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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 3 Apr 2012
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Doesn't like the cold!

3TB, 1994, 26k miles.

Minor issue, but would appreciate your views. Starting point: bike is starting and running well. In fact, it's never been better. Pull the choke, engine starts within 2-3 turns, good fast idle, choke off within half a mile and then runs and idles correctly. Ran fine all the way through last winter, too.

The last couple of mornings have been below zero, perhaps -3C. It's been a lot harder to start, firing 3-4 times and dying before getting going with the throttle cracked quarter of a turn and held there. That's not too bad, but once it's properly warm (after 10+ miles of 60mph plus) it's trying to die if I back off the throttle (e.g. changing down for a roundabout). This morning was warmer, and the bike started and ran fine.

It's not a big deal, especially as summer is coming, but it looks as though something has deteriorated since last winter. It feels a bit like carb icing, but I didn't think that was an issue with these bikes. Any ideas before I start tearing it apart?
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Old 3 Apr 2012
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Spark plug? one that would suit colder conditions?

Move to a warmer place?

Mezo.
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Old 3 Apr 2012
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increase pilot jet!
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Old 3 Apr 2012
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Quote:
Originally Posted by Mezo View Post
Spark plug? one that would suit colder conditions?
Plug was replaced 1200 miles ago, with DPR8EA-9 as always. I've had the same heat range plug since I have owned the bike, and this is a new issue, so I doubt if the plug is the cause.

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Move to a warmer place?
Good idea. When I win the lottery, definitely.

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Originally Posted by bacardi23 View Post
increase pilot jet!
As above, carb jets and settings haven't been touched for 3 years, and it ran OK last winter, which was a lot colder than this one.

Thanks for your ideas.
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Old 4 Apr 2012
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If the plug, ignition and lead are ok and you are getting a good spark, air filter is good and timing is good and the motor has good compression start looking at fuel flow issues:
- any water in the fuel? - try fresh petrol.
- is petcock working ok - is it vacuum operated?
- clogged fuel filter?
- any air leaks in fuel system?
- stuck or leaking float?
- is choke working properly (at carb)
-any gumming or deposits on idle screw or jets or in carb in general?
- worn jets?

good luck
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Old 4 Apr 2012
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Hi Gipper

Bike was recently serviced and is in daily use, so fuel should be good. And of course nothing fancy like fuel filters or vacuum petcocks I'd also rule out a problem with the choke as the near-stalling happens after 15 mins hard running, when the engine is thoroughly warm.

I'm more inclined towards the idea of an air leak, although I replaced the carb rubbers 2 years ago to cure a misfire and they seem good. As the problem seems to have developed over time, carb deposits or worn jets could also be in the frame. What's bugging me is that, as long as the temp is above zero, the bike is running well. In fact, but better than it ever has (starting, idling, power output etc).

Looks like the carb is where it's at. Unless it's compression - hadn't thought of that, and it hasn't been checked for about 2 years. Time for a look-see when I next get a day off. Thanks for the input.
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Old 4 Apr 2012
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Fuel filter

Hi there

You say nothing fancy like fuel filters but don't forget the little domed plastic mesh filter above the float valve. It gets clogged over time with all sorts of tiny particles of crap. It may be worth a look but I can't explain how it's linked to temperature.

Al.
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Old 4 Apr 2012
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Thanks Al. I haven't checked that filter (ever!). I'm pretty sure the issue isn't lack of fuel, though, as the bike will run at full throttle no problem.

The poor starting isn't surprising for an old bike in cold weather. It's only a concern because starting is normally instant. I'm more concerned with the behaviour when I shut the throttle at speed. I back off to change down, and if I don't get the clutch back in quick, it has almost stalled.
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Old 4 Apr 2012
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Try to increase your idle screw a hald or one turn...
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Old 4 Apr 2012
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Quote:
Originally Posted by acertainalias View Post
Hi there

You say nothing fancy like fuel filters but don't forget the little domed plastic mesh filter above the float valve. It gets clogged over time with all sorts of tiny particles of crap. It may be worth a look but I can't explain how it's linked to temperature.

Al.
Worth ditching that tiny fuel filter and fit an inline one upstream of the carb.
Outright speculation, but if there was to be some water in the fuel then it could, just possibly, freeze maybe at that tiny, useless-surface-area, too-hard-to-get-at filter?
(I had problems with brakes in cold weather years ago, but only in the cold weather - turned out to be moisture in the brake fluid).

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Originally Posted by BlackDogZulu View Post
Thanks Al. I haven't checked that filter (ever!). I'm pretty sure the issue isn't lack of fuel, though, as the bike will run at full throttle no problem.

The poor starting isn't surprising for an old bike in cold weather. It's only a concern because starting is normally instant. I'm more concerned with the behaviour when I shut the throttle at speed. I back off to change down, and if I don't get the clutch back in quick, it has almost stalled.
I concur with all of that logic & the starting issue is likely different from the running issue; the carb is the same as fitted to the TT600R I believe = it works in 2 stages, for slower speeds and then faster speed.
What you describe points to the 1st stage operation.
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Old 4 Apr 2012
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Quote:
Originally Posted by Walkabout View Post
Worth ditching that tiny fuel filter and fit an inline one upstream of the carb.
Hmmm. I've been meaning to do that for a long time. I like service parts that are visible and easily accessed

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Originally Posted by Walkabout View Post
Outright speculation, but if there was to be some water in the fuel then it could, just possibly, freeze maybe at that tiny, useless-surface-area, too-hard-to-get-at filter?
You could be right there. Fuel tank is emptied and replenished approx every 4 days, but over time I guess water could have got in there. I'll check that out.

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Originally Posted by Walkabout View Post
What you describe points to the 1st stage operation.
Definitely. Turning up the idle speed a fraction as bacardi23 suggests may well help, although the fact that it is happening now and not last year (when it was much colder) suggests a gradual deterioration in something rather than an adjustment in settings.

Couple of things to try - thanks.
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Old 4 Apr 2012
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Clean the carb, and tke things from there. As Bacardi says, adjust idlespeed aswell.
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