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setting up my 29mm smoothbores...

  • Thread starter Thread starter raceman
  • Start date Start date
R

raceman

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l bought my 78 GS1000 for a lot of reasons but the clincher was the mods it had. All the stuff l normally would do to a bike like this was already done,it has the suspension,brakes and engine bolt on`s like pipe and 29`s. l can`t help feeling like the carbs aren`t set up right though and l`m going in today to check them out. l have two sets of stock carbs also that l can get jets from if needed. The bike seems like it`s running on the fat side so l`ll do a plug reading first(it has Andrews/Dyna ignition) but l`m thinking it should pull cleaner at WOT...l`m at 7000 feet here also which needs to be considered. What jets are you guys running in your 29`s? and what needle position? any help appreciated. l`ve been tuning bikes for a while and l have some vacuum guages..
 
i have some 29's on my stock bike with K & N pods and a yosh exhaust. when i get home i can look at my notes on what i am running. in the mean time clean yours out real good and list what you have in them so we can compare.
 
VM 29's (Smooth Bore) - 1978 GS1000 1979 stock motor, K&N RC-2450 filter, Yoshimura 4 into 1 header and can

This is what I am currently running (it maybe differant for others)

117.5 mains (what came with bike)
.9 air jet didn't pull it to check
#20 pilot jet (changed from #25, to rich for me with #25's)
5dl31-3 needle
0.6 needle jet
1.5 throttle valve (cut away)
1/2 turn out on the mix screw

Looking at Sudco's spec sheet these carbs i have (A7) look like they were set up for a kz650, kz900 or a kz1000

sudco recomends for the GS750/GS1000 (A12)
#115 mains
.9 air jet
#25 pilot jet
5dl31-3 needle
0.6 needle jet
2.0 throttle valve (cut away)

sudco recomends for the kz650, kz900 or a kz1000 (A7)
117.5 mains
.9 air jet didn't pull it to check
#25 pilot jet
5dl31-3 needle
0.6 needle jet
1.5 throttle valve (cut away)

sudco recomends for the cb750 thru '78 (A2)
115 mains
.9 air jet didn't pull it to check
#17.5 pilot jet
5dl31-3 needle
0.6 needle jet
1.5 throttle valve (cut away)

About my carbs the 117.5 mains came with them, the carbs were mounted on the bike and at some point before me the bike liked the setup. i heared stories of the bike pulling wheelies down the street back in the day from a shop owner who knew the bike. I cleaned up every thing, carbs were nasty and tried running it with the #25 pilot jets but was to rich and not happy, what i found out was the 1.5 throttle valves cause a richer condition then the 2.0 which is what the bike is suppost to have so by dropping the pilot jet down one it should step everything down one notch. Which it did and the bike has been running good since, but keep in mind i think my motor is starting to get tierd so a fresh motor might like the #25 pilot with a 1.5 throttle valve.

make sure you have everything cleaned out real good, there is a little hole under the throttel valve as well as a paper gasket under it too. Make sure the hole is clean and the the gasket is replaced or in good shape. Also check your needles my orginal ones were less then desireable. Also make sure you have new o-rings, you can get almost any size you can think of from mc-mastercarr.

download this pic and save it, it is a real good referance point
http://www.sudco.com/Diagrams323728/expvm29.gif

-Ryan
 
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1977 gs750 w/29mm smooth bores
I have 125 mains
#6 air jets
.30 pilots
not sure of the slides
I also have vance & hines high lift cams & an 844cc kit, this may make a difference in my jetting to yours.
 
smoothies

smoothies

My engine is 1075, stage 1 yoshi cams, ported, o/s ex valves, race exhaust and 29's running

117.5 mains
17.5 pilots
needles on middle clip
air screws 1&1/2 out

but cant remember what else. Have a big flat spot at 4000-5000rpm which i cant dial out but might have a look at the 1.5 slide cutaway.
 
i'm using 140 mains. it works. i talked to a guy who said he tuned a lot of GS's and he said i could use even bigger jets. he also said to mount an oxygen sensor in the exhaust to help with tuning. seems like a good idea but it's gotta be expensive.
 
78 1000, ported head, 1085 pistons, degreed stock cams. I haven't run the motor yet, but with my mods my research shows I need to run 1.5 slides, 120 mains, 5DL3 needle, and 25 pilots. I'm at 5,200 ft. elevation. Suzukis usually run the 2.0 slides unless your head has been ported AND you have a big bore kit.
 
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Bruce - so are you saying the 1.5 cut aways are leaner then the 2.0 cut aways?

Sharpy, your setup seems kind of lean for the setup you have. Have you tried some bigger mains and fatter pilot?
 
first timer said:
Bruce - so are you saying the 1.5 cut aways are leaner then the 2.0 cut aways?

Sharpy, your setup seems kind of lean for the setup you have. Have you tried some bigger mains and fatter pilot?
Click to expand...

Oops, sorry. I mis-typed. Suzukis usually use the 2.0 slides. The 1.5's are richer. I edited my reply above. :oops:
 
lean?

lean?

first timer said:
Bruce - so are you saying the 1.5 cut aways are leaner then the 2.0 cut aways?

Sharpy, your setup seems kind of lean for the setup you have. Have you tried some bigger mains and fatter pilot?
Click to expand...

Been on a dyno and went from 125's down to what i have. I know it seems lean but runs great execpt for the hole in the middle. With 15th front and 40th rear sprocket can carry the front wheel in 2nd under acceleration.
 
sharpy, just a thought about your flat spot at 4/5K.
You don't say what throttle position it happens but I'm figuring just under 1/4 throttle or approx' 1/5? If so, that's right where the pilot circuit transitions into the cut-away. Some overlap of the two circuits there possibly creating the problem.
Sounds like you've tried many jet needle/pilot circuit adjustments? If you have tried the next pilot jets sizes on each side of your current pilot jets with no success and you've adjusted the side air screws using the highest rpm method, I'd have to blame the leaner 2.0 cut-aways?
If you tested again right at the problem point, but, found a downhill run to test, does it help? That would suggest it's lean also since it doesn't need quite as much fuel in the mixture when running downhill/lessening the load.
I'd tape and mark the throttle housing and grip to help assure the throttle position (minus cable slack) and this will help determine which jetting circuit you're operating in when the problem shows.
The jet needle starts having effect too at these approx' throttle positions but I'm thinking the slides, though they can be expensive if you don't have any. If you had the jet needles set well but did at some point try bumping up the needles another 1/2 position with a .022" spacer and it still had that problem, it would further suggest the slides are wrong for your bike.
 
Flat spot

Flat spot

KEITH KRAUSE said:
sharpy, just a thought about your flat spot at 4/5K.
You don't say what throttle position it happens but I'm figuring just under 1/4 throttle or approx' 1/5? If so, that's right where the pilot circuit transitions into the cut-away. Some overlap of the two circuits there possibly creating the problem.
Sounds like you've tried many jet needle/pilot circuit adjustments? If you have tried the next pilot jets sizes on each side of your current pilot jets with no success and you've adjusted the side air screws using the highest rpm method, I'd have to blame the leaner 2.0 cut-aways?
If you tested again right at the problem point, but, found a downhill run to test, does it help? That would suggest it's lean also since it doesn't need quite as much fuel in the mixture when running downhill/lessening the load.
I'd tape and mark the throttle housing and grip to help assure the throttle position (minus cable slack) and this will help determine which jetting circuit you're operating in when the problem shows.
The jet needle starts having effect too at these approx' throttle positions but I'm thinking the slides, though they can be expensive if you don't have any. If you had the jet needles set well but did at some point try bumping up the needles another 1/2 position with a .022" spacer and it still had that problem, it would further suggest the slides are wrong for your bike.
Click to expand...

Thanks Keith. Its not really a issue and as its a big 2 valve engine so i just "ride around" the problem but next time i have it apart ill have a look.
 
thanks for all the replies. l have yet to pull the carbs but to give you an idea of running condition the powercurve is totally flat.There really is no powerband shift at all as the revs build,just pulls evenly throughout. l expect more,not this flat. What happened was that the carbs were on a 28-K motor and were fine but the motor was getting tired.The PO replaced the motor with a 12-K donor and swapped over the tasty bits.l`m thinking the increased vaccum from the fresh motor is showing the carbs to be now dialed too lean. Perhaps a needle adjustment first will help?
 
Depends on throttle position.
As always, test at full, 1/3 and minimal throttle positions to see what the main, jet needle and pilot circuits are doing. Do what the plugs and performance say.
The 29's seem more difficult to jet correctly at lesser (1/4 and less) throttle positions than more opened throttle positions. The correct size cut-away, being the transition from pilot circuit to jet needle, seems to be a common problem. Your high elevation would add to jetting difficulty/optimum performance.
 
My set up;
#115 mains
.9 air jet
#25 pilot jet
5dl31-3 needle, set at middle position
0.6 needle jet
2.0 throttle valve (cut away)
Mix screws are around 1 3/4 to nearly 2
Vance&Hines exhaust and K&N pods

The bike seems to be a tad rich at very low speeds such as going through the subdivision and such, but not enough to foul them by any means. Not boggy at low speeds either. The appearance of low speed richness may be due to 30 year old bad valve seals, I don't know. They will be replaced soon. Kieth is right about the low throttle being the hardest to get right, at least on my bike, but it is close enough for me for now.
 
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thanks rock.Well l pulled and cleaned the carbs. l have ******97.5*****Mains! l can see a problem here huh? the other jets are about what others are running,pilots are 17.5,pretty lean also.l will set up the carbs per the Sudco info and what l`m getting from you guys.lf does run better since the cleaning though but lean on top.
 
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