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Plug reading with flatslide carbs - confused!

  • Thread starter Thread starter JimmyR
  • Start date Start date
J

JimmyR

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Today I finally got my bike running beautifully. "82 GS1100G, Mikuni RS36 carbs, K+N pods, new rings and reseated valves. The good thing is that the bike is running really well. Idles beautifully, is smooth off the line, effortless power everywhere on the throttle. I have 27.5 pilots, 127.5 mains, mix screws are 1/2 turn out. Power is silky smooth. Smaller pilots result in popping and farting in the carbs.

So tonight I thought I'd pull a couple of the plugs to see what colour they were. I thought they'd be the colour of creamy coffee. Nope - all covered in a fine mist of jet black. How can this be? If I lean it out at all it just gets cranky. I am obviously not cut out to be a mechanic! It took ages to get the engine running like this and now the plugs tell me it's running rich. What am I doing wrong?

FWIW carbs are new and definitely not dirty. Pretty sure I have gotten rid of the exhaust leaks now, and carb boots and O rings are all new.
 
What was your engine doing RIGHT BEFORE it was shut off? :-k

It doesn't matter if you just did a high-speed run to get away from local law enforcement, if you idled up the driveway and into the garage, THAT is what is going to show on your plugs.

To properly read the plugs, you need to to "plug chops". Mark your throttle to show FULL, 1/2, 1/4 and 1/8 settings. Find a place where you can run with the throttle setting STEADY for a while. Run the engine at the appropriate setting, simultaneously hit the kill switch and pull the clutch, coast to a spot where you can remove the plugs for readings. Repeat at the other throttle settings. Be sure to take paper or camera to record your findings.

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The color on your plugs is from pilot, screw or needle unless you spend most of your time wide open.
I havent read all of your posts about your carb setup. One thing I have heard over and over again on forums is that the float level is not set right from the factory. Have you set them to 17mm?
I have an A/F gauge on mine and with 17.5 pilots and screws at 1 turn I am getting ~11.5 A/F at idle, and 12-13 at cruise. Floats set to 18mm. No popping at all under deceleration.
My A/P is disabled also. My plugs have a nice tan brown color.
 
Thanks guys. The bit I am most concerned about is around idle and just off idle because that is where I have had the most trouble getting it right. And that is where the throttle was just before I shut it off.

Thanks gearhead - yes I have checked the float heights and they are spot on. Actually I have checked them a number of times.

The thing that niggles away in my mind is that the pilot jets I am using are quite a bit larger than Mikuni supply as stock. For my bike they suggest a 17.5 and I am using a 27.5. If I go down to 25 I get popping. Does the main jet have ANY influence at idle and just off idle? I am wondering if I go UP a size in mains to 130 maybe I could then reduce the pilots, or am I way off the mark here?
 
JimmyR said:
Thanks guys. The bit I am most concerned about is around idle and just off idle because that is where I have had the most trouble getting it right. And that is where the throttle was just before I shut it off.

Thanks gearhead - yes I have checked the float heights and they are spot on. Actually I have checked them a number of times.

The thing that niggles away in my mind is that the pilot jets I am using are quite a bit larger than Mikuni supply as stock. For my bike they suggest a 17.5 and I am using a 27.5. If I go down to 25 I get popping. Does the main jet have ANY influence at idle and just off idle? I am wondering if I go UP a size in mains to 130 maybe I could then reduce the pilots, or am I way off the mark here?
Click to expand...
Mains have nothing to do with the pilot and mixture screw unlike cv carbs. Is that popping at idle? just one cylinder? or under deceleration?
 
Thanks for the clarification. The popping I get is with anything lower than 27.5 pilots. It happens when I put any load on the engine at just off idle to about 1/4 throttle. After that it's good. BTW I have the jet needles at the second from the bottom notch. If they are on middle I get hesitation.

I have the mix screws at 1/2 turn out. Maybe I'll try a full turn out.
 
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