Plug Chop and Idle Mixture Screw Adjustment Methods

I've been collecting some words of wisdom from the GSR forum and I thought they were just too good to let fade off into the digital demise suffered by those of us who "always meant to back up our files".  Here are a few informative bits about tuning your carbs that I thought I might be able to use later.  Hopefully, you will find them handy, too.

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Adjusting Idle Mixture On CV Carbs Using The Throttle Response Method  (PDF File) by Mr. koolaid_kid (George Burroughs), edited by Mr. bwringer (Brian Wringer).

 Adjusting Idle Mixture, Using the Highest RPM Method

(by Mr. psyguy)

  1. Take off the tank, set up some form of remote fuel supply.
  2. Set the idle mixture screws at 2 turns out.
  3. Warm up the engine and let it run.
  4. Adjust the idle to approx. 900 rpm.
  5. SLOWLY turn the screw at carb #1 in - at some point, the rpm are going to start dropping and the engine won't run as smoothly.
  6. Start turning the screw back out until you get the highest rpm and a smoother sound from the engine.
  7. At some point, turning the screw out further won't make any difference (and even further out may again take away the smoothness and the rpm).
  8. You want your screw turned IN as much as possible without affecting the rpm/smoothness.
  9. Re-adjust the idle to 900 rpm.
  10. Stop the engine to let it cool down a bit or place a big fan in front of the engine and do this while the outside air is cooler (morning/winter).
  11. Proceed with carb #2.
  12. Repeat steps 4 through 10, as necessary, for all carburetors.
  13. When complete, re-adjust the idle knob for the correct rpm as per your factory manual (usually 1050 rpm +/-100 rpm). In any case, do not idle below 900 rpm as this may result in insufficient oil flow.

NOTE: (by Mr. Steve)

If you get no change when adjusting the mixture screws, I would suspect a tip broken off in the carb body.

Remove the carbs from the engine, run your finger into the throat, just under the mixture screws. If you can feel a point sticking through, it's likely stuck and broken off the screw. I have used the side of a Phillips screwdriver to roll it over the tip and pop it back up. You can then turn the carbs over and let the tip fall out. You can also verify by removing a screw from #1 or #3 for comparison. There should be a very fine tip on the end of each screw. (Replace screws as necessary.)

Plug Chop Primer for VM carbs
(by Mr. tkent02)

(Editor's note from BassCliff:  For CV carbs, reverse the order - see the Factory Pro link below.)

Learn how to do this, it's nothing dangerous. Three separate tests for three separate carburetor circuits. Before you start, put a little anti-seize on each plug, don't install the plugs very tight. No need to wear out the plug threads in the head, you will be pulling them out hot. Bring a pencil and paper; keep track of each plug at each test.

(For Fuel Screw Setting/Float Bowl Height) The low throttle chop: Just putt around a few minutes at about 20 or 30mph or so, keeping the throttle open, but just barely, in the correct gear for this speed. It takes a while to color the plug because there's not much going on inside the combustion chamber. This tells you about the mixture from the pilot circuit, the low-power stuff. Pay attention to how it runs and feels, you can tell rich from lean. If it runs well and the plug isn't black, it's probably doing OK. This one is probably the most important to get right, it sucks to have your engine screw up momentarily at the apex of a slippery corner.

(For Best Needle Clip Position) Mid range: Go out on a highway and run it at about 1/4 to 1/3 throttle for a while, up a long grade is wonderful, but not required, as it doesn't take that long to get a good plug read. The plugs will color faster than the low power check. This tells you about your mixture from the position of the needle. If it spits and pops, that would be rich, if it surges, wanders or loses power for a while and comes back on, that would indicate lean. If it runs smooth and nice, it is close, looking at the plugs will tell you exactly. You need to see a little color here, a mixture resulting in lean, white insulators will run fine, but will eventually burn things up.

(For Best Main Jet size) Full power is done in one hard blast, full-throttle through the gears like a drag race. Does not take very long at all to get the plugs colored, you don't need to do insane speeds. You do need to find a place safe to go faster, where you can see any cops, and yet still be able to safely pull over to remove the plugs. The mixture at full throttle is controlled by the size of the main jet.  You definitely need to see a color other than white; too lean here will burn things up in a hurry.

For each of these reads, you need to actually chop the engine. A few seconds under closed throttle as you slow down will taint your readings. Simultaneously hit the kill switch, pull in the clutch and close the throttle. This leaves the plug looking exactly as it was during the run at the power setting you are testing.

To get the mixture right at idle, you adjust for highest rpm, you are looking for the peak combustion temperature here. It won't hurt anything to be lean at idle, because there's not enough heat in the combustion to damage anything.

If the main circuit or even the needle circuit is too lean, you can burn up valves and even pistons from the excess heat.

If you are having problems with your low power running, I would start there. But still check the full-power read, a mixture a little too lean is very powerful, indeed, until something expensive burns up.

Here is an article on reading spark plugs from

Spark Plug Reading 101

Here is a PowerPoint slide show explaining carburetor theory by Prof. Paul Crovella at Morrisville State College.

Also see the very informative CV carb jetting article at Factory Pro

Thank you for your indulgence,


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