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SuperMod of the North
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Discussion Starter #201
OK, brief update. I actually had a very short ride. Just down to the end of the street to cruise around the high school parking lot for 10 min.. Brakes work, shifts nice up to third gear. Carbs are not right and plugs are crap. Carbs are running very rich I think. I tried to zip it up a bit and it choked on fuel under load at 3500. The carbs are not set up right. I am 3 full turns out in the fuel screw and up one notch on the needles. Going to put the needles back down and go 1/2 turn in. I think my other mistake was not setting the floats. They were brand new and it was a rookie move not to check and set them up. New plugs and throttle cable came yesterday and so did the pod air filters. I'll put it all together this weekend and see how it runs.
 

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I like to get the bike idling ok then tune WOT first. Because changes in main jet will affect your other settings making any changes you make possibly useless.
Run up and down the street WOT and guess if you are rich or lean. If you are not sure, a lean bike will jump forward a split second you close the throttle. Rich and you are spitting raw fuel and carbon coats everything.
Having a few jets either direction from current is good. I usually buy from here

They have a decent section on carb tuning i think, too.

Carb tuning is fun when you get it right. A b!tch when it's not.
All the above assume the engine mechanically is running well, too.
 

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Totally agree with making sure float height is set before carb rejetting. New filter and plugs.
 

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All right Tin! Had you even ridden this one yet? Either way- nice progress on the build!
Have fun, stay safe brother

I friend locally has a late 80, early 90's GSXR750- carbed and when he tried to go to pod filters, really gave him a hard time jetting/tuning it.
I;m sure you already knew this though.
 

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SuperMod of the North
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Discussion Starter #205
Great info JCW, thanks. The kit did come with two sets of main jets. I put the smaller 135s in to start and set the needle and fuel screw a little heavy. The larger jets were 145s. No I had never ridden it Todd, it was in boxes when I got it.
 

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Alright. Let's break it down.

If the fuel mixture is too rich, you may experience:
  • Dull and intermittent engine noise.
  • Performance worsens when the choke is on.
  • Performance worsens as the engine gets hot.
  • Removing the air cleaner improves the performance.
  • Exhaust fumes are heavy and black.
  • Spark plugs are black, dry, and sooty.
  • The engine runs better when it’s cold.
  • Acceleration is flat.
  • Throttle needs to be opened continuously to accelerate.
If the fuel mixture is too lean, you may experience:
  • The engine overheats easily.
  • Performance improves with the choke on.
  • Acceleration is poor or stumbling.
  • Engine doesn’t respond when the throttle is snapped open.
  • Engine speeds up when throttle is closed.
  • There is a lack of engine power, RPMs fluctuate.
  • The spark plugs are white or burned.
  • Popping through the carb when the throttle is opened.
  • Popping through the exhaust on acceleration.
  • The engine runs better in warm weather.
  • Performance gets worse with the air filter removed.
 
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Oh I'm not done...

Diagnose Motorcycle Carb Issues on the Fly
Accelerate and shift through all gears at the proper RPM. A properly tuned and carbureted engine will accelerate smoothly and quickly through the gears.
If the main jet is too rich, acceleration will feel slow or stuttery. Close the throttle from full to about ⅞ open when RPMs are over 4500.
  • If the engine accelerates, the main jet is too lean.
  • If the engine hesitates or misses, the main jet is too rich.
  • If the engine only slows slightly, the main jet is pretty good.
You can also try accelerating through the gears at full throttle.
  • An engine that backfires, misses, or dies has a main jet that’s too lean – try a larger jet size.
  • If acceleration is sluggish, sounds flat, or is unresponsive to throttle, the main jet is too rich – try a smaller jet size.
To test for lean or rich conditions at any throttle position, you can cover/restrict or remove the airbox.
  • If you cover the air box and running conditions improve, the carbs are too lean.
  • If you remove the air filter and the engine improves, the mixture is too rich.
If you can turn your fuel petcock completely off so that fuel does not refill the float bowls, you can also check for lean/rich conditions. Turn off the fuel and ride the bike.
  • If it starts to run worse as the float bowls drain, it is too lean.
  • If running conditions improve, it is too rich.
  • See below for how to check your fuel levels with the clear tube method.
If you’re tuning your carbs while the bike is stationary, remember to get the motorcycle up to operating temperature before making adjustments and set up a box fan to help keep the engine cool.
You can temporarily affect the fuel/air mixture and motorcycle performance on the fly by using hotter or colder spark plugs. Use a colder plug to lean it out, a hotter plug to richen it.
 
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And more fun...

Adjusting Your Carbs at Different Throttle Positions
If you remember what throttle position each component of the carb control, you can begin to make some adjustments depending on the running conditions you experience.
Idle to 1/4 throttle range
  • Turn the fuel screw out to richen the mixture.
  • Back the fuel screw in to lean the mixture.
1/4 to 3/4 throttle range
  • Raise the needle jet clip to lean the mixture.
  • Lower the needle jet clip position to richen the mixture.
3/4 to wide open throttle range
  • Use a larger main jet to richen the mixture.
  • Use a smaller main jet to lean it out.
Air Temperature, Altitude, and Humidity Conditions Effects on Carb Mixture
High air temperature, high altitude, and high humidity conditions require a leaner mixture.
Low air temperature, low altitude, and low humidity conditions require a richer mixture.
 
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But wait, there's more!

How to do a Plug Chop
If you plan on changing things on your engine – like the exhaust or air filter, you’ll likely need to test out new jet combinations to get your carbs running correctly.
The best way to see if your mixture is correct at certain throttle positions is to do a plug chop.
You’ll essentially ride the bike at the throttle position you want to test, stop the bike, and look at the spark plugs to decide whether it’s too rich or lean. Take a look at the charts that show which jets control which throttle positions and tune from there.
You’ll want to get the bike up to operating temperature when you tune your carbs, as a cold engine does not vaporize gas as well and will require a richer mixture to get going.
To decide if you need re-jet your carbs, you should start with a plug chop.

With any good controlled experiment, it’s a good idea to change one thing at a time and retest to get it perfect.
A plug chop will help you tune your carbs if you’ve changed things like the exhaust or air filter, or even top end components.
To perform a plug chop on your motorcycle:
  • Put fresh, new spark plugs in the engine.
  • Bring the motorcycle to operating temperature.
  • Find a flat, long, empty road.
  • Ride for about a mile at the throttle position you want to test.
  • Pull in the clutch, kill the ignition and throttle, and coast to a stop.
  • Remove the spark plug and inspect the color and note the engine’s performance.
  • Make changes as necessary.
 
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That should keep you busy for a while... Carbs are so much fun...
 
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SuperMod of the North
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Discussion Starter #211
Printing this out. All the stuff I needed to know! Thanks very much Spyder.
 

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Let me know when you run out. I have a vast collection of carb porn, LOL
 
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This is the one I was hunting for...

Follow steps in order....First, dial in:
  • 1. Top end (full throttle / 7.5k to redline -
    Best Main Jet must be selected before starting step 2 (needle height)!
    • To get the best, most even top end power (full throttle/after 7500 rpm), select the main jet that produces the hardest pull at high rpm.
      • If the bike pulls harder at high rpm when cold and less hard when fully warmed up, the main jet is too large. Install a smaller main jet and retest until you find the main jet that pulls the hardest at high rpm when fully warmed up. This must be done first - before moving on to the other tuning ranges.

    • If the bike doesn't pull well at high rpm when cold and gets only slightly better when fully warmed up, the main jet is too small.
      • In order to properly tune the midrange and low rpm carburetion, THE MAIN JET MUST FIRST BE PROPERLY SELECTED after 10 to 15 minutes of hard use!
        • Do not pay too much attention to the low-end richness when you are changing main jets - you still need to be using the main jets that produce the best power at high rpm. You will deal with the low-end / cruise later - after step 2.
  • 2. Midrange (full throttle /5k-7k)
  • Step 1 (Best Main Jet) must be selected before starting step 2!
      • Select best needle clip position
    • To get the best power at full throttle / 5k-7k rpm, adjust the needle height, after you have already selected the best main jet.
      • If the engine pulls better or is smoother at full throttle/5k-7k in a full throttle roll-on starting at <3k when cool but soft and/or rough when at full operating temperature, it is too rich in the midrange and the needle should be lowered.
      • If the engine pulls better when fully warmed up but still not great between 5k-7k, try raising the needle to richen 5k-7k.
      • If the engine pulls equally well between 5k-7k when cooler as compared to fully warmed up, the needle height is probably properly set.
        • Do not pay too much attention to the low-end richness when you are changing needle clip positions - you still need to be using the clip position that produces the best full throttle / 5k-7k power in conjunction with the main jets (Step 1) that produce the best power at high rpm. You will deal with the low-end / cruise next.
  • 3. Low end (full throttle / 2k-3k)
  • Step 1 (Best Main Jet) and Step 2 (needle height) must be selected before starting step 3!
      • Float height (AKA fuel level & how to..)
    • To get best low-end power, set float height (fuel level) so that the engine will accept full throttle, without missing or stumbling, in 2nd gear from 2.5k to 3k rpm at minimum.
      • Float heights, unless otherwise specified in the installation guide, are measured from the "gasket surface" of the carb body to the highest part of the top of the float - with the float tang touching but not compressing the float valve spring.
      • If the engine has a "wet" rhythmic, soggy area at full throttle / 3k-4k rpm, that gets worse as the engine heats up, lower the fuel level by resetting the float height 1mm greater (if the original was 13mm - go to 14mm). This will lower the fuel level, making full throttle / 2k-3k rpm leaner.
      • If the engine is "dry" and flat between 2k to 3k rpm, raise the fuel level.
      • Example: change float height from 15mm to 14mm to richen up that area.

    • REMEMBER, since the main jet WILL affect low speed operation, the MAIN JET has to be within 1 or 2 sizes of correct before final float setting.
      • Warning: If the engine is left with the fuel level too high,, the engine may foul plugs on the street and will be "soft" and boggy at part throttle operation. Adjust Floats to raise/ lower the Fuel Level.
        • Base settings are usually given if a particular application has a history of fuel level criticalness. The Fuel level height in the float bowl affects full throttle/low rpm and, also, richness or leanness at cruise/low rpm.
      • Reference: a bike that runs cleanly at small throttle openings when cold, but starts to show signs of richness as it heats up to full operating temperature, will usually be leaned out enough to be correct if the fuel level is LOWERED 1mm. Check out and RESET all: Suzuki (all), Yamaha (all) and Kawasaki (if low speed problems occur). Needless to say, FUEL LEVEL IS EXTREMELY IMPORTANT!!!
    • If there are low-end richness problems, even after lowering the fuel level much more than 1.5mm from our initial settings, check for needle wear and needle jet (part of the emulsion tube). See Worn Needle and Worn Needle Jet diagram. It is VERY common for the brass needle jets (in the top of the "emulsion tube") in 36mm, 38mm and 40mm Mikuni CV carbs to wear out in as little as 5,000 miles. Check them for "oblong" wear - the needle jet orifice starts out round! Factory Pro produces stock replacement needle jets / emulsion tubes for 36mm and 38mm Mikuni carbs. Click here
  • 4. Idle and low rpm cruise
    • Fuel Screw setting (AKA mixture screws)
      • There is usually a machined brass or aluminum cap over the fuel screws on all but newer Honda. It's about the diameter of a pencil. Cap removal details. Newer Honda carbs have no caps, but use a special "D" shaped driver, usually supplied in the carb recal kit. We do have them available separately, too. 800 869-0497 to order -
    • Set for smoothest idle and 2nd gear, 4k rpm, steady state cruise operation. Set mixture screws at recommended settings, as a starting point. For smoothest idle, 2nd gear 4000 rpm steady state cruise , and 1/8 throttle high rpm operation. (pj tuning information)
    • Pilot fuel mixture screw settings, float level (but, you've "fixed" the fuel level in Step 3 - which you have already done!) AND pilot jet size are the primary sources of mixture delivery during 4000 rpm steady state cruise operation.
      • If lean surging is encountered, richen mixture screws (turn out) in 1/2 turn increments. Alternative pilot jets are supplied when normally required.
      • Pilot fuel mixture screw settings, float level and pilot jet size also affect high-rpm, 0 to 1/8 throttle maneuvers. Too lean, will cause surging problems when the engine is operated at high rpm at small throttle openings! Opening the mixture screws and/or increasing pilot jet size will usually cure the problem.
        • NOTE: A rich problem gets worse as the engine heats up.
          • If the throttle is lightly "blipped" at idle, and the rpm drops below the set idle speed, then rises up to the set idle speed, the low speed mixture screws are probably set too rich: try 1/2 turn in, to lean the idle mixture.
        • NOTE: A lean problem gets better as the engine heats up.
          • If the throttle is lightly "blipped" at idle, and the rpm "hangs up" before dropping to the set idle speed, and there are no intake leaks and the idle speed is set at less than 1000 rpm, the mixture screws are probably too lean: try 1/2 turn out, to richen mixture. Be sure there are no intake leaks and the idle speed is set at less than 1000 rpm!
 

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SuperMod of the North
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Discussion Starter #214
Damn Sir. I would appreciate it if you would please make a thread in the Tech and Performance section with these posts? If you do I would like to sticky it. There is great information there that won't show up on a regular search because it is buried in this thread.
 

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SuperMod of the North
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Discussion Starter #216
Excellent. Thank you Sir.
 

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Glad you finally got to go for a short ride!

Isn’t there a tool for carb tuning? Reading through Spyders posts makes me think it sounds really hard to tune a carb engine.

Does this help at all?


 

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Yes, there are many "generations" of Mikuni (as well and other brands) carbs. The guide I posted covers all 3 generations of CV (Constant Velocity) carbs. Later BST Mikuni's have a TPS (Throttle Position Sensor) int hem, but the tuning procedure is the same. AND, that guide is a general, as well as universal, procedure for any carburetor. Hell, even if your are rebuilding a Holley 850 Double Pumper on your 84 Camaro... the principles are the same. you still have 3 circuits. Idle (pilot), mid (needle) and main.
 

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ANYWAY, let's go back to your yellow clear coat issue... My boy just bought an '01 750, with a kinda crappy paint job on it. The tank was the worst part of it, and you you clearly see the white Suzuki decal severely yellowed. Today, I pulled out my 6" variable speed random orbit polisher, and hit the tank with some cutting compound. It ended up taking that yellow clear off with very NONE effort, and left behind a VERY bright blue finish. Also left the white vinyl decal looking brand new. I know you (and another user) tried to wet sand with POOR results. I would try this. Bet it works out for you... I can post a few pictures of the products I use, and the results tomorrow, if you are interested....
 
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SuperMod of the North
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Discussion Starter #220
That is a great explanation Oz. It does help. I may come out of this knowing how a carb works if you guys keep this up, thank you.

Yes, pics of the orbital sander trick would be great. I have one and I think I have cutting paste as well. To be honesty I tried to mask and paint the decals...nuff said....don't do that either.
 
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