JETTING FOR PIPE AND CARBS

royfisk · Posted: June 11, 2006, 2:27 pm Post subject: JETTING FOR PIPE AND CARBS

Hi all; Just a note I am trying to find a good starting point for jetting my carbs which are 38's off of a 81 and the cams are off from the same motor, my motor is a 75, I am running a 2 into 1 kirker pipe and stock air box. Any ideas?

grizld1 · Posted: June 11, 2006, 9:30 pm Post subject: Re: JETTING FOR PIPE AND CARBS

If you have the original carbs from an '81 XS650 you don't have BS38's, you have BS34's ('course somebody could have retrofitted BS38's to an '81 just by installing the right boots for the carbs). If they fit the boots on your B-model, they're 38's.

BS 38 carbs from different years ran at least four different carb bodies and as many different jetting combinations, and assuming the prior owner hasn't botched things completely, you can usually ID the carbs from items that seldom got altered. If you find Z-2 neeedle jets and 502 needles, you probably have 78-79 carbs. Those took 135 mains and 27.5 pilots in stock form. 75-79 carbs took a Z-8 needle jet and 4M1 needle; mains were 122.5, pilots 45. Earlier carbs ran Z-6 NJ's, 4JN19 needles, 127.5 or 130 mains, 42.5 pilots.

Your Kerker will require at least a 3-step increase on the mains and possibly 1 step on the pilots. A good manual on carb tuning will help you a lot. Sudco's Mikuni Tuning Manual is written with reference to aftermarket non-CV carbs, but the principles are the same.

jayel · Posted: June 12, 2006, 6:53 am Post subject: Re: JETTING FOR PIPE AND CARBS

find out what you have in the carb(s) for jetting now then you might call Kerker and see what they recommed, but it still just a starting place with the change in the carbs and cams, i don't know if the earlier head had different valves, combustion chamber, porting all of these will effect what you need to jet at, it's trial and error but at least you can start with a educated guess

grizld1 · Posted: June 12, 2006, 7:45 am Post subject: Re: JETTING FOR PIPE AND CARBS

If the cam from the '81 is stock it won't make much difference--1 degree difference in opening and 1 degree in closing on both intake and exhaust, which is negligible. Valve train's virtually identical.

royfisk · Posted: July 12, 2006, 6:15 pm Post subject: Re: JETTING FOR PIPE AND CARBS

Thanks for the info. I got it all together and talked with previous owner of the pipe and carbs, actually the whole top end, he informed me that the whole package had been dyno tuned in 1998, with all same parts I have except he used pods instead of the box. This bike now has pods and runs slick..

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