increasing spark plug gap

The thin wire iridium electrode not only lets teh spark jump easier (on one plug with a waste spark as Pete already mentioned) but the thin wire electrode doesn't quench the initial flame kernal as does a thicker heavier conventional center electrode. This helps even on the 'wrong' side of a waste spark ignition.

The Autolite #63 is a hotter heat range than the NGK #7. This gives better carbon burn off than a colder plug but is less safe at high speed or with a powerful engine. I ran NGK #6 (hotter than #7) for a while and they ran so clean I thought I might try them full time. The colder #7 has not given problems so I stuck with these as I take the old girl over 100 every now and then. I still run points and the original coils so non fouled plugs are of more concern with my ignition.

I have not had the plugs out in 2000 miles. Maybe I ought to check them! :^)

Tom Graham
 
Spark plugs operate over a heat range, I'm not sure the exact Degree range but it is something like 700°F to 1100°F. Using this range as example, if a plug never gets over 700° it is too cold because the carbon fouling builds up. A hotter plug is needed. But the oposite end of the range is very dangerous if the tip of the plug gets over 1100°. During extended heavy throttle a plug may get so hot it turns into a sort of glow plug. Detonation and pre ignition will then damage the engine. The trick is to find a heat range plug that runs inside the range under all conditions. IOW, no matter if a plug is hot or cold it has to run in the same heat range. It is the engine that 'picks' the plug.

A stock XS650 has proven to use the heat range of a #7 NGK (or equivelent) under most all conditions. So, a 750 kit with higher compression, bigger carbs, free flowing exhaust can use the #7, if it isn't used any different than the stock engine. But if the souped up XS is using this power in a sustained way there may be a chance the plug over heats. That's why many tuners go with a colder plug right off the bat, to be safe under the most gruling conditions.

Like I said above, I think I use my basicly stock XS rather mildly and I considered a hotter plug. I also blast to over the ton sometimes so I pulled the hotter #6 NGKs I was using to sort the bike with and put in the stock colder #7. I've now ran about 5000 miles with the number 7s and feel these are a good compromise. Not too hot, not too cold.

My bike came with BP8ES and promptly fouled these once I got teh bike running. I skipped the BP7ES and put in BP6ES to sort out the bike. I loved these hotter #6 plugs as they ran hot enough to stay very clean even if I never got off city streets. Once I was ready for the freeway and fast blasts on back roads I went with the stock spec BP7ES for safety.

Here is a wonderful article by a man revered by every knowledgeable person on the forum, Gordon Jennings.
http://www.strappe.com/plugs.html

Tom
 
I've been wondering about my spark plug gap as well. I just took them out of the box and put them in. I'm running a banshee alternator, Boyer Blue Box (Micro-Digital Ignition System) and Gill ignition coil, 5K ohm spark plug caps. Would any of this be a reason to try increasing my plug gaps?
 
Tomterrific said:
I put big generic car coils on my Norton in 1975. Since the cars these coils were on used a .035" sparl plug gap that is what I gapped my plugs at. Starting became super simple and the minor fine jetting I chased all those years was instantly corrected. At teh time I credited the hotter spark but now I know it was as much the larger gap of the plugs that the hotter spark allowed.
Click to expand...

I have read this thread several times now, but this paragraph, especially, has me anxious to try .035 on the Iridiums instead of the book's .028. Seems like a simple tweak that has a lot of good science and experience behind it.

TC
 
I might try it and see how it does. Back in the day my brother had a N stock NHRA record holder and he would side gap and index plugs worth a try.
 
(Hi, John! :) )

I don't know about side gapping/indexing (I was under the impression that it only yielded discernible gains in racing applications), but I did re-gap my Iridiums at .035 yesterday afternoon. I was busy cleaning up the yard after the hurricane, so I have not had time to fire her up yet.

In the process, though, I noticed that my right plug looked quite good, actually, but the left was a little dark. It'll be interesting to see if there is any lightening of them at .035. The dark left plug may be due, I suspect, to the fact that I have only one carb, with a slightly longer right intake runner, so the left plug is closer to the fuel source. Maybe a charge density inequity? Just a theory.

TC
 
T Cat He would cut the ground srap back alot further than the example on the opel site witch fully exposed the electrode. Then bent it down and gaped to the side of the electrode. This was a small block chevorlet witch has a quench combustion chamber the plug index was so the ground strap was facing towards the center of the cylinder. The XS 650 has a hemi chamber so the plug index would have the ground strap facing the exhaust valve. This was a drag race car and any advantage on the street I don't know. Its eazy to index mark the plug with marking pen and shim as needed. Nice Day for a ride the Quiet after the storm. :bike:
 
scabber said:
The XS 650 has a hemi chamber so the plug index would have the ground strap facing the exhaust valve.
Click to expand...

Our bikes have hemis. :D

(Oh, but in theory - and practice - wouldn't you want the flame to face the intake/fuel source? I don't know the physics of it... just seems logical though.)

TC
 
Facing the intake valve is the usual way i hear to index a plug but I'm not sure of the advantage. There is the open side getting blown clean with freash mixture for a good solid fire. But there is also some thought, by me, that indexing toward the hot exhaust valve would have the flame front burning out the most detonation prone area before it has time to heat and explode. The only real advantage I can definative state is when larger valves and a high lift cam is used and the plug needs indexed so the ground electrode doesn't touch a valve.
 
Let me clear this up I was referring to where the strap is attached to the plug so the open side would be facing the intake valve. Sorry I guess it was misleading. I did have a great ride today. I have my plugs gaped at .040 wiith the pamco ultimate coil works great. :D
 
scabber said:
Let me clear this up I was referring to where the strap is attached to the plug so the open side would be facing the intake valve. Sorry I guess it was misleading. I did have a great ride today. I have my plugs gaped at .040 wiith the pamco ultimate coil works great. :D
Click to expand...

Ah, there we go... that's what I was thinking, John. Thanks for the clarification!

I gapped my Iridiums at .035 the other night, and then took her out yesterday afternoon for about an your or so. It'll be interesting to get a few more miles on the plus and have a look at them.

Tell you what, though... it's so nice to finally have a nice stable (if lopey from the 2-into-1 intake) idle, instantaneous throttle response, a good hard pull, and stone-steady performance at speed. I'm telling you... ever since I did that single-carb conversion and got that VM dialed in, three seasons of vexing fuel delivery problems seem to have evaporated like water on a hot griddle.

TC
 
TeeCat, any updates on your iridiums gapped at .035?

I have been only getting like 30mpg (horrible, i know!) with iridiums, pamco, green monster coils, etc.

My right plug looks perfect, my left plug is a bit sooty black, just like yours were. Let me know!
 
I'm running the 'Kung-Fu' black TCI coil & NGK Iridiums gapped at .050". Not having any troubles with this setting. Thought the old-timer's way to go about it was to open the gap until the top end started falling apart, then back them off a bit. I'm not having any of that trouble, super easy starts, idles really well.
 
My iridiums always ran sooty black. Changed to autolite 63's@ .035 (Pamco ultimate) without doing anything else and now a nice light tan color. A little hotter plug I think.
 
The way I understand it is the larger gap, doesn't spark as fast under higher RPMs.

But if its keeping up, works good...then, no problem.
 

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