I have a 113 cu. inch S&S with a shorty G and today I noticed that the plug on the rear cylinder is much cleaner (whiter)than the plug for the front cylinder which is somewhat black except for the very tip which is tan. The ignition is I.S.T. so I presume this is not a timing problem. I believe the I.S.T. does not allow any timing modifications. Ignition was factory installed. Any idea what causes this? (Left plug in image is for front cylinder, right one is for rear)
Best regards
Adrian

 

Rear cylinder runs hotter which will make it run leaner and the plug will be whiter.

 

It's typical of all Harleys, In the old days some guys would run a slightly hotter plug in the front for that reason. If the rear is not white with an eroding tip it will be fine. I occasionally swap front to rear like you would car tires, but it isn't really necessary.

 

Carbs are known for this, usually, if everything is tuned good, the rear cylinder will get a better burn than the front cylinder will and here's why;
The intake valve opens on the rear cyl, the intake air traveling through the carb and intake runner will have a velocity up to 650 feet per second. As this air travels through the venture it picks up fuel from the emulsion tube (this air is called the signal in carb terms the stronger the signal the more fuel it's able to pull up out of the float bowl) this atomized fuel and air mixture is traveling to the rear cyl at 650 fps into the combustion chamber as a good mixture. All of a sudden the intake valve slams shut and this column of air and fuel does what......well it doesn't just stop......for every action there is a reaction, so that column of air bounces off the closed valve and heads toward the frt intake valve and finds it closed so it bounces of that valve and makes it's way out the carb as a pulse wave. This pulse wave has a slight signal to it so it picks up more fuel as it goes out through the venture and when the air hits the atmosphere it bounces off it and heads back into the carb but this time since the fuel is heavier it has momentum so80% continues out the mouth ofthe carb as a spray called 'stand off', if you ever rode your carbed bike with out an air filter on it your leg would be soaked in gas and this is where that gas comes from. The air and 20% ,or so, fuel mixture that bounced off the atmosphere heads bask down the throat of the carb and guess what .....it picks up more fuel and down the runner it heads. This time all the valves are closed, (because of HARLEY'Sfiring order of315 (R) and 405 (F) so the air slows way down to almost a stop, when the air slows the fuel gets too heavy for it so the fuel falls out of suspension and lays in the manifold as a little puddle. Just about now the frt intake valve opens and the air once again rushes into the frt cyl, but this time the fuel is not atomized so it kind of rolls or drips into the combustion chamber, the signal coming through the venture of the carb starts to pick some fuel but since fuel is heavier than air it takes time to get the fuel into suspension now it all heads into the frt cyl, then it starts all over again. Now you ask why is the plug black, well the fuel that puddled and dripped into the frt cyl burned much slower and less efficient just because it wasn't in a gasses form so the it made more soot as if it was richer, these larger droplets of fuel can also cause detonation from after gas left in the cylinder forthe next compression stroke. The frt cyl doesn't get that fresh atomized charge as the rear gets and there is your color change from one to another. If the AFR is way too rich or not tuned properly you will see the rear cyl blacker that the front.
The way we get around this in racing is to tune the intake track.....this means make it longer so when that pulse waves comes back into the runner toward the frt and rear cylinder it is timed to hit the valve as soon as it opens and the fuel won't fall out of suspension and actually add in cylinder fill for more power. We can tune for the 2nd 3rd or 4th pulse wave, the 2nd wave is more powerful than the 4th.The intake on our Pro Modified Sporty was 19 3/4' long, (a great place to hang your helmet). A stock Harley intake track is about 11 3/4'.
Sorry

 

Good explanation, I assume the velocity stack is tuned to improve the flow to the front cylinder as well, being an integral part of the intake track. Thanks for the info.

 

That is correct...you can add length to the intake track behind the carb or in front of the carb. The measured distance is from the intake valve stem to the atmospheric pressure (end of carb). This is good for both culinders, not just the front. We used to have a little trick we did to favor the front cylinder by angling the carb and runner back 3 degrees so the front runner would be a straighter shot to the valve.

If you would like the formulas for 'intake track tuning'I can post them for you.
Doc

 

I've never been a big fan of the 90 degree turn type air induction add-on systems. They can cause air distribution issues, such as directing more high velocityair to the rear cylinder causing a more lean A/F ratio for that cylinder. Some have overcome some of this issue byinstalling internaldeflector plates in the 90 deg turnto helpimprove air distribution.

 

Does this effect change with rpm? At high flow it would seem the pulsing would change.

 

I agree with you GearGrinder....the biggest problem with the 90* induction is every time you change the direction of inertia flow, it slows it down....this effects cylinder fill. People want to get the tuned length of a runner correct for thier build but don't want it to stick out so bending it 90* is their way of meeting their goals....the more bends the more restriction. The best induction manifold (runner) is one that yo ucan open the throttle plate and see nothing but intake valve like the V-Rod or S&S Pro Modified heads for good examples.

 

Seems like the ideal is to run 2 front heads with seperate intakes like the sporty racers do.