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VM off idle stumble

  • Thread starter Thread starter althomas101
  • Start date Start date
A

althomas101

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Hello, on my GS1000 I'm getting a stumble at a very small throttle opening just slightly off idle. It almost feels like it is intermittently dropping a cylinder if I hold it in that one throttle position. As soon as you roll off all throttle it goes away, or if you roll on any at all it accelerates smoothly. I'm sure I don't have any air leaks, as I've changed the o-rings and did a complete carb tear down and rebuild. Other than that one throttle position stumble it runs perfectly. As I have never owned a slide carb bike before I don't know if this is just the nature of the beast or if I need to revisit carb syncing , or valves. Any advise would be appreciated.
 
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Where/how did you set your pilot fuel screws?
No, this is not the nature of the beast, a perfectly tuned set of VM carbs should run..... Perfectly.
 
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If you have adjusted your valves and synched your carbs properly it sounds like an adjustment matter as 'tkent02' is refering to. Fuel screw should be around 5/8th - 3/4of a turn out and your air screw double that 1 1/4 - 1 1/2. Those are starting points of course, yours may be more or a bit less than that. One other thing is what do your plugs look like?
 
I think the pilot fuel screws are at 1 1/4 or 1 1/2 turns out I can't remember. Air screws are at 3/4 turns out. I have the floats set at 27mm @ 26mm it was peeing in the garage. It has #15 pilot jets, what ever air jets come in the carbs, 100 main jets, needle clip in middle position. I was thinking about trying richening the pilot fuel screws, but other than that one throttle position it doesn't seem lean, and if I turn in the air screws any more it seems to run rich and the idle will lower.
 
Well there's the first thing to fix. Is the bike a basic stock setup or have you modified things? if it's stock, use the stock settings and go from there. You are supposed to have double the air to fuel not the other way around. Your floats are out as well, if they were leaking it just meant the float needle was stuck, maybe from being dirty or it needed a 'smack' to free itself up. I've found that after rebuilding carbs I often get one or two float needles sticking, I just smack the side of the float bowl with something wooden and they usually always unstick. I always figured it was from not being used or something? Should be around 24mm give or take a 1/2mm if memory serves right. If you can sort out a clear tube to measure the float height it's far more accurate. You can pinch the brass tube from your carbs that holds the choke cable to the carb rack. Fits right in the float bowl thread and the right size clear tubing fits over it tight.
 
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Sounds like you either have your settings between air and fuel pilots flip-flopped or you are confusing one for the other. Like others have said, fuel setting's about half of the air settings. For example, right now, I'm toying with 1/2 turn on the fuel pilots and 1-1/2 on the air.
Your symptom sounds somewhat similar to what I'm experiencing. Pull your plugs. Are they sooty? Clean em real well, if so, and see what happens. Does it pull without stumble, or is it still there? After a while, does it come back? If so, pull the plugs and see if they are sooty again. These VM carbs are new to me, and while I'm not new around an engine, most of my work has been with cars.
 
Thanks for the advice, the main thing I was looking fore was vallidation that it shouldn't behave this way. I need to get some new plugs to do chops with but if I take a ride and pull in and let it idle for a bit, they look slightly grey, I don't think I'm far off with the mixture. Btw what kind of mileage shoul I expect? On my first ride I only got 34 Mpg, but I was running it really hard :lol:
 
Rash, same here I can tune a holly like nobody's buisiness. I've also worked a bunch with CV carbs too, but I've heard that slide carbs are touchy and you can operate them too fast. But mine seem to like fast throttle movements just not really slow ones. I'll double check my settings, and I know I'm not confusing the two circuits. I started with 1 turn fuel and 2 turns air but it was super lean. I do know I have some exhaust leaks due to rust holes and a PO drilled the baffles.
 
Well you were close Rob...

Pilot is at 3/4 to 7/8..no more.

Mixture screws are at 2 out as the start point.

Also check the timing once she is able to run decently again.
 
One other note..as per the manuals directions..set the slides at 1MM past the top of the carb throat.This is done with the screw at the back of the rack to the right side of the throttle linkage. Right side if your sitting on the bike that is.

You set the screw so that when you are at full throttle, the bottom rim of the slide is just above the top radius of the carb bore.
 
Your barely off idle stumble is the fuel screws. Don't get the lean, rich notion about them, since it is lean where it is set now, "I have to open them up". They work in conjunction with the air screws, and each affects the other. Start out at just a little over 1/2 turn out. Properly working fuel screws seem to set how far in or out the air screw will achieve the highest idle. Set the fuel screw so the highest idle on the air screw is about two turns out, and then tweak the fuel screw until your 1,500 to 2500 rpm stumble goes away. Once you find the proper range where it is happiest, the fuel screw will have the range to adjust it perfectly. Each adjustment of the fuel screw requires an adjustment of the air until you are just about right on. I am using the equivalent of 132.5 main jets. At 1/2 turn out my highest idle on the air screws were almost closed and had the off idle stumble. I opened the fuel screws out about another 1/16 to 1/8 of a turn and the air adjust for highest idle moved to about two turns out. At this point, I was able to tune out the off idle stumble. It doesn't take much adjustment on the fuel screw to make a large change.
 
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