Hi there.I'd like to start off by saying that this has been an ongoing situation for me, and I have got a lot of helpfull advice off the Hubb which has helped me chip away at my problem XT. A while back I had running problems with my 4PT, which left me stranded on the side of the road in the middle of winter a few times. I have since thrown a lot of money at it (as you do when you not a mechanic), with only a moderate amount of success. I have so far changed: the Coil, with lead and cap
cut out the sidestand switch
cut out the clutch switch
installed K&N filter, DEP silencer, and DynaJet stage 2
swopped the CDI unit to prove it
installed new chain and sprockets
all wiring checked and terminals greaced
changed the spark plug about 4 times

The chain and sprockets actually made a big difference, because they were exagerating the flat spot which has been there all along. I have had the bike at a Yamaha Main dealer, twice, and my local bike shop, twice. I would like to take this oppertunity to say my local guys were WAAAAY better than Yamaha, but still didnt get to the bottom of the problem. The bike problem is as follows. When I'm riding along at slow speed, from about 2-3500RPM the bike feels like its starving for fuel, just a little. There is a pop or two sometimes too. If I pull the choke out just a little it comes right, but the bike doesnt like this when really hot. If I give it a handfull of throttle and it shoots past this point, it seems ok, and its ok on full throttle cruising. Idling is sweet too. Its just that initial bit after pull off. Since the choke kinda fixes it, and chokes enrich the fueling temporarily, it must be starving for fuel just a bit. Is this something to do with the fuel/air mixture screw? And if so, how come the bike shops I went to didnt see this? Is there a bike mechanic in London who knows a lot about singles? Can I just turn the screw and hope for the best?

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hi mate just check plug gap and elimanate that mine used to do that also try another garage for petrol. you may have checked these before but its the simple things that sometimes we miss good luck

Adjust you CO screw out a bit, that should richen it up a bit.

Otherwise, get it on a dyno, so you can see the fuel/air mixture over the whoole powerband..

Have you checked the inlet rubbers for splits, de-lamination, pin holes yet?

It may be a long shot but easy to check.

Cheers from Oz.

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I'm not getting older; I'm just progressing through LIFE!! Harleys do not have a throttle- they have VOLUME control!!!

Dyno is the way to go you may have an air leak which the dyno should spot. Gibson exhausts (see web for their address) are having a dyno day on Saturday and its only £10 a run and there is free food you could take advantage of that they are just off junction 28 of the M25 so easy to get to.

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Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did do.

Thanks for the response. I think Dyno is the way to go. I get petrol from multiple stations, plug is good. Rubbers are new looking, but that could be something to look at.
A mate at work has said something about lifting the needle one notch. Without a dyno this would be like shooting in the dark though. I need a tuner, just running it on the dyno is only half the job. Thanks again for the replies. Cheers

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rule of thumb for carbs in basic terms are mixture screws are for idle to low revs, needle adjustments are for the transition from low to mid revs and the main jets are for full revs

dyna needles tend to be more tapered than standard to give that smoother transition so i'd go up 1 notch or even beter if you can go half a notch some kits have tiny washers for those .5 increments

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XT 630 SUPERMOTO

GSXR750L KMX200

RF900 & GSX400X Impulse Projects
3.9 V8 Rangerover

I just did a little experiment. I rode home with the choke pulled out just a little. It seemed to almost cure the problem. The problem is definitly lean running, from low to medium throttle. Thanks for the info Wolfzero, that is what I need. What do you rekon I should do? The idle is good, so probably the needle position. Do I put the little clip one notch down to get the needle to raise a little more?

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yep clip one notch down will raise the needle but like i said if your dyno kit came with those tiny washers they are nomaly half as thick as the gap between the grooves in the needles so you can raise the needle by half a clip by putting one washer under the clip as you fit it into the slide but dont worry if you aint got em you just might be ok going a full notch and a little mixture adjusting as the idle mix might alter slightly

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XT 630 SUPERMOTO

GSXR750L KMX200

RF900 & GSX400X Impulse Projects
3.9 V8 Rangerover

Any shop with a dyno will (should) be able to explain what the dyno read out means and be able to tell you what is causing the problem and how to sort it. I use the dyno at Gibson exhaust and they can tell me exactly what to change from the read out, but that should be the case with any shop with a dyno.

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Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did do.

unfortunatly thats not always the case some dyno owners are just that

if your looking for a dyno go for one attached to either a carb or exhaust specialist preferably both as thats what dynos were developed for and if your really lucky the guy running it should be in his sixties an ex racer with a passion for perfection

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XT 630 SUPERMOTO

GSXR750L KMX200

RF900 & GSX400X Impulse Projects
3.9 V8 Rangerover

Is it popping a bit between low gear changes, and upon deacceleration? It should if its lean.

The CO/fuel-screw affect the whole area, just not much in the top. But i needed to adjust mine after the change of primary jets.

hence the should, i can read a dyno sheet and its not rocket science, modern dyno's do most of the work for you older ones require a bit more mechanical knowledge or trial and error for some. If they own a dyno they should have done a course on how to use it, but i agree thats not always the case.......

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Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did do.

It is easy to check, use a spray bottle (atomiser) with a bit of fuel and spray onto inlet rubbers.

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I'm not getting older; I'm just progressing through LIFE!! Harleys do not have a throttle- they have VOLUME control!!!

I did the same with a hose pipe once and sure enough steam came out of the exhaust

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Yamaha TT600RE 2004
Yamaha YZ250 2 stroke 99