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GS 750 classic racer jetting advice needed

  • Thread starter Thread starter Harry 66
  • Start date Start date
H

Harry 66

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Hello.I have a question to the carburator wizzards. This is a dyno run from my GS750 1977 Uni filter pods, open exhaust. Pilotjet 15, Mainjet 117.5, fuelscrew 2 turns ,air screw 1.75 turn, needle one notch from downside. So needle can move one higher.
It runs not bad however runs a bit lean on the lower revs. Float level standard . What should I do to make it a bit better?
Dynorun GS750.jpg
 
Make it a bit richer, right?
Raise the needle a notch = tiny bit richer.
120 main jets = a bit richer.
126 main jets = some what richer, with option to lower needles.
 
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The needle controls the jetting up till the slide is almost fully open. Since on a cv carb, the slide doesn't open until the rpms are high enough, then it looks like raising the needle would be a good start to correct the low speed leaness.
 
I am thinking that with a #15 pilot jet the fella has VM26 carbs. I suggest you mention if you have the stock air box, stock exhaust, stock motor, etc. If you have most any mods and the bike is running lean at lower rpms, then the pilot jet should be bumped to #17.5. Raising the jet needle will pick up fuel flow at about 3500-4000 rpm on your carbs and that is the transition point from the pilot circuit to jet needle and that will help a tad as well. Raising a jet needle one slot on VM26 carbs is not a huge amount of different due to the gradual taper but it is a little. The pilot jet will be more helpful. I could get a little more specific but I can't read that dyno chart very well.
 
Agreed on a 17.5

the fuel screw should be about 1 turn out, the air about twice that.

It needs more fuel down low

You might have better results with APE or K&N filaters
 
Jet pilot 17.5 it is then.
Next is fiddle with timing of ignition, it's now on factory settings. Should I gaine power if I advance it a few degrees?
Bike is running bit rich on higher revs so it runs cool.
 
UNI pods... rich at higher RPM? How do you know it is rich? The dyno run? If so, have you checked coil voltages? If you spark isn't what it should be, caused by low coil voltages, that will cause the fuel mixture to not combust fully. I would try 17.5 pilot jets, clip to 4th slot and 112.5 main jets for your set up. I am going to defy popular wisdom and suggest 1 3/4 turns out for the pilot mix screw and about 1 3/8 turns out for the air screw. Last dumb question... how did you oil the UNI pods? Most of their stuff says to saturate them with their own filter oil. Pod air filters that have to be oiled sometimes change the air / fuel mixture depending on how they were oiled.

I would also leave the timing alone and only fiddle with the carburetors but again, how is the voltage at your coils (I don't care if you have new coils or old, poor spark is often caused by poor voltage).
 
Checked spark and is good. Coils have good feed from battery. With the dyno run air fuel mixture is monitored by exhaust gasses. I had the dyno run at Ten Kate racing. They ride world championships I might think they know what they are doing. Except.... there is not much knowledge left of older machines so for carburators we have this site (-;
 
Looking at the ex gas trace, it's lean on the pilot jet, very lean when it transitions to the needle then slightly rich up top.
If you are using VM26's, in my experience they respond better on track if they're slightly rich down low.
I'd go straight to a #20 pilot, lift the needle one notch and try that.
To lean it out up top, once youre happy with how it's running, yes, bump the ignition up 2 -3 degrees.
I'm assuming it's running a decent performance pipe ?
 
I order some pilotjets and try that first. Spark is good and filters are lightly oiled with foam oil. I agree to with a slight rich fueling, there is more torque and runs cooler.
 
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What was the voltage at the coils when you just turned the key on and bike wasn't running?
 
Your pilot fuel screws should be more like 3/4 turn out from lightly seated. Two turns out is way too much. Stick with the stock #15 pilot jets and work on getting your main jets the correct size so and needle height will control your lean condition. Pull your two top vent hoses off if they are there.
 
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tkent02 said:
So much wrong advice.

Are they CV carbs, VM carbs, or something else?
Click to expand...


I wouldn't normally bite at something like this - but...yes, if you haven't done it, don't respond.

Setting up VM's for roadracing with modified motors is not like setting them up for street use. You want them very slightly rich all the way to top end for best response. With pods and an open pipe the suggestions I've made I know will work - because I've used the exact same setup on VM26's before.
Also the old 2 valvers are not sensitive to total spark advance - within a range of 4 - 5 degrees anyway. I've used the advance curve to smooth out carburation glitches which were not solvable any other way due to no alternative needles being available.

Feel free to disagree - but please post pics and a history to support your view...
 
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GregT said:
I wouldn't normally bite at something like this - but...yes, if you haven't done it, don't respond.
Click to expand...

Sorry I didn't specify, your posts are right on. Crap about mixture screws and other CV carb concepts are what got me going.
 
Put in 20 pilotjet. Adjusted airscrews with Colortune each carburator separate.
It runs great now . Revs up from 1100 to 11000 without bogs or splutter with a nice blue spark.

Final settings are: Uni pods, 4-1 open exhaust, 20 pilotjet ,117.5 main jet, needle clip on lowest notch, fuel screw 1.5 turns out air screw around between 1.5 and 2 turns .

Thank you all very much for the advice.

Great site
 
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