Jetting on Suzuki GS

I acquired a 1977 Suzuki GS750 a while back and I am now getting it on the
road. The previous owner completely gutted the exhaust and it has pod
filters on it. Needless to say, it is way lean; after 4-5K is really gives
up. I am trying to figure out what size main jets I should replace the stock
(102.5s) with. These take the large round mikuni jets. All else on the bike
is stock, as far as motor/drivetrain. I have a set of 115s and they don't
seem to be big enough. I am in the process of thoroughly cleaning all the
carbs now (berrymans carb dip) but I want to have her running this weekend.
Any tips/ideas on what size jets to get? The nearest dealer with these type
of jets (suzuki dealer) is 40 minutes each way.

Thanks

 

Maybe your idle jets are really plugged up. Try spritzing out the jets
with Berryman's B-12 ChemTool. If that doesn't clear the jets out,
you'll have to remove and gently clean them with the finest sewing
needle you have. Don't even TRY to remove an idle jet if you don't have
a small slot screwdriver that fits the jet perfectly, you'll bugger the
slot and be unable to remove it. You have to push down hard on the jet
while you're turning the screwdriver...

Do you understand how the jet sizing numbers go? The hole in a #100
main jet is 1.0 millimeters in diameter. The hole in a #125 main jet is
1.25 millimeters in diameter. If you work out the A = pi X radius
squared, you'll see that the area of the hole in the #125 jet isn't
25% larger than the hole in the #100 jet, the area in the #125 main
jet's hole is 56% greater than that of the #100 jet...

So you could easily be passing 1/3rd more gas through the #125 jet...

Mikuni made VM29 smoothbore carbs for 750cc Suzukis doing all out road
racing, and they had #140 main jets, but you will probably be riding on
a public highway, not wide open throttle on Daytona and you'd be
blasting great clouds of black soot out your tail pipes...

I bought a set of used VM33 smoothbores from another rider who was
interested in drag racing. They had #135 mainjets, and those jets were
too large even for my GS-1100 that I road raced at Willow Springs. I
went down to #130, then down to #125...

Those VM33's are now on my GSXR-750 and it has lots of power, but not
really controllable power for slow stuff...

You won't be able to use jets any larger than about #125, so there are
four more sizes you would have to buy if you want to experiment:
117.5, 120, 122.5 and 125. That's 16 jets at about $4.00 to $5.00 each,
about $80 worth of jets and you'll be stuck with 12 jets that are the
wrong size and you can't take them back because you've used them....

That sucks, doesn't it? It would be nice if you knew somebody who's
been through all that jetting business before and has a box full of
brass that he might let you borrow...

Bzzt! The Berryman's carb dip is too thick, it won't penetrate all
those little teeny passages. Use the Berryman's B-12 ChemTool in the
aerosol spray can to spritz out all the jets...

If you remove the idle mixture screws and squirt B-12 through there,
you should see it coming back through the pilot air hole, back through
the idle jet, and it should be passing through the idle passages
downstream of the slides or butterflies, whichever you have...

Are the carbs CV carbs or slide valve carbs? The idle mixture screws
on CV carbs can be considered as adjusting fuel flow, so turning them
counterclockwise makes the mixture richer. The idle mixture screws on
slide type carbs can be considered as adjusting air flow, so turning
them clockwise makes the mixture richer...

If you don't get all the crud out of your idle jets and passages, you
can screw around with changing jets all day and never get anywhere...

Do you have a race track in your area? The cheapest way to get speed
tuning parts is to hang around the pits, talk to the racers, and find
out what old brass they might have in their tool box. Once the
jet that's too large or too small goes back into the jet box, it rarely
comes back out. If you're really nice to some old racer, he might even
*give* you his old jets...