Here at EC, we are best known for our carburetors, especially Tillotson-Style Diaphragm carburetors. As an authority on carburetors, we receive many phone calls, emails, and messages asking for help tuning or troubleshooting them. Most issues start before the carburetor with the fuel tank.
Whenever there is a problem, we first ask, "What fuel tank are you running, and how is it vented?" There is often resistance to answering this question, and we usually have to follow up with a picture or video request.
This scenario happened this past weekend when Red Beard's Garage had an issue with the Super Ghost engine he built. The engine ran flawlessly on the dyno, making good horsepower for a mild engine with only a .265 lift camshaft. Red Beard gave the engine to TyrannoSilas_Rex(follow them on YouTube and Instagram) for their Coleman go-kart, but the engine never performed as it did on the dyno. It ran terribly.
Like many, the presumption was that it was a tuning problem and that the carburetor needed to be re-jetted. Since the engine seemed to be running rich, they changed from a 105 main to a 95 main, a considerable jump in size that helped but didn't resolve the issue.
Red Beard called us to ask if we could help with the issue. We discussed the tuning changes they made and what steps they could take to resolve what was thought to be a rich tuning problem. Until Red Beard sent us a video of the kart running. It was breaking up as if it was on the rev limiter, which indicates a lean condition(we're going to explain how a plug can read rich but actually be lean). That's when we asked about the fuel tank.
The fuel tank was a stock Predator tank mounted on the rack of the go-kart above the engine with no fuel pump inline. The biggest issue with fuel tanks is the vents. Red Beard switched the tank, and the engine ran as it should. We can't stress enough the importance of proper fuel tank venting for your performance small engine, which may help you tackle your tuning issue.
About Fuel Tank Vents
Fuel Tank Vents balance the pressure inside the fuel tank. Gasoline can become a vapor inside the fuel tank as it is agitated and temperature increases, increasing pressure. The pressurized fuel goes through the fuel pump or carburetors, leading to a rich condition. We don't have fuel pressure regulators on these engines. A vacuum or negative pressure is created as fuel is used, preventing fuel from flowing and creating a lean condition.
To help you better understand the importance of fuel tank vents, I've included examples that illustrate the mechanisms at work.
1. Red Plastic Fuel Jugs
If you've ever used your standard fuel jugs to fill the fuel tank of your lawnmower, car, boat, mini bike, or go-kart, you may notice that the fuel flow slows down as you pour, especially if you have the jug tilted to a high degree. You'll see the sides of the fuel jug suck in as it pours. That is because most fuel tanks are designed to be spillproof and lack a vent or, in some cases, have a cap over a vent by the handle. Fuel weighs about 6 lbs a gallon, and even with a 5-gallon fuel tank, which would be about 30 lbs of fuel, it's not enough weight or pressure to overcome the vacuum caused by poor ventilation. You must tilt the jug back down, allow air to fill the jug, and start again.
Refueling is crucial in motorsports, especially endurance or high-mileage racing that requires pit stops. Research Penseke's Fuel Tower or look at the fuel cans NASCAR uses. Even today's standard fuel jugs have new designs to help fuel flow by routing vents through the spout or with a check valve. Fuel Jug designers know the inconvenience of poor fuel flow.
2. Shot Gunning Canned Drinks
What would be the fastest method if you were to chug a drink? The answer to this trick question is the one from the cup or glass. When it comes to canned drinks, there is no vent other than the mouth of the drink, so as you tilt it upside down, it doesn't pour very well and must stop to pull in more air. If you shotgun it, you are now drinking through a puncture in the side of the can and pop the top for your vent, and you can drink it much faster. If you notice, the puncture hole is smaller than the vent. However, you can't drink it as fast as from a cup or glass. You may be unable to keep up with the drink flow because it is open and not restricted by a smaller vent. Vent size matters.
3. Shaking a Soda Can
We've given examples of vacuum, but we also need to explain increased pressure. What happens if you shake a soda can or bottle before opening it? It builds pressure in the can and usually spews all over when opened. They cannot vent because we're trying to keep the CO2 carbonating the drink in the can. If that can is agitated or is heated, Co2 wants to escape the liquid and expand, but it can't escape the can until a vent or an opening is made. The same thing can occur with your fuel in your fuel tank. Vaporized fuel occupies more volume than liquid fuel, and in a sealed container, this will increase the pressure inside the container.
Stock Fuel Tanks
Hopefully, you started to piece these thoughts together, making it easier to explain fuel tank venting issues. Stock fuel tanks vent through the cap or, more commonly, have a vent offset on the top of the tank with a hose recirculating it to the stock air box.
[This stock fuel tank vents through the fuel cap. The middle vents the tank through the outside diameter of the cap. These caps can be fairly restrictive and can clog over time.]
Red Beard used the more common vent, which is offset and separate from the fuel cap. We've pulled the vent out of the tank to show you how this vent works. Many may assume it's only a fitting, but it has a check valve built into it, so if the fuel tank is upside down, it will close itself off to prevent spills.
[The check valve needs to be replaced by a 1/4 fitting]
While you may see engines advertised for go-karts and mini bikes, especially since many of them have these engines, they are not designed for them. The primary design function of these engines is for stationary power equipment.
The check valve can float, and if fuel sloshes around due to rough terrain or vibrations, is tilted, or is overfilled, it will close off and restrict or cut off fuel flow. The vent is small, with only a .125" or 1/8" hole. If you don't think that is small, try blowing up a balloon through that size hole. To make it worse, the check valve can further restrict the vent due to the tapered spike that closes off the vent line.
[In some cases, you overfilled your tank, and it's closing off the vent.]
Other issues caused by stock fuel tanks are the location of the tank's fuel outlet and the carburetor's fuel inlet. Most aftermarket carburetors will not sit low enough for fuel to gravity feed from the tank to the carburetor. Either the fuel outlet is below the carburetor's inlet, or the fuel line can't make a sharp turn downward to the carburetor; in both cases, fuel is going uphill to the carburetor and will need a fuel pump inline to work.
Aftermarket Fuel Tanks
Aftermarket fuel tanks can run into the same issues, as in many cases, the fuel tank is vented through the fuel cap. Plastic fuel caps that have the vent molded into them may be very restrictive because they don't have a check valve to close if the tank is sideways or upside down, so they are very restrictive to prevent fuel from leaking all over the tank and rider.
[The 2-quart tank has a rubber gasket with two pin holes that vent to the plastic cap. The cap has a small .125" hole that has a plastic burr on the inside and outside of the vent hole, which further restricts ventilation. If you have this style tank, you must drill out this hole or add a 1/4 fitting.]
[The GPS Unitank has a similar cap with a rubber gasket with three pin holes but has an additional plastic washer that channels the vent through two channels to the outside of the washer. Then, it is vented through the top of the washer on one side, where the washer presses into the cap. Then, another single pinhole through the cap. We recommend removing the washer between the rubber gasket and the cap, then drilling out the cap and adding a 1/4 fitting.]
Other times, there is a hose on the fuel cap with a check ball vent. These check valves can be found on Amazon, and if you read some of the reviews, you will see how unreliable they are. Some give details of how it has affected the tune of their engine. What should be a one-way check valve is often stuck and doesn't vent at all or checks both ways, not allowing vapor out and not letting atmospheric pressure in.
[GPS also sells a similar billet cap; the cap should work fine without the cheap check valves on it. We do prefer to have a 1/4 fitting vs the 1/8"]
Motorcycle Tanks
In many cases, we're talking about cheap replacement tanks found on eBay and Amazon that were adapted to a mini bike. Generally, good tanks functioning properly are not an issue, but like many mass-produced, inexpensive things, not all work as they should. These are usually vented through the fuel cap, and it's hit or miss whether they work well enough or not to prevent pressurizing or vacuum.
[This tank has a cap similar to the vented caps used on some stock fuel tanks. They vent through the middle to the outside diameter underneath the cap. They are hit or miss for venting but can be clogged over time.]
[This tank has a locking fuel tank. Interestingly, it doesn't seem to have a vent. However, it doesn't seal against the cap's gasket and only sits on the tank. So it may not vent except if it tips over and vibrates, which means it will leak fuel.]
Kart Racing Fuel Tanks
The best vent you can have is without a check valve. The fuel tanks specific for kart racing don't have check valves and are usually vented separately from the fuel cap. Usually, there is only a fuel line coming off the tank to prevent debris from entering the fuel tank, and some sprint karts will have a catch that can be added to this line to catch any fuel that may exit the fuel tank. That catch can be vented to the atmosphere without a check valve or restriction. Our carburetors are blueprinted using these style tanks with intentions for racing applications.
[This is our fuel tank on one of our personal karts. It's a 1/4 fitting with a hose and no check valve. We don't have any issues with any of our carburetors or engines with this setup.]
What are the symptoms that may indicate I have a venting problem?
- Off-idle hesitation/bog(usually a rich condition): If the fuel tank isn't venting, it may increase the fuel pressure, which overrides the fuel pump and carburetor inlet needle.
- Other possible Off-idle hesitation/bog can be caused by big ports, oversized carburetors, and camshaft overlap when there is low vacuum/reversion or airspeed; the carburetor may hesitate to pick up fuel(lean condition). In some cases, the idle may be too low, and you need a vacuum or airspeed at a higher idle to overcome the drawbacks of large ports, big carburetor, and camshaft overlap. Idling the engine higher may be problematic for those with low-engaging clutches, especially CVT, where engagement can be as low as 1800rpms, which may cause the vehicle to move without throttle. The problem here( is either a fuel signal or poor ventilation.
- Rev-limiting is very common. The carburetor runs lean and starts breaking up before you reach the top end. The difference between a lean condition and a rich condition is that a rich condition will act like you hit the kill switch at wide open throttle and bog down. The lean condition will act like a rev limiter and stutter like you have a bad ignition or rev limiter or are floating the valves. The fuel is being restricted or cut off, which will cause the carburetor to run lean.
- Excess fuel is coming out of the bowl vents. When the fuel becomes pressurized, the carburetor's inlet valve has trouble closing off, so fuel overflows out of the bowl vent, especially when idling.
As you can put together, many of your "tuning" symptoms are not because of the carburetor but the fuel tank, so if you have had issues with carburetors in the past, it may not be because of the model or the tuning. In many cases, you can make a big change in jetting, but there is very little change in performance, often indicating a venting problem. Even more so, the symptoms are the same if you switch to different carburetors.
Why will my engine only run on choke?
Whether it's a stock carburetor or an aftermarket one, many times, it's due to poor ventilation, usually caused by the check valve in the vent. It could be as simple as overfilling the tank or creating a vacuum in the tank due to the restrictive vent.
There are two types of chokes: enrichment valves, like what comes on most of your PWK, PE, and VM slide carburetors, or a shutter, like many of your stock carburetors. The shutter restricts atmospheric pressure and airflow to create more vacuum(negative pressure) for fuel to inject into the air stream from the open circuits of the carburetor, usually the main jet circuit, which is why a main jet can help your stock engine start better. The enrichment valve is an additional circuit to allow more fuel to enter the engine. Both are generally doing the same thing, except the enrichment valve may not see the same amount of vacuum and may not work as well in some cases.
Regardless, the engine running on the choke either increases vacuum or negative pressure to overcome the pressure in the fuel tank or tries to get more fuel from the bowl to run properly. Usually, the engine doesn't start well, will not stay running, is slow to accelerate, or stalls. Fixing the venting issue is one step in resolving this problem.
Can my plug be dark or black if my carburetor is running lean?
Yes. Most do not fully understand how to read spark plugs. Most will pull the plug, read the color, and make changes according to what they think they see. However, they are seeing the results of the engine at idle and low speed, not the full range of the carburetor. Their conclusion is to lean the carburetor.
The carburetor runs rich at low speed and idle partly because of the fuel system's poor ventilation and changing pressure. At idle, the fuel may be pressurized at the tank and override the carburetor's inlet valve. There can become a tug of war in pressure and fuel flow. First, the tank has too much pressure, causing the rich condition, but at the throttle, it can create a vacuum, which causes hesitation or the engine stalls. So you have a rich idle mixed with a lean condition, making tuning extremely difficult.
In some cases, small jets can help by restricting the fuel flow through the carburetor when the fuel is pressurized so the carburetor runs better at low speed but then becomes leaner at higher speed or during longer runs and may not turn rpms, so larger main jets are used in an attempt to balance out the fuel curve. Tuning this way isn't a resolution as it makes it harder for the engine to start, hinders performance, and will behave erratically. You'll see examples of this online with the Knock-Off VM22 carburetors. They run small pilots and huge mains, and everyone runs something different. That said, you are at the mercy of the fuel tank. If the pressure or vacuum changes, the carburetor is in and out of tune.
Hopefully, this information will serve you well and help you fix any issues you may be having. We're working on adding more troubleshooting and tuning information to our site and blogs in the future.