increasing spark plug gap

Any advantages/disadvantages to increasing the plug gap dimension? I have the ultimate coil with autolite 63's. I know Pete said that the setup will jump like a 1"gap but would .040 or .050 be too much?

A larger gap creates a larger kernal of spark. This larger kernal ignites the fuel charge better.
I have the Green monster coil and run my plugs at .038 to .040. Works ok for me.
Some plugs are designed for wider gaps, they have a longer ground electrode that allows a larger gap.
If you widen the gap too far the elecgtrode sets at an angle and won't spark where it should. the spark will jump to the narrowest spot. Off to the side, not out the end.
Leo

How do you decide what gap works best?

As you increase the gap, you may experience hard starting. Back it off till you get good starts and good gas mileage.

Of course, like everything internal combustion, there have been numerous tests with the benefit of increasing the plug gap. It has been found that after .045" the returns of larger gap drop off. If using NGK and a coil with enough punch, look for a -11 at teh end of the spark plug number. This corresponds to .11mm which is .044". Most all cars using electronic ignition use ths gap.

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.

Curious, I remember a 70's Buick with early HEI used .080" gap. Now that is a big gap!

Tom Graham

jchrisk1 said:

How do you decide what gap works best?

Click to expand...

Its a combination of things.Pamcopete mentioned a couple.

You know the gap is right when:
-Bike starts instantly
-Gas consumption is good, these bikes get 55 mpg/ 23.5 km/L easily
-Bike idles and accelerates well
-A blue spark, not yellow
-tan spark plug

When I bought my Accel coil, the manufacturer said to use 0.035" gap on plugs. I know they do lab testing, so that's what I have used, and it works great.

Tom,

I think that the number after the part number is just the gap that the plugs are set at the factory. Not necessarily the ideal gap for your particular application.

I use Autolite #63's set to .035" right out of the box. I could launch into a detailed reason why I use a .035" gap, but the truth is that is the way they come in the box and it is a good gap to use with the 17-6903 "Ultimate" coil, and the results are also good. Quick starts, smooth idle and reasonable mileage.

I think that I also may have a spark plug fetish. I know that I tried just about every suitable plug for my trusty '78/E, changing plugs every week or so, but finally settled on the Autolite #63's which I also use in my '81/H with good results. I ran a series of tests a couple of years ago to see what was the best coil / plug combination with a PAMCO ignition on a stock bike:



I think that my spark plug fetish probably started in my teens when I tried to keep my '49 Chevy straight 6 on the road on the cheap. I even got a part time job at a gas station on weekends so I could get gas at a great price and I could use the lift to work on the car's underbelly, but the greatest advantage was that the station had a sand blaster type spark plug cleaner and I would take my 6 Champion spark plugs out of the Chevy and blast them on Friday night so the car would be ready for my date on Saturday. Spark plugs didn't last long in those days due to the lead in the gas and low tech engines, and they were expensive, so everybody would have their plugs sand blasted as part of a "tune up". I could do that every week, but then I discovered that the sand blasting wore the electrode down, so I just blasted them every two weeks instead of every week.

Since then, and now that I can afford to buy brand new spark plugs, I change them often, even when they don't really need to be changed. I love the simplicity and elegance of spark plugs. We think we know all about them, but there are engineers who have written their Masters thesis on spark plugs. Very complex dynamics going on there.

I didn't mean to mislead anyone Pete. I just wanted to report the research I knew about. Given a powerful enough ignition, a gap up to .045" has shown to be beneficial. For our bikes with more powerful coils I'm sure .035' is a fine gap.

I have the same spark plug fetish as you do Pete. I've read every spark plug report I've come across. Sir Harry Ricardo did spark plug experiments and wrote about them in his ground braking work, "The High Speed Internal Combustion Engine", 1931. For such a simple inexpensive part a spark plug sure does allot.

Tom

retiredgentleman said:

Its a combination of things.Pamcopete mentioned a couple.

You know the gap is right when:
-Bike starts instantly
-Gas consumption is good, these bikes get 55 mpg/ 23.5 km/L easily
-Bike idles and accelerates well
-A blue spark, not yellow
-tan spark plug

When I bought my Accel coil, the manufacturer said to use 0.035" gap on plugs. I know they do lab testing, so that's what I have used, and it works great.

Click to expand...

55 mpg... EASILY????????????????!!!!!!!!!!!!??????????!!!!!!!!!!!!!!!!!!!!!
My 75 xs has struggled to get 40 mpg at a sustained 55-65 mph..... (stock 17/34) but 55mpg? really? I knew I needed to clean the carbs and tune everything, but my God, how rich has my machine been running?

That's because you're in South Bend!! Kidding. I used to live in Niles.

I am at .035 now with the 63's and was just wondering if a larger gap would do any good/harm. I used the iridiums that came with the Pamco ultimate pkg but they seemed to always run a little on the dark side. Without changing anything except going to the 63's I now have a nice tan color and about 54mpg. I think the 63's are a step hotter than the iridiums though.

The Pamco has a sturdy enough power transistor to run a fairly powerful coil so .035" is a great gap for that setup. Heat range is super important and insulator color is no longer a way to read plugs. Perfect mixture and heat range should have the plug insulator clean with a carbon ring inside. The BP7ES plug has proved itself to be a good all-a-rounder given proper mixture, timing and general engine health.

My 77 gave 60mpg to Mid-Ohio and back. I'm very happy with that mileage. I have stock carbs and the stock points ignition.

Tom Graham

grinder,

I thionk that .035" is about ideal. You may experience high speed miss if you open it up to .040".

Did both Iridiums run dark? The magic of the Iridiums really only works on the negaitve plug wire. Dual output coils produce a positive voltrage on one of the wires. You can actually determine which is the negative wire and just install the Iridium on that wire. Use a #63 on the other wire. This reduces the cost of relpacing the Iridiums as well.

Here is the way to determine which wire is the negative wire:

AceofSpades,

Here is a list of things that can cause high speed performance problems as well as bad gas mileage:

1. Low battery / charging system. A low battery will produce a low ignition voltage at the plugs which is most evident at higher RPM's and not all of the fuel will be burned.
2. Timing. Did you use a timing light to set the timing?
3. Stuck or no advance. When checking the timing with the timing light, did you check for maximum advance at 3,000 RPM.
4. Binding advance rod. Check out this video:


5. Fouled plugs. Did you install new plugs? Check them after you got it running?
6. Plug wire not making contact in the coil. Measure the resistance from plug cap to plug cap to ensure that both plug wires are making contact in the coil.
7. Brakes dragging.
8. N/A
9. Tire pressure. Pump up to 32 PSI for better gas mileage.
10. Drive chain too tight. That will rob you of power to the wheels and burn more gas.
11. Enricher plunger stuck partially on.
12. Enricher (choke) left on or partially on.
13. Weak or loose advance springs. Weak or loose advance springs cause the advance to use up some of the available movement because you have to essentially retard the advance to get it on the idle advance mark, so there is less movement available at higher RPM's resulting in poor power and gas mileage.
14. Timing chain needs adjustment.
15. Cheap or bad gas.
16. Low octane gas.
17. Water in the gas.
18. N/A
19. N/A
20. Try a run with a full tank of high octane, fresh, expensive gas.
21. Blocked fuel cap vent.
22. Incorrect sprockets. What RPM are you getting at 60 MPH.
23. Rear tire rubbing on brake rod or swing arm.
24. High wind resistance. Are you a large person?
25. Carbs not synced.
26. Blockage in the muffler(s)
27. Dirty or blocked air filters
28. High altitude
29. Very hot air, like 95+F
30. Very cold air, like 32F
31. Low oil
32. Really dirty oil
33. Gas in the oil
34. Weak TCI magnet in rotor. Unplug Reg/Rect to test

AceofSpades said:

55 mpg... EASILY????????????????!!!!!!!!!!!!??????????!!!!!!!!!!!!!!!!!!!!!
My 75 xs has struggled to get 40 mpg at a sustained 55-65 mph..... (stock 17/34) but 55mpg? really? I knew I needed to clean the carbs and tune everything, but my God, how rich has my machine been running?

Click to expand...

Yes, any XS650 should get 55 mpg ( 23.5 km/L) or better. Pamcopete just gave a large list, but I'd say the main reasons are carbs not tuned correctly, carb internal parts worn, and weak ignition.

I averaged 56 mpg (24 km/L) on a long 5 day trip, and that was through the mountains at elevations between 3400 and 5400 feet ASL. If my bike was at sea level and on level ground, I suspect I would be getting better than 60 mpg.

The NGK BP7ES plugs work very well.

Guys, great thread.

Your posts have me wondering whether I should try the Autolite 63s. I'm running a PAMCO (with one dual output coil) and Iridiums, gapped at .028, with my single carb setup. The bike runs so well that I'm afraid to make any alterations in jetting, air screw setting, timing, etc., but the plugs always seem to be dark (but dry... not fuelly or oily). So on the principle that you only want to change one thing at a time, I'm wondering if trying a set of 63s gapped at .028 or wider would improve my plug color before I make any change to the air screw. Or is my stock gap of .028 just way too small, perhaps?

TC

AoS - I get 60 mpg stock everything on my '81. And in 5th gear only 3/4 of the time!

My bike had a slight pulsing feeling or erratic feeling like bad sync. New plugs fixed it. The old ones looked basically ok but probably the gap was eroded. Didin't measure them.

TeeCat said:

Guys, great thread.

Your posts have me wondering whether I should try the Autolite 63s. I'm running a PAMCO (with one dual output coil) and Iridiums, gapped at .028, with my single carb setup. The bike runs so well that I'm afraid to make any alterations in jetting, air screw setting, timing, etc., but the plugs always seem to be dark (but dry... not fuelly or oily). So on the principle that you only want to change one thing at a time, I'm wondering if trying a set of 63s gapped at .028 or wider would improve my plug color before I make any change to the air screw. Or is my stock gap of .028 just way too small, perhaps?

TC

Click to expand...

A wider gap exposes more of the mixture to the spark and creates a larger initial kernel of flame which in turn ignites more of the total mixture, so it's possible that a wider gap (.035") would help with the spark plug color. It definitely helps with gas mileage.

Pete, thanks. I had .028 stuck in my noggin and it just wasn't at the top of my consciousness that a larger gap might help improve my bike's combustion. I'll have to give .035 a try.

And do you think I should try the larger gap with the Iridiums, since I have another set, or am I sort of just wasting money with the Iridiums? Sounds like 63s are well liked by many here as well.

TC

TeeCat,

The Iridiums do produce a more efficient engine as my above post shows. They do come at a very good price from MikesXS in the kits as well. the question comes up when it is time to replace them.