Bonneville America Forums Forums Technical Forums Performance Mods Carb adjust question

The mains, pilots, and needle position were set up optimally by my service dealer after a number of dyno runs. The pilot adjustment screws are about 1.75 turns out and carbs are balanced (checked with manometer). When checking plugs, my left is near perfect but my right appears rich (noticeably darker). Wouldn't the correct move be to adjust the right carb screw in slightly? I did this today and plan to run a few hundred miles and check again.

It had been argued by at least one person on this forum that the plugs are no indication of how the carbs should be set. It seems logical to me that plug condition is a definite indicator of running condition. I can't sense or really see on the tach much change from slight adjustments. As I said the bike is running great. The bike had been running too rich. Months ago I turned the screws in a 1/4 from 2 turns out. My mileage went up slightly and the plug condition improved. I'm assuming on many bikes the pilot screws may have to be slightly different for best results.

Mine are set at different turns out and they seem right to me. Every carb is going to be different just as every motor is a little different...what could it hurt to find out by cranking that right mix screw in a tad?

Quote:

---

plugs are no indication of how the carbs should be set

---

It make sense to me too that how the plugs look are a direct response to how much or little fuel a carb is delivering. That's what carbs do, deliver fuel, so I would be doing exactly what you are doing Bob.

Good luck with that one and maybe you can let us know if adjustments make the difference as we suspect

The plugs are a very good indicator. Only time it becomes a bad way to check is when the engine gets worn and needs rings etc.. since then you are burning oil too.

There are many reasons why you can't use plugs for the type of fine tuning that usualy raises this issue. Racers have specific drills for checking plug color, none of which applies easily to a street motor. Let's say you ride for 500 miles, then pull the plugs. They're light tan. What does that tell you about each of the metering circuits in the carbs? What does it tell you about the ability of the carbs to produce the correct A/F ratio under various conditions, including intermittant acceleration and idle? What does it tell you about the color of the plugs just before you decelerated and shut off the motor? The answer is "nothing."

It tells you that you are jetted pretty well for what ever RPM you were riding at mostly, that's what it tells you. The amount of time it takes to decelerate will have no visible effect on the color. Then there are other ways to determine if your pilot circuit is close or not. Granted it isn't as accurate as a dyno but for the last 30+ years it has worked really well for me and millions of others.

Bob...Ill preface this with "I am no expert but I am also mechanically inclined and apt". OK...that said I used plug color to get my carbs tuned after initial jetting and then again after swapping pilots.
one carb is adjusted about 1.5 turns more than the other and my plug color was about perfect when all was said and done. The bike runs fantastic and strong.

Now....the above method is the old method that I and a lot of others grew up with.
If you want to get more exact then your talkign dyno runs with A/F equipment and such. Itll be by far more accurate but also more $$.

Heres my thread (with pictures) about the similar situation Plug color

Hi Guys, If both carbs are setup identically the airmaix screws should be the same,,lots of guys seem to forget how important the float levels are to this sort of carb.....
Some guys say they have lots of popping on overun and others dont, this is because low float levels make it run leaner and hence the popping and vice versa...
extensive playing has shown me that the fuel level needs to be 2mm above the carb bowl/body joint with both carbs exactly the same,,,both idle mix screws should be the same if the air injection system is removed.The problem with different air mix screw settings is that each carb will react differently in different revs/throttle openings which means they are not being fully optimised..Also any air leaks in intake or exhaust systems has an adverse effect on these carbs.hope this helps yas, cheers, Tadpole

Well Dog, I guess we won't ever resolve this. I know from experience on the track that reading plugs requires A) brand new plugs and B) cutting the ignition prior to decel. You can say decel doesn't matter to color, but I know better. Sorry, not trying to make this personal, but for all but the coarsest of tuning, this is simply sloppy, backyard mechanics. It might make the owner feel better, but it won't achieve proper tuning in a street motor.

if this helps, I have just made a video on carb tuning and ...once I get a chance, will be making a vid on using a color tune product; a device which ensures the right amount of fuel is being delivered for the optimum burn. The device replaces the spark plug and allows you to adjust the burn(fuel) based on color seen through a glass sight into the cylinder. Will post as soon as I get a chance!

could you not just rev the motor at stanstill?