M8 idle delay
Stellar HDF Member
Joined: Aug 2020
Posts: 3,174
Likes: 2,221
From: College Station, TX
Agree with the post above, S&S. FuelMoto has a nice one too, along with a CNC ported one but with a mostly stock bike that is unnecessary.
Either way, do the ignition on off cycle as describeds abvoe first.
Paul
Either way, do the ignition on off cycle as describeds abvoe first.
Paul
Stellar HDF Member
Joined: Aug 2020
Posts: 3,174
Likes: 2,221
From: College Station, TX
S&S has a standard alimunum manifold and a CMC ported performance one. There were no installation instructions for the standard one, but there is for the CNC ported one. I did not see a recommendation to perform a tune on the engine, but if it were mine I would,
See attached S&S document.
Paul
See attached S&S document.
Paul
Road Warrior
Joined: Dec 2005
Posts: 1,276
Likes: 1,076
From: SE SC
I did all of stuff through FuelMoto. I have a PV3. Told them what I had added (for me, pipes, intake and throttle body. They sent me a new tune. Flashed it. Smooth like buttah.
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Stellar HDF Member
Joined: Jul 2017
Posts: 3,336
Likes: 1,281
From: Ky
i tried blipping the throttle today, didn't notice idle holding high, but it was less than 15 miles and temperatures were in the 70's
Cruiser
Joined: May 2023
Posts: 121
Likes: 153
From: Florida & Michigan
First off, what the OP described is pretty much normal when coming to a stop. Idle around 1100-1200 and then gradually returns to around 850-900. Now if it stayed high (usually much higher) then I would say warped manifold/vacuum leak. Harley makes an aluminum manifold identical to the stock plastic one for the M8s. It also is around $200
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Stellar HDF Member
Joined: Jul 2017
Posts: 3,336
Likes: 1,281
From: Ky
First off, what the OP described is pretty much normal when coming to a stop. Idle around 1100-1200 and then gradually returns to around 850-900. Now if it stayed high (usually much higher) then I would say warped manifold/vacuum leak. Harley makes an aluminum manifold identical to the stock plastic one for the M8s. It also is around $200
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Advanced
Joined: May 2012
Posts: 89
Likes: 28
From: NC
First off, what the OP described is pretty much normal when coming to a stop. Idle around 1100-1200 and then gradually returns to around 850-900. Now if it stayed high (usually much higher) then I would say warped manifold/vacuum leak. Harley makes an aluminum manifold identical to the stock plastic one for the M8s. It also is around $200
checked it and said it's normal..... even test drove another brand new fatboy at the time and it
did the same as what torkmizer said above......... my dealer said only fix it to buy a tuner...
kind of pissed me off .... and pissed that harley got rid of the idle screw on my stock bike
Club Member
Joined: May 2018
Posts: 2,373
Likes: 2,309
From: Tampa Bay Area
Thats not my experience with my M8 (117ci).
A delayed settle to idle (from high rpms to low) indicates a lean idle mixture - whether this is the idle air/fuel mixture settings or a manifold leak, there is more air in the mixture than should be.
For the record my M8 has pipes, cam, high-flow filter and a PowerVision tuner - not a stock tune - but the same principle applies.
A delayed settle to idle (from high rpms to low) indicates a lean idle mixture - whether this is the idle air/fuel mixture settings or a manifold leak, there is more air in the mixture than should be.
For the record my M8 has pipes, cam, high-flow filter and a PowerVision tuner - not a stock tune - but the same principle applies.
Last edited by Old Sport; Apr 18, 2026 at 02:09 PM.