Back pressure.
6th Gear
Joined: Mar 2025
Posts: 10
Likes: 8
I have a 2004 road king and just newly installed some straight pipe fishtails. Love the sound but the bike doesnt like em. Under low idle 10-30 mph. It spits and sputters. The straight pipes dont have enough back pressure. My question is What is the cheapest and easiest way to fix this? Not sure if a $500 tuner will fix this issue. Any ideas. Thank you.
I am going to use the Dyno Jet Power vision and allow it to self learn on my 05 i just picked up. It is in no way perfect but it is very effective. I do not believe V&H supports the older twin cam management anymore
Yep I started with a 2010 Ultra Classic and the original unlocked Super Tuner. Built a basic tune that ran decent, then got a V&H FP3 because I wanted to calibrate the speedo and used that SE tune as a base for the FP3 to auto tune. That cleaned it up even more. I was impressed with the user friendliness of that FP3. Really seemed like a do it yourself basic tune. Don't know if such critters exist for the M8s or not.
Grand HDF Member
Joined: Sep 2017
Posts: 3,716
Likes: 919
From: Southern Virginia
V&H offers the FP4 which goes up to 2024 models. The V&H tuners get a lot of flak from the HP maximizing types. Mine's been great, the autotune got my bike running with all the power I can use and it stays cool even on the hottest days. Yes, it's running a bit richer than it could be, but I've always prefered my bikes to run a little sweet. The engines last a long time and don't cook my leg (once the cat's been removed).
Last edited by Hoyt 1911A1; Feb 23, 2026 at 08:38 AM.
That's good to know. I wonder if the FP4 can deal with spark knock on an upgraded cam? That was my main gripe with trying to keep warranty. Harley sold me the SE 447 cam and tuner so I could keep the warranty, but the tune that thing applied had HORRIBLE spark knock on it. I was able to use my PC and take the "swat a fly with a sledgehammer" approach to rolling back spark advance to eliminate most of the spark knock, but I know it cost me a little torque and isn't nearly as precise as it should be.
(562) 921-7461
help@vanceandhines.com
Grand HDF Member
Joined: Sep 2017
Posts: 3,716
Likes: 919
From: Southern Virginia
I don't see why it couldn't. V&H has amazing tech support for the FP tuners, too, you could give them a call to ask.
(562) 921-7461
help@vanceandhines.com
(562) 921-7461
help@vanceandhines.com
Do you know if the FP4 used the existing map in the ECM as a base or do you select your options (intake, exhaust, and in my case cam) and start from their canned tune?
Novice
Joined: Feb 2026
Posts: 16
Likes: 1
Essentially a washer on a bolt
Last edited by renardanderson9; Feb 28, 2026 at 12:16 PM.
I'm not familiar with the FP4's workings, as my old twin cam uses the FP3, but for the FP3 the stock map is stored and a canned map loaded, then autotune can optimize it from there.
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From: Florida
A Tuner isn’t going to fix a bad exhaust design. It may compromise and mask symptoms through tuning but it can’t magically fix a pipe with reversion issues. I’d still try the bolt trick and then mess with the tuner.
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