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I hope I'm posting this in the right place..........
I have a stock 500 (except for a mystery pipe) and I'm about to install a VF2 reed cage and an FMF exhaust, I also plan on removing my airbox lid for better breathing. Currently, the bike runs on the rich side (stock carb, stock pilot jet, 370 main jet, needle is raised one clip position). I was wondering what you guys thought would be a good starting point for jetting? Elevation would be around 500ft and 70 degree temps.
Thanks in advance.
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Where are you located? Elevation and outside temperatures.
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Where are you located? Elevation and outside temperatures.
Elevation would be around 500ft and 70 degree temps. Upstate NY. -
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raising the needle richens the mid throttle. what you got sounds like a great starting point other than the needle being raised. Does it idle? I'm guessing this is an 88+.
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raising the needle richens the mid throttle. what you got sounds like a great starting point other than the needle being raised. Does it idle? I'm guessing this is an 88+.
Its an 89, and idles like a champ as is......
I will be sure to replace the needle position back down one when I start throwing the new parts at it. -
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do a plug chop after lowering the needle at around 1/2 throttle. If it looks like a paper bag around the electrode, you're good. If not, try dropping it again. Once you get the needle worked out, test the main the same way. It's a lot of work, but a tuned zilla is well worth it.
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When you say plug chop at half throttle.....you want me to run through the gears at half throttle then shut down the motor and check the plug? Sorry for my ignorance.
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exactly. dont forget to pull in the clutch after shutdown and dont let out till it stops.
I guess i should clarify about the main too. When doing the plug chop for the main, run it at 3/4 to full throttle before shutting down. -
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exactly. dont forget to pull in the clutch after shutdown and dont let out till it stops.
I guess i should clarify about the main too. When doing the plug chop for the main, run it at 3/4 to full throttle before shutting down.
Thanks for that info bud. After its all said and done i will post back the results in case someone else new to these bikes has the same question for a bike with similar mods. -
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i thought if you just put it in 1/4 - 1/2 - 3/4. in those positions and run it. this will tell you if it runs away, then its lean, if it starts to bog then its rich.
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in extreme cases, yes. But a slightly off tune bike may not show much signs of racing or bogging.
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Motor will act differently when load is not applied. May sound good and crisp revving it in "N". But once in gear it could be lean.
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i was told before to ride it down road, put it at 1/4 thrtl ride it if it revs up or takes off it may be lean.
but if it holds it good, and so on -
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It was suggested to me to goto the lowest elevation, and find a slight inclined road.
When I raced the Baja 1000 we went to the road near mikes sky rancho. Tested numerous needles/jets. In higher elevations we had a little blubber but we didn't sieze anywhere on the course. -
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So I bolted up the new pipe and reeds and modded my airbox lid for a lot more flow . Only took four kicks to start up. The bike runs crispy, idles good. It was about 60 degrees this past weekend when I fired her up. All i need to do now is plug chops. It sucks living in NJ, no where to test or ride.
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Start off rich everywhere and slowly work on each circuit until it just starts to run smooth without much misfire. Be careful trying to jet it by looking at the spark plug. It is real dangerous to try to tune 1/4 through 3/4 throttle positions by plug chops and monitoring the spark plug. Don't become another victim of the current fad of cutting spark plugs apart trying to get an accurate plug reading. Remember when you look at a spark plug, you are not reading AIR FUEL MIXTURE or THE LEVEL OF POWER BEING PRODUCED...you are just see the operating temperature of the shell, ground strap, center electrode and the center wire. The mixture does affect the operating temperature of all of the parts of the plug but so does the type of plug, the heat range, water temp, type of head and the length of time the engine is running near peak power. The length of time the engine spends under hard load is very important when setting the main jet.
A LT 500 does not spend enough time near power peak doing plug chops on a level road unless you have it geared for around 100 mph. Typical gearing on a LT 500 limits your top speed to less than 80 mph. On a level road the engine will easily over rev 500 to 1000 RPM past the power peak in 5th gear. That is not the part of the power curve you want your main jet set for. Lean main jets will almost always make the engine rev a little higher at the expense of making it too lean in the part of the power curve that produces the majority of the acceleration. You really need a long steep uphill for setting the main jet or running it on a dyno. The spark plug needs to experience the heat of combustion in the "right" part of the power curve long enough to reveal any detonation issues and any critical overheating of some of key areas on the spark plug.
We have barrels full of burnt pistons from customers trying to jet their carbs by "reading" their spark plugs.
Remember a two stroke that is just a little rich runs real well.
Remember a two stroke that is just right runs real well.
Remember a two stroke that is just a little lean runs real well.
Remember a two stroke that is real lean runs real well until it seizes or burns the piston. -
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Thats a heck of a write up. Great info!!!!
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That's golden info. Thank you for taking the time to share that.
What say you about my current jetting for the new setup?
I called fmf just to see what they would say and they advised that stock jetting will do fine with just their pipe bolted on. I asked if the air box lid was to be removed and they said that their systems do not specify anything about the air box.
I know I can't run stock jetting cause of all the air now being pumped into the motor.
I was ecstatic when the bike fired up with zero issues. I wish I don't have to mess around too much with jets. I just want to ride, damn it. But I will do what I must to get her to run healthy -
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So a question then to MotorGeek, what is a TRUE was to test jetting scientifically then other than just how it runs and rides?
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So a question then to MotorGeek, what is a TRUE was to test jetting scientifically then other than just how it runs and rides?
There is not any real scientific equipment that I have found that can tell you when a two stroke has the best power air fuel ratio. Instruments that can measure performance like stop watches, radar guns with acceleration plotting ability, dynos, timing clocks and speed traps at the drag strip are some instruments than can tell you when you are making optimum power. You just have to make enough runs with different jetting to see which one runs best.
An 02 sensor like we use on 4 strokes will not tell you what mixture will make the most power.
An egt gage will not tell you when your engine is making the best power. Every engine has a different egt where it make the most power.
A water temp gage or a spark plug base temperature gage will not tell you when you are making the most power.
An air flow meter and a precise fuel flow gage like we use on the dyno will not tell you when you have the mixture that makes best power.
Reading a spark plug will not tell you when you have the mixture that makes best power.
Looking at the top of the piston will not tell you when you have the mixture that makes best power
Trying different main jets on the on the dyno or roll on side by side drag racing on a long paved steep up hill will quickly tell you if you have the best power main jet.
As I lean the main jet and power increases I look at the piston crowns and spark plugs for indications of detonation and overheating of the piston crown . A well developed engine package will not usually hurt the piston before you reach the jet that will make max power and max acceleration for brief periods of full throttle. An engine that has a good working cooling system and a properly designed piston will allow the engine to run at max power for longer time periods before reaching critical temperatures.
I wish there was a jetting for dummies instrument but I have not seen or used one. You have to use all of your 5 senses and painful past experience to tune two strokes. -
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Awesome write up and advice thanks again!! There is definetly a lot that goes into a proper running 2 stroke engine lol!!!
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An air flow meter and a precise fuel flow gage like we use on the dyno will not tell you when you have the mixture that makes best power.
Not sure I understand this one. So why wouldn't it? -
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An air flow meter and a precise fuel flow gage like we use on the dyno will not tell you when you have the mixture that makes best power.
Not sure I understand this one. So why wouldn't it?
The airflow and fuel flow meter will tell you how much air and fuel is passing through the engine at any given RPM. Using this information, a theoretical air/fuel mixture can be calculated.
Not all of the air and all of the fuel that passes into an engine will make it into the combustion chamber. This statement holds true for 2 and 4 stroke engines. Some of the raw air and fuel passes through the engine during the scavenging phase into the exhaust system and may not get a chance to experience the combustion process. How much of the air and fuel are trapped in the cylinder each engine revolution is known as the engine's trapping efficiency. An engine that has 100% trapping efficiency does not allow any fresh air or fuel to escape into the exhaust system. The trapping efficiency on most two strokes is very poor. On a two stroke engine, the trapping efficiency is usually the highest (best) at the torque peak and the worst at the lower RPM where the torque dips just before the engine “comes on the pipe”..
The exhaust system and the ports are having a serious mismatch at the RPMs where the torque dips just before the engine “comes on the pipe”. At this transition RPM, there is a lot of fresh air and fuel passing through the engine into the exhaust pipe while much of the exhaust has not been scavenged from the cylinder. At this transition RPM the airflow and fuel flow meters may show the air/fuel ratio to be theoretically optimum, the O2 sensor is often showing off the scale lean and future jetting changes will show a power increase in this RPM range when the main jet is severely leaned down. By trial and error, the main jet that produces the best power can be found using the dyno or other performance measuring devices..
Every two-stoke engine design has a different personality (trapping efficiency, combustion efficiency, scavenging efficiency and thermodynamic efficiency). The design of the exhaust system and how well it works with the rest of the engine design has a major effect on these listed efficiencies. It is next to impossible to determine what these efficiencies will be when the engine is on the drawing board, but can be determined with sophisticated test equipment once the engine is produced. Much can be learned about different design efficiencies within a particular engine by looking at the exhaust composition with a multi-gas exhaust analyzer. A series of dyno runs on different engines using different jets can show what jet will produce the best power. The unfortunate truth is the optimum jet on each of the different engines will not produce the same numbers from the measurements taken with an O2 sensor, a 5 gas analyzer or the air flow meter and fuel flow meter. It still requires field-testing, dyno testing or some other method to gage performance to find the jet that produces maximum power.
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Nice to get schooled in a readers digest fashion.
Great info to boot -
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Well motorgeek then it is time to develop an adjustable main jet on the handlebars ....
and be a millionaire $ $ $ $ $ $ ;-)
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