Bonneville America Forums Forums Technical Forums Performance Mods spark plug insight

Here are 2 pics of my plugs on my '04 SM. left plug is from the left side and right plug from the right. I have 2,000 miles on it. I get a loud backfire (sounds like a gun shot) every once in a great while from the left side exhaust pipe. I have bubs slash cuts on - 130 mains - 42 pilots at 3 1/2 turns - stock air box. I'm no pro at judging spark plugs and i did look at the chart to see if they are looking good but all the pictures look basically the same to me. They're obviously different colors so i'm guessing a carb bablance is in order? Possibly air/fuel adjustment? Opinions appreciated. Thanks all.

-Josh

Turn the right pilot screw out a full turn and balance the carbs. be sure to cap off the holes if you have a crossover tube working.

Your right cylinder is running too lean. The left looks OK. Make sure you have no vacuum leaks.

Those plugs are just begging to be side gapped!

PD, do you really think the stock ignition system can tolerate that? I am switching to Iridium,s to lessen the load on the ignition system. We might have some fun with a high output coil and a large flame kernel(.045/.050),but it seems no one makes a HO coil yet?If you go to wide on a stock coil isn,t it also possible to have the flame extinguish?? Al

Side gapping is a style of gap, not necessarily distance of gap.

There are two ways of doing it: one is called "tee gapping" and the other is my favorite, "side gapping".

The former involves shortening the ground electrode so that its end stops right over the middle of the center electrode, when viewing it straight down from the bottom.

The latter, side gapping, is the cutting short of the side electrode so that the complete center electrode is visible when you look straight down at the bottom of the plug.

I use wire nippers/side cutters to shorten the ground electrode. You normally have to bend that electrode up to be able to clip it. Then, I take a points file (remember those?) and dress the distorted end where I clipped. Make it look like the original end.

The downside to these mods is that they shorten the life of your sparkplugs. BUT, plugs last so much longer now that I don't think it's an issue of importance.

OK, here's the answer to your question: I regap the plug to whatever gap I want. In the case of a stock Bonny ignition system, go with .035". You'll have to put your feeler gauge in on about a 45* angle to get a propper reading on a side gap, but a straight-in-from-the-side measurment is what to use on a tee gap (since the side electrode end is hanging half over the center electrode).

Yes, these bike ignitions need some help in the form of a higher output spark box. The 360* standard firing Bonny, T100 and Thruxton engine is easier to do than the Speedy and America, but someone who is well versed in automotive igntion systems could do it. No doubt MSD in El Paso, TX could make a box, but I don't think the market is big enough, and the box would probably be fairly expensive.

The main reason one would want a "hotter" electronic ign. box is that he could then widen the spark gap to something serious, like .065". That bigger gap gets the flame front going better. Think of it as starting a bonfire with a plummer's blow torch (another "remember those"?) as opposed to a matchstick.

Hey Papa,

A Dremel with a cutoff wheel works really good for this. It leaves a fairly straight edge where diagonal cutters leave a V. Lots less filing...

I chuck them in my vise and carefully hit them with the Dremel. I always buy extra and carry them with me.

I get plugs for about $2 apiece. That's cheap enough that I don't worry about 'minor surgery'!

Points file! plumbers torch! I think I am having a flashback!!!

Phil, you da man! Haha on me, I even have a Dremel out in my garage (got it for model airplane use years ago).

SUBJECT: Side Gapping Spark Plugs!
How to site:
http://performanceunlimited.com/documents/plugsidegapping.html

Member
G@ry J@mes

That's an excellent article, Gary.

Thanks.

Papa,

Do you see any obvious benefits to your side gapped plugs? I have some iridiums and felt a bit of improvment. But i might have to try side gapping my old(but still very new)stock plugs and see if this is really worthwhile.

Dale, side gapping a plug is about like converting to Splitfires (which are made by Autolite). Sometimes people don't see any advantage, but others do.

A friend of mine is a prototype machinist at the GM proving grounds over in Mesa, AZ. Some years back, a GM engineer went to him with a few sets of stock AC plugs and asked him to split the stock side electrode to emulate Splitfires. (He did this using a very thin slitting saw on a vertical mill.) My buddy told me that the engineer said they didn't see any performance advantage. Very interesting.

There are some stock plugs on the market today that have multi electrods - 2, 3, even 4 side ones. Notice that they all terminate just short of covering the center electrode (when viewing straight down on the plug's bottom). There's also a design that has two side electrodes going beside! the center electrode. Makes sense to me!

But, having said all that, I must emphasize that I feel this is a minor item - easy to do in anyone's garage. What really is important for Bonny twins is to get a decent high output ign. box. I can't say that enough.

A high output box would allow one to open up the plug gap for better combustion initiating. That box, and a high output coil (or two) would really help the Bonny twins make more power. Heck, it would even help lower emissions, but I'm not concerned with that, as bikes put out so little smog in the overall scheme of things.

Say, Yoshi. Did you ever get the lean condition in your right cylinder, sorted out?

One last side gap bit of info. Projected nose plugs, like the ones pictured above, show less of an improvement from side gapping than do non-projected nose plugs (since the projected nose is kind of hanging out there in the combustion chamber in the first place).

Colder plugs, as are usually found in race motors, are not a projected nose design, so side gapping works well there.

OK, enough already. I'm 60 years old and it's almost 2:30 am. Beddy-bye time for grandpa!

Yep went with one more turn out on the air/fuel screw and all is good. I still do get a loud backfire every once in a while when i let off the gas but i still need to balance the carbs and all that good stuff. Its winter so i should have some time here soon. Thanks for all the help.

-Josh