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Discussion Starter #1
Hi Guys (and Gals)

Just spent the day playing with the fuel system on my 99AT. I have fitted the new Facet pump I have had on the shelf for ages and stripped down the carbs looking for a niggly fault that has been bugging me.

Basically it starts runs and accelerates fine but out on the open road at any sort of cruising speed it kind of hunts like it is a bit starved of fuel, ta any time if I whack it open it goes like the proverbial off a shovel but slowing down when the fault is apparent requires more than usual roll off of the throttle to get decelleration. Then it takes a big handfull to get it up to speed again. I suspected a dodgy diaphragm but they are both in tact. I could find no other faults apart from one of the choke cables was a tiny bit sticky.
Having refitted the carbs I idly pushed up the slide on one carb with my finger and it sprang back damped by the diaphragm then I tried the other and it stayed up!! I tried the first one again and it too stayed up this time.
I took the cap off the diaphragm and checked the spring again and then noted that with the slide pushed up the diaphragm is kind of folded into a sort of pentagon shape which can sometimes lock itself in the up position against the force of the spring. This surely can't be normal? Could this be why the throttle is not responding as it should?

Advice please anyone?
Is the remedy new diaphragms/slides?
Dare I ask how much they are?
:confused::(
 

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Not a lot of help Gromit but Airwolf helped me strip the chokes a couple of weeks back and the left side(sat on the bike) was very slightly rusty. Removal of both made a big difference in running smoother. After replacing a knackered coil!!

The bike was feeling hesitant for a while on a steady throttle and the mpg has improved now. It definitely won't hurt to remove the chokes while you're there
 

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I took the cap off the diaphragm and checked the spring again and then noted that with the slide pushed up the diaphragm is kind of folded into a sort of pentagon shape which can sometimes lock itself in the up position against the force of the spring. This surely can't be normal?
Pushing the slides up is not the normal way they operate. The slides are pulled up by a reduction in pressure above the diaphragm so pushing them up will not have the same effect. When the engine is running, the pulses produced in the inlet tract make them far less likely to stick but you could try cleaning the slides and the carb body.

The hunting and symptoms you described could be a number of things from mixture screw settings through to dirt in the carbs or even improperly seating choke plungers. If you've given them a thorough clean and set the mixture screws back to spec, hopefully the symptoms will be gone.
 

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Discussion Starter #5
Pushing the slides up is not the normal way they operate. The slides are pulled up by a reduction in pressure above the diaphragm so pushing them up will not have the same effect. When the engine is running, the pulses produced in the inlet tract make them far less likely to stick but you could try cleaning the slides and the carb body.

The hunting and symptoms you described could be a number of things from mixture screw settings through to dirt in the carbs or even improperly seating choke plungers. If you've given them a thorough clean and set the mixture screws back to spec, hopefully the symptoms will be gone.
Having looked at the part numbers of the diaphragms etc and checked out the prices £87 each plus VAT from David Silvers I decided to think again. However, David Silvers also list a complete bank/pair of carbs for £149 plus VAT. Unfortunately this is for an S model AT not the X that I have. The Diaphragms etc are though the same part No. So I ordered a set and they are indeed a complete bank of two carbs with all ancilliaries that lkook identical to the original carbs. Although some components in the parts list are different part Nos between the S and X models There can't be much difference in the specs of the engines, can there? I note the S model carbs have a 118 Front and a 120 Rear main jet whereas the X has a 115 front and rear. As I have already fitted a K and N air filter which is supposed to pass more air I'm sorely tempted to stick the whole brand new plot on and see what happens. What do you think? at least the jets in the new carbs are bigger not smaller so whats the worst that can happen. I guess the new carbs will need a bit of adjustment to set mixtures, tickover and synchronisation etc, but how hard can that be?
 

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Hi grommit, if i were in your position i would'nt hesitate to put them on. :thumb:

At the worst you can take them off again if needed, and now you 'got your hand in' it wont take long :cool::)
 
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