Kdx idling inconsistent.

[JZ05220r]

First off I know I have been posting a lot about the Kdx. I apologize. I am very close to having it dialed in but their is one issue with the idle that is bothering me. On cold start the idle is pretty steady at about 860 to 900rpm. When warmed up and I pull the clutch in at a stop the rpms hang as high as 1200rpm, then the rpms will always drop to about 650 rpms nearly dying within about 5 seconds. If I try to turn the idle up when it’s low it doesn’t do anything. I have done a leak down test and it had only lost 1psi in about 8 minutes. I have played with the float. Tried messing with the the air screw. Going up a size on the pilot jet. No change. I am clueless at this point.

[JZ05220r]

Seeing that I have just about tried everything I double checked the compression and it topped out at about 155 psi. I looked at the spark plug that has about 3 hours on it. Looks/more so seems lean throughout the midrange and maybe a bit too rich on the main. The ground strap has some bright tan colored finish.

[KDXGarage]

Where did you read that you could read the needle and main jet after a few hours?

What is your expectation of idling?

[JZ05220r]

Where did you read that you could read the needle and main jet after a few hours?

What is your expectation of idling?

I do agree with the fact that jetting can’t be read on a plug with multiple hours. But I think that I can still tell if the engine is generally running lean. My main jet is alittle high based on the standard size recommendation from RB. Before all the mods have been done my plugs were always dark brown after a multiple hours. I just need it to idle steadier without hanging up randomly and also dropping very low randomly, it’s actually becoming irritating. I think I should have no problem with idle issues as it performs so well at low rpm to begin with and considering that everything is sealed and in good shape mechanically at this point. Thanks

[SS109]

You need to follow the jetting guide and dial in the pilot circuit first, then your main, and then you go to the needle. Lastly, the strap won't tell you crap except maybe whether you have detonation or not.

[JZ05220r]

You need to follow the jetting guide and dial in the pilot circuit first, then your main, and then you go to the needle. Lastly, the strap won't tell you crap except maybe whether you have detonation or not.

It’s difficult for me to set the air screw based on the jetting guide as the idle keeps dipping down even if I set the idle slightly fast.

[JZ05220r]

I’m just going to try try a 38 pilot and see what happens. Maybe it’s just loading up quickly idk.

[SS109]

And what happens at different air screw settings with your current pilot jet? Also, make sure your float is set right before doing anything. Set to 18mm.

Oh, make sure your idle speed screw isn't set too high. It's impossible to dial in the pilot jet if set too high.

[JZ05220r]

And what happens at different air screw settings with your current pilot jet? Also, make sure your float is set right before doing anything. Set to 18mm.

Oh, make sure your idle speed screw isn't set too high. It's impossible to dial in the pilot jet if set too high.

Pretty sure I can turn it pretty far out past 3 turns and it still runs and it won’t run at 0-1/2 turn out if I remember correctly. Float height is with in 15mm +- 2mm spec. I will take a video of me messing with it. Thank you

[bufftester]

Your float height is way off. Getting a consistent idle is necessary to get the jetting correct. If the idle is all over the place then there is an issue in either the carb or fuel delivery

[JZ05220r]

Your float height is way off. Getting a consistent idle is necessary to get the jetting correct. If the idle is all over the place then there is an issue in either the carb or fuel delivery

I have the fuel level tube and made sure the level was within 2 mm above the bowl mating surface. Should I just ignore that? My gas cap breathes fine and my petcock works fine.

[bufftester]

Ahh OK measuring it externally that way is more accurate than setting it per the normal procedure so you should be fine.

[SS109]

And what happens at different air screw settings with your current pilot jet? Also, make sure your float is set right before doing anything. Set to 18mm.

Oh, make sure your idle speed screw isn't set too high. It's impossible to dial in the pilot jet if set too high.

Pretty sure I can turn it pretty far out past 3 turns and it still runs and it won’t run at 0-1/2 turn out if I remember correctly. Float height is with in 15mm +- 2mm spec. I will take a video of me messing with it. Thank you

I don't care what the service manual says, plus or minus 2mm is a huge swing on these carbs. Set it precisely. I set mine manually by pulling the float bowl but, since you're doing the fuel method, try setting it exactly at the mating surface or 1mm below and see if that makes a difference.

As for idle speed, can you hear the difference at different needle settings? Do you have a tach installed if you can't physically hear the difference? 1 turn out should not idle the same as 2 turns out and so on. You gotta be methodical on stuff like this.

[JZ05220r]

And what happens at different air screw settings with your current pilot jet? Also, make sure your float is set right before doing anything. Set to 18mm.

Oh, make sure your idle speed screw isn't set too high. It's impossible to dial in the pilot jet if set too high.

Pretty sure I can turn it pretty far out past 3 turns and it still runs and it won’t run at 0-1/2 turn out if I remember correctly. Float height is with in 15mm +- 2mm spec. I will take a video of me messing with it. Thank you

I don't care what the service manual says, plus or minus 2mm is a huge swing on these carbs. Set it precisely. I set mine manually by pulling the float bowl but, since you're doing the fuel method, try setting it exactly at the mating surface or 1mm below and see if that makes a difference.

As for idle speed, can you hear the difference at different needle settings? Do you have a tach installed if you can't physically hear the difference? 1 turn out should not idle the same as 2 turns out and so on. You gotta be methodical on stuff like this.

I’ll try setting it even with the mating surface. First I want to try a nearly brand new number 7 slide I had on my old carb to see if maybe a worn slide was the problem.

[kdx633]

my bike acted like yours, go 2 mm below mating surface.

[John_S]

With the #7 slide you'll be starting the jetting from scratch. I recently put a 7 on my 35 and only have 2 rides on it but its a work in progress. I was riding with friends so I got it good enough but didn't have the time to mess with switching out jets. Before the 7 I had the standard 35 AS 4.5 slide with 18mm float height, 40 pilot, air screw at 1.5, and the needle on the 2nd clip from the top.

First ride I had to go all the way down on the air screw to 1/4 out from fully seated before it wouldn't lean bog.
The second weekend I had a 42 pilot and my air screw was best at 3/4 out from fully seated.
I thought I had a 45 pilot on hand for this weekend but don't, I have a 48 so I'll try that.
The mid range was lean so I needed to go to the 3rd clip on the needle.
I may need to go to the 4th clip but won't until I get the pilot and air screw pretty dialed in.

The good news is that my idle and air screw are much more reactive to changes. I lowered the slide with the idle screw about 2 whole turns and little changes make a difference. The air screw is very sensitive now which I don't mind. It's easier to see if the pilot is correct. It'll be idling good and another 1/4 turn out ramps up the idle and it hangs there before coming back down. Very easy to tell it's too lean. I leaned out the slide but am having to add a lot of fuel to the pilot and needle circuits so time will tell if it really makes a difference or just runs the same but with different size jets than the 4.5 slide. I wish my pilot jet was correct because the throttle response and idle are so good with the #7 and the air screw in the 1-1.5 range while in neutral but it lean bogs while stabbing the throttle while riding so it's not there yet. I'm still hopeful the slide change is going to be a good thing.

[JZ05220r]

With the #7 slide you'll be starting the jetting from scratch. I recently put a 7 on my 35 and only have 2 rides on it but its a work in progress. I was riding with friends so I got it good enough but didn't have the time to mess with switching out jets. Before the 7 I had the standard 35 AS 4.5 slide with 18mm float height, 40 pilot, air screw at 1.5, and the needle on the 2nd clip from the top.

First ride I had to go all the way down on the air screw to 1/4 out from fully seated before it wouldn't lean bog.
The second weekend I had a 42 pilot and my air screw was best at 3/4 out from fully seated.
I thought I had a 45 pilot on hand for this weekend but don't, I have a 48 so I'll try that.
The mid range was lean so I needed to go to the 3rd clip on the needle.
I may need to go to the 4th clip but won't until I get the pilot and air screw pretty dialed in.

The good news is that my idle and air screw are much more reactive to changes. I lowered the slide with the idle screw about 2 whole turns and little changes make a difference. The air screw is very sensitive now which I don't mind. It's easier to see if the pilot is correct. It'll be idling good and another 1/4 turn out ramps up the idle and it hangs there before coming back down. Very easy to tell it's too lean. I leaned out the slide but am having to add a lot of fuel to the pilot and needle circuits so time will tell if it really makes a difference or just runs the same but with different size jets than the 4.5 slide. I wish my pilot jet was correct because the throttle response and idle are so good with the #7 and the air screw in the 1-1.5 range while in neutral but it lean bogs while stabbing the throttle while riding so it's not there yet. I'm still hopeful the slide change is going to be a good thing.

Thank you for that info! I will say that the number 7 I put in may have made a difference in the 1/2 ride I took the other day. It didn’t hang at all but the idle did dip a few times. When I got back I idled it for a bit and messed with air screw and it ended up steady without dipping.

[JZ05220r]

my bike acted like yours, go 2 mm below mating surface.

If it keeps up I definitely will. Just got a 91 Kdx and I have my work cut out for me on this one so I’ll be putting in a lot of time to get it restored the best I can. The frame has some pretty decent corrosion on the frame rails below the seat and the wiring is going to be a pain after the frame is cleaned up and I go to put it back together.