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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 30 Nov 2009
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Engine not slowing
Recent work (20k mile engine): new exhaust headers, new inlet rubbers, cyl head and piston decoked, valves reground. Only work on the carb was to take off the float bowls to check float heights, which were correct.

The bike now runs fine, and seems to have a bit more power and be a bit smoother than before (one of the inlet valves was showing signs of blow-by). But there are three things not as good as before:

1. Starting takes longer on the button - it was almost instant before, now it takes 3-4 seconds of turning

2. Bike will occasionally stall during slow running, e.g. in traffic, when the throttle is snapped on and off

3. Revs take longer to return to idle when throttle snapped shut. Before, with a closed throttle the revs went back to idle immediately, now they die down slowly over about 5 seconds. This is a problem, as it means there isn't as much engine braking.

The throttle quadrant is snapping back onto the stop, so it isn't a sticky cable. It all says 'weak mixture' to me, but I can't think why. Anyone got an idea?

(Forgot to add: plug reading is mid-brown, perhaps slightly darker than ideal.)
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Last edited by BlackDogZulu; 30 Nov 2009 at 21:01. Reason: Plug info
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  #2  
Old 30 Nov 2009
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Sounds like its running a bit lean - smart moneys on inlet rubbers or some other air leak or pilot jet blocked????
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Old 30 Nov 2009
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Tight valves?
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Old 30 Nov 2009
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Thanks guys. Valve clearances were all reset, and the inlet rubbers were new and installed correctly. I agree, it all sounds like a weak mixture, but I can't see why. The plug certainly doesn't look weak. The carburation was damn-near perfect before and, as I said, all I did was check the float heights, so a blocked jet would be a coincidence.

I'll give it a good thrashing tomorrow and report back
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Old 1 Dec 2009
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Pigford is right. Sounds like its running lean. Have you sprayed the intake manifold?
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Old 1 Dec 2009
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Yep, sounds lean.

In checking the carb float height. Did you make an adjustment?

When you put the head back on was the cam chain a bit stretched. i.e. Have you inadvertently advanced \ retarded the valve timing?
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Old 1 Dec 2009
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My buddy's xtz660 did the exact same thing after a rebuild.

We will work on it the upcoming weekend, my plan is to go 1-2 sizes up in idle jet.
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Old 1 Dec 2009
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If your model has the twin carbs - secondary which is a CV ????, check the diagphram for splits/holes/cracks !
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Old 3 Dec 2009
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Thanks for all the suggestions! Answers:

No adjustment made to floats. All I did was take off the float chamber, checked the heights, bang on, put it back.

Camchain was cock-on when I put it back - in fact, I think it's better than before, as I freed up the adjuster and the engine is noticeably quieter and smoother than before. It's certainly not a tooth out.

Apart from slightly slower response to the starter, and this reluctance to die when the throttle is shut, the engine is running better than ever, with a small but noticeable increase in power. On a long uphill on my way to work, it is 5-10mph faster with the same throttle opening.

I'll try the inlet spray thing and double-check the valve clearances. If that doesn't work, I'll pull the whole thing off and have a look. Diaphragm first. But the plug certainly doesn't look weak, and the bike doesn't ride like it either.

Onwards and upwards. Cheers.
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Old 3 Dec 2009
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Quote:
Originally Posted by BlackDogZulu View Post

No adjustment made to floats. All I did was take off the float chamber, checked the heights, bang on, put it back.

.
This is not the best way of doing it on these old bikes as over time the "buoyancy" of the floats can change. Better to get a silicon tube and connect it to the drain nipple underneath the float bowl and open the drain screw. Bend bend the tubing gently back up against the body of the bowl and if memory serves me correctly the fuel level within the tube should be +- 5-8 mm below the mating surface of the float bowl and carb. (check the manual whether this should be done on a straight and level or in situ on the bike. My manual for the 3AJ says on the level, but for the 550 the measurement is in situ on the back side)
Drop the tube slightly below this line and ensure that the fuel easily and consistently flows (i.e. no blockage up line of needle and seat).

If you don't have any of this tubing go to your local pet shop, they stock it as an aerator for goldfish / aquariums and the like.
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Old 3 Dec 2009
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Funnily enough, I just bought a length of this a few days ago. I'll give it a go. The manual said to measure the float height with the spring not compressed, and juggling the float so that the needle was in contact but not compressed was a bit of a carry-on. And yet the measurement was exactly right to the mm. So I didn't adjust anything, as it was fine before.

I'll do the tube trick anyway, just to see. Thanks.
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