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VM 1/8 throttle circuit?

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I've been having a hard time tuning in my 78 GS750 with a stock airbox and 4 into 1. I have installed a wideband O2 on my bike so this may be me asking too much of my carburetors but I'd like to see if there's anything I can do about it. When cruising between about 45 - 60 mph almost exactly 1/8 throttle I get a lean spot. The O2 reads 15.2-16.0 which is far too lean for my liking. Everything else is perfect. I get 12.5 at full throttle, and 13.8 or so cruising at 75 mph, even 50 mpg on long trips, I just can't get the 1/8 throttle figured out.

So what exactly is going on at this point? Is it the idle circuit? Needle? A combination? My idle AFR hovers around 12.0 and my idle is great. If I turn my fuel screws out some, my idle does increase but the exhaust starts to smell noticeable rich and the idle peaks at around a 11.0 AFR and my fuel screws end up at about 1.5 turns out. I tried moving my needles up a half clip position with a shim and it had no effect at 1/8 throttle. So what gives? Do I have to live with a very rich idle?

I currently have a 102 main, 15 pilot, needle at 3rd position with a shim to make it half a clip richer.
 
The screws on the side of the carb throats, not the ones on the very bottom, are the MIXTURE screws. Turning them IN will RICHEN the fuel to air mix. Out, obviously, will lean the mixture. You may try starting there and set the bottom ones where they were initially. Typically you set the bottom PILOT screws at around 7/8 out and forget about them.

I also agree with yanking the needle shim from under the clip and get things back to standard set up. If youve got a shim thick enough to take up half the distance between those slots on the needle you are way way off base already. Typically some .002 or .005 shims are whats needed.

Do some looking around here for shimming washers.

https://www.superiorwasher.com/washers/6/Shims.html
 
Last edited:
chuck hahn said:
The screws on the side of the carb throats, not the ones on the very bottom, are the MIXTURE screws. Turning them IN will RICHEN the fuel to air mix. Out, obviously, will lean the mixture. You may try starting there and set the bottom ones where they were initially. Typically you set the bottom PILOT screws at around 7/8 out and forget about them.
Click to expand...

I hear the 7/8 number a lot for the bottom fuel screws. But when I attempt that on my bike it barely idles even with the side mixture screws all the way in. I think Greg may be right, I have a set of 17.5 pilots maybe I'll try those and pull the shim
 
I pulled the shims and went up to the 17.5 pilots jets and got the same result. After some more playing I got it rich enough to be safe but that left me with the bottom fuel screws right around 1/4 turn and the side mixture screws at around 3/4 of a turn. I had to stop tuning because of rain so I don't know for sure what it does when I'm cruising but it still seems to have a lean spot around 1/8 throttle. Is this just a inherent problem with the VM carbs?
 
The1970's said:
I hear the 7/8 number a lot for the bottom fuel screws. But when I attempt that on my bike it barely idles even with the side mixture screws all the way in. I think Greg may be right, I have a set of 17.5 pilots maybe I'll try those and pull the shim
Click to expand...
Chuck only mentioned half of his usual advice. Start with the fuel screws (on the bottom) about 7/8 turns out and the mixture screws (on the side) about double that (1 3/4 turns out). With the mixture screws all the way in, you'd be super rich!

This advice was enough to get me running with pods and stock exhaust so it should be pretty close for you as well.
 
hannibal said:
Chuck only mentioned half of his usual advice. Start with the fuel screws (on the bottom) about 7/8 turns out and the mixture screws (on the side) about double that (1 3/4 turns out). With the mixture screws all the way in, you'd be super rich!

This advice was enough to get me running with pods and stock exhaust so it should be pretty close for you as well.
Click to expand...

The problem I'm having is that chucks advice seems to work for most people and I can't figure out why it doesnt work for me. Even with the side mixture screws all the way in my O2 was reading 19.0+:1 and the bike ran really bad. Turn the fuel screws out some and the idle smoothed right out. For some reason the ration of double the fuel screw doesn't work out for me.
 
The1970's said:
The problem I'm having is that chucks advice seems to work for most people and I can't figure out why it doesnt work for me. Even with the side mixture screws all the way in my O2 was reading 19.0+:1 and the bike ran really bad. Turn the fuel screws out some and the idle smoothed right out. For some reason the ration of double the fuel screw doesn't work out for me.
Click to expand...

Have you got a vacuum port open to air on at least one carb body ? Check if any of the open hoses or breather holes are pulling a vacuum when the engine is running. A damp finger over the end/hole should tell you.
 
GregT said:
Have you got a vacuum port open to air on at least one carb body ? Check if any of the open hoses or breather holes are pulling a vacuum when the engine is running. A damp finger over the end/hole should tell you.
Click to expand...

I think the only one that has vacuum is to the petcock correct? I have 3 hoses coming off my carbs I think 2 are vents and the one to the petcock. I get vacuum at the petcock and not the other 2. I also don't see any crack or feel and leaks in the hose to the petcock.
 
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