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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 24 Feb 2005
JFL JFL is offline
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Morning Backfire

Problem: When cold, the bike starts OK, drives OK for about 5minutes then stutters and eventualy stalls when coasting to a stop (it can be avoided by tweaking the throttle constantly). After about 10minutes it seems to be OK again! Whats up?

More info:
I've moved to a place nearer my work (3 miles away!) so the bike is doing a lot of small journeys, usually in slow traffic. In the recent cold snap we've had in London, the bike has been slightly more difficult to start (backfiring, stalling on throttle) but does so fairly quickly. It then behaves fine until I hit about 5minutes in and acceleration goes lumpy and the bike wants to stall when not in gear or with clutch fully in. After about 10minutes it seems ok again. The problem doesnt happen on the way home (the bike is in an underground garage- ie its warm). PS if you've read this far- is it a problem that the bike is now making lots of small journeys?
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  #2  
Old 24 Feb 2005
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I have a similar problem with my Thunderace which also occurs about 5 minutes after starting my motorway journey. It is OK on a steady throttle at say 85 mph but when I throttle on it splutters and nearly dies then finally catches again and carries on. Like you, no problem going home in the evening. I wondered whether it was caused by a small amount of water in the tank resulting from overnight condensation in damp conditions? I keep my bike overnight in an open garage. During the day at work it sits outside.
Neil.
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  #3  
Old 24 Feb 2005
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I think you gentlemen are suffering from Carb icing especially in this weather.

The treatment is to add some I think Silkolene fuel treatment to the fuel until it warms up again in March.You can get it from bike shops just ask.

Charlie
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  #4  
Old 24 Feb 2005
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The Thunderace problem is definitely carb icing. The XT maybe not. My Tenere had the same symptoms a couple of weeks ago both going to work and coming home (25 miles each way). My first thought was carb icing but I have never suffered it before on this bike and the return journey was definitely above freezing, even allowing for carb venturi effects, etc. However, I do have in line fuel filters from each fuel tap. I haven't cleaned them for years (and many thousands of miles) and my last major trip was UK to Timbouctou and back winter 03/04. I think they are contaminated and would not allow sufficient fuel through to the float chamber whilst moving. When I stopped I reckon enough fuel trickled through to fill the float chamber and allowed me to continue until it emptied again. Haven't had a chance to check this out yet (rubbish weather at weekends) but that's my theory for the moment!
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  #5  
Old 25 Feb 2005
JFL JFL is offline
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Well it was snowing this morning and it was slightly worse than normal... the carb icing?

Having said that, I think it could well be a fuel problem...

Well, until it gets warmer, I guess I'll be revving the throttle at all the traffic lights sounding like a bonkers biplane and spooking crossing pedestrians! Thanks for your help.

[This message has been edited by JFL (edited 25 February 2005).]
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  #6  
Old 25 Feb 2005
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Probably condensed water from air intake and carb ice, lying in the bottom of carb.
Try draining carb to see if that is it..
Open drain screw, let poor down.
Water is heavier than fuel, can accumulate.
Store bike on dry place, when turning off engine, switch on choke to the fullest, cough, switch off, keep choke until morning, start, better now?

Matt
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  #7  
Old 25 Feb 2005
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Carb icing ... I've heard of it but never experienced it before - although I have had the experience I've described with an FZR600R too, and my son still has it with that bike. If carb icing is what's happening it does not seem to correlate with outside temperature particularly. I rode in to London this morning in snow most of the way (after the bike had spent two nights and a day in a very cold open garage) and there was no problem. I assume the phenomenon does require a particular combination of conditions - presumably damp, followed by condensation and then low temperatures. Is it well understood and is there a good explanation somewhere on the Web?
Neil.
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  #8  
Old 25 Feb 2005
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The venturi of the carb lowers the pressure due to the shape of it and Boyles law so as the pressure in the venturi drops the temperature has to to allow an increas in volume. Therefore this can happen at positive temps anything below about 7C. Fall in temp cuases the water in the air to freeze in the carb this inhibits the fuel evaporation so the engine doint go. Stop for a few mins and the engine heat thaws the carb and your off again for about 2 miles unntil it freezes again.

Get this fuel treatment and all will be well Demon Tweeks part number 600150389 Silkolene Pro FST, not the cheapest but they will get it to you quick.

I know because the wife had a ZZR600 and if the carb heating curcuit got blocked it iCed in milliseconds.

Charlie

Charlie


[This message has been edited by braindead (edited 25 February 2005).]
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  #9  
Old 25 Feb 2005
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carb icing is quite common, 4cyl kawasakis are well known for it. a lot of bikes have carb heaters installed. it doesnt occur at really low temperatures like you'd expect,its the temp difference across the carb that makes it ice up. if the air temp is a few degrees above freezing, the venturi inside the carb lowers this to below freezing and water is condensed onto the metal. this forms ice and it doesnt take very much to spoil the shape of your inlet, throwing the fuelling out.

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  #10  
Old 25 Feb 2005
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I never had carb icing problems with the XT, so would not think of that as the first cause.
Important to know is that with lower temps the mixture gets leaner so with the choke off you might just 'pass the edge' at which the engine runs fine.
Another common cause for problems might be that the choke plunger (left side of the carb, where the calble goes in) does not fully return to the 'choke off' position initially. Then, when engione stalls and you kick it a bit around it does return to the choke off position because of the vibrations. Common problem with the 3AJ types as dirt and water flows into the bore of the carb via the choke cable. Remedy is to remove, clean bore and plunger.

Auke
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  #11  
Old 25 Feb 2005
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Thanks, all, for the interesting info. In view of the cause as described it is surprising that carb icing doesn't happen a great deal more often.
Neil.
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  #12  
Old 26 Feb 2005
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I've only experienced the problem once, when it was very cold.
I took the grip of a screwdriver, knocked the floater case a little, then it started immediately.
3 possibilitys,
condensed water from air, fuel etc.
-inlet valve freezez, knock with screwdriver grip!
-jet hole freezez, open throttle and rev it!
-water in floater case making fuel level unreliable, bleed carb!

I guess its a british problem as air is cold and humid over there these days.
Maybe an occcasional 0,5% mix of full synthethic 4 stroke oil in fuel would cure, its also good for plenty of other reasons when humid, as avoiding tank rust etc.

Matt



[This message has been edited by Matt595 (edited 26 February 2005).]
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  #13  
Old 14 Mar 2005
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Quote:
Originally posted by greynomads:
The Thunderace problem is definitely carb icing. The XT maybe not. My Tenere had the same symptoms a couple of weeks ago both going to work and coming home (25 miles each way). My first thought was carb icing but I have never suffered it before on this bike and the return journey was definitely above freezing, even allowing for carb venturi effects, etc. However, I do have in line fuel filters from each fuel tap. I haven't cleaned them for years (and many thousands of miles) and my last major trip was UK to Timbouctou and back winter 03/04. I think they are contaminated and would not allow sufficient fuel through to the float chamber whilst moving. When I stopped I reckon enough fuel trickled through to fill the float chamber and allowed me to continue until it emptied again. Haven't had a chance to check this out yet (rubbish weather at weekends) but that's my theory for the moment!
Finally checked filters this weekend - left hand filter totally blocked and right hand a bit less blocked. Replaced them both and over 50 mile test ride no problem.
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