Here's a weird thing that happened Today.

Was putting some finishing touches to the xj 650 4kO getting it ready for a run out tomorrow, fingers crossed.

So had a set of plugs 100 miles on them at most, gave them a clean gapped them, took out the old plugs 2 outside cylinder's had NGK bp5 3 electrode plugs middle 2 had champion plugs.

Fitted 4 x NGK BP7 Es plugs ran really badly, idled at high revs and was back firing.

Swapped the plugs back ran fine, what about that, weird, Eh!

G.

 

Those BP7ES plugs are the correct ones, they (some or all) must be bad, a mis-firing plug(s) will make the engine out-of-synch and then you get all sorts of weird issues as you describe from the out-of-synch condition.........

 

Funny thing is they came from a bike I could never get to run right, rebuilt carbs shaft seals the lot, still bad, changed carbs no change, then resealed inlet manifolds, still wouldn't run, till I eventually gave in and stripped the engine, that project stalled so bought another bike, ran great till I put those plugs in it.

Told you it was a weird day.

Remember just cos the plugs are new don't trust there good!

G.

 

ohm out those bad plugs tell us what you get for readings.
spark tip to cap end and cap end to body. see if the bad plugs show up with a meter test

 

a regular DVM won't do any good for sparkplugs. they use about 1.5 volts to test and can't know what happens when twenty thousand volts come along

 

so a high pot tester would be needed

 

A meter can be used. It won't necessarily be conclusive that the plug is good because of the higher voltages of the ignition system, however a reading with the DMM is a sure indication that the plug is fouled.

 

New set of plugs were cheaper than multi meter, so ordered them.
Be here in a few days.
Went out on the old plugs, ran fine.

Happy Days.

G.