Need jetting baseline please - RS34s on modded GS1000

[KEITH KRAUSE]

I don't see why you're thinking of main jet changes when the problem appears to be pilot jet related.
Seems to me it's loading up a bit at idle and a blip of the throttle produces dark exhaust. I'm not familiar with the carbs you're using but when you say the pilot jets, at 17.5, are stock, what does that mean? They are aftermarket carbs or are they used on certain models?
If not specifically matched to your model/motor, the "stock" 17.5 pilot jet may be too rich for your set up and the next smaller PJ would probably work.
Something doesn't sound right here. You install a leaner main jet and it then idles cleanly. "Blipping" the throttle can be done in varying ways and also in an unnatural way, such as literally trying to gag the carbs and pass too much fuel than the motor can combust at that moment. Now if you're just revving it in a normal way and get dark exhaust then you have a problem.
I don't see how the main could cause the problem though or have the effect on idle as you describe. Most likely the PJ or float/fuel level and/or an incorrect air jet.
These carbs have AIR (not mixture) screws? Then they should be set using the highest rpm method.

[Billy Ricks]

They're RS flatslides Keith. Just an idle mixture screw.

[80GS1000]

I don't see why you're thinking of main jet changes when the problem appears to be pilot jet related.
Seems to me it's loading up a bit at idle and a blip of the throttle produces dark exhaust. I'm not familiar with the carbs you're using but when you say the pilot jets, at 17.5, are stock, what does that mean? They are aftermarket carbs or are they used on certain models?
If not specifically matched to your model/motor, the "stock" 17.5 pilot jet may be too rich for your set up and the next smaller PJ would probably work.
Something doesn't sound right here. You install a leaner main jet and it then idles cleanly. "Blipping" the throttle can be done in varying ways and also in an unnatural way, such as literally trying to gag the carbs and pass too much fuel than the motor can combust at that moment. Now if you're just revving it in a normal way and get dark exhaust then you have a problem.
I don't see how the main could cause the problem though or have the effect on idle as you describe. Most likely the PJ or float/fuel level and/or an incorrect air jet.
These carbs have AIR (not mixture) screws? Then they should be set using the highest rpm method.

Thanks for your response.

These are Mikuni RS34 carbs, which are aftermarket. They came from the factory jetted with 130 mains, needle on the 3rd clip from the top, and with 17.5 pilots.



I know it's against conventional wisdom but installing leaner 122.5 main jets cleaned up the richness in the idle to 1/4 throttle range at the expense of making the mid to upper range too lean. Installing 125s makes the bike pull hard in the mid to upper range but makes it too rich at idle to 1/4 throttle. Adding choke at idle to 1/4 makes the engine load up and want to stall, indicating an overly rich condition I think.

By blipping the throttle I just meant giving it throttle at idle to say 1/8th throttle quickly and then letting go of the throttle.

I'll try installing leaner pilots and see how that goes. Failing that, what's the best way to adjust the float height?

Trying the highest idle method had little to no effect which makes me think that the pilots are too rich...? I had the pilot screws turned in all the way and then 2 turns out in 1/4 turn increments and it had no effect on engine RPM.

I may have the terminology wrong - the pilot screw is by the float bowl on the engine side of each carb.

[chef1366]

I'd lean the needle one notch and run the 125 mains.

[rapidray]

First thing, lean the needle 1 clip position & go to the 122.5 mains. You may even need to go to the 125 mains after leaning the needle. You need to work on each area of the jetting one at a time so you KNOW how the changes you make are affecting the engine. Ray.

Good thinking Bill! Ray.

[chef1366]

First thing, lean the needle 1 clip position & go to the 122.5 mains. You may even need to go to the 125 mains after leaning the needle. You need to work on each area of the jetting one at a time so you KNOW how the changes you make are affecting the engine. Ray.

I'd lean the needle one notch and run the 125 mains.

[rapidray]

LOL!! I hope you are doing well & had a good weekend! Ray.

[KEITH KRAUSE]

OK. Seems you're only complaint a couple posts back (when running the 125 mains) was some dark exhaust after blipping the throttle off idle. By blipping it, you meant opening the throttle by 1/8.
At 1/8 throttle you are not on the main jet yet, at least on any carb I've seen. Is there something different about the flatside design?? I just can't understand how a smaller main could effect the idle or the 1/8 problem you describe.
Now as for the smaller main creating worse issues at wider throttle positions, I understand that.
As for these flatsides coming from the factory "pre-jetted" or "stock", what model/motor/mods is that jetting supposed to work with?
For main selection, the general rule of thumb is to choose the main that delivers the highest top speed, as long as that doesn't create any bog during acceleration. You said the 125's did just that and the plugs looked good too. So I think those mains are good.
As for jet needles, you also said the performance/plug reads were good so I think they are good where they were set.
I think there may be some form of compensation jetting happening if you install smaller mains and the pilot circuit mixing improves? Compensation always results in one thing appearing fixed but something else shows a problem.
Can you show an illustration of the insides of these carbs? I'd like to see how the different circuits are regulated.
I still think the dark exhaust off idle shows that the pilot circuit is allowing the bike to load up a bit. I've always said around here that the jet needle has an effect much sooner than typical charts show but blaming it for a 1/8 throttle problem seems wrong too. On the VM's we're familiar with on our 70's bikes, the cut-away could be a possible problem because it transitions the pilot circuit to the jet needle. Do the flatsides have a cut-away or equivalent? Otherwise, a smaller pilot jet seems the way to go, unless there's ANY chance the fuel level in the carbs is too high. A too high level can cause exactly your problem you described with the 125 mains and the idle/dark exhaust. The bike has a harder time burning excessive fuel at lower rpm's/lesser throttle openings than at wider openings/higher rpm's.
The other thing it could be is the incorrect air jet. If the fuel isn't atomized well by the air jet then the bike can load up. If ALL float levels are correct (and you should know/check) and the air jet size works in other applications similar to yours, then I'd try a smaller pilot jet and keep the 125's. By all conventional wisdom your bike would then pull hard up to top end as you described with the 125's and run leaner at idle to hopefully stop the loading up.

[80GS1000]

Thanks for the help guys. It turns out I was too rich on the pilots.

Temps were in the 60s here today so I got the GS out for a ride. Went down to size 15 pilots from the stock 17.5s and the bike flat out rips though the rev range. No stumbling or flat spots. Very fast revving and great torque - tan plugs too. The front end gets light in the first 3 gears if you get on the throttle hard.

So as a possible reference point for someone else, here's the setup:

Mikuni RS34 carbs at 4500 feet elevation

125 mains
needles 3rd position from the top
15 pilots
accelerator pump turned off