Help! 3 cylinder yamaha is driving me mad

[neels]

Because it's supposed to be a 4 cylinder.

I guess it's partly my own fault for buying someone else's 'unfinished project', which always leads to tearing out of hair to some degree.

Anyway, no. 2 cylinder on my xj550 doesn't seem to want to run at low revs, and I'm almost out of ideas. So far I've done the following;

Replaced spark plugs (needed doing anyway)
Replaced plug caps (also needed doing anyway)
Removed carbs, adjusted float levels (which were waaay out), checked diaphrams.
Swapped plug leads with no.3, no change
Swapped coils from 1&4 to 2&3, no change.
Moved the vacuum hose to the fuel tap from no.2 to no.1, no change.
Checked compression, all 4 around 140psi

It may be doing something once it's up around 5-6000rpm, as a bit of plug lead pulling while riding seemed to cause similar slowing when no.1 and no.2 were pulled, but once stopped it was back on 3 again.

I'm thinking I need to pull the carbs again and have a closer look, but thought I'd see if anyone can suggest something I haven't thought of.

Ta,

[Kickaha]

Removed carbs, adjusted float levels (which were waaay out), checked diaphrams.

You should have cleaned them while you had them apart, they are well know for carb problems especially if it hadn't been run for some time and there's a well documented "proper" way to clean them out (according to the merkins)

http://xjbikes.com/Forums/viewtopic/t=2908.html
http://xjbikes.com/Forums/viewtopic/t=1640.html

Rear brakes shoes have also come apart if they're old

[neels]

You should have cleaned them while you had them apart, they are well know for carb problems especially if it hadn't been run for some time and there's a well documented "proper" way to clean them out
Rear brakes shoes have also come apart if they're old

Cheers, sounds like another evening in the garage then, it had been sitting for a while....

I've been pointed in your direction by a couple of people, should really pop up the road for a chat sometime, if I can get it running well enough.

I think the previous owner mentioned new rear brake shoes, if they fall to bits then they would probably have been replaced when the rear end was stripped to have the wheel painted, alloy bits polished and brake arm rechromed.

[Kickaha]

You could try what i did to one I bought that was only running on three and dump a bottle of injector clean in a tank of gas, took about 50-60km and it came onto all four

I still I stripped and cleaned them later on though

[lone_slayer]

try some start ya bastard or similar on the intake mannifold (on my gsxr250 its 4 rubber boots from the carbs 2 the engine) see if it makes a difference it could be a bad air leak the first time i had this issue the bike was only running on 3

[MSTRS:Me]

So - it's not spark and it's not compression...

What's left to check?

Carb #2 jets are clear, needle isn't worn, butterfly opens in sync etc
It is sealed against the inlet manifold.
There is no blockage in the inlet manifold.
Both valves on #2 cylinder are opening properly.
There is no blockage in #2 exhaust.

The colour of #2 sparkplug might give a further clue as to what is/isn't happening.

[neels]

Carb #2 jets are clear, needle isn't worn, butterfly opens in sync etc
It is sealed against the inlet manifold.
There is no blockage in the inlet manifold.
Both valves on #2 cylinder are opening properly.
There is no blockage in #2 exhaust.

The colour of #2 sparkplug might give a further clue as to what is/isn't happening.

Carb jets *looked* ok when I set the float levels, needle looked good, I've roughly set the throttle butterflies all the same.
Seems to be sealed to the manifold OK, actually runs better with the vacuum port open
Whipped the cam cover off last night, valve clearances pretty good and valves opening
Was thinking of taking off the no.2 header for a look, given how the rest of the bike has been put back together there could be something left in there blocking it.

No.2 plug is coming out black and sooty, which with the above suggests a lack of air.....

[MSTRS:Me]

...

No.2 plug is coming out black and sooty, which with the above suggests a lack of air.....

Well - more fuel than is needed, anyway. I don't know how the enrichener (choke) system works, but maybe that carb is 'always on' ?
Have you checked the main jets, and they are all the same size? Or at least 2+3 are?
Is the slide working properly? I know you said you checked the diaphragms are all good, but maybe the slide isn't lifting as much as the others and/or maybe the needle isn't seated right?

[Katman]

I know you said you've tried swapping the vacuum hose from No.2 to No.1 but did you put a fresh plug in No.2 when you did it?

It certainly sounds like a leaking fuel tap diaphragm.

[neels]

I know you said you've tried swapping the vacuum hose from No.2 to No.1 but did you put a fresh plug in No.2 when you did it?

It certainly sounds like a leaking fuel tap diaphragm.

That was my thought, but have swapped plugs (and everything else) and no.2 still not going properly, no.1 now has the vacuum hose and the plug comes out clean.

[MSTRS:Me]

I know you said you've tried swapping the vacuum hose from No.2 to No.1 but did you put a fresh plug in No.2 when you did it?

It certainly sounds like a leaking fuel tap diaphragm.

How does that affect #2 carby (or any of them for that matter) if all is well inside said carby?

[neels]

How does that affect #2 carby (or any of them for that matter) if all is well inside said carby?

The vacuum hose for the fuel tap, which connects onto the vacuum port on the inlet rubber.

If the fuel tap was leaking fuel it would be fed down the hose directly into the inlet for that cylinder.

[MSTRS:Me]

Ah. Thank you. Wouldn't have thought of that, since tis more normal for one of those taps to just not release fuel...

[neels]

Back on 4

I don't know how the enrichener (choke) system works, but maybe that carb is 'always on' ?

Right on the money, no.2 enricher valve lever was bent, meaning it was slightly on all the time. Noticed half way through stripping the carbs so finished the job anyway, gave it a tweak to match the rest and now she's purring like a kitten.

Well, a slightly asthmatic kitten anyway.

[MSTRS:Me]

Brilliant. Good guess, huh?
They all were...