Too slow or too fast idle??

15 replies to this topic
  • Thumpin_426

Posted October 03, 2001 - 07:12 AM


Is anyone else having problems controlling their idle? Mine is either too slow ( 1000 rpm ) or too fast (2500 - 3000 rpm). It seems like when I try to adjust it up, the rpm jumps quick. There is no in between. I can't seem to get it close to 2000. As soon as I try backing it down it falls from 2500-3000 to 1000. What do you think???

  • Guest_Guest_*

Posted October 03, 2001 - 07:15 AM


Have you ever had the slide out of the carb? It is very common to put the plate back on the slide upside down. This will cause erratic idle. The "pointed" end of the plate goes the opposite way from the "pointed" end of the slide.

  • Thumpin_426

Posted October 03, 2001 - 09:10 PM


No, the carb has never been out of the bike with the exception of changing the jets and the BK Mod. Even then it was done on the bike.

  • Ramrod

Posted October 03, 2001 - 09:39 PM


My idle is high - in fact when I hit the gas it will idle fast for 5-10 seconds, then it will lower really low ... this is where I've run into fouling problems or usually it just dies out. I see other guys who can leave their bike on the stand idling, but not mine. What gives?

  • mikeolichney

Posted October 03, 2001 - 10:23 AM


Ramrod: when your bike has the hanging idle, you are too lean on the pilot circuit. Back out your fuel screw (up to 3 turns out), in 1/4 turn icrements until the hanging idle vanishes. If no improvement at 3 turns out, go from a 42 to a 45 pilot, start at 1 turn out, and do the same.

  • Ramrod

Posted October 03, 2001 - 10:37 AM


Thanks Mike, I'll give that a try tonight, and hopefully my idle will get dialed in once and for all ...

  • vznx1w

Posted October 03, 2001 - 10:53 AM



Having the idle too rich will also cause the idle to "hang" before settling to normal idle. If you're fouling plugs this is likely the case. If going richer--as previously suggested--doesn't work try going leaner instead. I'm running a WB E-series exhaust w/ stock jetting and had to turn the fuel screw down to only 3/4 turn from seated to correct my hanging idle. My plug fouling and poor hot starting problems disappeared.

  • mikeolichney

Posted October 03, 2001 - 12:14 PM


vznx1w: I remember when I went too rich on pilot/screw the idle would be low after throttle was chopped, and then speed up. It was pretty subtle but that is what I remember. Anyone else see this?

[This message has been edited by mikeolichney (edited October 03, 2001).]

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  • DaveJ

Posted October 03, 2001 - 02:01 PM


I don't know if this is your problem, but perhaps it will at least get you thinking or lead you in the right direction.

During one of my take-it-all-apart moments, I split the two halves of the carb.

In the middle of the two pieces is a complex route of circuits sealed by a thin rubber gasket. Looking a lot like a miniature automotive automotic transmission valve body (dang, that was long to say...hope someone can relate to that).

So here's the problem, the re-assembly of these two components is beyond tricky. I literally had to make a special clamping device to get the two halve to seal evenly. With this, I can imagine that a very small percentage of the carbs that come from the OEM may have minor leaks because of this design.

The point to all of this is that when I didn't get the two halves sealed correctly, I suffered these high and fast idle problems as posted. It would take anywhere from 15 to 30 seconds to get the bike to idle back down after a quick rev. Starting was more difficult, and I’m sure the mixture was way off. Full throttle worked.

Is this your problem? I don't know. But try the easy fixes first, and if you fail to come up with anything else, let me know and I'll explain further how to resolve this issue.


  • Taffy

Posted October 03, 2001 - 02:50 PM


i feel the description that the mixture is lean is correct and that you can go at it two ways. you can either put in a larger PJ and then he'll get that bwooooaaar noise on hitting the throttle because it's rich and won't rev SHARPLY or he can drop the PAJ by buying the Pilot Air Screw from sudco.

if hick gets involved he'll tell you the same thing.

i'm down to a 40PJ and hick runs 38 or 40 along with #75/85 PAJ.

anyone who followed my "snap and 3/4 throttle..." (WR side) knows that i've been up to 50PJ and #140 PAJ and had got it wrong. by my own admission. but now i realise the folly of running large pilots.

i hope that many of you will take the opportunity to reduce the AIR to get the ratio right another time.


  • vznx1w

Posted October 04, 2001 - 05:00 AM


Your observation is the exact opposite of what I observed. I have tried pilot jets 42, 45, and 48 and the fuel screw settings anywhere between 1/2 and 2 1/2. Anytime I get richer than the 42 and 1 turn out I get the hanging idle. I wonder if there is some variation in carbs that explains why mine works different.
Regardless of this anomaly, the bike now runs and starts great. When I ran the 48 pilot I was fouling a plug every ride, since leaning the idle I've been running the same plug for the past 26 rides.

  • vznx1w

Posted October 04, 2001 - 08:32 AM



My observations are the opposite of yours and many of the others who have posted on this topic. Makes me wonder if something wierd is going on with carb, although the bike runs and starts great--and I haven't fouled a plug since using these settings (25+ rides).

I have run pilot jets 42, 45, and 48 with fuel screw settings up to 2 1/2 turns. The only way I get rid of the hanging idle is to run the 42 at 3/4 to 1 turn on the fuel screw. NOTE: Running this lean at idle forced me to adjust the BK screw for a longer pump duration (than BK's .3 sec recommendation) to retain good throttle response. However, that makes sense as BK recommends a richer pilot as part of his package.

I originally started out with the BK setup (richer idle, higher needle, richer main) but this was too fat for my bike. I suspect that Timmy's bike has other modifications that require the increased fuel...or maybe the difference is just that Ferry keeps it pinned longer than us old vet guys!!

My 2 cents

  • Jason_in_KC

Posted October 04, 2001 - 08:42 AM


BK's settings could be a little too rich for most of us..he did state that Ferry's practice bike had a full race DSP exhaust. Most of these aftermarket exhausts do require richer jetting. Hope that helps.

  • mikeolichney

Posted October 04, 2001 - 09:51 PM


vznx1w: I set my pilot up using a digital tach. I set the idle at 1800 rpm and adjust for max idle speed by using the fuel screw. If the peak speed is not between 1/2 and 2 1/2 turns on the screw I change the pilot, and do it again. If the idle ends up being above 1900 rpm I set it back to 1800. This works really well.

My bike is stock (yes I'm an old guy too and the bike is plenty fast for me stock). At 5000ASL I end up with a 40 pilot in the summer, and a 42 in the winter. I have a jetting chart that works well for the 426, I think Motoman393 has it on his website if its of any interest to you.

Interestingly enough, I also have a WR250. It wants a 45 pilot, up from the stock 42 that sea level guys might want. So clearly bikes respond differently. My speadsheet works great for the 426 in the pilot, but not at all for the 250F.

  • Boit

Posted October 04, 2001 - 03:56 PM


This has evolved into one of the most interesting and beneficial threads I've seen in a long time. As I've read thru the posts, it becomes clear that NO two 426's are exactly the same. Jettings are pretty much a ball park figure but do seem to follow a general trend, as they should. What impresses me is how some guys have experienced some very unusual problems and have been able to persevere and solve them. And better yet, they share them with us.

As far as Ferry's bike goes, his engine really doesn't compare with ours in the sense that his jetting would apply to a bone stock 426. I would love to know what has been done to his cylinder head. No doubt, that thing has been massaged to flow much higher numbers than ours due to the roadracing experience and knowledge at their disposal. Our heads are massed produced and have inherent casting flaws that differ from one head to the next.

So much info to absorb, so little time.

  • Mr_Toyz

Posted October 04, 2001 - 04:45 PM


In mikeolichneys defence, I think the Maloneys fouling plug statement confused the whole issue.

Ramrod, I think you have two separte isses to deal with, hanging idle and fouling plugs. Fix the plug issue first. You may even think about going back to stock settings and starting over. Remember to only make one change at a time. Hey...the throttle issue could be as simple as a dirty throttle tube or cable..... :)

Oh, hanging idle = too lean
slumping idle = too rich

As for Thumpins original question. You may have to try and adjust your idle in conjuction with the fuel screw. Look in manual. My guess is that adjusting them together will solve you problem.

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