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jetting for 750-to-850 conversion

  • Thread starter Thread starter gssuzukixxx
  • Start date Start date
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gssuzukixxx

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Last winter I put 850 jugs and pistons on my '77 GS750. I stuck the original non-oring intake boots on and the carbs without any changes to jetting. The bike ran OK but I thought a bit flat in the mid range 3500-5500rpm.

Now this winter I am doing a complete cleaning of carbs, all new carb. orings and new intake boots/orings. I also want to change the jetting to '79 GS850 specs.

BTW I am running the GS750 cams. Also I am at 3000 ft above sea level.

Right now the carbs have the following GS750 jetting:

needle jet: 0-6 (assuming from engine # but cannot find any stampings)
jet needle: 5F21-3 (it doesn't have the "-3" stamped on the needle)
jet needle clip is set at 4th groove from top (bottom groove) and there is just the clip...no shims.
main jet: 97.5
pilot jet: #15

The spec for 79 GS850 is;

needle jet: 0-6
jet needle: 5DL36-2
jet needle set at 2nd groove from top
main jet: 102.5
pilot jet: #15

How can I confirm the needle jet number? I have checked every bit of it for stampings but cannot find.

Shouldn't the needles have shims to at least take up any play in their mounts? Right now the needles have some slight up/down play.

Any other constructive comments will be greatly appreciated.

Brian
 
Should also mention that I am running stock mufflers and air box.
 
Throw a set of 102.5 or 105 mains in it and adjust the fuel screws.
If it runs rough or rich in the of idle to mid range, drop the jet needle one click.
Z1 enterprises sells the main jets for $1.74 each.
Report back after the main jet swap please.

Daniel
 
Thanks for the comments and clarification on the needle numbers.

I have a set of 102 main jets so I could put those in. And I will put the needle clip in the 3rd position where it should be.

How about the needles. Would there be any advantage in getting 5DL36-2's to replace the 5F21-3's?

Brian
 
OK here is what I did:

Put in 102.5 main jets
move the clip on the jet needle to 3rd groove from top
put pilot fuel screw at 1.25 turns out
set all fuel levels to 2.3-3mm down from mating surface.
synched the carbs

It is still winter here but a mild day so I took it for a blast around the block. Runs real nice, idles smooth. About the same mid range acceleration. Much better pull above 6000 RPM though.

Will do more testing in spring.

Thanks for all the help.

Brian
 
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