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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 20 Jan 2008
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What does it do???xt question

On the side of the slide carb on my 3aj, theres a unit called a coasting enricher assembly. Can somebody please tell me what it does??????

Help anyone
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Old 20 Jan 2008
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Black thingy

Here in the states we call them a "anti backfire valve" . When you shut the throttle at speed it creates a high vacum and actuates this diaphram actuated valve. On most bikes it restricts the air bleed to the idle jet circuit which richens the mixture there by lessening backfires out the exhaust pipe. If you soak your carb in a tub of super cleaner and don't remove the rubber diaphram it will be eaten away putting the carb into a rich/plug fouling mode.
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Old 20 Jan 2008
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Thanks Bill, I was wondering if this could this be a contributing factor to the problem with my bike. The bike seems to run fine through the gears, but if I'm doing about 70, at neutral throttle, the bike "skips" every now and again - I was thinking fuel starvation though.

I want to check my fuel level, and in the yamaha workshop manual, it says between 5-7mm. Wheres that measured from? Can anyone help on either of these points.
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Old 21 Jan 2008
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Level

The 5 - 7 mm is, with horizontal carb (meaning, when carb is installed, you have to lift the frontwheel), the distance between fuel level inside float bowl and the edge of the connection surface of the bowl to the carb. And then, only when the engine has run a bit to stabilise the level. You can see the level inside the bowl by attaching a transparent hose to the drain of the bowl and opening the drain screw.

Bettter to measure in a different way: remove carb and bowl. Turn carb upside down. Measure between underside float and connecting surface to carb. To be 25 - 27 mm, or 26 - 28 mm depending on model. The XT is not very sensitive to this BTW.

Auke
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Old 21 Jan 2008
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yeah, Auke is spot on with that....

Quote:
Originally Posted by aukeboss View Post
Bettter to measure in a different way: remove carb and bowl. Turn carb upside down. Measure between underside float and connecting surface to carb. To be 25 - 27 mm, or 26 - 28 mm depending on model. The XT is not very sensitive to this BTW.

Auke
The Coasting enricher stops it cutting out when you 'throttle off' from 'giving it some Bigtime!'.......if that wasnt there...... the engine would die, and you would find yourself restarting every time you stop at the lights/ junctions/ etc.

FLOAT LEVELS yeah, XT aint too fussy on that!
I just "guestimated" mine at about23-24mm and it is OK. I didnt have internet connection on at the time and neither did I have the Manual handy,so i thought that looks about right, measured it, 23.5mm in actuality...... but it back together

She runs sweet as... so never bothered to readjust it....... if it aint broke n all that

The XT600 IS sensitive to Mixture & Idle settings though.

Martyn
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  #6  
Old 21 Jan 2008
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Generally speaking a coasting enricher really comes into it's own when throttling off at speed and sustaining a fair amount of speed over a distance (long downhill etc) where the motor could otherwise lean out to the point of siezing.
Truth of the matter is this was added to the carb setup in on the XT550 K model for the first time. The prior XT550 J did not have a siezing issue tho, but one 'orribly bad backfire that would blow the back off the end can. There after great fun could be had terrorising motorists.......ahhh the good ol days :-)
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  #7  
Old 21 Jan 2008
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hmmmmm 10 paces & fire!

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Originally Posted by bruken View Post
Generally speaking a coasting enricher really comes into it's own when throttling off at speed and sustaining a fair amount of speed over a distance (long downhill etc) where the motor could otherwise lean out to the point of siezing.
Can't agree with the SIEZING bit Bruken!

If it was a 2 Stroke, then most probably Yes...... but a 4 Stroke is lubed via the Oil in the Sump or from the Oil-pump not Oil in the Fuel, so theoretically it wouldn't Sieze even if you were at altitude in the French Alps on a long-down-hill-off-throttle-engine-braking-run..... as long as the engine is Ticking over..... which is what the device is meant to do, it keeps the engine from Cutting out.

Martyn
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Old 21 Jan 2008
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Ok, thamks guys, but how do I adjust the mixture????????? The only adjustment I can see is the pilot jet, (and also the idling speed). When i was out on the bike yesterday, goes like a stabbed rat, but on the open road doing about 75mph, every now and again it felt like it was going onto reserve. You know the kind of feeling that it's just about to run out of fuel, but very slightly.

So i've cleaned the tank, fuel taps, fuel pump and carb. What the hell should i try next????
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Old 1 Feb 2008
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hm yeah, they do that sometimes!

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Originally Posted by Dave O View Post
Ok, thamks guys, but how do I adjust the mixture????????? The only adjustment I can see is the pilot jet, (and also the idling speed). When i was out on the bike yesterday, goes like a stabbed rat, but on the open road doing about 75mph, every now and again it felt like it was going onto reserve. You know the kind of feeling that it's just about to run out of fuel, but very slightly.

So i've cleaned the tank, fuel taps, fuel pump and carb. What the hell should i try next????
Hi Dave, its always hard trying to diagnose bike prob's especially fuelling probs over the net... but im gonna have a stab anyhow.

under the Carb's at the front, ( sort of nearer the engine) is a mixture screw. that is the left hand Carb as you are sat on the bike.
that should be out about 3-4 turns from fully home ( fully home means Finger TIP tight just about touching tight) or you will knacker the end if you tighten it too much.

worth doing is making sure there is no dirt in your fuel, rinse the tank out, fit a fuel filter, and make sure your Plug, HT lead, and Cap are in good nick, or better still, replace em, its not a lot of dosh and wont harm anything.

another thing, its a royal pain in the Ass but worth checking! remove carbs, take out floats ( will give you a chance to check levels when you refit anyway ) and remove the needle valve...... there is a little filter, tiny thing it is, that just clips to the top of the needle valve assembly, (you have to use a phillips screwdriver to undo a screw to take the needle valve assemble out) but that filter can get clogged with little bits of debris, fluff, hairs etc....... clean that and give the carb a blow through with an airline.
then reassemble.

My money is on that little filter...... its a bitch to get to, and so well hidden most people miss it.

Martyn
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Old 2 Feb 2008
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Thanks Martyn - (things already done)The carbs got standard jetting, so I set the mixture to the workshop manual. Cleaned out that filter above the needle valve, it was full of crap.

Now I'm gonna change the HT lead and cap. The bike was running ok today, but I did get one massive backfire when throttling off heavily. F**k it was loud. Do you thing it's worth trying another CDI, jus for the hell of it?
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Old 2 Feb 2008
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Quote:
Originally Posted by Dave O View Post
Thanks Martyn - (things already done)The carbs got standard jetting, so I set the mixture to the workshop manual. Cleaned out that filter above the needle valve, it was full of crap.

Now I'm gonna change the HT lead and cap. The bike was running ok today, but I did get one massive backfire when throttling off heavily. F**k it was loud. Do you thing it's worth trying another CDI, jus for the hell of it?
If you mean it Backfired after givin it some welly then snapping the throttle shut really quick?

If so....... it is probably sucking air in at the Exhaust Manifold.. there should be round copper sealing Rings ( one time use only) there that squish up n seal, sometimes a previous owner will remove the manifold and not put new sealing rings in..... still runs OK but can suck air and ignite unburnt fuel under the conditions you describe.
Replace them and seal the exhaust to Muffler joint to and it should cure the backfire problem.
I had the same problem on a Honda CM125 twin 23 years ago!

Martyn
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Old 2 Feb 2008
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Quote:
Originally Posted by Dave O View Post
On the side of the slide carb on my 3aj, theres a unit called a coasting enricher assembly. Can somebody please tell me what it does??????

Help anyone
These sort of add ons are pretty common - my DRZ400 had the same device attached to the side of it's FCR slide carb. It's not essential to the running of the bike, it's just to comply with noise/emmisions requirements.
Problem is they can and do malfunction, causing all manor of fueling problems. I've removed mine and blanked the air holes in the carb after it started sticking and the bike refused to idle properly.
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Old 3 Feb 2008
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Martyn - you f**king legend. When I rebuilt the top end I used the old copper sealing washers. God knows why - I've got a set of new ones in a box! I guess I'll definitely be changing them now and taking the bike for a spin. Thanks again. I'll let you know what happens. Dave O
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Old 10 Feb 2008
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The ongoing saga - Ok, so now i've fitted new exhaust gaskets and used a tiny bit of sealant. Put in a new coasting enricher assembly and fitted a new needle and seat (plus filter) above the floats. The carb rubbers are good and sealed. The rubbers from the air filter are nicely sealed. The fuel pump and tank (incuding taps) are clean.

I haven't yet changed the plug cap and lead. What else can this be, help, I'm tearing my hair out. By the way, no more backfiring, but still the same original prob!!!!!!!!
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Old 12 Feb 2008
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hmmmmmm

Quote:
Originally Posted by Dave O View Post
The ongoing saga -

I haven't yet changed the plug cap and lead.


but still the same original prob!!!!!!!!
eliminate, eliminate, eliminate...... its not expensive Dave, worth a try..
at least for peace of mind, ( and if now't else gives you something to do eh?)

Martyn
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