Do I need to rejet or remap my fuel system if I change my exhaust?

Unfortunately, there is no definite answer to this question. In some cases where the motorcycle is running properly, you do not have to make any sort of adjustment when changing the exhaust. This is especially true when adding slip-on mufflers. The short mechanical reason for this is that changing the exhaust does not directly add (or remove) air or fuel to your engine. So there is no reason to adjust your air/fuel ratio to compensate for the change (which is what rejetting/remapping does!). That being said, many motorcycles come from the factory with a lean air/fuel mixture and do benefit from an adjustment. All aftermarket exhaust manufacturers do recommend rejetting/remapping.

West End Motorsports Recommends:

1. If you think your bike has a lean or rich condition already- Remap/Rejet

2. If you feel your bike is running correctly- It will not hurt your motorcycle to put on the exhaust and give it a test run. If you install the exhaust and experience poor throttle response, backfiring, popping on deceleration or excessive heat, then go ahead and rejet or remap. If you install the exhaust system and are satisfied with the results- Great…keep riding! West End Motorsports will not tell you to buy or not to buy an aftermarket fuel system. We simply present the facts as honestly as possible (we have no reason to lie to you- We sell the entire line of aftermarket jet kits and fuel processors!), but the decision is yours. Ultimately, making an adjustment can optimize performance but is often not necessary.

Do I need to rejet or remap my fuel system if I change my air intake?

Absolutely! When you change your intake (adding a free flowing intake or filter) you will need to remap your fuel injection or re-jet your carburetor. The short mechanical reason for this is that changing the intake directly adds to the amount of air going to your engine – so you must adjust your air/fuel ratio (add fuel) to balance the change.

For carburetor models: Most air kits state they come with jetting but they only include larger main jets. We recommend buying a Dynojet jet kit separately to take advantage of the Dynojet needle which has a finer taper and will give you better throttle response.

If your motorcycle is fuel injected you will need to remap your fuel system. This can be accomplished with any aftermarket fuel processor such as a Dynojet Power Commander or a Cobra FI2000 Tripot Fuel Processor. Look in the Engine/Performance category for your model for pricing. There should be several different brands available for your particular application.

In either case since you are adding significantly more air to your air/fuel mixture with these kits, adding more fuel is required. Not rejetting or remapping your air/fuel mixture could result in excessive heat buildup, causing significant damage to your motorcycle.

Watch West End Motorsports video Blog on this subject. Click here

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147 Responses to Do I need to rejet or remap my fuel system if I change my exhaust?

  1. Big Jon says:

    I have a 91 883XL and I’m thinking of replacing the factory exhaust with a set of 1 3/4 drag pipes. Can I keep the original braether cover and just drill/tap 3 holes in the cover to giveit the air flow it needs to breathe? I am running a M-Twins air filter in it and have already tapped a few holes on the back side of the filter box close to the jugs. What should I do to stay in my budget after installing these pipes?

  2. admin says:

    As long as your not drawing in unfiltered air, your plan should work. You will need to rejet to richen up the fuel mixture. ~Wiley

  3. Big Jon says:

    what size Jets would you recommend Wiley?

  4. admin says:

    The kit for your bike would include a range of pilot jets from 42-44 and main jets 170-175.

  5. Max Bellman says:

    Thanks for the info! I wasn’t going to bother installing a processor and air kit but your article proved that I should! It’s what I get for listening to a shade tree mechanic. I’ll be calling to order more from you soon! Max Bellman

  6. darik stedman says:

    What all would it take to put different exhaust on a 1987 honda super magna? Would I need to rejet ?.

  7. admin says:

    You should rejet (PN 1112.003 Dynojet Jet Kit) and add a High Flow air filter (K & N High-Flow Air Filter [Part # HA-7598].

  8. bill says:

    I have a 2013 street glide and I just installed thunderheader slip on mufflers
    I also took out the cat, everything else is stock.
    what I want to know is, someone that does these minor modifications
    do they normally remap or run it as is, I just started riding this year
    so I really wouldn’t know what to look for as far as how the bike should ride
    it pretty much rides the same, maybe a little rougher idleling because of the
    mufflers, but it does not pop or backfire, the throttle response appears
    to be okay. I noticed when I first turn it on, it idles around 1200 rpm
    but after I start riding and happen to stop and let it idle its around 1000 rpm
    thanks in advance!

  9. admin says:

    If you’re just changing the mufflers you should have no problems. But to get the most out of your engine add a high flow air filter kit and a fuel processor.

  10. chris anderson says:

    I just put a set of v&h slip on’s on my 09 sportster custom 883 now it has a lag when I take off (poor response to acceleration) but It only happens some of the time. what would cause this

  11. admin says:

    Hi Chris, Your engine is sensing the loss of exhaust back pressure due to the less restrictive mufflers. You are also probably running leaner and hotter. The addition of an after market fuel processor and a high flow air filter kit will remedy your situation.

  12. Dennis says:

    I just bought a 2013 SG, I want to add V&H slip ons and a replacement K&N air filter, do I need to remap my bike. I don’ tplan on any other changes to the motor.

  13. admin says:

    If the SG means Super Glide, then yes, you will need to add a Fuel Processor. Your bike will run super lean if you don’t. I recommend the DynoJet Power Commander V for ’13 Dyna Glide [1020-1880]
    Click here to buy the DynoJet Power Commander V

  14. craig donaghey says:

    looking for advice, I have a Harley Davidson road king custom 2007, standard bike, but I wish to fit a set of vance & hinse slip ons, can the bike the run if I just fit these or does it need re-mapped or a fuel pack. thankyou

  15. admin says:

    Hi Craig, When installing slip-on mufflers you shouldn’t have to remap as long as you keep the stock air cleaner in and the stock header pipes. Unless your bike starts running hot or backfiring, you should be fine with just installing the mufflers. ~ Wiley

  16. Chad Schuster says:

    I have an 08 Electra Glide Standard and was thinking of putting slip ons on. The guys that sell the pipes said I do not need to remap unless I go larger than a 2″ baffle. Does it matter what size the baffles are?

  17. admin says:

    Hi Chad. The baffle size might be an issue. Your bike runs pretty lean stock and the large baffle mufflers might make it run leaner, also hotter. Is your airbox and air filter stock? If so you might be able to get away with it. I recommend upgrading the air filter to a High Flow filter and install a fuel processor to get your bike to run better and cooler. ~ Wiley

  18. Stephen Busuttil says:

    I have a Harley v rod muscle 2012. It has the topless airbox,Super tuner SE Pro kit and V&H competition 2 into 1 pipe. I have just changed the exhaust to stock headers with V&H competition slip ons with quiet baffles. Do I need to remap.Worried about doing damage if I ride without remap.
    Thanks

  19. admin says:

    Anytime you change to a high flow air intake and filter and replace the exhaust with performance system you should always install a fuel processor. Your bike was set up to run lean from the factory and the components you added will make it run leaner and hotter. Your bike will run richer and cooler with the addition of a fuel processor.
    I recommend a DynoJet Power Commander.
    http://www.westendmotorsports.com/dynojet-power-commander-vrod-p-28922.html ~Wiley

  20. Andrew Moore says:

    G,day i live in Australia ive got a 2013 softail fatboy 103 ci ive put some s&s slip on mufflers and havent yet done anything to the air cleaner , the bike does pop a little bit when i back off.what can i do to remy this your expertize would be appeciatted.

  21. Denson Davis says:

    Hi I have a Harley Davidson 48 2012 model .To get a better sound i drilled out the Baffle End caps .It sounds great and no real issues .Do i have to remap the Engine
    I read several articles saying you should and shouldnt i am confuse please reply

  22. admin says:

    Unless you changed to stock air filter to a high flow air filter, you should have no problems. ~ Wiley

  23. Richard Cranium says:

    My brother suggested I might like this web site. He was entirely right.
    This post truly answered all my questions about my Honda Rebel 250. You cann’t imagine simply how much time I had
    spent for this information! Thanks! Dick

  24. Norm says:

    I have a 2010 Honda fury and I just installed the cobra speedster swept exhaust and am planning on installing the cobra powerflo air intake kit do you sagest I remap the air/fuel if I do the air intake kit?

  25. steve masala says:

    I have a 2013 road king full boar 2inch baffles and k&n air filther did not change filter box or cover. also i have a set of 2 1/2 inch baffles also.Will I have to remap

  26. admin says:

    Typically with just slip on mufflers no remap is needed. But when you replaced the stock air filter to a high flow filter you leaned out the engine starving it for fuel. So in your case a fuel processor is needed. I recommend the DynoJet Power Commander V for ’13 FLHR [1020-1877].

  27. Karl says:

    Hiya. I have a 1999 WideGlide,and just purchased V&H short shots. I have stock air filter and everything stock.Will i need to rejet? will it harm my bike if not,if it feels ok? If i do need to,what size jets please? Many thanks. Karl,England.

  28. René says:

    Hi there. Can you tell me what mileage normally is on a new Harley when it comes from the factory? have been told a lot of different Things from 6-25 miles, but belive there must be an average from the factory testing ect ect. one dealer says he will guarante zero miles on the clock on a new bike, but with testing how can this be possible???
    are really enjoing this site, keep up the good Work :-)

  29. admin says:

    Yes, all Harley’s are test ridden at the factory. So to be promised one with 0 miles is a little fishy. Every bike they test is going to be different so the best way to find your answer is to go to your local dealer and check the miles on a few new ones. Get an average and that will be your answer. ~Wiley

  30. moe says:

    Hi admin,
    I just bought a 2014 sportster 48 and im about to add antee laf drag pipes, and I was looking at the kuryakyn air cleaner hypercharger. Do I need to remap it or just adjust the fuel flow?

  31. admin says:

    Hi Moe,
    If you are going to change to a high flow exhaust and air cleaner set up, you will definitely need to manage the fuel and richen up your fuel/air mixture. I recommend the Vance & Hines Fuelpak 3 Autotuner
    Fuel Injection Management System Part # 66005.
    The unit recalibrates your ECM by Flash Tuning from your smart phone. Click here to check out the Vance & Hines Fuelpak 3 Autotuner.

    Vance & Hines Fuelpak 3 Autotuner
    ~Wiley

  32. Paul C says:

    I have an 08 EG Ultra running a Big Sucker Stage-1 AC, Dresser Duals, 48H cams, and RCX-Exuast 4” slip-ons. The bike has been dynode at a HD dealership. I keep getting DTC codes: P0132 Front O2 sensor high (rich) and P0152 Rear O2 sensor high (rich) in the P ECM Electronic Control Module [EFI]) category. I cleared the codes 4 times riding 50 miles each time. I also noticed that my fuel economy has degraded to 32~32 MPG from 42~45 MPG. I have a set of Monster Rounds that I want to put on to replace the RCX-Exuast 4” slip-ons. Will putting on the Monster Rounds help lean out the bike or will I need to Re-map/Dyno the bike again?

  33. admin says:

    Hi Paul,
    The EXCALIBUR exhaust you have on now is about the same baffle restriction as the V&H Monster Rounds so I think the engine will run about the same. Install the Monster Rounds and see how it runs. You should remap to get your MPG back on track or install a fuel management system like Cobra’s FI2000R PowrPro Tuner. Click here to view the Cobra FI2000R PowrPro Tuner.
    ~Wiley

  34. Panta says:

    I just purchased a 2014 harley davidson nightrod special. I want to get rid of the huge stock exhaust and get a deeper sound. I am interested in installing the Dan-moto highwayman 2-1 full exhaust system. They suggest you adjust fuel mapping. Can I just install the exhaust or is re mapping or other components necessary to prevent any damage to the bike?

  35. admin says:

    The installation of a full exhaust system does require the installation of a fuel processor. A high flow air filter should be installed as well to benefit from the added low and mid range torque and added horsepower. Check out the DynoJet Power Commander V for 2014 V-Rod [1020-1557] and K & N High-Flow Air Filter for V-Rod ’02-Up [HD-1102]
    ~Wiley

  36. Paul C says:

    Hi Wiley – just an update – I removed the Excalibur slip-ons and put on the Monster Rounds. Initially there was no noticable difference in the ridability or performance of the bike. However, after a new set of plugs and about 220 miles later, the P0132 is gone and has not retuned; the P0152 intermittently returns. Lastly, fuel economy is much better, approximately 42 MPG. I expected some deceleration popping but there isn’t any at all, I’m happy! I’m still researching fuel management systems, they’re quite expensive

    thanks,
    Paul

  37. Paul C says:

    I’ll look for a different fuelpak, that one won’t work with my bike since it’s an 08.

  38. admin says:

    Paul, there is a lot of choices on the market. But all of the fuel processors we sell will get your bike to running within normal parameters. Give us a call if you need to discuss what’s best for your machine. 800-520-8525 ~Wiley

  39. Joe says:

    Hello.

    I have a 2004 Sportster XL1200 (Carbureted) and will be switching out my Vance and Hines Straight Shots for some V&H Short Shots. From what I’ve read here it sounds like if I don’t touch my air cleaner, I don’t have to worry about rejetting. But if I were to change the air cleaner, would I need to rejet? And if so what size is recommended? Thanks.

    Joe

  40. admin says:

    Hi Joe,
    If you were to install a high flow air filter kit, I would recommend the Dynojet Thunderslide Jet Kit, part number DJ-8136. The kit includes all of the jets, needles, clips and other necessary parts for CV carbs, plus the Thunder Slide itself provides unparalleled performance gains in both horsepower and mileage…the faster the slide opens, the quicker the throttle response. Click here to check it out on our web site. ~Wiley

  41. steve musto says:

    I have a 2009 883 Sportster low, that I am adding Vance & Hines straight slip ons . Will I have to re-map or change air cleaner or make any other changes? Thanks in advance!

  42. Ryan says:

    I have a 2013 street glide just bought a set of thunder harder slip ons do I need to get a power commander or do anything else before I install them like add a power commander or re jet it

  43. avrajwytboy says:

    2014 flstf, stage 1 ac, screamin eagle slipons, already tuned. Runs great. Don’t like how it sounds. Slipons seem to have cats in the baffles. Wanna change baffles. Will this be an issue?

  44. admin says:

    Changing the baffles in a set of slip-on muffles should have no effect on the way the engine performs for the most part. If you feel the engine is running too hot then you will have to re-tune your fuel injection settings. ~Wiley

  45. admin says:

    Installing a set of slip-on muffles should have no effect on the way the engine performs for the most part. If you feel the engine is running too hot then you will have to add a Power Commander V or a Cobra PowrPro2000R. ~Wiley

  46. admin says:

    Installing a set of slip-on muffles should have no effect on the way the engine performs for the most part. If you feel the engine is running too hot then you will have to add a fuel processor and you should add a high flow air filter to get the bike to run at it’s potential. ~Wiley