Loading...
Podcast

Episode 14: Carburetor Tuning

 

Motorbike Mondays Logo

Episode 14: Carburetor Tuning

Episode 14 is the second part in our series on Carburetors. If you haven’t listened to Episode 13, I would recommend doing that first. ¬†Assuming you are familiar with all the different components in your carburetor, and it’s general operating principles, going through these steps will help you tune your carbs and get your bike running perfectly.

Here are our notes for the show below. These are the most important points to follow for tuning the carbs correctly.

Make sure to check out the General Jetting Guide here for killer info on jetting your carbs for mods.

Carb Tuning
reasons to rejet/tune

*modified intake and/or exhaust
*change in cylender bore or stroke
*head work

-Rich comditions:

*backfire when accelerating
*black to dark brown plug color
*engine noise is dull or muted
*worsens when choke is on
*worsens when engine gets hot
*strong fuel odor
*smoke from exhaust

-Lean conditions:

*backfire when deaccelerating
*engine runs hot
*improves when choke is on
*poor acceleration
*white to light brown plug color
*lack of power
-Prep

*make sure ignition timing is set
*make sure valves are set
*make sure carbs are synced
-1st-

-Float

*Parts:
*Tang
*Float
*Float needle
*Float seat
*With carbs upside down measure top of float to carb gasket surface
*make sure the spring is not compressed
-2nd-

-Main jet
-3rd-

-Needle
-4th-

-Pilot jet
-5th-

-Mixture screw

*When the engine is warm, set idle speed higher than desired speed
*Start at 1.5 turns out
*Make adjustments .25 to .5 turns at a time
*Set to the highest RPM position
*Set idle speed back to desired speed
*Repeat previous steps
*Adjust idle to desired speed

**If mixture screw is more than 3.5 turns out or seated, replace pilot jet with a larger or smaller jet

 

7 comments
  1. mitch

    Hey guys,
    Thanks for the podcast. I feel like no matter how many tuning procedures I have read, your talking through it and having an actual conversation about it really helped me to dispel a lot of the mysteries that have always clouded my understanding of carbs.
    If there was one classic example of an easy first carb to try to rebuild/tune to cut ones teeth, what would that be. I would like to one day do a bobber build and the only reason I never even attempted was most that had a frame that could be modified easily enough all were prior to FI and I just wanted to avoid the carbs.
    As a background, I built a bit of a CR out of a 77 GL1000 and it was a very negative experience that I would like to blame on carbs. Might have been unwarranted but thats where I place the blame in my mind, so all carbs get the blame and I avoid them.
    So I need a first quick win. Can you suggest what bike would provide me the biggest chance of getting that win?
    thanks,
    Mitch

  2. Evan

    Hey Mitch,
    Really glad that it helped. Sometimes we’re not sure how well we’re actually explaining stuff, so it’s great to hear that it’s working. I would say an easy carb is any old mechanical or direct-slide Keihin or Mikuni from a single cylinder or twin cylinder bike. These are very simple carbs to start with. What happened on the GL1000? Just never ran right? I would look for a small cc 70’s twin, like a CB/CL350/360. That would be an excellent starter bike!

    1. mitch

      Yeah it just never ran right after I started having problems. It ran like a scolded dog for about 6 months and I put it in the garage for a few winter months and guess I didn’t drain the bowls well enough or missed some and it just never ran right. I sent the carbs off to a guy who specializes in the old gl stuff and he rebuilt them and it ran well for about a month.
      The problem it was having was no power at about half throttle up so without having the bike to run all the process you guys provided I don’t know more than that. Im just guessing some sort of synch issues, I never felt well about my bench synching of them and I think they were close when the guy sent them back so they ran well then something was loosening up and coming out of sync.
      Whatever the case, Im going to work my way back up. Ill start out with something a bit more straight forward and work up to the old big four.

Leave a Reply

Your email address will not be published. Required fields are marked *