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Fearless Keyboard Warrior
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115 Posts
Discussion Starter · #1 ·
I'm having a hell of time getting the TM40 tuned on my Virago XV920. The bike is running/pulling great at "around town" speeds (under 40mph or so, 1/4 throttle, up to 1/2 while accelerating), but once I get on the freeway the bike falls on its face and goes super lean and coughs out of the carb if I try to cruise at 60mph at a little over 1/4 throttle or if I try to accelerate. If I crack the throttle to get the accelerator pump going, it goes a little better, but starts to lean out again once the squirt of fuel wears off. My plugs are about as clean as they can be if I pull over and check them after a highway run; they're bright white, so the bike is definitely lean.

Also, once I hit the main jet at 3/4 throttle the bike still goes lean and falls on it's face. That seems really strange considering it's a 165 main, which should be more than enough for this bike.

Normally, I'd just toss another needle in, but something seems "off" with my bike since this is a common upgrade for these bikes and others who use this carb don't need a richer needle, same with the countless people who run this carb on their Sportsters and don't need any different parts.

Current settings are -

Pilot screw: 2.5 turns out
Pilot Air Jet: 1.1
Pilot Jet: 17.5
Needle jet: Y-6
Needle: 9DJY01 (on notch #4 of 5, putting it on 5 (richest setting) doesn't help much)
Main jet: 165
(Jets and needle are the factory parts that came on the carb)

Other pertinent info:
Ignition is C5 digital ignition (I binned the factory TCI), currently with the blue and brown wires grounded (most aggressive timing profile, recommended by C5)
BIP single carb manifold
I've checked for air leaks, and I'm pretty positive that there is no false air getting sucked in
Exhaust is factory, but the baffles are drilled out to open it up a bit
Spark plugs are factory BP7ES
Compression is 150 in both cylinders
Carb is new, clean, and the floats are at the factory settings

Any ideas?



 

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Remi's Dad
Joined
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17,593 Posts
You mention floats. Is it adjustable on this bike?

I usually test with a clear tube and threaded male barb adapter outside the carb to be absolutely sure the bowl is filling properly. Here is an example. http://kzrider.com/archive/viewthread.php?tid=14015
Also make sure fuel lines arent restrictive and fuel filter is not slowing flow too much

If your float is filling properly id look at adjusting clip position/taper of the needle
 

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Is my bike ok?
Joined
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15,025 Posts
Is the rear of the fuel tank raised quite a bit? Looks like you'd have to pretty much keep the bike full for the petcock to see good fuel flow, unless you've removed the stand pipe from the petcock.

Standard list of fuel flow checks

- filters: inline or in-tank
- tank venting properly
- carbs venting properly
- pinched/collapsed lines
 

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Fearless Keyboard Warrior
Joined
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115 Posts
Discussion Starter · #4 ·
I'm thinking it's the float level. I think it's time I confirm it's set correctly. I'm running out of possible causes at this point. I'll report back.
 

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Fearless Keyboard Warrior
Joined
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115 Posts
Discussion Starter · #5 ·
Man, what a headache, but I found the real cause of my issues, there were three of them.

1. A small air leak on the boot for the rear cylinder
2. A clogged up pilot jet
3. A malfunctioning petcock that wouldn't flow enough fuel to keep the carb fed

Got it running pretty well now. Got a flat spot around 1/8 throttle and another flat spot at full throttle, so I ordered a larger pilot (#20) and a smaller main (155) and hopefully that will sort things out. It's rideable in the meantime though.
 
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