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bigl

Air screw adjustment

6 posts in this topic

I am fairly mechanical, but carburation is a mystery I rather leave to others.  I suppose I need to change that, therefore the following.

 

Since my recent carb rebuild, the bike was great.  No popping, hanging, surging or anything bad.  However.  I occasionally start the bike during the week to keep the carb with fresh fuel.  Since last week, I have noticed the idling to be a bit off.  The revs will drop real low and sometimes just cut out.  On throttle the beast is fine.  Today I had to run around a bit on the bike and thought I was running out of fuel at one stage.  This afternoon another run - after toppping the fuel up - gave the same symptoms - mostly pulling away from a stop. 

 

Before I go and strip and clean the carb, how often does the airscrew need adjustment?  On my IT200, it gets adjusted during the ride as the day heats up.  South Africa is entering winter now and the ambient temps have dropped +/- 5degC in the last two weeks.  Could this be the cause?  I am sea level.

 

Cheers

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To begin with, it is a fuel screw on an FCR carb, not air, so the whole tight/loose in/out thing works backward; turning the screw out makes the idle circuit richer.

 

Second, the stock screw is a pain in the butt to access, especially on anything later than an '04, because the screw was capped over to prevent the owner from tinkering with it (for smog control reasons, may not be true on one sold in SA).  If it's capped, you'll need to remove the float bowl to tap the little plug out of the bowl from the inside. 

 

Then, follow this link to reset it:  http://api.viglink.com/api/click?format=go&jsonp=vglnk_14271338602096&key=fd4d0c8927dbf2331e83bbda32ac7c21&libId=i7m6nc92010004is000DA13cn2vi6nerx6&loc=http%3A%2F%2Fwww.thumpertalk.com%2Ftopic%2F378677-common-threads-look-here-1st-mods-maintenance-common-issues-and-faqs%2F&v=1&out=http%3A%2F%2Fwww.thumperfaq.com%2Fjetting.htm%23PJ&title=Common%20Threads%3A%20Look%20here%201st!%20Mods%2C%20Maintenance%2C%20Common%20Issues%20and%20FAQ%27s%20-%20YZ%20400%2F426%2F450%20-%20ThumperTalk&txt=http%3A%2F%2Fwww.thumperfa...%2Fjetting.htm%23PJ

 

As to how often they need tweaking, that depends.  The older models seemed to be affected by the slightest changes in ambient conditions, at times, while the newer carbs are a little less fussy. 

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The free mods have been done and I could make the fuel screw easier to adjust.  If we were gambling, what are the odds that the fuel screw is to blame.  It seems a bit lean when the idling drops, as there is some popping.

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Check your gas tank vent hose.  You say it happens when you have a full tank.  It could be a problem.  However, the fuel screw and the pilot jet are more often the cause.

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Check your gas tank vent hose.  You say it happens when you have a full tank.  It could be a problem.  However, the fuel screw and the pilot jet are more often the cause.

Tank full and empty.  I doubt the vent is the problem.

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Tank full and empty.  I doubt the vent is the problem.

If you want something done right.....

 

The o-ring around the fuelscrew was miles to big.  It is a wonder it ran at all.  There was also no throttle stop screw.  It was cut flush to the body.

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