Main jet and pilot screw adjustments for altitude and temperature

The official Honda XR500 shop manual offers a handy chart which helps determine the optimal main jet size for altitude and temperature (averages).  Refer to the Honda chart below to find C, the correction factor.

Altitude and temperature main jet correction factor C chart.

If C is 0.95 or below, lower the jet needle one groove and screw the pilot screw out 1/2 turn.  Jet needle position and pilot screw adjustments are not needed if C is over 0.95.

To determine the main jet size, multiply the standard main jet size (155) by C.

For example, at a temperature of 28C (82F) and an altitude of 1655 m (5430 ft, which happens to be the altitude of Boulder, Colorado where I live), C is 0.95 and carburetor recommendations will be as follows:

  • Main jet, 155 x 0.95 = 147.25 (#148 main jet is the closest, part number 99101-357-148)
  • Jet needle, 3 –1 = 2 (2nd groove)
  • Pilot screw opening, 2 1/4 – 1/2 = 1 3/4 (1 3/4 turns out)

Note that the above is for a stock engine.  In this project we have a few modifications which will likely require a slightly “richer” carburetor setting due to the small increase in displacement and the Supertrapp performance muffler.

Optimal carburation is dependent on several other factors besides altitude and temperature, such as engine temperature, engine tune-up, load, RPM, and throttle position for example.  The following chart helps us visually understand which carburetor component is impacting mixture with respect to throttle position.  Yes, the chart is in French, now hold your colorful comments for a second, I’ll translate:

  • On the horizontal X-axis we have throttle position from idle to fully open.
  • On the vertical Y-axis we have the carburetor mixture-impacting components, showing at which throttle position each one is active.
  • Orange: needle position
  • Green: cylindrical part of the needle
  • Yellow: conical part of the needle
  • Blue: throttle piston chamfer
  • Darker red (left): idle air screw
  • Red (right): main jet
  • Purple: Pilot screw

Chart indicating which carburetor component is impacting mixture at increasing throttle position.

This chart can help trouble shoot carburation problems and understand which component should be tuned in situations where the engine is not running properly at some throttle position.

Next step, let’s compare XR and XL stator & rotor.

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11 Responses to Main jet and pilot screw adjustments for altitude and temperature

  1. I’m impressed by such detailed research. You better have the bike running at the first kick! I could have used a guy like you when I was racing endurance . 4 stroke carbs are a real pain especially if you have a battery of 4.
    Keep up the good work and I can’t wait your report when you attack the chassis part.
    Good luck!

    • Mike says:

      Thanks for the encouraging words Roland — I wish I could have been there with you helping tune your endurance engines at the Bol d’Or, 24 hours of Spa or Oss in Holland, or in Nivelles. Not many other things would have given me such pleasure.

      I’ll button up the XR engine within the next few weeks and then will focus on the chassis. I’ll make a few modifications to help the overall handling, especially at higher speed.

      • guillaume says:

        bonjour moi je suis surpris par ton travail et je me pose plein de question:
        1- tu dis avoir nettoyez ton moteur avec une brosse mettalique ,laquelle? car ton travail est vraiment bien réussie;)
        2-quel est la couleur noir sur ton moteur pour qu’il rézsiste a la chaleur ?
        j’ ai aussi une xr500 et j’ aimerais la refaire comme la tienne et je suis sur tahiti
        merci pour ton coup main

      • Mike says:

        1. “La brosse métallique” est en fait une série de brosses métalliques manuelles de duretés différentes et de tailles différentes. Rien de magique vraiment, la tâche était de systématiquement nettoyer les pièces diverses, à la main.

        2. La peinture (spray) utilisée pour le moteur est une enamel mi-mate qui résiste aux hautes températures. Voici un lien qui t’apporteras plus d’information:–engine-enamel-ceramic-ford-semi-gloss-black-12-oz-aerosol-by-duplicolor-part-de1635.html

        Bonne chance avec ton projet!

  2. Hey would you mind letting me know which web host you’re using? I’ve loaded your blog in 3 different web browsers and I must say this blog loads a
    lot quicker then most. Can you suggest a good internet
    hosting provider at a honest price? Cheers, I appreciate

    • Mike says:

      As you may see in the URL for this page, this blog is hosted by WordPress — — which is a blog hosting service, not a general Internet hosting provider. For the latter, a recommendation would have to be based on requirements such as price/budget, load, SLAs, services needed, geo coverage, hosted vs. colocation, dedicated vs. shared/virtualized, etc.

      If you’re an early stage startup, I would recommend Amazon Web Services (AWS, where you pay for what you use and have a long list of available services, including geo coverage. If you want the cheapest service, you can try Godaddy ( Anything in between will be based on other requirements. Hope this helps.

  3. Jonathon says:

    Mike, just found your blog and your XR is looking mighty sweet. I just put in a new 91mm Wiseco piston on my ’80 XR 500. I am having trouble though with it either not starting, starting on full choke only and/or bogging down when I can get it off of choke and am able to ride. Any suggestions? Thanks, and again your XR looks awesome!

    • Mike says:

      What you experience can be due to a number of things but is likely an air/fuel mixture related problem. Before focusing on the intake system, verify the following just to make sure you’re not going down the wrong rabbit hole.

      1. Verify your cam timing to make sure you’re not off by one tooth.
      2. Verify that you’re spot on with your intake and exhaust valve gaps.

      Then move upstream to the intake system:

      3. There could be an air leak downstream of the carburetor. Remove the carburetor and then the rubber intake boot. Verify that the gasket/o’rings are soft and provide the necessary sealing. Use gasket silicon paste if needed or replace parts.
      4. Clean the carburetor. This is easier said than done. If your carb float chamber was full of water for a while, the oxidation may be massive and a real challenge to reverse. Of all the steps, this one will be the most time consuming and challenging but is likely the one that will bring you what you’re looking for.
      5. Assuming that you have baseline jets and needle position?
      6. Tune the float position and verify that the float valve is not worn, replace if needed.

      I’ve had serious trouble with the carb which came with the parts. It was massively oxidized and no matter how hard I tried to get it back to specs with ultra-sound and specialized chemicals it never worked properly (difficult to start the engine, difficult to keep it running). At the end I decided to try another “clear” carb and bought one from eBay. That did it. Here is a link for what’s available now.

      Good luck!


      • Jonathon says:

        Well, I appreciate your suggestions. I have done all the aforementioned, and I was not really looking forward to breaking down my carb again as I had already taken it apart, cleaned and blasted the jets with compressed air. The inside of the carb was already in really good shape for the age of the bike.

        My problem was this…I didn’t have enough fuel in the tank. So simple and so silly, but it came from not wanting to drain a full tank again and again just to take it off to start tinkering with other sections. As my friend put it, “they don’t tell you that in the manual.” That would be the reason it sounded like it was starved for fuel.

        Anyways, I’m glad I have found your site. Your XR is looking so sweet, and mine is up and running. The charts are especially helpful too. Take care and happy riding


      • Mike says:

        Always love a happy ending to a troubleshooting. Especially when there is silliness involved that will make for great memories and inevitable teasing later on by your friends!

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