plug chop

this may be a daft question but here goes any way.living in the mountains of nc i rarely get into 4th gear(75 mk111) so is wot in 2nd or 3rd the same as 4th as far as main jetting goes?thanks to all for your help.dave

dvd said:

this may be a daft question but here goes any way.living in the mountains of nc i rarely get into 4th gear(75 mk111) so is wot in 2nd or 3rd the same as 4th as far as main jetting goes?thanks to all for your help.dave

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I reckon third would be okay as long as the motor is under load.
Dave

To expound a bit from what Dave noted, the main thing is to be able to keep the engine under WOT load long enough to get the plugs to color up. It doesn't take but a few seconds to change their color (unless they're really fouled), which is also why you need to immediately chop the throttle and kill the engine after the hard pull. ANY time at a light cruise or idling will change the plug color, ruining your chance to properly read a max effort jetting.

But without lead in the fuel you will find it really hard to read the plug colour.

For testing the main jet(s): find a road where you can pin the throttle in third or fourth gear, 10 seconds should be sufficient, Interstates are good for this. Once the engine responds, gets the bit in it's teeth and starts pulling, abruptly shut the throttle, if the motorcycle briefly surges forward with a burst of power your mains are too small.

If the motorcycle simply shuts down your main(s) are either just right or a tad rich. At this point a plug chop may help you decide. You can go the next smaller main(s), but be careful. A chassis dyno with EGA is, hands down, the best way to tune, but in the Mikuni tuning thread Full Auto posted a note about employing a wide band O2 sensor which sounds like it would pay greater dividends over the long haul.

Best

Proper plug chop requires abruptly shutting throttle, pulling in clutch and killing ignition, more or less all at one time.

You can get it done in third gear with the help of an up hill and drag the brakes a little bit to keep you from revving too high and going too fast.

Nater_Potater said:

To expound a bit from what Dave noted, the main thing is to be able to keep the engine under WOT load long enough to get the plugs to color up. It doesn't take but a few seconds to change their color (unless they're really fouled), which is also why you need to immediately chop the throttle and kill the engine after the hard pull. ANY time at a light cruise or idling will change the plug color, ruining your chance to properly read a max effort jetting.

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Yes, a new spark plug won't color at WOT . But reading plugs for WOT is not the color but looking very closely with a bright light down inside the insulator, the ground strap and shell...The plug color charts in old manuals do not apply here...
This plug is from my non Norton T brand Brit street bike after 10 miles of high power loading on back roads.Non ethanol pump gas..The mixture is actually slightly rich...There's also very slight signs of detonation, a few tiny dull black spots from oil deposits..The timing was backed off two degrees and no more spots and no loss of power...
Buy a doctor's Otoscope or magnifying plug reader to have a good look..



Second photo is a plug from my Triumph 650 land speed race bike after several runs don the 1-1/2 mile track at WOT. The fuel is leaded 108 octane race gas...



Here's some info on reading plugs at WOT...

https://www.dragstuff.com/techarticles/how-to-read-plugs.html

thanks to all. it seems my 220 mains are a bit small and my #3 throttle slides are rich.will make adjustments one at a time.thanks again.dave

The main jets operate at full throttle. If you fit a warmer plug when jetting the mains, you will stay safe. When you read he plug, you should look right down inside the plug at the porcelain just above where it meets the metal, there should be a black ring there about 2mm wide. The different heat range plugs conduct heat at different rates. A hotter plug conducts heat more slowly. So when you are jetting, it keeps you slightly rich. Then you change to the colder plug. If you are racing, you would normally jet so that the black ring just disappears. But it is not really important because on most race circuits you are usually not on full throttle for more than a few seconds from about a third of the way down any straight I usually leave the mains a bit over-rich and concentrate on getting the needles and needle jets right. They are far more important. When you road race, you usually feed the throttle on relatively slowly from very early in corners. A common mistake is to do point and squirt - get the bike upright, then whack the throttle fully open. Everything happens too late and too slowly because you lose vacuum almost completely. I ride my Seeley 850 as though it is a two-stroke, except it has more torque and handles better, so it does not need to be upright coming out of corners.