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The Garage Motorcycle Related Discussion (Yamaha XS650, other makes and models, and anything else motorcycle related including tools and gear)

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Old 06-13-2011, 10:39 PM   #1
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Default valve clearance and spark gap

I'm running a 73 engine with 34mm 1981 cv carbs and a pamco ignition. Also Air pods and strait pipes with baffles but no packing. My question is what do I set the valves at the tx650a/xs650b at in .002 ex .004 or the 1979 and later in .0024 ex .006 setting? I ask because Im going to get less fuel with the 34mm cv's. And what about the gap settings do I set at .024-.028in or the later settings at .028-.032in? Anyone know?
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Old 06-14-2011, 12:24 AM   #2
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Default Re: valve clearance and spark gap

Since you have a newer, stronger ignition and coil, you can use the later larger plug gap. It's accepted practice to use the larger valve clearances for your bike as well.

'78E Standard and 4 Euro twins
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Old 06-14-2011, 06:19 AM   #3
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Default Re: valve clearance and spark gap


5twins is correct. In general, you can open up the spark gap with an electronic ignition system.

The spark gap will depend on which kit / coil you have with the PAMCO. If you have the 17-6822 coil, then .028 to .032" will work. If you have the "Ultimate" 17-6903 coil, then you can open up the gap all the way to .040" . This will produce more power and better gas mileage, but may result in harder starting on a cold morning with a low battery, unless you kick start.

Here is a chart showing the results of a series of test runs that I did with my stock '81/H with various coil / plug combinations. In the final run, the Iridiums were gapped to .040" but I did notice a little harder start first thing in the morning.

I am currently running Autolite #63's gapped to .032" with the 17-6903 coil and I like the overall performance and better starts in the morning. The other thing to consider with the Iridiums is that only one of them really works as advertised do to the positive voltage on one of the plug wires in a wasted spark system. There is a way to determine which is the positive wire and only install the Iridium on the other wire to save a few dollars.
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Old 06-14-2011, 07:22 AM   #4
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Default Re: valve clearance and spark gap


Check out the double needle ND / Denso plugs. Purty sweet! Unfortunetly, now only sold in Japan.

The ND plugs use a .04mm iridium center which is even better than the .06mm of the NGK but doesn't last any longer than standard plugs.

Tom Graham
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Old 06-14-2011, 08:55 AM   #5
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Default Re: valve clearance and spark gap

Sweet thanks guys. That helps a lot! Now I just have to get the carbs sorted..
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