reduce the main jet

Hi,
here in Italy we have a "green Gasoline".
Green means without lead (and without ethanol).
So this gasoline has a lower octanes and detuned less.
I thought (but you know i am not a technician but only a fan) that it not needs any more to have a lot of gasoline if it dont fires at all.
So, i thought was an idea (may be bed) to reduce the main jet.
Ciao.
Piero

The main jet should be chosen based on the results of a "plug chop". Which means you run full throttle in 4th gear then pull the clutch in and kill the engine. Coast to a stop and check the color of the spark plugs insulators. A dark plug is rich, a white plug is lean. There are color charts on the web to show you what to look for.

Generally, with a Commando, too small a main jet and you won't be able to reach 100 MPH. Too big and the sparkplugs will be black and sooty and foul easily.

Be careful - full throttle on a Norton can be scary.

It depends on the octane rating of the fuel. If it is lower than usual you might need to increase the main jet size to stop detonation, or piston eating. In the end a plug chop is usually worth doing before you cop engine damage. When you read a spark plug, there should be a 2mm wide black ring on the porcelain, right down inside where the porcelain meets the metal. That is the indication that the plug heat range, and mixture are correct. With petrol, I always use the manufacturers' specified ignition timing and jet to it. With methanol, in a standard compression engine I usually advance it about 4 degrees beyond standard, and jet to that. If I am running 12 to one comp ratio, I use the standard ignition timing as for petrol. If you have ignition problems they can give the same symptoms as incorrect mixture, so the first thing to do is make sure your ignition system is reliable, and adequate for the comp. ratio you are using.

We only have unleaded fuel at the pump in NZ. Generally you will find the unleaded fuel will mainly turn the plug insulators dark. [ the leaded fuel could turn the insulators white if running lean of close to lean. ]
Main thing to test for leanness in the "OLD DAYS" was to wind the engine up on full throttle, then back I t off slightly. If the engine went better when backed off, it was generally considered to be running lean.

I have run a 50/50 mix 110 leaded racing gas and 93 pump gas for many years.

I resently discover a Sunoco station down the sells 90 octane puregas and decided to give a tankful a try.

My bike is somewhat modified with Stage 1 cam, big pipes, big pumper carb and a head that has been tweaked a bit. Needless to say, it has (as promised) a rather lumpy idle and is not realistic to try to get it much below 1500 rpm. No big deal here, it is what it is.

I am in the middle of the tank of 90 puregas and now have a rather lovely idle at 1000rpm. No other changes were made. The plug are only slightly darker but still a nice golden brown. I cannot say I notice any difference in its performance, except for the nice idle.
And it doesn't smell as good.

This Sunoco station is bringing in a 100 flavor but I think it may have a small precentage of corn, like 5%.

I cannot recall what effect octane has on timing. It seem that the lower octane has had a slight retatding effect. Yes/No?

Running the pump gas which is currently available in the UK / Europe (up to E10 content depending where you buy )

I have recently tested a stock T120 on a dyno and found the factory setting with new carbs give less than perfect results , main jets need to be increased by 5-10 % to correct the AFR
the fuel to air ratio graph under load with E10 pump fuel shows a lean mixture at half to full throttle with stock jets ( 190 ) change the jets for 200 problem solved
ignition timing also needed to be retarded by 2 degrees to stop Knock under load as this new fuel does burns quicker

The dyno guy was very informative marking throttle positions rpm air temp etc so the runs were like for like when we made changes

On old air cooled motorcycles Without the aid of a ECU to compensate for fuel quality / ign timing etc he advised to increase factory main jet settings by min 5-10% ( UK) to compensate for fuel quality

in theory hot climates have less dense air so jet sizes need to be decreased and cold climates have dense air so jet size needs to be increased
but air cooled engines do not follow the rules hot climates promote more cylinder heat due to lack of cold air for cooling , so it is often wise on a road engine ( hot climate ) to run slightly rich when cold to compensate for increased
temperature

kerinorton said:

Main thing to test for leanness in the "OLD DAYS" was to wind the engine up on full throttle, then back I t off slightly. If the engine went better when backed off, it was generally considered to be running lean.

Click to expand...

Still Works and I still do it......and the logic is pretty inescapable, if you shut off the air, say to 7/8th throttle, you are still running on the main jet with less air, that last 1/8th throttle to full will need more fuel.

Last time I did it was a GSXR750F about 2 years ago, it actually accelerated as I gently rolled back the throttle, but it only needed to go up one jet size....

Colour reading does not seem to be as accurate as it was...and its a long time since I actually did a plug chop in the traditional manner

I've had similar issues with Mikuni VM34 set up and am contemplating doing a plug chop as described here.

With the key and clutch lever on the same side of the bike, I'm wondering how I'd actually reach down and turn the key off while holding the clutch in without coming to grief?

freefly103 said:

I've had similar issues with Mikuni VM34 set up and am contemplating doing a plug chop as described here.

With the key and clutch lever on the same side of the bike, I'm wondering how I'd actually reach down and turn the key off while holding the clutch in without coming to grief?

Click to expand...

It's not that critical. Pull in the clutch, slow to where neutral is obtained, or just come to a stop and shut it off. A few moments of idling will have little effect if any. Just shut it down as soon as it's safe and do your check.

freefly103 said:

I've had similar issues with Mikuni VM34 set up and am contemplating doing a plug chop as described here.

With the key and clutch lever on the same side of the bike, I'm wondering how I'd actually reach down and turn the key off while holding the clutch in without coming to grief?

Click to expand...

That's one of the reasons I'd like to fix my kill switch, which has been jumpered for the last 35 years or so ago.