Randakk’s Jetting Advice For Vintage Honda CV Carbs

Whenever I’m asked about “jetting” issues on vintage Honda carbs – here’s my strong advice:


For the Vintage Hondas we support: (GL1000, GL1100, GL1200, GL1500, DOHC-4s, CBX, CX500s, FT500, etc.) and any similar technology motorcycles, I strongly recommend:

  • Retain the OEM air box and filter. This is vital!
  • Keep the OEM jetting specs exactly as delivered by Honda.

For the vast majority of owners, this will result in the best performance with the least chance of negative “unintended consequences.”

If you bike doesn’t run well with the OEM jetting (and air filter!), then something else is probably amiss,

The “leaness” associated with modern oxygenated fuel is minimal and easily accommodated with a slight tweak on the idle mixture fuel screws found on Keihin carbs for the models we support: open them slightly to enrich the idle mixture. (If you have carbs with idle mixture air screws, turn them in for more richness.) 

Also – you can run virtually any exhaust you wish with the stock jetting so long as the OEM air filter and air box are retained.

I specifically recommend against changing the idle fuel jet (typically #035 for most models).   Why is that?

Whenever additional richness is added to a well-calibrated idle circuit, rpms necessarily drop.  To compensate for this physical fact, the curb idle speed must be adjusted to bring the idle speed to the desired level.  This adjustment necessarily opens the throttle plates a bit more than the designed target opening. So what you say?

This simple change can begin to recruit the progression circuitry into the task of maintaining the desired curb idle speed.  This is especially true if carb synch is not perfect …and we all know it rarely is!  Progression circuits are designed to assist engine speed transition purposes …not to maintain a steady rpm.  This is done through the simple introduction of generous extra richness.

If progression circuits are recruited into idle maintenance, all sorts of undesirable symptoms can emerge:

  • unsteady idle with oscillations and surges
  • excess idle sensitivity to temperature changes
  • uneven mixtures that lead to fouled plugs on one or more cylinders
  • unpredictable and variable, vague throttle response

Always evaluate idle settings after a complete warmup – 20 minutes riding (minimum) of actual riding.  Most people don’t have enough patience for this and adjust prematurely.  The results are less than optimal.

GL1000s are a special case.  They do benefit from a special fix for the off-idle “glitch” that plagued the earlier models.  However, the preferred fix still retains the stock idle fuel jet combined with a slight change to the idle AIR jet.

No argument intended or necessary, but I stand by my advice. When asked for advice, I give MY ADVICE.  People are free to consider and make their own decision.

Important distinction: my advice is aimed at AMATEUR MECHANICS with generally limited experience to the task at hand. It’s based on a best-practices approach refined through providing kits and coaching on thousands of successful carb rebuilds.

I’ve learned: amateur mechanics always benefit from ruthless elimination of unnecessary variables and tasks.

In my own work, I implement all kinds of crazy carburetion “tweaks” that I would never discuss or publish for amateurs (or even for what passes as “pro mechanics” in today’s dealerships.)

Changing from #035 to #038 idle jets is inconsequential and would do not harm. My point is that it is also unnecessary. Ditto for any other jetting changes for that matter.

Further, people are not universally skilled at reading, comprehending, vetting and implementing advice gathered on the internet.  Someone might read “up a size” on jet size, then search the meager commercial choices available that day on the internet and learn that the choice jumps from #035 to “050.  So they install the #050 jets and then wonder why their bike runs so poorly and fouls plugs! 🙂

Ride safe!

12 thoughts on “Randakk’s Jetting Advice For Vintage Honda CV Carbs

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  1. Ive got a unique issue that im not 100% actually correlates to a jetting problem but figured id seek your advice. 83 gl1100 sat for 3 years, came with rebuild kit so i tore down and cleaned carbs and rebuilt them along with thorough cleaning of fuel tank. Adjusted and synced per your manual. I get a great idle and starts up with choke just fine and warms up quickly. My initial off idle response is great too but after about 1/4 throttle i get bad bogging/stuttering/coughing. It doesnt matter if i roll into it easy or fast, when i get to this point in the throttle it does it. Giving the bike 3/4 to full throttle it will take off and run out great. This is not an rpm based problem as it will happen at any rpm when i am in the throttle range. Double checked and my slides are cleaned and smooth, and i have found no other intake leaks anywhere. Didnt know if you may have run into this before. Any advice is appreciated!

  2. I have a 1979 GL1000. Has not run in 37 years. Cleaned and installed 4 carb kits. Gapped and lubed the points, installed new spark plugs, and a few other things. Bike runs like a scolded cat!! Runs and idles well. Will do a carb sync at some later date once I get the tool to do it. Right now.. My question is this.. Even so my bike runs well, I know it is running fat. In 3rd gear using high revs slowing down coming out of the mountain twisties I can get it to pop out my exhaust. I live in Colorado and I am at 6000 feet and its only up from here. I have been riding a lot and having a blast on this old bike. Getting only around 30mpg or so. Another indicator the bike is fat. Was wondering what the stock main jet size would be. Thinking I should go 4 o5 5 thousands smaller to lean it out some. Thoughts on this is appreciated. Not looking forward to pulling the carb bank out again as I have my throttle cables hooked back up.. What a pain that was!! Any way.. Suggestions are appreciated. Thanks

  3. I have an 84 gl1200 with open headers it seems to bog a little when its given gas then catches up and runs fine what should i do

  4. I have a 78 with rebuilt carbs. Runs and idle well. I want to improve the first 500 rpms from idle. The initial off idle is bogged a bit. Question is given the dynamics at off idle – pressure and air flow as the throttle opens would I be best to tackle this as is done with 77’s with a air jet change to the idle circuit (side of carb fix-easy) or mod the idle post jet. Would an air restriction or more gas supply be the better answer? Seems in a lean situation gas is the answer as air and flow are plentiful. Which approach is most valid…thxs kenai

    1. Lee: None of that is usually necessary with ’78 or ’79 models.


      1. opening the idle mixture screws 1/4 – 1/2 turns (out).

      2. Clean the CV slides.

      3. Check the ignition advancer to make sure it is “free.”

      4. Check/set point gap and static timing.

      5. Switch to higher quality gasoline.

  5. I have a 1979 cx500 that someone previously removed the old exhaust cross pipe and welded on straight pipe I am gonna add 8 inch slip in baffles but wondering on jetting and what to change main jet to. It is stock as of now but including running stock airbox and filter. Would like to get close for a starting point. I live in Iowa to get point of reference for altitude if makes much difference.

    1. With the stock airbag and an OEM air filter, the stock carb jetting should be fine regardless of exhaust setup. You may need to tweak the idle FUEL mixture screws to a slightly richer setting by turning them OUT a bit.

  6. I have a 1975 GL and have a problem I can’t seem to solve. One of the floats intermittently sticks, flooding the box and eventually on the engine. I’ve had these carbs in and out more times than I can count over a 3 year period, checking settings, etc.

    Any advice?

  7. So I recently bought a 77′ GL1000 with 63k miles (fresh timing belts recently) I bought it from someone who hasn’t really ridden it, and I think it was purely a project for him to fix up and sell for profit. I was concerned about that the bike has been sitting, but he says it always had gas stabilizer in it.
    Anyway, despite the gas stabilizer, the throttle response is not good, especially at low load when turning the throttle it kind of bogs down and hesitates. From 3000 rpms and up the bike appears to run very well. The hesitation at 2500 rpms I assume is the factory setting problem you have described elsewhere, and I will look into that (install #4 washer, etc).
    When the bike warms up – after 10-15 minutes, it idles at 2500 rpms! During warm-up it idles around 1000+ rpms. Still, even with the high idle, it hesitates when I take off from stand still. Like I said, above 3000rpms it runs very well.
    WHere should I start investigation for these problems ?

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