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Old 11-25-2009, 01:53 AM   #1
Basri OP
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Why spark plug turnes to black ?

Hi, here is my problem;

96 R80GS, right side spark plug always burns (goes black) after few kms, after replacing with a new one or clean it.

Cleaned both carbs (Bing CV) and replaced diaphraghms.
Replaced spark plug cables and caps.

When syncing the carbs with Twinmax, I can center the needle, but somehow don't like the sound of the bike, something going wrong.

I have no problem with throttle stop screws.

When syncing, left side idle mix screw works fine, I start from 1.25 turns open, screw it in, find the point engine is almost dieing, screw it out, find the point engine is almost dieing, fix it somewhere in between two extremes, where you get the best noice.

But when it comes to right side idle mix screw, when I screw it in, rpm increases, when it touches the bottom, I get the highest rpm.

Is this the way it should be ? Or something wrong with my right carb ?
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Old 11-25-2009, 02:09 AM   #2
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Yep, something is wrong with your rh carb.
It should have it's 'sweet-spot' at around1-1,5 turns on the idle mix screw, just like the left.

You have a blockage or leak somewhere in the idle control passages, cleaning the carb and checking the O-rings is your next step.

If you got access to it, have the carb's cleaned ultrasonic and rebuilt them with a set of new gaskets and O-rings.

Paul.
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Old 11-25-2009, 04:18 AM   #3
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Something is definitely not right with your right carb. As said above, check idle circuits, but the other problem spot can be the choke/enrichener.... check to make sure that it is disengaging completely,

As said, pull the carb, pull all the adjustment screws, jets, etc and at least use some compressed air to blast out the little orifices. Cleaning those itty bitty holes will likely solve your problem.

Good luck.
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Old 11-25-2009, 04:39 AM   #4
Wirespokes
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Definitely something wrong. Sounds like an air leak - bad ORing on the idle adjust screw, idle jet, choke shaft (not that common), or the choke body gasket.
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Old 11-25-2009, 05:25 AM   #5
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If three people say "definately something wrong with right side carb", than definately something wrong with right side carb.

I will tear the carb and check all above mentioned points.

Thank you all for valuable advices...
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Old 11-25-2009, 07:50 AM   #6
StephenB
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Maybe you want to check the easy things first before going in again deep: maybe a severely rounded idle mixture screw tip or the o-ring is damaged ...

Swap the idle mixture screws and see if the problem moves.
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Old 11-25-2009, 08:48 AM   #7
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Just saw your original post again, and a black plug indicates too much fuel. ORings are easy to check. Rebuilding the carb isn't necessarily needed. Most of everything is accessable with the carb mounted. The enrichener ckt will require removing the carb.

These carbs are actually very simple once you understand them (isn't that the way it normally works?), but they are very simple. I'd assume all passages are clear since a plugged passage would create a lean condition, and that's the main reason for rebuilding. And it sounds like the bike was doing fine till recently when something changed? I'd seriously consider the choke gasket or an ORing. Make sure the choke is returning to full off position, the cable is allowing the butterfly to return totally... little stuff like that.
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Old 11-25-2009, 12:39 PM   #8
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Opened up the right hand side carb and finished the job 10 minutes ago.

Only visible problem was little holes on idle jet screw was blocked and screw needs a new o-ring.



Opened the holes,put the old o-ring on (I will order a pair with some other o-rings and gaskets)

Cleaned all other things checked the idle circuits.

Thanks to Advrider forums one more problem solved

Now rhs idle mix screw responds to my setting just like the left one and bike sounds much smoother.
I guess it is done for now

Will see the spark plug after few miles tomarrow.

Thank you all one more time for valuable advices.
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Old 11-25-2009, 02:54 PM   #9
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You didn't put THAT old ORing back on???!!!! And it WORKED???

I've had to replace them when they still looked ok at a glance but were just starting to split near the screw head. I can't believe it worked at all with that mangled piece of rubber!
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Old 11-25-2009, 03:30 PM   #10
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It works because he opened up the holes where the air enters and then for a certain setting of the mix screw.

At least there's respond from the mix screw.
That's a different thing to working as it should.

I long made it a habbit to clean all every year, you'd be suprised how much crud comes out the vapouriser part where the needle hangs in.

And the needle and vapouriser tube do wear over time, no bad thing to check and replace.

Paul.
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Old 11-25-2009, 09:19 PM   #11
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Quote:
Originally Posted by Wirespokes
You didn't put THAT old ORing back on???!!!! And it WORKED???
There is a misunderstanding.

I put, old, worned out 0-ring back on.
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Old 11-26-2009, 05:52 AM   #12
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That's what I thought you said. And, by the way, the ORing goes in the groove between the head and the threads (in case there was any question).

That ORing pictured is such a mess I can't imagine it doing much better than no ORing at all!
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Old 11-26-2009, 11:34 AM   #13
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Quote:
Originally Posted by Wirespokes
That's what I thought you said. And, by the way, the ORing goes in the groove between the head and the threads (in case there was any question).

That ORing pictured is such a mess I can't imagine it doing much better than no O-Ring at all!
That picture is not my screw, just a picture I found on net.

My o-ring even worst.

Anyway, after a 30 miles ride (home-work-home) today, spark plug colored to coffee with lots of milk, beige or very very light brown, which is good.

No more black spark plug.

I think we found and fixed the problem.

Next step is to replace the o-rings and gaskets on both carbs...
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Old 11-26-2009, 03:44 PM   #14
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Well done!
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Old 11-26-2009, 08:45 PM   #15
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Excellent!
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