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Big Bore Changed Cams Hanging Idle, Jetting Question


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2006 DRZ400SM. 434 Cylinder works big bore, Stage II hot cams, MRD exhaust, E model CDI, FCRMX39 Carb I bought from TT a few years back.

I have hanging idle issues. I had my fuel screw stuck closed? Don't know how I did it but I had to take it off to fix it.  So I have it at 1.5 turns out for when I reinstall it, I plan to modify jetting first as needed, then try her out.

Anyway I have the following Jetting

Slow Air Jet = 200
Main Jet = 158 I think I need a 160-165?
Slow Jet = 45
Starter Jet = 80
Needle = EMP 3rd clip from top.
Wire vs. O-Ring mod.

It falls a little flat on top since I put in the smaller cams I had to 9.58 lift old Hot Cams before.
So I think I have the thing about right just looking for advice.

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Don't be too concerned with the main jet BB's suck more air so the mixture richens a little compared to standard bore.  I had to go up one size on the slow jet to get stable idle with no popping on overrun and no hanging idle.  The fuel screw is about 2 and 1/4 turns out.  The slow jet is 46.  It sounds like you could wind the fuel screw out a bit more 

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I went down to the 158 with the smaller cams.  Lift went from 9.58 mm to 9.05 mm or so.  Just wondering if I need more main or I am out room since I am not opening the valves as far.  The bike runs basically the same, but falls flat at 92 mph vice 98 mph before.  Trying to sort it out.  I could go up a size on the slow jet.  I am thinking my air screw being stuck closed could have been hurting me as well.

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If you lost miles per hour, verify the throttle is opening the slide 100% .  Turn the throttle grip to the stop and then try to rotate the throttle wheel counter clockwise.  If you are running standard SM gearing or taller, it should be able to crack 100 mph indicated on the stock speedo.  There isn't much margin of error to get the cables set so the slide tops out before the the throttle tube.

The old cams were a more RPM biased setup. You'll notice a slight difference in power but it shouldn't limit maximum speed. 

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Carb opens fully, speedo is correct as it is a Trailtech vapor calibrated against GPS.  I am geared taller than stock, don't remember what ratio but figure it out.

Bike pulls solid all the way to the top, just doesn't hit the rev limiter in 5th like the other cams.  So I figure that is the price of less lift.  

As to the bottom end, I guess I will just put it back together as is.  With the ability to adjust the idle air screw now maybe I can fix the hang.  I am most interested in whether when  I changed the cams and dropped the main from I think a 160-162 to a 158 I went to far?

Also, are the slow jet and slow air jet properly sized for what I am running?

Thanks for the input.  I used to have all this written down and since I am old, I "misplaced" it.  So maybe I will find my notes, but I thought I would take the forum short cut in the mean time.

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2 hours ago, TLRDoug said:

Carb opens fully, speedo is correct as it is a Trailtech vapor calibrated against GPS.  I am geared taller than stock, don't remember what ratio but figure it out.

Bike pulls solid all the way to the top, just doesn't hit the rev limiter in 5th like the other cams.  So I figure that is the price of less lift.  

As to the bottom end, I guess I will just put it back together as is.  With the ability to adjust the idle air screw now maybe I can fix the hang.  I am most interested in whether when  I changed the cams and dropped the main from I think a 160-162 to a 158 I went to far?

Also, are the slow jet and slow air jet properly sized for what I am running?

Thanks for the input.  I used to have all this written down and since I am old, I "misplaced" it.  So maybe I will find my notes, but I thought I would take the forum short cut in the mean time.

Your speedo is more accurate than the stock so comparing numbers is apples to oranges.

For the most part your settings are similar to what I used in Ohio around 700'.  Put the old main in and see how it does.  The engines runs better when it's on the rich side vs lean.   

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Got the mixture adjusted she just idles beautifully at 1500 or so.  Rev it and she comes right back down.  So the stuck mixture screw was all that was wrong on the bottom.  I haven't found my old jets, I had a 165 in with the big cams, if I can't find one I will order a couple and play with it.  Thanks for confirming all my thoughts.  Sometimes I forget that I know a lot of this stuff, just forget it for a while.  Lately I have been remapping Fuel Injection on my KTMs and V Stroms so I am off on my FCR knowledge.

Hopefully someone else has this issue and did what I did and apparently stuck the mixture screw.  Easy fix once I pulled her apart.  I did get pics of my jets and stuff so I can remember what is in there next time.

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I’m running fcr slant 39 with 158 main. In sm trim with 15/39 drive i saw 110 mph indicated with sm speedo gear. I’ve tested slower highway speeds with gps and it’s only off 1-2 mph. Had to tuck way down to get there. 94 bore, single base gasket, e cdi, hc 2003 cams. No tubes in the wheels 22/24 psi f/r.

The flexible extended pilot screw adjuster was $30 and money well spent. Don’t really have to adjust it in central CA for seasons.

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