Choked Up Black Plugs

[Hedgehog]

Changed the plugs, the old ones were black and sooted up.

Sometimes a small amount of black smoke is emitted from the exhausts when reving, especially from cold when the chokes out.

The bike is also a bit heavy on it's fuel consumption.

The carb is a Mikuni HR42, which seems to need some minor adjustment.

At first glance without removing anything, there does not appear to be any screws to twiddle in or out, apart from a small knob at the end of a cable coming out under the air filter. If I'm not mistaken this is the idle adjustment.

I would be greatful to hear from anyone who can tell me what I need to fiddle with, to stop the bike running a bit rich.

Hedgehog..

(P.S. I've just got me new set of tools, so I can't wait to get fiddling)..

Hi mate, try this site it will give you the info you need.
http://www.mikuni.com/fs-carburetor.html
Noj

[BrunoKatz WW1FighterPilot]

Mr Hogg sah!
You`re right about the twiddly fing on the bit of pipe sticking out the back, it is the idle adjuster. Now seems to me that one is running extremly rich to soot yer plugs uo like that. On the rear of the carb body, above the float chamber join, is a brass screw sticking up at 45degrees, this is yer pilot air screw, just look round the back of yer air cleaner backing plate and you`ll see it. Start her up on the choke fully out and idle till you burn yer digits on the pipes. Then shut the choke fully and hold a fast idle on the throttle grip. [use the friction screw to hold it until you got yer helmet etc on]
Now go for a short ride until the engine idles around 900 rpm with the slide closed. Pull into yer local church society meeting room car park, give it a few gentle revs then let it settle down to idle.
Now with a flat screw driver, turn that air screw in slowly until the engine begins to stumble, then, turn it out a few turns very slowly until it again begins to stumble. Yoy need the screw to be set half way between the two stumbles. This can take some time and experiance to set accurately. Put your head close to the carb and listen, give the engine a rev between each adjustment and let it idle again. By this time the engine is going to be bloody hot which will give you a false reading so, go for another ride to cool her down before making further tweeks. Hopefully, this will sort out your low speed mixture.
If the plugs are still sooty after this, then you need to change your jets and/or adjust the stroke of the accelarator pump. Dunno wot yer bike is but, Mine`s an Evo 1340 and i`ve installed jets as follows: pilot jet-no.20 main-no.160. needle-no.97 set in the middle slot and the pump jet is a no. 50. If you have a Mikuni handbook, all the instructions as to changing jets etc are in there. If you don`t, then you`re ferked, you need to get one. If you ride down here to Sussex, i`ll sort it out for you.
Happy tuning, Bruno
PS- I`m sure i`ve left something out, but no doubt S.T. or Kiwi Dave will finish off wot i`ve started!

[S.T.Chapman]

who set it up?

[S.T.Chapman]

sorry you cant just slap a carb on and expect perfection
bolt on bits do not give performance

[Chromer]

Oh my feck ... i wandered in here by mistake looking fer a bottle of chrome polish and got lost

.... it looks so fecking technical

I 'm out of ere

Chromer

[Hedgehog]

Originally Posted by S.T.Chapman

who set it up?

Simon,

The carb was apparently installed by Riders of Bridgewater down Bristol way. I have subsequently had it to Thunder City who solved the original starting problems.

I am told the bike is running at it's optimum performance, though it is still extremely heavy on fuel, and would appear to be slowly choking the plugs.

H.

[BrunoKatz WW1FighterPilot]

Of course, it maybe that the choke slide is sticking open, in which case check that the choke cable is operating properly. You should have tiny bit of free play with the slide fully home. There is a right angle elbow in the cable, check this to make sure the vinyl outer bit has not hitched up on the elbow. If i knew wot bike you had, maybe i could help you some more. Good luck.

[S.T.Chapman]

Originally Posted by Hedgehog

Simon,

The carb was apparently installed by Riders of Bridgewater down Bristol way. I have subsequently had it to Thunder City who solved the original starting problems.

I am told the bike is running at it's optimum performance, though it is still extremely heavy on fuel, and would appear to be slowly choking the plugs.

H.

apologies for the out burst,
sorry I've seen it so many times where other people do do as I described.

quite possibley the choke has stuck, if that had happened though it would be running like a pig.

the method Bruno has so elequently describe regards setting the pilot air screw is perfectly correct.
black sooty plug's denotes obviously an over rich over fueling mixture.
frankly that is outside the range of the pilot air and is being affected by a more severe fault.
starting from cold is a usual give away, if it starts without any choke from cold it denotes an over sized primary jet (or a stuck choke), unusual, as unless someone has changed it, because they normally are stock at No25.
another thing that could cause over fueling is an unvented tank, just check that the tank vent pipe is clear and not trapped any where, which can cause pressure to build up in the tank.
when its warm undo the filler, you will soon know if there's pressure in there...

anyway purely for your amusement take a gander at this....

[Hedgehog]

Thanks fella's,

I knew it must be something to do with the air/fuel mixture...

The bike is a 96 Wide Glide.

One other really silly question, presumably screwing the air screw inwards reduces the air flow into the carb. Would that suggest screwing it out allowing more air in would create a leaner mixture?

Though how does that affect fuel consumption?

Hedgehog.