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Jetting Anyone?

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Here is info I gathered from various sources. This appies to all carbs and bikes even though I cover premixes for 2 strokes. I believe this all to be correct and to be the nuts and bolts of the jetting issue all in one place. What I don't go into is the exact proceedures of setting the carb, as each carb manufacturer has it's own specs and sizes. Even two bikes exactly the same in every way may have to be jetted completely different. Oftentimes, guys will add-on every performance product available but they don't all play nicely together and become a nightmare to jet correctly. Cyl mods, reeds, pipes, etc all need special tuning. What if the pipe manufacturer suggests to go 2 sizes leaner on your main, but the pros who did your porting say to go 2 sizes richer??? Everything is trial and error when trying to get aftermarket equipment to work well together. Sometimes what you gain from the new fangled reeds you just installed, will be taken away by the mismatched pipe your running. and say, "What the heck, I don't see any gains!" If I am wrong about any of this or if you have anything to add, feel free.

A few things to check that affect jetting. Go through the list and fix any problems you find before attempting to adjust you carb. Beleive me it will save you a lot of time a frustration in getting your bike running right. DON"T ASSUME ANYTHING!!!! Check it anyway you may find something went bad, was misinstalled, overlooked, etc.. Just because you had something "professionally done" does not mean thay they could have forgotten or missed something.

I once had a local dealer who I never used before, do a bore for me. Bike would always have a problem somewhere and could not get it right. I dismissed the bore job because it was "professionally done"...WRONG!!! When I decided to finally pull the cyl. The rings never seated correctly and instead of a nice smooth shiny bore, I had stripes!!!! The machinists equip was out of alignment. They did make good on it an replaced the cyl. with a brand new one because it could not be bored again. All I'm saying is don't leave anything to chance. Like the carpenters always say, measure twice, cut once. Thery just don't make any good 2x4 streachers anymore. Ha! Ha! Ha!

* Air Leaks - Will make the bike sluggish and may produce a pinging sound. This occurs when the mixture becomes lean from the air leak.

* Right side crank seal - Leakage here will cause your spark plug to carbon foul and make you jetting seem too rich.

* Coolant leaks - Can cause misfire or popping sounds at the exhaust. This will cause a loss in peak HP and make it run as if it were too rich.

* Reeds - Broken, cracked, or non-sealing reeds will make the bike hard to start and have loss of torque.

* Silencer packing - Blown out or spoog soaked packing will make the bike run flat at high RPM's.

* Power Valve - Carbon siezed or gummed up exhaust valves make the bike run flat at low rpm's and makes your slow-speed jetting seem too lean.

* Coil - Weak coil will cause high speed misfire. make sure coil is properly grounded. Hot glue, silicone, or rubber dip make good sealing agents around mounting surfaces and bolts keeping moisture out to prevent rust and corrosion.

* Fuel inlet needle and seat should be replaced every other year. A worn one will cause fuel to travel up the pilot jet (slow jet) and into the engine making the jetting seem too rich.

*Float level - Too LOW a float level makes the jetting seem too lean and performance sluggish. Too HIGH a flost level makes the jetting seem too rich and may even start spewing gas from vent tubes and overflow.

Surging - Is mainly caused by a too lean condition especially in the pilot jet. But first check for air leaks at carb joint boot, reed block, cyl. base, crankcase, etc. before going a size larger on the pilot.

There are 3 circuits that control air:

1. Air Screw

2. Slide

3. Air Jet

There are 4 circuits that control Fuel:

1. Pilot Jet also known as Slow Jet

2. Needle Jet

3. Jet Needle

4. Main Jet

From throttle closed to 1/8 open is the idle circuit and is controlled by the Pilot Jet and Air Screw.

From 1/8 to 1/4 throttle transitioning into midrange is controlled by Pilot Jet, Air Screw, and Slide.

From 1/4 to 1/2 throttle midrange is controlled by Slide and Jet Needle.

From 1/2 to Full open throttle (WOT) is controlled by Jet Needle, Needle Jet, Main Jet, and Air Jet.

Real world test.....In 2nd or 3rd gear, roll on the throttle slowly from 1/4 to 1/2 open.

If it is slow to respond and bogs (Booooowah sound), then the carb jetting in that area is too Lean. If it makes excessive smoke and a crackling sound or seems to be running like the choke is on, The carb setting is too Rich. Make adjustments to slide and Jet needle.

Performing the same test in 3rd or 4th gear from 1/2 to Full open (WOT) will tell you about the top end jetting wether too Lean or too Rich. The areas to be adjusted are Jet needle, Main Jet, and air jet. When set correctly, doing a throttle chop test should result in a nice tan spark plug reading.

Premix ratios must always be consistant and variations will affect your jetting. It is better to have your carb jetting set correctly than to adjust your jetting through altering premix ratios. But if you must, here is the theory of how premix ratios affect carb jetting.....

First, the ratio (like 32:1) means 32 parts Gas to 1 part oil. Many get confused on this because they add the oil to the Gas and not Gas to the oil. They think, "How may parts of this oil do I add to my Gas?" I am not keen on the oil bottle marking graduations and do not feel they are accurate. Get yourself an oil ratio mixing cup/beaker or something that measures in "cc".

2 US Gallons equals 7,570.82 cc

2 1/2 US Gallons equals 9,463.52 cc

5 US Gallons equals 18,927.05 cc

If you take one of the above quantities, say like 7,570.82 cc (2 Gallons) and divide by your desired ratio (like 32:1), then 7,570.82 / 32 = 236.588 cc of oil to be added to the 2 Gallons of gas making your 32:1 ratrio.

Always go by the oil manufacturers mixing recommendation for their brand and NOT by what your owners manual tells you....unless you are using the owners manual recommended oil.

Many get confused on Lean or Rich mixture. They think LEAN means MORE GAS and LESS OIL. And RICH means MORE OIL and LESS GAS. The opposite is true.

30:1 = 30 parts Fuel to 1 part oil (less fuel) Leaner

40:1 = 40 parts Fuel to 1 part oil (more fuel) Richer

More Fuel makes the mixture RICHER, More Oil makes the mixture LEANER.

If you RICHEN the mixture (30:1 instead of 40:1) there will be more oil and less fuel which will LEAN the Carb air/fuel ratio jetting.

*You Richen the carb jetting by reducing the premix ratio (less oil)

*You Lean the carb jetting by increasing the premix ratio (more oil)

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