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VM Throttle Valve Cut Away?

srsupertrap

srsupertrap

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Past Site Supporter
Since moving to Colorado (Elevation 5150 Ft) I found that my GS1000 29mm S/B jetting is slightly off in 1/4- 1/2 throttle opening (rich). I believe this symptom resides with the Throttle Valve but I am a little weak on the subject and I was wondering if the following statement is true for the VM29mm Smooth Bore carbs.

"The larger the cutaway, the leaner the mixture (since more air is allowed through it) and the smaller the cutaway, the richer the mixture will be."

If this statement is correct ( I am not sure if it is) my symptom would make sense because the S/B's were set up as the VM29-A2 Series which used the 1.5 T/V Cut Away. Changing to a 2.0 T/V will make it leaner.

Setup: GS1000 Wiseco 1085 10:25 cr, Carbs (Above), Supertrapp 4 into 1, K&N's Pods, Andrews S1
 
correct you are on the right track . as far as the slide cut out detail.

I'd go smaller on the pilot and bigger on the air bleed instead of getting different slides.

consider this

also if those are older carbs the needle may have a wear groove that is allowing a sloppy fuel control situation. (uncontrolled fuel)

high elevation brings out problems that are not present in lower altitudes.. it is less forgiving to tunning specs.
 
Trippivot,

That advice makes sense and probably much cheaper. I will examine the needle & seat to see if its contributing to the issue.

Regarding the air bleed (air jet right?), the 29 S/B series use a BS30-97 series .9 air jet. Never swapped out the air jets before so what type of step should I take? Next size up is 1.0 followed by 1.1 etc.

Thanks,
 
At that altitude you're getting around 90% of the airflow of a sea level bike, so in theory you would need to down jet by 10% across the board.

Adjusting the needle height changes the fuel flow at half throttle by somewhere between 5% and 8% for each groove and between 3% and 5% at 1/4 throttle.

Slide cutaway is 8% - 10% per size at 1/4 throttle and 4%-5% at 1/2 throttle.

You should be able to tune in the slow run by leaning out the airscrew alone.

I think I'd be tempted to play around with needle height first.
 
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When I moved to Colorado I had to move the Needle Jet from clip position # 4 to # 3. It's been set @ position # 3 for awhile.

The airscrews are currently backed out to allow max flow. I adjusted this with the carbs removed and when shining a light down the air jet passage, backed the airscrews out till the thru hole in the screw lined up with the air passage. That was Fall 2007 and I still have the described initial symptom.

I am inclined to increase the air jet size & lower the pilot jet size from 25 to 20 but I could use some guidance on the step of the air jet from .9 to ?

I hope I am not confusing the term air bleed with air jet.
 
I guess I'd go smaller on a pilot fuel jet first and experiment with a air screw adjustment that was closer to 1-2 turns, not wide open.

Until a idle mixture screw adjustment really has control of the idle per cylinder it is out of adjustable range.
 
If your backed right out on the airscrew I'd be tempted to drop 10% on your pilot jet - if your starting at 25 you can go a straight 10% to 22.5 (2.5 increments available between 10 and 60).

Similarly if you've shifted your needle lower already it's certainly worth looking at the air meter jets. If your currently running 0.9 I'd miss out 0.95s and go straight to 1.0 - I've got them listed over here at increments of 0.05 at those sizes.

If you've got a colourtune kit you can check out low throttle openings nice and easy. An advantage I've found is that you just change settings on one carb until you get the right burn on that cylinder and then repeat the settings on the others. (handy if you haven't got 4 jets to play with)

Bear in mind that these changes are really only having a significant effect on the area around the 1/4 throttle opening and lower - towards 1/2 throttle the slide and needle start to swamp effects from the lower systems. And you may have to swap slides and 2.0 is the way to go - max over here is 2.5 and I'm sure it will throw out the rest of the carb settings (pilot system 'finishing' too early etc).
 
Great Advice Guys . . . .

I will have to place an order with Sudco in CA. for the VM Pilots so it will probably be a little while before I have an update.

Thanks again
 
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