Hey Everyone! My name is John. I bought this spirit 750 3 years ago, and fell in love. Ive had some issues getting it to idle smooth. Just figured I'd come one here and do some research. Im excited to join this community and learn more about bikes!
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2001 Spirit 750
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John Condie New Member
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TJ Admin Staff memberModeratorMember who has the rights to delete posts, and ban members form forum if they violate the rules.@John Condie hey John! welcome to he forum and congratulations on the bike! (3 years ago LOl)
what have you tried as far as addressing the idle issue?
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John Condie New Member
@TJ Hi TJ. Ive cleaned the carbs multiple times and tried to sinc them. The mixture screws dont really respond. when I turn them unless I turn them super lean, then they die. Ive checked the diaphrams, and they are a little cracky, but I cant see light through them. I just ordered your stock rebuild kit. to replace those diaphrams and possibly replace the main jets/jet needles (I have a stage 1 jet kit in right now). I also have Seafoam running through the tank right now while Im waiting for the package.
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TJ Admin Staff memberModeratorMember who has the rights to delete posts, and ban members form forum if they violate the rules.
@John Condie you just told me your diaphragms are cracky... thats not acceptable for the carbs to function.
So lets break this down real clean-
Your key symptoms are as follows
Mixture screws have little to no effect
Engine only reacts when turned very lean (then dies)
Carbs have been cleaned multiple times
Diaphragms slightly cracked, but not visibly torn
You’ve attempted sync
What that combination points to
1. Idle circuit still restricted (most likely)Even after cleaning, the pilot circuit can still be partially blocked.
If screws only affect the engine at the extreme (fully lean), it means:
The circuit is barely flowing
You’re only seeing effect when you completely shut it off
Very common on these carbs:
Pilot jet looks clean but isn’t
Emulsion/pilot passages still varnished internally
Important detail:
Spray cleaner alone often does NOT clear the pressed-in passages.2. Idle speed set too high (very common with this symptom)
If the throttle plates are open too far:
The engine is running on the transition/needle circuit
Mixture screws become mostly irrelevant
Why this fits:
You still get a response when going extremely lean (you’re finally starving it enough to matter)
Otherwise, changes seem minimal or nonexistent
3. Vacuum leak (possible, but not primary suspect)
A vacuum leak would:
Lean the mixture across the board
Reduce sensitivity of mixture screws
Why it’s less likely as the main cause:
You do get a response when going very lean
A major vacuum leak usually causes:
Hanging idle
Erratic idle
Difficulty syncing
Still worth checking:
Intake boots (especially if original) replace them asap. a slight bump to the carbs can create a leak in old boots wreaking havoc on your diagnostics
Vacuum caps/ports - replace all old caps
4. Diaphragms
Cracked-but-not-through diaphragms typically affect:
Midrange
Slide lift under load
They do not control the idle circuit, so they are unlikely to be the cause of unresponsive mixture screws but if they are already cracky then they will likely fail under use and leake you stranded during a ride.
What I would verify next (in this exact order)
Step 1 — Force it onto the idle circuitBack IDLE speed screw down as low as possible (barely running) the idle screw is not the air fuel mixture screws!
Then adjust mixture screws again
If they suddenly respond → problem was throttle plates too open
Step 2 — Confirm pilot circuit flow physically
Do not rely on “it looks clean”
Remove pilot jets
Verify you can:
See clean, perfect round light through them
Spray cleaner through and see it exit from:
mixture screw port
idle discharge ports
If not → circuit is still restricted
Step 3 — Vacuum leak check
Spray carb cleaner gently around:
Intake boots
Carb joints
Watch for RPM change
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