81 XJ650 - Runs with resistive spark plugs?

Second thread for today, racking up those internet points. Recently ordered some new spark plugs from Len, since I have no maintenance history for my 81 XJ650 Maxim and figured the ones in the cylinders might be original.

Len sent me the stock non-resistive plugs, NGK BP7ES. After some twists and turns through the post office, they arrived and I set about replacing mine. And yet.. lo and behold, mine are BPR7ES, the resistive variety. I’m 99% sure my bike has stock coils and plug wires too… plug wires hard-wired to the coils, and the wires are numbered 1 through 4.

So, what’s the deal here? Bike seems to run fine. Can it run with resistive plugs in resistive wires? Or does it have aftermarket ignition without my knowledge?

 

I installed the non-resistive plugs today and took her out for a ride. I experienced basically no difference. Also took a picture of the coils while I had the tank off. Certainly looks stock to me…

 

IMO running resistive plugs with resistive caps is hit or miss. You really should pull the caps and measure (just unscrew from the wires).
I ran resistive plugs for a couple hundred miles, then one plug failed right after an oil change and I was running on 3 out of 4. Put in non resistive plugs and I was all good. Get used to measuring a plug resistance cause that will tell you if the plug is bad or not. Clean to test them.
I have a bag of bad plugs pulled from 2 XJ650's and an XS650.

You say no difference, were you having a problem before?

 

Resistor spark plugs reduce radio frequency interference (RFI), preventing issues with radios, ignitions, and electronics, mainly for FCC compliance. They don’t generally affect engine performance.

For example, in my old boat with non-resistor plugs, I could hear the engine whine through the FM radio—spark plugs throwing off radio waves picked up by the antenna. Installing resistor plugs reduced the interference, though not completely.

The resistor slightly lowers spark voltage, but the effect is negligible, especially in older bikes. I’ve had no issues running them in my ‘80s bikes & boat...

Unless your spark is already weak, resistor plugs won’t affect performance. If you do have weak spark, clean connectors and switches and consider an ignition relay mod to ensure full voltage to the ignitor and coils.

 

The bike was running fine before. I changed the plugs because I suspected that the ones that came with the bike were very old — I had noticed corrosion on some of the crush washers. In fact, after I posted this thread, I started having running issues with the new plugs, on cylinder #2 only.

I suppose it’s good to know that resistive plugs won’t necessarily harm the running of an old bike. I do need to check the condition of my spark setup though, since I’ve been having some misfiring issues (posted in different thread).

 

Electrical problems could be the spark plugs, the plug caps, or the plug wires (assuming the ignition coils, the TCI, and he pick-up coils are good). If you run the engine at night (darkened area), then you will be able to easily see if there is any voltage leaking (blue sparks) that may be an issue (or you can just reach down there and feel around over everything....if there's an electrical leak you'll know it!).

In the ELECTRICAL SECTION of our parts listing you'll find the specs and testing procedures for the component parts (except the TCI, which really can't be tested):

https://www.xjbikes.com/forums/threads/xj-parts-catalog-section-d-electrical-system.44641


Of course, this assumes that the misfire is due to electrical issues, rather than carb-related


BTW, resistance in the plugs, plug caps, or wires (stock wires are zero-resistance) serves to "slow down" the transmission of electrical energy in the circuit....it may not decrease the voltage by much overall, but it "spreads out" the delivery of whatever voltage is available over a longer period of time. Increase the resistance high enough, and you'd be able to see a nice long spark emission (maybe 1-2 second's worth) but it would be very weak, whereas with the stock resistance levels (anywhere from 5,000 - 10,000 ohms) you'll get a very brief, very bright spark at the plug.

Extended spark times are useful if the engine is running very lean (to control emissions) or under very high compression or any other situation where the spark time may "end" before all of the fuel in the cylinder has been ignited.

 

How very interesting. Reading the coils section of that FAQ, I may already have found a problem. I tried checking the secondary coil’s resistance with my multimeter with the caps on, expecting to see about 21 kOhm resistance (5k cap + 5k cap + 11k internal), and I got no connectivity at all. The caps themselves tested good when removed, 5k each. I didn’t try the secondary coil with the caps off because I wasn’t sure I could get a good connection with the meter, but I will try that tomorrow. If the coil is good, then it means one of the caps has a bad connection.

 

Maybe.....it's sometimes difficult to measure secondary resistance with a basic ohmmeter, that's a lot of windings/resistance to try to push a tiny current (that the ohmmeter provides) thru all that wiring inside an ignition coil.

Note that if the coil is bad (the one that fires both the #2 and #3 sparkplugs) that you would expect to see problems on both of those cylinders, not just 1 of them (same thought applied to the pick-up coils). If you want you can always swap the 2 coils (but now the #1/4 plug wires will go to cylinders #2/3,etc.) and see if the problem "follows the coil" or not.

 

Welp.. seems like it’s not the coils. Finally got a good resistance reading with my multimeter. 21 kOhm with both caps, right where it should be. I must have been getting bad contact before. And the misfire on cylinder #2 is still there.

The misfire also gets worse as the bike warms up, which I assume means it’s something besides the ignition circuitry. Probably another carb issue, dishearteningly enough.

 

I found the problem. Going to post this here, in case anyone in the future has the same problem and stumbles upon this thread. Turns out, I had vacuum leaks in the other three cylinders. When I pulled the carbs off to investigate, I noticed that the butterfly for cylinder #2 was almost completely closed. When I synched the carbs, I must have needed to close butterfly #2 so much to maintain synch that it essentially cut off air flow to the cylinder.

Replaced my throttle shaft seals, and she now fires on all cylinders. When I did my carb work over the summer, I listened to that little voice in my head that said "nah, you don't have to replace those. They're fine."