78 GS 1000 VM carb jetting

[Guest]

Hi Guys

I did a lot of searching on this forum which has helped me substantially - many many thanks for this

I got my bike (78 GS1000E) with a Harris 4-1 exhaust and velocity stacks (now changed to pods)

I had a lot of reliability issues which are now fixed and I have a nice set of rebuilt / balanced std size VM carbs which I am tuning with jets (std pilot 15 and mains 120), I have checked for air leaks and also the clearances (valves / timing) and everything is good. In fact the bike now run quite nicely - idles well and pulls quite nice.


What I am looking for is feedback on the fine tuning side of jetting and have some questions

Do the pilot jets govern the throttle between idle and 1/4 open irrespective of rev range ?

What conditions would I move up a size on the pilot jets - + 5 main jets or simply how the bike responds ?

When changing the main jets would it cause an issue not to fine tune the fuel mixture screws or are these mainly fine tuning related - ie I would still feel the affect of moving a jet size without changing the mixture screws or not re-setting the mixture screws would comply negate any change of main jets ?

[chuck hahn]

If its solid throughout the throttle range and youre happy with how it pulls..no stumbles or other issues..you fine tune using the side MIXTURE screws. these screws meter in air to the mixture. Turning them IN will RICHEN the cylinder and out will LEAN a cylinder.

My 1000s have the bottom PILOT screws set at just over 3/4 out. I call it about 7/8 out and they are left alone. What I have found is that somewhere between 1 1/4 out and 1 1/2 out on the MIXTURE screws is where they seem to be the best. I have come to these settings using my Colortune and actually seeing whats happening in the jugs. I tune at 1,500 RPMs and then run it up to 4,500 and then readjust again if I see the dreaded ice blue flashes in the Colortune.

If you dont have a Colortune, I hope what I have found is of some value.

Check out some of the videos. Note that CV carbs dont work well with a Colortune. Vms are awesome though.

https://www.youtube.com/watch?v=WnBJNQom5hs

[Marty]

Hi Chuck

Yes I have the colortune tool which I use to fine tune the mixture screws which works well. However it does not allow for all conditions / loads - so this ends up with the feel of the rider.

I am interested in understanding if the pilot jets mainly affect the idle or show up as being an issue through idle problems

Having tried 120 main jets the bike ran well with some hesitation (small) on low revs, 5th gear opening the throttle from 5% onwards, this got much worse when I went up to 125 main jets. So I guessing the hesitation could be a rich issue which has been magnified by moving to 125 mains. So I am now thinking of trying 117.5 to check if 120 was too rich (colortune and plugs looked ok) the other option might be that 120 is perfect but I need to go up a pilot jet size as it was lean hesitation on low throttle?

[chuck hahn]

Pilot jets act on the idle and from there up to "around" 1/4 throttle. They also supply fuel to the circuit as the main jet needle starts to take over.

[Marty]

Tried the 120 jets on a longer ride and it is definitely running rich, plugs dark and exhaust smelling rich, the bike was still performing pretty well so can't wait to get it just right.

Have gone down a size to 117.5 and will try them out and report back.

[Spyderman]

hello marty
the pilot jets are from idle to 1/4 throttle
the needle jet & jet needle (work together) and cover from 1/4 to 3/4 throttle position
Main jet covers from 3/4 to wide open throttle (WOT)

the needle jet is the brass tube that the main jet screws into
the jet needle is the thin needle that hangs down from the slide and is adjustable by moving the clip to 1 of 5 different positions

each of the 3 circuits do overlap each other
fine tuning of the jetting is not governed by the RPM of the engine or the speed of the motorcycle only by throttle position

so
Do the pilot jets govern the throttle between idle and 1/4 open irrespective of rev range ? YES

What conditions would I move up a size on the pilot jets - + 5 main jets or simply how the bike responds ?
your colour tune does help you a lot however learning to read your plugs will tell you what your engine is doing in each throttle range by the colour of the electrode in a particular part of the electrode the ideal colour is a light golden tan all the way down the electrode from tip to base

hope that helps a bit

[Marty]

hello marty
the pilot jets are from idle to 1/4 throttle
the needle jet & jet needle (work together) and cover from 1/4 to 3/4 throttle position
Main jet covers from 3/4 to wide open throttle (WOT)

the needle jet is the brass tube that the main jet screws into
the jet needle is the thin needle that hangs down from the slide and is adjustable by moving the clip to 1 of 5 different positions

each of the 3 circuits do overlap each other
fine tuning of the jetting is not governed by the RPM of the engine or the speed of the motorcycle only by throttle position

so
Do the pilot jets govern the throttle between idle and 1/4 open irrespective of rev range ? YES

What conditions would I move up a size on the pilot jets - + 5 main jets or simply how the bike responds ?
your colour tune does help you a lot however learning to read your plugs will tell you what your engine is doing in each throttle range by the colour of the electrode in a particular part of the electrode the ideal colour is a light golden tan all the way down the electrode from tip to base

hope that helps a bit

Thanks Spyderman that does help greatly. I have gone up 8 sizes in the main jets from 95 to 115 - I did try 120 and 125 but there was obvious signs of running rich - both exhaust smoke and the plugs. I am now trying out the 115 mains and the exhaust smoke seems to have cleared and the plugs are looking more normal in colour although I want to try them on a longer ride.

I am now convinced of moving up a pilot jet size as it seems a bit hesitant up to the 1/4 throttle opening.

There does seem to be 2 types of Pilot jets for sale of a slightly different size - can anyone advise the right spec for the 26mm VMs ?

[hannibal]

I believe the two options are visibly different. I have the pilot jets that are shown in basscliff's VM tutorial. Hope this helps you distinguish between the two types.

[hannibal]

I was thinking of needle jets when I wrote that, but JetsRUs should have the pilot jets listed and help you figure out which you have. Wasn't aware there were multiple types of pilot jets in VM carbs.

http://jetsrus.com/a_jets_by_carburetor_type/a_jet_kits_mikuni_selection.htm

[Big T]

Generally, you don't need larger pilots until the fuel screw is about 1 1/2 turns out.

I think the bottom ones in the JetsRus link are the correct ones. Look at the jet with a magnifying glass, the model should be on there

Have you tried raising the needles?

[Spyderman]

marty the pilot jets your looking for are the VM28/486 #15 (stock) #17.5 (is what you may need to got to

but first try playing with the air screws on the side of the carb body (not the ones on the bottom - those are the pilot fuel screws leave those at about 7/8 th's of a turn out from lightly seated)

the air screws can be touchy on some bikes. 1/16 to 1\8 of a turn can make all the difference in the world as to how the bike reacts and runs at the idle to 1/4 throttle range and each carb will need a different setting on the pilot air screw
set them all the same and then ride it make a minor change on all then ride it and then once you get it where it needs to be you can tune each one for best performance