main jet test

What is a valid test of the main jet ? Difficult to test because it means wide open throttle, screaming it down the freeway, way over revving, for at least a mile or so. I need a better method.

A valid main jet test is to run it on the main jet and see how the plugs look as has been done since the bikes were new. Having said that, a few things come to mind -
1) start with a clean set of plugs and you don't have to go as far.
2) Rather that blasting down the freeway find a long uphill grade.
3) Why bother ? Mid range is where these bikes really get used (I'm guessing that you are not racing if you are doing your tuning on the freeway). Get the mains close, but focus on a clean pick up and a satisfying mid range.
4) If you really must, cough up the dough for a few dyno runs.

Greg

seattle##gs said:

What is a valid test of the main jet ? Difficult to test because it means wide open throttle, screaming it down the freeway, way over revving, for at least a mile or so. I need a better method.

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The following test methods for sizing a main jet are from a Mikuni tuning manual, where the twist grip travel has been marked, with a piece of tape and marker, in 1/4 throttle increments so the operator knows the relative slide position at any throttle position. This should definitely get you in the right ballpark.

Roll-off Method:
This method is a good way to get the main jet either correct or within one size of correct. It is based on the fact that that as the throttle is closed, the air/fuel mixture richens momentarily. This normal enrichening can be used as a diagnostic tool. The test is started with the engine running at an rpm high enough to ensure that it is “on the cam.” Open the throttle fully and let the engine pull for several seconds. Then, quickly close the throttle to about the 7/8ths position. If the engine seems to gain power, the main jet is too small (lean). Fit a larger jet. If the engine hesitates as the throttle is rolled off, the main jet is too large. Fit a smaller one. When the main jet is correct, the engine will continue to run smoothly and evenly as the throttle is closed. Note that a main jet that is far too rich or lean may cause the engine to misfire at full throttle.

The roll-on method could also be employed depending on your locale and circumstance.

Roll-on Method:
This is the method used by tuners at race tracks and is generally the most accurate way to set the mainjet size as it reflects the needs of the engine under actual riding conditions. It is also the most dangerous and we do not recommend it for that reason. The test consists of making fullthrottle runs between two points or markers on the track. The jet that gives the highest speed at the second marker is the best main jet. The gear selection and speed must always be the same at the first marker when full throttle is applied. The distance between the markers must be enough that small differences in performance can be detected.

seattle##gs said:

What is a valid test of the main jet ? Difficult to test because it means wide open throttle, screaming it down the freeway, way over revving, for at least a mile or so. I need a better method.

Click to expand...

Wideband 02 sensor.

Spark plug color doesn't tell you much unless you have a tank full of leaded fuel. And then when you switch back to unleaded -the jetting will be wrong again.

Or just start too big and go leaner a little at a time until the bike pulls up to the desired RPM. But do this with cold plugs....Jim

for running an O2 sensor ...most require welding in a bung on the pipe close to the exhaust port. Is there one that stuffs into the end of the pipe, mount the box on the handlebars and go for a ride?

So...get on the freeway late at night when the traffic thins out, put it in 2nd gear, no higher than 3rd, whack the throttle on wide open, ....the motor hits 5000 then back of to 7/8 throttle and feel what happens?

seattle##gs said:

for running an O2 sensor ...most require welding in a bung on the pipe close to the exhaust port. Is there one that stuffs into the end of the pipe, mount the box on the handlebars and go for a ride?

Click to expand...

You would need to weld in a bung. If the O2 sensor samples the air in the muffler instead of the headpipe it will be contaminated with ambient air and will not give a good reading.

If you are running an NGK 8 grade plug then you can look at the side electrode for an indication of mixture.
When it's right the side electrode will be burned clean down to about halfway to the point where is is connected to the base of the threads after a hard pull.

seattle##gs said:

What is a valid test of the main jet ? Difficult to test because it means wide open throttle, screaming it down the freeway, way over revving, for at least a mile or so. I need a better method.

Click to expand...

Why would it involve over revving?


If you're scared to open the throttle, you maybe don't need optimum mixture at full throttle.

If you know what the factory specified main jet size is, go one size over and forget it. As long as the mains are over-rich you should never burn a valve or a piston - unless your ignition timing is wrong or you have a blocked tank breather. For most of your riding the carburation is metered by the needle jets and needles - getting them right is critical - the rest does not matter much. If you find the motor coughs as you change up and down through the gears, it is probably due to the needle jets being too small or the needles are too low. If you have erred on the rich side, the motor will be slightly sluggish. Induce the cough then raise the needles one notch.

book time on a rolling road

rolling road with gas analyzer shoved up the exhaust pipe is the best way AFR can be checked at different RPM and adjusted accordingly by re jetting moving needles etc

take all the jets in your working range with you and make the required adjustments in your time slot a small cash investment may save your pistons and valves from a lifetime of torture

If the bike is basically stock then just start with new stock jetting. That will likely get you 95% of the way there. If you have a lot of mods then you might have some work ahead of you. These Amals / Nortons are so forgiving though for a near stock bike.

I'd be reluctant to run a commando at full throttle on a dyno to get the main jets right. Better to find yourself a long uphill piece of road and do a plug chop. Air temperature affects the jetting. What temperature would you set the air supply to, when the bike is on the dyno and how would you control the revs so you don't lose the motor ? In every dyno run I've seen on Youtube, max revs only occur in short bursts.

Acotrel running a bike on the dyno at full throttle is in effect the same as going down the road at full tilt rolling roads have in built resistance to mimic real world road conditions
a short throttle burst at max rpm will tell you the mixture settings via the exhaust anlylizer no need for guess work running uphill with a thermometer in your pocket
air temprature / pressure are also right in front of you on the screen on most modern rolling roads

So you have a calibrated exhaust analyser and some scientist has told you what he thinks the optimum composition of the exhaust should be ? In the end it comes down to maintaining the optimum combustion temperature through a balance of mixture, ignition timing and comp.ratio and the black ring right down inside the plug tells you if you've got that right , and that changes with the plug's heat range. The composition of the exhaust gas depends on the components in the fuel. I've heard guys claim there is an optimum air to fuel ratio and they know what it is - too many EXPERTS !
One thing not many scientists will tell you is that engineers should stay away from our black boxes.

Acotrel i make no claims to be an expert

Guy`s that claim to know The correct AFR ? a ratio where all fuel burns during combustion process leaving no excess this will change on an hourly basis depending on air temp / moisture content etc
with a motor fitted with carburettors this can not be changed on the move ,modern ECU controlled fuel injection is programmed to change mixture / timing settings according to load and atmospheric conditions / altitude
all i suggest is to optimize your running by using a rolling road ,set the base line settings and work from there by using an exhaust gas analyser it will tell you if your running is rich or weak
UK pump fuel now contains 5 -10 % ethanol depending on supplier this effects mixture and through experience i always start with main jets 10% bigger than factory and adjust from there
common sense tells me to run a tad on the rich side so any deviation in fuel quality or dramatic temp swings will be ( safe ) compensated for

Doing a plug chop is not an exact science unleaded fuels leave no / little signature on new plugs by the time the black ring appears you may have a hole in a piston ( if your base settings are way out )
i have seen guys rebuild motors run down the road for the first time and melt pistons due to bad timing / mixture settings

The common sense approach using both old school techniques and modern tools if available

With a Commando engine you will always need to jet on the rich side of maximum power to avoid piston damage.

How rich depends on the piston in use.

Most engines will do OK with full throttle mixture around 12.8 or 13 to 1.

Maximum power seems to be about 13.3 to 13.5 to 1.

To darken a plug's insulator immediately with unleaded fuel requires a mixture around 11.5 to 1.

Disclaimer
These readings are using my measuring equipment. Different setups may give different results.

When I get particular, I do plug chops to find the right main jets using plugs which are one range hotter than I use when racing. It is not the colour of the porcelain in the plug, but whether the black ring is there right down inside where the porcelain meets the metal. If the plug is hotter than normal and the ring just disappears, it will return when the colder plugs are fitted. In any case the discussion is purely academic, my main jets are usually a couple of sizes too big, because I don't race on big circuits where you are on full throttle for long distances If you do a plug chop, fit larger mains then do another and the black ring in the plug does not get darker, you are probably metering off the needles at full throttle - not good. At anything over 3/4 throttle, the main jets should do the metering.
With a four-stroke, if you want best performance, you are in the same realm as with a two stroke. The black ring in the plug should just disappear as you jet down. Usually that is somewhere I don't choose to go. What you lose in power by running rich on the mains is bugger-all.

Other than the .5 percenters who tend to take over threads whenever the door is ever so slightly ajar, Main Jetting at WOT is somewhat relative.
On a single 36mm VM, I have gone with a 240 all the way to a 320 successfully. For us regular people it really is more of an economy vs performance adjustment. 240 for economy, 320 for performance.

How many of us rely on WOT, rarely even going there unless it's an emergency? Tune for where you where you spend your time. That is, if you spend most of your time at WOT and need it to serve you well, then tune to where you spend your time. Otherwise it's relative, being an economy vs performance choice.

Yes, choice. There is a variance. With Amals, 180 all the way to 280 and beyond depending on your riding habits and how often you spend your time at WOT. Maybe 180 to 280 is a bit broad, but with a stock 750, 200 to 240 is not out of bounds. With an 850, 220 up to 300.

If you are a moody sort, it's a simple matter of pulling off the side of the road, screwing out the bowl plug, turn out the main with the main jet tool and pop in the main jet of choice. Go hit your Ton, submit for another patch, put your regular main jet back in and cruise on down the road.

It is rare that a motorcycle can successfully be dual purpose. With some modern motor cars, you can simply flick a switch.

What started this..on the Britbike forum there is an Amal expert who says over and over, with exclamation points, to START jetting by determining the main jet required. This seems almost an impossibility but I figured I would try here and see if anyone had the magic method.

What I have done for the past 30 some years is to accept the manual's suggested main jet size and tinker with with the other jets. This is primarily using Mikunis. I am an average street rider and never on full throttle. Unless it is a racing bike few of us ever reach full throttle.