GS 750 classic racer jetting advice needed

[Guest]

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

[Buffalo Bill]

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.

[TxGSrider]

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.

[geol]

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.

[Big T]

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

[Harry 66]

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.

[geol]

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).

[Harry 66]

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 (-;

[GregT]

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 ?

[Harry 66]

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.

[geol]

What was the voltage at the coils when you just turned the key on and bike wasn't running?

[tkent02]

So much wrong advice.

Are they CV carbs, VM carbs, or something else?

[Steve]

So much wrong advice.

Are they CV carbs, VM carbs, or something else?

According to the original post:
- 1977 GS750 (stock bike came with VM carbs)
- classic racer
- UNI pods
- open exhaust

.

[OldVet66]

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.

[Harry 66]

VM carbs original slides.
Voltage at coils 12.3Volts