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Old 07-26-2013, 07:54 AM   #1
Fundy Rider OP
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Joined: Jul 2009
Location: St. Stephen New Brunswick
Oddometer: 233
Idle jets and IMS settings

Been having an idle issue as of late. Hung up and then will drop. Into the carbs at this moment and been reading a fair bit about jetting and issues.

One thing that just doesn't make sense to me is when I see the 45 idle jet used with the stock 50 air jet the Ims screws seem to be in the area of .75 to 1.25 and with the stock 42 and stock 50 air jet the setting for the ims seem to be in between 1.75 and 2.5.

Yes there are some that may contradict this and have taken elevation into account also but this seems to be the pattern. Running Factory Pro needles(not sure of position yet) and only have the front out with a 175 main.

Trying to deal with the idle issue thinking I was rich I took the srews out to 3 with the 45 and AIJ 50 and needed to choke and then went the other way to 1.25 and she fired right up but still had the idle issue.

In seems to contradict logic with the bigger pilot jet. Why have the ims srews turned out less with the Bigger Pilot? Thinking more gas should mean more air to balance things out.

Just trying to understand this here and not asking for help with the idle issue on this thread.
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Old 07-26-2013, 08:33 AM   #2
Head2Wind
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Joined: Mar 2006
Location: NorthWet Washington
Oddometer: 2,473
the "IMS" (Idle Mixture Screw) is a "FUEL" screw, not a "AIR" screw.

since it is a "FUEL" screw, more turns out equals more fuel.

for a given engine configuration, if running #42 pilot and lets say 2.5 IMS turns out and then were to go to a #45 pilot (again nothing with the engine configuration has changed) I would anticipate setting the IMS at 1.5 (+/-) to get the idle fuel correct.

We also need to consider the fuel height in the float bowl. It greatly affects the idle fueling and can be used to fine tune this area of operation.... More fuel height (in our case a lower measured number, lets say 2.5mm opposed to 3.5mm) will deliver more fuel (richer). 4mm measured float height would be less fuel (leaner).

these carbs tend to leak fuel past the needle jet at idle and because of this they are a real PITA to get the idle mixture nailed AND make it so that the engine doesn't go lean on fast tip in and then also at high manifold vacuum steady state cruise (highway speeds, usually around 3.5k-4.5k RPM)
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Old 07-26-2013, 08:43 AM   #3
Fundy Rider OP
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Thanks

I have read countless times that they are an Air Idle mixture screw and a few times been told the other way. Even the Ktm manual calls it an Air Idle Mixture screw.

It never made sense the way the bike ran after countless times of adjustment.

Now coming from someone as yourself I can go back to applying logic and get this sorted.

Thank you for clearing this up for me.
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