VM 29 Smoothbores an stock '78 GS 1000, jetting?

[Anonymous]

Hi,

I recently managed to got an NEW set of vm 29 Smoothies at ebay (here in germany). Naturally they are some years old, but have been unused so far. They also have an accelerator pump assembly on them, which is rather unusual for these carbs. So, basically I?m a very lucky guy :-)).

I know for sure they aren't RS, stock VM or other carb types.

My bike is a stock '78 gs 1000, 100.000km, motor rebuild but with stock pistons (first oversize), stock airbox, stock exhaust. Just electronic ignition, new coils, oil cooler. Intake manifolds are new for i had to change them by size compared to the old VM26SS carbs. Bike ran ok with these old carbs, so ignition, valves and all that other stuff should be ok.

First i tried the carbs with some aftermarket pods (no K&N), but the performance at low and top end wasn't any good, the bike just sputtered around.

Then I went for the stock airbox, and the performance when opening the throttle is amazing, the plug readings are really optimal.

My problem is the idle circuit. Bike will start hard with yoke, you can set the yoke so that it runs ok on idling till its warm. When it is warming up, and you close the yoke, there is much stuttering, bike dies off etc. No popping, no backfiring thru exhaust.

When i remove the airbox cover, things just dont change. Spark plug readings just don't give any real hints, cause with pilot air screw out a half turn, they look good, out 1 1/2 turns, they look a bit lean, but not much. Things are a bit better when air screw is turned in to maximum (1/4 turn).

So i'm supposing that idle circuit is to lean and I have to richen it up. Am i right? Should I change the pilot jet? To what size? Is there anybody out there with the same configuration and a carb jetting that ist running well on idle?

Jetting now, like recommended and factory default for these carbs on gs1000 (according to mikuni/SUDCO manual), is:

Main 115
pilot jet 25
Jet needle 5dl31-3
Needle Jet O-6
Main Air Jet 0,9
Cutaway 2,0

ThanX a lot for any advises to the "old" continent
Michael
Germany

[robinjo]

Check out this thread, very usefull. http://www.thegsresources.com/_forum...highlight=vm29

And welcome!

[Anonymous]

News

Thank you for your tip, robinjo!

Now I have dismantled the carbs and have some strange news:

the jets are not like factory specs, main jet = 100, pilot jet = 15. No wonder it didn't start with pods on and is running good (except of idling) with standard airfilter (and exhaust).

So my suspicion on too lean a pilot jets seams right, I will go to 20s, I think.

The pilot Jet assembly is a bit strange. On the bottom of the small channel near the main jet (carbs inversed) there sits the pilot jet, but above it, there is another jet looking much like a main jet but smaller, with a sign on it saying "50", I suppose this is the size. You have to uninstall this jet first before you get access to the real pilot Jets. So you have two jets in the pilot circuit (plus Air screw, plus Pilot fuel Screw).

Has anyone out there seen such a type before?
Michael

[jeff.saunders]

RedRooster, I do not think you have 29mm smoothbores.

It sounds like you have a set of 26mm carbs - probably from a Kawasaki Z650 (1977) - these carbs have a 50 jet over the top of the pilot jet, they run a #15 pilot and often a 100 main jet. The 50 jet directly over the pilot jet is a very strange arrangement - this bike is the only bike I've seen them on, although it is possible other bikes/makes had these type carbs.

Here is a link to some 29mm smoothbore pictures on our web site.

http://www.z1enterprises.com/reference/z1-carb-guide.aspx

Scroll down for the 29mm pictures.

Key features for the 29mm carbs
Single fuel feed (between 2 & 3 carbs).
Brass floats
Filter over on the float needle valve assembly
Most have a 17mm drain plug directly under the main jet (there are some early exceptions to this)
Small square shape in the casting on the lower RH side of the carb bodies - just above the float bowl.
The main feature is the smoothbore jet block.

Do you have any pictures of your carbs?

[Hoomgar]

I was thinking they didn't sound like smooties either. He has them off so simply looking inside should be easy.

Look for part #87 in this pic:
If you have that part then you have smooth bores, if you do not then they are not smooth bore carbs.