poor running 1984 XT600 34L.
The original symptoms was:

Won't run at full throttle
Idles bad when hot.

Thinking that this would be a doddle to fix I bought the bike.
Well, not it has turned in to quite a headache. So I need some of your wisdom.


The symptoms are now the same.
Here are two videos, one is when the bike is just started. Then it will run with no problems, when it gets to operating temperatures, it misfires, cuts out etc.
When it does this, it can be hard to start it again.

http://youtu.be/HGipgiK3Xj0
http://youtu.be/_FgNRz8LKdk

Well I have done the following.

Cleaned out the carbs
Measured Pickup
Measured Source Coil
Measured Ignition Coil
New rubber intake boots incl. o-rings
New sparkplug
New CDI
Checked the ground-connections for CDI, Coils etc.
Checked Valve clearences

After all this the symptoms are exactly the same..!


I am thinking that this is Ignition related since it gets hard to start after cutting out because it sucks in fuel while stumpeling and therefore drowns.

Please let me know if you have any good ideas to what this might be...


thank you
jonas

Bad stator likely. Testing them does sometimes show them to be good but once hot they short out. Try doing the same tests, once cold, once hot and compare.

Hi -

I have done a test like that measuring the resistance in both pickup and stator both hot and cold.
And everything was within spec.

The ignition coil was within spec aswell

Just because you've cleaned and tested something doesn't mean you should necessarily cross it off your list.


Any chance of getting another carb off another bike and trying it out ?

Cold engines and hot engines have VERY different fuel/air requirements.

It could be running very rich and at high temperatures, trips it up.

How is the choke and pilot curcuit ?? Do you need choke to start it ??

What jet sizes are you using? What altitude are you at? Could be waaaay too rich.THUMPTHUMP

Hi Ted

It's true!
As it is right now it will start without choke. It did need it before.
The thing is, I havn't been able to adjust the air / fuel screw since I can't get it to idle.

Last time i cleaned out the carb I went through pilot / choke systems with a piece of piano wire and they were 100% clean.

I am thinking that this might be my ignition coil that is going wrong. It is afterall a momentary fault. So I guess I won't be able to measure it....

I sent this to a few mates who had XT or similar in the past.. one of them just emailed this..

Have you changed the air filter or is the one fitted a cheap after market one? Once had a Honda with a similar issue - as you ran towards full throttle the foam compressed making it run rich to the point of misfire and cutting out.

FYI, dragging steel wire through aluminium carbs isn't a great idea. You can do damage and make things worse. However, lets assume you were super careful and everything is okay.

Well, In my experience a big 600cc air-cooled single usually always needs a decent amount of choke to get it going. Even in hot countries. If it starts without any choke in a bleak, cold British winter then I think your jetting is out.

Take it all back to standard book settings with the jet sizes and pilot screw and see how it goes from there.

Do you know what your screw setting and jets are at the moment ??

I know that steel vs alu or brass is bad. So yes, I was super carefull.
As it is now the jetting is standard.
And the air/fuel screw is 1,5 - 2 turns out. I havn't felt a big difference when turning the A/F screw. But then again, it will not idle long enough to adjust proper.
I have tried turning in and out for 1/4 turns then starting.

Hi
it might be worth just taking the end off as on the older xt they quite oftern blicked the silencer up and this would xause the engine to run bur cut out soob after starting skip

There is a capped off nozzle on the left carb boot between the carb and motor. Take that cap off and see if it clears up. It'll let more air in and if it's running extremely rich there will be a very noticeable difference. I have one carb that is screwy and always ran overly rich, Till I had the time to make smaller jets I had that cap off(put a foam over it to somewhat filter the air). That carb needs several sizes smaller jets than all my other XT carbs. It would puff black though when I twisted the throttle, but idle like crap(had the mixture screw bottomed out even) and top end surged badly but would rev out.

I don't think it's carb related but you never know. I should need a choke to start definitely when properly jetted.

Jonas has posted the problem last week on my forum, have a READ.

Nothing wrong with posting on here as well Jonas, we post on all forums so more brains will hopefully fix the problem.

I think the clues are when the bike gets to operating temp, electrical components can fail when hot so its leading towards your coils breaking down.

Possibly your trigger coils are giving mixed signals to you CDI?

Mezo.

News..!

Hi Everybody.
Well, after being fed up with this bike i decided to try all over again. But this time focus was on fuel and carbs.

So I pulled the off yet again and cleaned them and put them in my small (read too small) ultrasonic bath.

Everything was clean.

On assembly I saw that the small disc that contains the piston for the Fuel Enricher maybe was the wrong way in.
I have not been able to find any info on the, but it will fit both way.
It has a high lip on one side, and a small rounded on the other side.

Can Anyone tell me if this picture is right?
http://s879.photobucket.com/user/Mot...x384x.jpg.html

Anyway, I turned it around.
But the carb on the bike and fired it up. Cold idle was perfect, as it was before.
But it didn't seem to misfire.. Rolled around the carpark and nothing, just a bit of adjustment to the A/F screw.

I was not able to do a test ride tonight, but will try tomorow.

Could all this be related to a bad Fuel Enricher / Air cut-off valve?

Hi.

I don't know how it's meant to look on the XT but yes this cut off valve can cause you problems if it's assembled incorrectly or is the air hose is damaged or kinked etc.

I pulled this off the net because I can't be bothered typing it ;)

It's for a YZF but it's the same principle.



"The Air Cut Valve lets extra air flow to the pilot jet during most running conditions, but not under deceleration. The carburetor has 2 air passages to the pilot jet and one (the ACV passage) closes of during deceleration. This richens the pilot circuit on deceleration to reduce popping. The Air Cut Valve "cuts" the air passage off by using vacuum to pull opposite the spring and diaphragm. When you back off the gas at high revs, the ACV richens the air/fuel mix to stop the bike from popping and banging through the exhaust. In the perfect world, the air cut valve would ONLY richen the circuit on high RPM deceleration (to prevent backfire).

How It Works:

The center hole with white plastic valve leads directly to the pilot jet air circuit, along with the pilot air jet. The diaphragm spring and pin normally hold the valve open.
The hole on the left leads straight into the intake manifold for vacuum. This pulls the diaphragm pin away from the center hole on deceleration, closing it.
The hole on the right leads to the airbox side of the slide. It allows atmospheric pressure air to flow thru the passage in the center when the vacuum is lower, and leans the pilot circuit. The WRF's have a bigger pilot jet(42-45) than the YZ's(40-42) to compensate for the added air.
The ACV simply cuts the air to the pilot jet in half when using engine braking.
Unfortunately, the spring tension against the vacuum at mid throttle position also richens the circuit and keeps changing. Some riders have suggested that it is hard to set the fuel screw by ear because the ACV constantly changes the mix. Others have indicated that reading the plug color and jetting the pilot circuit are also more difficult due to the ACV. Especially if you have YZF timing on your WRF, it's a good idea to modify the ACV, so jetting will remain more consistent."




Read more here...

http://www.thumperfaq.com/acv.htm

I had played around with the valve on one of mine when I didn't know which way was right, it didn't make any difference to how it ran. I think it still does the same thing either way except maybe have less/more spring tension. I would think something else was the main issue that changed from the cleaning.