Topic: 73 CB750K plug fouling / Exhaust restriction

[mrmach5]

I'm battling some carbon plug fouling on #1 and 4 . 2 and 3 look great.
Good compression , stock air box, electronic ignition, timing spot on, 105 main jets, 35 pilots needle in the 3rd slot. Bike runs nice and idles good but after a short test drive I can tell those two plugs are fouling and will only get worse. Seems like its right around 2200 rpm where there is the slightest hic-up.
I've replaced the coil, firing 1 & 4 , new wires and plug caps as well. ND X22ES-U plugs. All four carb idle mixture screws are at 1 turn out.
Nothing I seem to try seems to have any effect. So crazy question , the exhaust is a brand new Italian made HM300 reproduction 4 into 4  set.
If there were too much restriction, causing excessive back pressure, could that contribute to the plug fouling? I could remove the baffles on those cylinders to see if it has any effect?

[calj737]

I doubt the exhaust is your cause. I would tend towards an ignition problem. Normally I’d say condenser, but you list “electronic” ignition. Is it perhaps an old Dyna unit? There has been a spate of reports on them failing on one side only after the motor gets hot.

You could try swapping the leads around to setup up the current 1/4 cycle to fire the other side and see if the problem follows.

[scottly]

Carbon fouling usually indicates a carburation problem: too much fuel/too little air.
Please post pics of your spark plugs.

[Mark1976]

Carbon fouling usually indicates a carburation problem: too much fuel/too little air.
Please post pics of your spark plugs.

+1

[calj737]

Carbon fouling usually indicates a carburation problem: too much fuel/too little air.
Please post pics of your spark plugs.

There is no doubt that is typical. However, since the malady is attached to a coil pair (1&4) while the other pair is “perfect” seems that carbs may wouldn’t be the issue. But too little info yet to make any diagnosis.