It’s simple, deliver air and fuel in the right quantity, compress it, then ignite it with a spark at the right time in a well maintained set of cylinders and you’ll have a perfect orchestra of intake-combustion noises playing harmoniously whilst you prepare to ride off into the sunset.
Whilst it may be simple, it doesn’t mean that it’s always easy. You often need to take a step back to the basics to figure out what needs to be done.
Let’s start by understanding what actually happens when you press the starter button and your motor begins to turn over (four stroke cycle);
- The first step is the intake stroke; this opens up the intake valve(s) to draw in air and fuel. The exhaust valve(s) are closed.
- The second step is the compression stroke, where all valves are closed and the air-fuel mixture becomes compressed within the cylinder.
- The power stroke starts when the piston reaches top dead centre. The spark plug fires, and ignites the compressed air-fuel mixture to drive the piston back down.
- The cycle ends with the exhaust stroke and the gasses are expelled out of the exhaust valve.
- The cycle continues over and over again until the engine is shut down.
For a visual representation of the four stroke cycle refer to the diagram below.
It’s clear to see that for any engine to run efficiently all of the components need to be set correctly. This means setting your valve tappet/shim clearances to the factory specification to ensure all valves open and close at the correct time. It also means that they remain closed when they’re supposed to, thereby providing the engine with the necessary compression capacity. The spark plugs and electrical leads need to be in good condition to provide the energy required to ignite the volatile mixture, and so on. If you’re buying a new motorcycle, read our guide on the Where To Start When It Just Isn’t Starting for more details on the fundamental checks required to ensure you get the best chance possible with your new project.
Setting timing, adjusting the valve clearances, and swapping around spark plugs is a typically straightforward task and requires you to read the owners manuals to follow precise instructions. However, tuning the carburetors on a vintage bike can leave a lot of people scratching their heads.
We hope that by reading this guide and starting to understand the principles behind the carburetor mechanism that you’re able to tune your build correctly and relatively hassle free!
The Perfect Air-Fuel Mixture
In a fuel injected engine all the hard work is done by electrics that measure air velocity, engine speed, crank-position and so on to deliver the correct quantity of air-fuel mixture. To get more out of the air and fuel delivery you will need aftermarket upgrades or a remapping of the ECU. We won’t get into the specifics of this, other than to say that whilst the air-fuel mixture isn’t set by you, you should do the fundamental checks to ensure that the correct amount of air is being delivered in each cylinder. This means correctly balancing throttle bodies and ensuring there are no vacuum leaks in the intake set-up that could bypass any sensors. If you’re suspecting fuel delivery is an issue, it’s worth removing the injectors and checking the spray (caution – do this away from the bike and without a chance of igniting the fuel).
If you’re working with an older build then it’s likely you have a carburetor (or a series of them). We could go on to explain how a carburetor works on Bernoulli’s principle of static v dynamic air pressure, but many others have already written about this in great detail. If you’re more of a visual person, refer to the highly entertaining video down below. It does a fantastic job of explaining how a carburetor works by showing slow-motion footage of the internals of a transparent one.
Having a general understanding on how carburetors work will give you a better chance at tuning them, whether that be the original Keihin carbs fitted to your bike or aftermarket Mikuni flat slides, because they all work with the same principle in-mind.
Here are the top 10 tips on tuning
STEP 1: Check the bike (valves, timing, compression, electrics etc.). Before you move on to setting your air-fuel mixture and tinkering with your carburetors you need to ensure that you’ve checked and adjusted all the necessary components that will ensure your engine is operating as it should. Don’t skip this step. It’s worth going through it each and every single time you do any major work on your bike. If you skip it, you leave yourself open to becoming incredibly frustrated and second guessing your work.
STEP 2: Understand how your carbs work. Whilst all carbs work under the same principle, how they deliver the air and fuel mixture can be slightly different. So be sure to read up on exactly how your particular bike’s carbs work and what some of the common problems are. There is likely to be a wealth of knowledge on forums and online groups about the nuances of each setup. It’s also handy to get a cross-section view (similar to that below).
STEP 3: Check for vacuum leaks at the intake boots (especially if you’re reusing the older boots on your vintage bike). We do this by spraying starter liquid or brake cleaner around the intake boot. If there is a vacuum leak then the engine will inadvertently fluctuate in rpm as more fuel is sucked in via the vacuum leak. Rough running and hunting (rpms fluctuating or not settling) can sometimes indicate vacuum leaks.
STEP 4: Be sure your carbs are clean. This is mostly relevant for older carbs that have been sitting around for a while and are gunked up by old fuel, dirt or grime etc. It can also be relevant when installing new carbs because of the test fluid that is run through them by the factory. Film, dirt or grime can and will block jets or accelerator nozzles. Be sure that everything is clean, the floats aren’t stuck, any spray nozzles spray consistently and so on. A tell-tale sign that a jet may be blocked is your exhaust is cold on one cylinder despite a consistent set-up between all cylinders. You can double check this by pulling the choke and seeing if this heats up all cylinders. If it does, remove the bowls and inspect for dirt and residue. If it’s unclear, remove carb (or carbs if they’re in series) and start to carefully disassemble and check each of the jets.
STEP 5: Ensure your throttle cables are set correctly. This is particularly important on push-pull setups where the return cable could partially leave open the butterfly or slides. This will result in an incorrect idle setting, and engine hunting. Be sure the accelerator cable has some play in it, about 1-2mm. However, the return cable needs to be tight so the valves/slides are fully withdrawn when you get off the accelerator. If you’ve just moved to a different setup then be sure that you have the necessary adjustment in the cable to set it correctly. If you don’t, you may need shorten your cables accordingly.
STEP 6: Be sure your carbs are synched. Depending on your make of carbs you can do this by using vacuum gauges or by bench syncing. Vacuum synching can be done with a set of vacuum gauges or a carbtune tool that allows you to adjust all carbs according to a reference point and equalise all based on the one. Mikuni flat slides allow you to bench sync them by making sure the slides lift at the same time, are in the same position when closed and when fully open. This is because the height of the slide within the bore can be measured unlike those with a butterfly valve. This ensures that each cylinder receives the same volume of air when the throttle cable is resting, and when it’s pulled.
STEP 7: Set your fuel delivery based on your filters and fuel. How well your carbs work will depend on the air intake system. For example, a common café racer modification with Honda CB750s is removing the rear air intake box. This box does three things; 1) filters the air via an internal filter, and b) moderates the air volume before the carbs c) allows some crankcase re-circulation. If you remove it and slap on a set of pod filters and expect it to run well, you’ve missed the point. The pod filters will allow significantly more air into the system, and beyond the amount of fuel the jets can deliver. Re-jetting is often needed. It’s also necessary that the engine is tuned with the fuel that you will be using. So if you tune it with 91 Octane, then don’t fill up with 95 Octane later down the track.
STEP 8: Set your idle. Before moving on any further, the idle needs to be set to the manufacturer’s specification. Start with the idle adjustment screw almost completely out. Progressively tighten the idle screw clockwise until the bike no longer wants to stall (as this brings the rpm up) and you have a smooth running engine at the desired RPM.
STEP 9: Check your air-fuel mixture is correct by inspecting your spark plugs after some extensive running. If the mixture is too rich, you’ll be cutting yourself short on performance. If the mixture is too lean the engine could be running hotter than it’s designed for, which will eventually lead to burnt valves. When inspecting your spark plugs you’re looking for a caramel brown colour on the insulator. White indicates lean, and black (sooty) indicates rich.
STEP 10: Set your bottom end, mid-range, and your top end. You can do this by riding the bike and observing whether there is any sputtering or popping at different speeds and rpms. Most carbs have different set ups; however traditional set-ups include different jets for different rpm ranges. “Sputtering” typically indicates too much fuel, and popping noises indicates a lean mixture. Adjust the mixture screws as necessary to dial things in and be sure to keep an eye out on the spark plugs. If you can’t get the right response despite adjusting the mixture screws, you may need to go through the process with different sized jets.
If you’re dealing with Mikuni carbs (we highly recommend them), then you will only need to do this for the single primary jet as the design of these is greatly simplified. But be sure to read your Mikuni guide when setting them up.
Bonus Step: Be patient. A well-tuned engine will reward you in the long run and be a joy to ride.
Where to from here?
If you’re looking for other guides and technical articles be sure to visit our blog section on our website. If you’re looking for more tuning advice or in need of fresh Mikuni carbs for your ride then get in touch with us and see how we can help by emailing us by using one of our contact pages on the site.
For parts related to all things custom motorcycles be sure to check out our online store.
Until next time!