Friday, August 14, 2020

Adjusting to the modern world…. Re-jetting vintage Honda carburetors

 For those of us who were alive and involved with 1960-80s Honda motorcycles before the advent of “unleaded gas” and “10% alcohol gasoline,” we remember that Honda motorcycles were carefully jetted for maximum power, economy and reliability, right out of the crate.

                                            Honda CB72 carburetor with "power jet" function.

Reviving those 40-60 year-old motorcycles generally requires fuel system cleaning and a careful inspection of the carburetors. Before the introduction of CV (constant velocity) carburetors on the DOHC CB/CL450 twins, the vast majority of Honda’s carburetors were made by Keihin and were simple throttle-valve units, consisting of a float valve, float, main jet, idle jet, carburetor slide and tapered slide needle. Changing the fuel calibration of the basic designs to accommodate various versions of each application was generally required when the exhaust systems were changed from CB=Street to CL=Scrambler. With mostly identical engine specifications between two series of machines, the change from a low-mount to high-mount exhaust causes the carburetors to act differently and so Honda’s engineers carefully made appropriate changes to needle taper, main jet and sometimes the idle jets to bring a smooth fuel delivery to each application.

For the most part, Honda offered many sizes of main jets, idle jets and needles to re-calibrate the carburetors, as needed. In the early 1960s, Honda changed the basic shape of the float bowl from a rounded shape to one that was more squared-off on all sides. There was no need to change the calibrations as the body shapes changed. When owners began to alter either the intake or exhaust system for more “performance” the original calibrations are no longer valid. Changes to the intake or exhaust systems often require a change in the jetting of the carburetors due to an increase in airflow or alterations in the intake or exhaust tract length. Changing the tract lengths changes the resonant frequencies of the systems which can enhance or detract performance from the original settings.

For example, the difference between the 305 Honda Super Hawks and Scramblers lie in both the exhaust header pipe lengths, muffler lengths and the length of the intake to air cleaner runners. The engines have the same pistons, cam timing, valve sizes and carburetor bodies, but the standard main jet of a CB77 is #135 while a CL77 runs jets down around #130. Even the pilot jet sizes change from #38 to #42 between the two models. 

The Scrambler’s long, long, header pipes have a much different resonance frequency that bounces a pressure wave back to the exhaust port that can affect the incoming intake charge in the cylinder. The longer intake and exhaust tracts of the Scrambler enhances some of the mid-range torque, while the shorter CB intake and exhaust lengths enhance higher rpm performance on the road or race-track. Scramblers are geared shorter than CBs, so feel “more powerful” than Super Hawks, but the CB77s will hit nearly 100 mph vs about 85 mph max on a Scrambler. While 305 carburetors with 26mm slides have 2.0mm cutaways, the CL72s have a 3.0mm cutaway but the CB72, with the same-sized carburetor has a 2.0mm cutaway. You have to look over each item carefully, if you are tempted to start mixing and matching Keihin carburetor parts from similar models.

Be aware that prior to 1967 all of the Honda carburetor jets were JIS thread pitch and thus not interchangeable with later ones. Main jets in ISO dimensions have a ring marked around the outside edge of the jet, whereas the JIS threaded jets were left plain. This can cause great confusion when you have a model like the 1965-69 S90, CL90, SL90 models. which bridged the 1967 changeover gap. When people start trying to mix-match a box of loose parts from models like these, problems will arise.

Fast-forward to the 21st Century and most all markets have succumbed to at least 10% alcohol-infused gasoline which is fairly unstable in long-term storage and has less fuel energy that pure gasoline. In order to compensate for the loss of fuel energy (and a slightly different fuel viscosity), the rule has become that bumping the carburetor main jets up at least one size (5 to 10%) will equalize the air-fuel ratios that these old engines were used to seeing. After setting up stock CB77 Super Hawks to original stock settings of #42/135, I found the bikes to having flat spots and seemed to be struggling at highway speeds. Swapping the #135 main jet for a #140 pretty much cured the mid-range/top end power problems and even helped the normally cold-blooded Super Hawks to warm up quicker and have better overall throttle control.

Similarly, I have had to re-jet a number of CB400F Super Sport fours from #75 to #80 main jets, in order to overcome the same lean conditions. I made that change to a bike that I sold to a young man from NY. He flew in to San Diego and drove the newly rebuilt machine all the way back to NYC in 4 days, running it at 70-80 mph for most of the trip. When I first drove the bike, after a careful check over (and it had an electronic ignition), the bike struggled at freeway speeds, feeling flat and struggling. A simple step up in the main jets solved the problem and allowed a successful cross-country journey, which I think might have suffered a melt-down due to the original lean jetting coupled with alcohol gasoline.

Bear in mind that by the mid-1970s, the EPA was beginning to lean on motorcycle manufacturers for their part in the increases of carbon monoxide and hydrocarbon that the world was experiencing. Motorcycles were carefully calibrated for emissions during start-up, idling, off-idle performance and at full throttle, just enough to keep them from seizing their air-cooled pistons during extended riding conditions. I think we have all seen the idle mixture screw limiters that were required to keep owner’s from destabilizing the factory settings. Lean mixtures due to slide needle taper designs were another part of the equation and soon owners were putting little thin shims beneath the needle heads to raise them up slightly to enrichen the fuel ratio mixtures at part-throttle.

1980s Kawasaki motorcycles were famous for requiring 5-minute warm-ups, just to get the bikes down the driveway due to lean idle/part throttle calibrations required to keep the engines clean enough to pass the EPA regulations. Kawasaki was always pushing the envelope on performance machines, but the more power they make, generally, the more pollution is created in the process.

In order to enhance engine performance, companies like “Dyna-Jet” began to develop “Stage” kits to help owners get a handle on carburetor calibration issues with either bone-stock machines or for ones that had a 4:1 aftermarket exhaust or replacement air cleaner element or full system. These were sold under the guise of being a “racing” product that was not to be installed in production street bikes, especially in strict-California emissions zones. Dyno tests generally showed some improvements to horsepower, throttle response, torque readings and overall drive-ability with installation of the kits, but obviously the previous emission calibrations were out the window and any measured emissions were substantially higher than those that were create at the factories.

The days of carburetors are numbered due to the introduction of efficient fuel-injection systems and computer-controlled ignition advance curves. But in the meantime, we have to do our best to work with what we were dealt with from the factories dating back sixty years. So, to begin with, we have to clean, clean, clean the carburetor bodies and all associated components, then start to make educated guesses about what the engine will need in the way of corrected fuel-air ratios at all engine speeds and conditions.

Just assume that the use of alcohol-gas is going to make the engine run lean with stock OEM jetting settings. Once the fuel passages are all clean (ultra-sonic cleaners are great!), then a careful look at each internal component is next and then consider obtaining some slightly larger sized main jets to begin the calibration experimentation. Multiple test rides will give you a “seat of your pants” feel of what the bike is trying to do and what it wants in the way of fuel needs. When you feel the flat-spot or performance plateau/misfire, try to pull the choke up slightly to see if the problem gets better or worse. If it improves, you need to richen up the mixture. If it gets worse, lean it out again.

Reading spark plugs is somewhat of a black art, as well, but an initial look at the overall appearance of the plug tip will tell you if it is running lean (all white tip), rich (black and sooty), fuel/oil-fouling (wet end) or has a nice tan insulator without a lot of build-up of carbon or oil.

The bike should start up with sometimes full choke for just a moment until it is running, then reduce choke to around 1/2-3/4 while the engine warms up. Backfiring back through the carburetors/air filters is a sign of a too-lean fuel condition. If the bike fires up with no choke, it is generally running too rich at idle and off-throttle. The engine should idle down normally after about 4-5 minutes of operation time and the air-mixture screws should cause the engine to change idle speed and quality when they are turned in and out within a quarter-turn of normal. Always make the final idle mixture adjustments when the engine is at full operating temperature.

If the idle mixture screws are not responsive, then you have issues with jet sizes, blocked air/fuel passages inside the carburetor body, air leaks at the intake manifolds or warped carburetor bodies at the flanges. Ensure that the suggested float levels are use without fudging more than a millimeter or so. If you have fuel running out of the bowl’s overflow tube, you either have a float valve not sealing, a float level that is too high or a crack in the brass tubing of the bowl overflow tube.

While Honda bolted carburetors directly onto the cylinder heads in the early years, they began to recognize the high-frequency engine vibrations could cause fuel frothing and inaccurate metering at various engine speeds. The new-generation 350s and 450s used rubber-fused manifolds to secure the carburetors to the engines with both damping qualities of the rubber, as well as some heat insulation features which helps to stabilize metering in both hot and cold driving conditions.

As an example: The little 1960s Honda 160-175 twins had solid-mounted carburetors, bolted to aluminum intake runners, which were then insulated from the head with phonolic blocks and o-rings. But, when Honda revised the 175 twin engine up to 200cc in the early 1970s, they created rubber intake manifolds to improve fuel metering and vibration dampening of the carburetor bodies.

Some of the most confusing carburetor sets are featured on Honda CB/CL350 twins. There were several different carburetor body styles and 4-5 different calibrations suggested for the bikes, depending upon whether they were CB or CL versions (and the early SL350s, too). In the case of the 350s, there were different camshaft timing periods between CB and CLs, so that, alone, is enough to cause changes in calibration settings. These early CV carburetors featured both a primary and secondary main jet to help with the transitions from part throttle to full throttle conditions. Sometimes, all the changes needed were to bump up the primary main jet to the next size in order to smooth out the throttle response. One of the problems with 350 twins is that they vibrate quite a lot and the original mufflers for both CB and CLs tended to either break or rot out prematurely. Replacing the damaged components with OEM items (of which there were several types) was often costly and the parts were hard to find due to demand. Owners resorted to use of other slip-on muffler systems which had different lengths and back pressure features, all of which caused metering problems back at the carburetors.

Many of us “old-timers” know that the carburetors are the LAST thing to blame/adjust when engine performance is suffering. Setting up carburetors to operate correctly requires a sound foundation in several aspects of the engine’s systems. Go back to the beginning: Compression test, valve adjustment, camchain adjustment, ignition timing settings, spark advancer function, coil/condenser/point conditions, spark plug caps and even the correct spark plugs are all to be checked and confirmed before diving into the carburetors.

If you have a low-compression engine that is using oil and fouling spark plugs, the carburetors probably are not the root cause of your engine performance issues. If the point contact faces are all pitted due to age/miles or a bad condenser, you will have erratic ignition output at the coils. If the coil leads are crispy and cracked, the sparks might be jumping to somewhere else than the gaps of the spark plugs. Spark plug caps should have no more than 5k ohm resistance measurements and should be screwed onto the spark plug wire ends which have been trimmed back to bare wire.

Aged condensers cause points to arc randomly or excessively which adversely affects ignition coil output. Make sure that you are using a fully-charged, load-tested battery and that the charging system output is keeping the battery charged during operation. Engine vibrations can cause loose connections in the wiring harness and individual connectors to loosen or disconnect altogether. When Honda started putting ignition switches up in the center of the fork bridge, the pull on the switch connector often causes the switch connection to come apart, partially or completely. Bikes with KILL switches need to have the switch contacts checked and the wire connections secured so the coils get a steady feed of voltage.

All of these aspects of motorcycle function and maintenance can come into play causing performance issues that are first attributed to “the carburetors.” Obviously, if the carburetors have been sitting for months/years with old fuels in the bowls, then they need careful cleaning and scrutiny for normal function. Carburetor work is made more difficult with the alcohol in the fuel, which attacks most rubber parts, especially float bowl gaskets! If you drop the bowl off of a carburetor and the gasket is not the original glued-on version, chances are that the gasket will swell up and will NOT fit back into the carburetor body grooves. Cleaning with soap and water then drying in the sun or gently with a heat gun will generally allow them to reclaim their original shape and size. If you are doing a lot of jetting or carb work, plan on getting several sets of bowl gaskets to have on hand for quicker service procedures. 

To review:

Test drive evaluation…

First, start the engine and listen to it to hear if it is going to need a prolonged period of “warm-up” or if it will take enough throttle to drive off after a minute or two of running. Run the bike up gently with slowly increasing throttle openings. Listen and feel if the engine is taking throttle cleanly or if there is a hesitation or misfire occurring at certain throttle openings. While driving, if it is safe and easily accessible, pull up the choke lever or knob slightly during the time when the misfire or roughness is occurring. If you feel a surge of power or increased smoothness in the engine performance, then you will need to enrichen the fuel mixture at that point.

Off-idle hesitations can be resolved with either an increase in the size of the idle mixture jet and/or raising the needle slightly with shims. If the hesitation is more in the ¼ to ½ throttle settings, then you can raise the needle a little bit more and/or increase the main jet size slightly, one step at a time. Remember that if you increase the main jet size, you have enriched the whole fuel ratio from around ¼ throttle all the way up to WOT. If you get a good full-throttle response, but the mid-range is sagging and the plugs start looking dark and sooty, start to lower the needle back down one step at a time.

It’s a balancing act, but one well worth pursuing in order to maximize the engine’s performance over a wide rpm band and to conserve fuel that might be wasted in an engine that is running excessively rich. Pull a spark plug or two and look at the tip to see if they are getting a nice color on the ends which indicates proper jetting.

Excessive rich conditions can wash down the oil on the cylinder walls, leading to ring wear or even piston seizures in extreme cases. Excess fuel will contaminate the engine oil, as well, leading to lubrication breakdown. There is a whole chain of events that happen when you start to “adjust” the carburetor settings, so think it through first, before you begin wholesale changes. On many bikes, just removing the carburetors is a huge pain, so you don’t want to be in the position of having to do it more than once or twice, at the most.

Take time to understand each stage/function of a carburetor, so any changes you make are done with caution and thoughtfulness about what you are trying to achieve. Carburetor wizards are getting harder and harder to find these days, now that fuel injection is becoming more and more common. Take time to learn the craft for yourself and to help your friends who are in need of timely and educated repairs.

Bill “MrHonda” Silver



  1. Another awesome article Bill! Thanks.

  2. Great read, and I'm saving this to re-read, and read again. Thanks Bill!

    1. I'm with you 👌 What a great article! I am now wondering if I should have the changed the mufflers and air filters at all on my part way restored Cb72/ Cafe racer. I may have opened a big can of worms after reading this 😳 Thanks Bill 😊