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Hi all,

i have an 85 xv1000 (yamaha virago) that is constantly fouling the plugs with a dusty dry black soot. seems to be running rich but i cannot find the cause. to date i have done the following:

replaced needles, seats, main jets, pilot jets, and all other carb parts that came with the standard carburator kit, thoroughly cleaned the carbs,
new air filter
new oil
new carb boots, from engine to carbs
tested the coils , resistance is within haynes spec, seems good
spark will jump a 1/4" gap, so it seems strong
pilot jets are 2-1/2 turns out from soft seat

bike starts excellent and runs strong on new plugs, but 100kms on a new set and she starts sputtering a little, soon after it wont start until the plugs are replaced, i am only getting about 120kms per tank of gas (15litres)

anyone with any thoughts?

thanks

Paul

I can only tell you what I would do, I would do a compression test and a leak down test. It is possible your valve seals are bad as well. Does it smoke when you first start it up and then clear up? If it smokes what color is it?

Have you checked your air filter?
Its unlikely, but could restrict your air intake.

as mentioned , the air filter is new... no issues with it as far as i know. no smoke that i can see, it does blow some black soot out the pipes when rpms increase.

the compression was tested as well, within spec according to haynes, i think i had 148lbs on one jug and 135lbs on the other, if i am not mistaken, spec said anywhere from 130 to 150 is acceptable. the bike only has 22,000kms on it! I am at my wits end with this, i almost have the bolts worn out on the carbs from removing them and re installing them... what is this leakdown test you were speaking about?

thanks guys,


Paul

I'd expect oil consumption to show up with careful observation. Oil consumption would point to rings or valve seals. Your compression results are close enough to set aside for awhile (I think).

Poor fuel economy suggests over rich mixture. (which soots plugs also)
Rich can come from choke misbehavior (enrichener circuit leaking) or wrong jetting or wrong needle position.
It can also come from blocked air path causing more suction on the carb thus pulling more fuel (classic lawnmower style choke). But if you don't have a paper towel stuffed down an intake tube between the air filter and the carb.... then this has been checked with new filter.

I've not messed with CV carbs much but I understand that jets can be changed and needle positions also can be changed.

Perhaps looking into what the stock main jet size is supposed to be and then comparing it to your jets is next? Also know that people monkey with jets sometimes so even if your jets are marked to match stock, they could have been drilled out sometime. (that's the kind of stuff you might get with used bikes)
Is it possible the idle jets have been removed or tinkered with?

Has it ever run right for you? Or did you buy it this way?

The carb/ carbs is not working properly or wrong there really is no other explanation with the info posted. Black smoke is fuel to much to be more precise. As stated if compression is good and no blue smoke on start up (valve seals) and black smoke on accel your intermediate range is dumping to much fuel. If your sure the carb is working properly and you bought the bike used I'd try to replace it with another one either someone jetted it for an exhaust, or it is just the wrong carb.

Just a little more thinking about this-

If both plugs are equally sooted then 98% the problem cannot be oil. Seals and rings don't wear or fail that evenly. The remaining 2% probability could be something like crankcase ventilation drawing oil vapor into the intake to be distributed evenly. This can be easily checked by disconnecting the vent and seeing what's going on there (pressure? clouds of oil vapor? lots of oily slime in the vent tube near the carb?). Can even ride it unvented if you block the suction to the carbs/intake manifold.

It could be as simple as badly set float height. Or damaged float needle/seat allowing too much fuel all the time.

But to me the biggest hints are black soot and on BOTH plugs (if that's actually true?).
There's something common to both cylinders which narrows the list of possibilities. (if true)

Does this bike happen to have the kind of petcock with a vacuum line to shut it off when the bike isn't running? (if I recall correctly my CB900F had such a rig)

If so, the diaphragm that engine vacuum sucks against to open the valve could be perforated and lets fuel go down the vacuum line and into the intake manifold. That would cause it to run rich, would supply fuel to both cylinders and would give poor fuel economy.

If there isn't such a mechanism on the petcock, is there an equivalent mechanism on the fuel line someplace else?

I pulled up this picture of a petcock repair kit for Virago (not your year). It has a diaphragm. If your petcock or fuel system uses those part I think that's your problem.

(hopefully moderators will allow the link to stay as it's part of the discussion since I don't have a picture of anything Virago)
http://www.ebay.com/itm/K-L-Fuel-Petcoc ... 088wt_1147

Whoah yeah that could be a culprit, I had an old Yamaha XS 650 that did just that good call!!!!

Question is - how long until the OP returns to get the answer?