still problems with rear cylinder carb ?????

[Kato]

Firstly are you sure its carb related, given the symptoms you describe I would be looking at the pickup's & Kokusans as Ped says.

If that's not it then other possibilities

Diaphragms have you checked them swapped the between carbs etc
Atomiser / emulsion tube whatever you want to call them, did you change them ? they wear oval and make fine tuning next to impossible especially with new needles

Lastly consider that those Mikuni's were notorious for having porous castings

[ibduke]

you have everything on the bike to dignose without spending money electrically, just swop stuff over if it transfares the fault its electrical. But usually it wouldnt run ok further up the rev range if the fault was on the electrical side.
I said before make sure there are no air leaks on that inlet manifold side, rubbers, pipes etc it wont take alot to knock the idle circuit off. I'd expect possible idle jet size around half a millimeter..0.5mm, so it wont take alot to knock the cylinder off.
Running the bike induce fuel if it goes on two cylinders its fuel related.
Diagnose.

[slob]

Spraying a little EasyStart round the outside of the carbs usually shows up any kind of air leak pretty quickly

[Darren69]

Is it possible that the jet is partially blocked, by varnish with a pin hole so that if you pour liquid in it will come out but not when the bike is trying to pull fuel through it won't?

[ibduke]

If you can see through the idle jet WD40 should do it. If the jet is blind suck though it, ( jet end). If you have both jets infront of you, by sucking you should be able to tell yourself rather than blowing through it with compressed air. if there is dirt in a blind jet very rarely did you get the dirt out.
Still check the passage is clear between the idle jet and mixture screw,
look for the simple stuff first before you comit to anything

[ibduke]

Just a quickie, the jet ill not be the diameter of a pin, more like one strand from a bowden ckoke cable..............0.5mm ish.

[ibduke]

Had the bike been stood with fuel in or did the fault just appear.

[jerry]

Originally Posted by ibduke

Had the bike been stood with fuel in or did the fault just appear.

No it was under rebuild after Kat crashed it in 2018 so the tank was away for repair and repaint ..carbs were drained for cleaning etc ,, it was running perfect before and when I stripped it it was still running good after the crash ,, i cant get to it this wek but will run it with the other bikes carbs all the lectronics have been swopped too befor i took the bad set of catbs off ,, no worries I will win in the end

[jerry]

Update ,,

Put the spare carbs from 1997 M750 on and the 2000 bike runs fine so its definitely the Vertical carb on the 2000 bike ,, so I split the carbs again and the dud left hand carb is completely stripped and back in the sonic tank ,,,it is also being looked at by a carb specialist ?????

[Martin C]

Originally Posted by jerry

This is driving me mad,,, The bike runs but will NOT TICKOVER or run below 3k revs on rear cylinder ,front cylinder and carb are perfect,

I have rebuilt the carb 4 times now with new parts it has been thoroughly cleaned , in ultrasonic bath twice , blasted with air ,, each time I have fitted it the fault is still the same ,,it has new float , new gaskets, needle, airscrews, even the pilot air cutoff valve is new ,,,baffling ???
The right hand front cylinder carb is fine

I have put other spark plugs , HT leads , coils and igniter units they are from one of my other bikes but the fault persists so its not Electrical ,, There are NO airleaks I have now spent some 80 hours on thsi one problem and im run out of ideas ???? totally baffled

Pardon me for asking, but are you absolutely certain it’s a carb problem? My M900 once had a very similar problem that was caused by a poor coil wiring connection, almost no copper strands were intact inside apparently good insulation at the connector. At higher revs normal vibration was evidently enough to cause a better contact path.

[Darren69]

I think he's switched over the coils and the ignitors to rule out that issue?

[jerry]

Bike is now complete ready to MOT and Tax etc ,, it looking lovely but still maor issues with left hand carb its useless ,, missifiring lean not ticking over ,, Its not porous , there are No airleaks its been ultrasonicly cleaned 3 times everything inside is new , totally clean and refurbished and its been assembled exactly as the right hand carb same jets same settings on needles etc the right hand carb id fine but the left hand one will not work correctly ,, I am totally baffled as are my local carb specialist and ducati mechanics Paddy and Alf ????? as never seen anything like it ,the carb is perfect looks like new ,


we rode the bike down the road for 5 miles with the crusty looking carbs off the 750ss it ran perfect

the bike works with other carb sets off my other monster 750 and 750ss and none of the electrical components or valves are faulty ???

The carbs are back off the bike and on the bench the dodgy left hand carb is now essentially a new fully refurbed piece of junk ???

the 750ss is coming off the road and my other M750 is taxed now for the summer but all the money and work that has gone into the refurb bike which looks stunning is on hold due to this weird carb screwup

[slob]

https://www.ebay.co.uk/itm/164769297...iABEgJclfD_BwE

[jerry]

Slob,, I already have been given a very manky set from a 900 that I can refurbish that are in the same condition as the Ebay ones ,, but they dont have carb heaters which is a pain,,to make them look good is not easy or cheap , Im tired of wasting money on this fiasco and those carbs in the ebay ad are from a later bike than 1994 as they do have carb heaters which makes them 1998 up ??? and they will also need a lot of cleaning up to look good seriously if I had a spare £800 I can get a set of shiny used FCRs ...from a mate in Japan

[Sirc]

if you find it Jerry, could you please post it up? I haven't read thread in minute detail but looks like nearly all has been covered. Have you tried replacing the choke unit on the troublesome carb? With the airbox lid and filter removed, looking into L/H venturi, needle slide is obviously returning ok after a throttle blip?