Carburetor Adjustment & Troubleshooting Series- Part 1: dialing in the idle circuit

May. 01, 2005 By George Szappanos
Carburetor adjustment is another one of those great mysteries of engine tuning that some perceive as being a black art. And true enough, it is a bit of an art if you lack the sophisticated equipment to do it scientifically such as CO analyzers and dynamometers. And as an art form, it takes practice to get good at it.

Carburetor adjustment can be done by the garage tinkerer quite successfully, and has been for years. Unfortunately, it does take some trial and error and experience developed over time to do it well. The topic of carb tuning has been covered by just about every motorsport website, and has had countless books written on it. The intention with this installment in our carburetor series is to tackle the subject in the context of the asian mini-quad. Specifically, what are the pitfalls and troubleshooting techniques that are unique to these machines.

The most fundamental idea regarding carb adjustment is that there is an ideal air to fuel mixture for every engine operating condition. This is not only defined by the engine’s own unique requirements, but also by the air density (temperature, pressure /altitude, humidity). What this means is that not only must a tuner determine what the engine wants at a certain environmental condition, but that adjustments may need to be made to accommodate a colder day, a more humid climate, or a higher altitude.

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The idle mixture screw on the Asian mini-quad is located near the air-cleaner side of the carburetor (slotted screw on upper right hand of the photo).
Before getting into the environmental adjustments, the first step is to get the adjustments and jetting correct for that engine configuration. Anything that would affect the engine’s breathing ability will affect carb adjustment. Obviously, installing an aftermarket carb will require tuning, but so will changing to a different air filter.

Cylinder porting and a pipe will not directly require a tuning adjustment since they only affect the engine’s ability to create airflow. The carb really doesn’t know what’s downstream. However, it might care what’s downstream – for example, if a pretty hot cylinder is installed with lots of compression and/or lowend porting, then the carb might want to be a little richer, or fatter, to decrease peak combustion temperatures. Ideally, tuning should be done at the “near normal” conditions you expect to run so as to minimize the need for fine tuning when conditions change.

Whether stock or aftermarket, the first step in carb adjustment is to dial in the idle circuit. Recall that a relatively high vacuum exists downstream of the throttle slide when it’s closed, or at idle. A passage in this area is connected to the float bowl that allows fuel to be sucked into the engine while the throttle is closed. As the throttle is opened, this vacuum drops and the amount of fuel drawn into the engine through this passage decreases accordingly.

We can adjust this amount of fuel by turning the idle mixture screw. On these carburetors the idle mixture screw actually controls the amount of ‘bleed’ air that’s T’ed into this passage. The more the screw is turned out, the more the bleed passage is opened, and therefore more air instead of fuel that’s being sucked into the engine via the primary or idle circuit.

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The idle circuit air supply passage can become blocked with dirt and cause a rich running condition. This is the hole at about 7 o'clock on the carb inlet.
The question is just how do we know when the adjustment is correct. The simple answer is when you’ve obtained the highest idle rpm.

The highest rpm is achieved when the air-fuel ratio (AFR) is optimized. Remember that while this adjustment is being made the throttle is essentially fixed, that is, held at idle. We are not changing the amount of air being drawn into the engine. If the engine rpms increase, that would imply that the engine is doing a more effective job converting the fuel and the given amount of air to useful mechanical power. Too little fuel, and the revs are too low. Too much fuel, and the revs are also too low. Somewhere in between, and combustion efficiency and the revs are maximized.

With the engine off, turn the idle mixture screw in (CW) until the screw seats and count the number of turns. Note this number as a “fall-back” if you decide that your adjustments are not working and you want to get back to the baseline, or ground zero point. Next, turn the screw back to the baseline adjustment and start the engine and warm it up thoroughly, maybe even ride the quad a bit before starting the adjustment.

With the engine running at idle turn the screw in (CW). This has the effect of closing the ‘air bleed’ passage to the idle circuit, thereby allowing more fuel to be drawn up from the float bowl, richening the fuel mixture. Make 1/4 turn increments until you notice a drop in engine speed. Then turn the screw out (CCW) in 1/4 turn increments. You should notice the engine revs going back up, then dropping again. Take note of how many turns it took to reach this point. Divide this number in half to determine where the theoretical optimized point is. Turn the screw in to reach this point. You may need to experiment with this procedure a few times to satisfy yourself that you’ve found the sweet spot.

If by turning the screw you were never able to find the point where the revs peaked, i.e, the revs keep climbing/dropping until you bottomed the screw, or the screw came out, then you may need a different size pilot jet. As noted above, the idle circuit is fed by a passage that connects a port in the carb to the float bowl.

The amount of fuel drawn by this circuit is not only controlled by the idle mixture screw, but also by the size of the orifice in the pilot jet at the float bowl. If you’ve turned the screw in completely (air bleed totally closed) and the revs seemed to keep increasing up to that point, it would indicate that the engine is wanting an even richer mixture. Install a bigger pilot jet. If you’ve backed the screw completely out and the revs still haven’t peaked, it would indicate a need for even more bleed air, or less fuel. Install a smaller pilot jet.

If adjustment of the idle mixture screw doesn’t seem to have any affect, then one of two things is going on. The first most likely possibility is the air bleed passage may be blocked. This can occur if the air-filter has passed too much dirt which has gotten into the passage at the mouth of the carb. If this happens the engine will be running very rich, but can still run. The second possibility is that the pilot jet itself is blocked from dirt getting into the float bowl. If this happens, very little or no fuel will enter the carb.

It’s quite likely that engine will not even run since the primary ingredient in the recipe for combustion is missing – fuel. Engine’s generally have a higher tolerance for too rich than for too lean. If it seems you need quite a bit of throttle to keep the engine idling (no longer using the idle circuit for fuel supply) then this could be your problem.

Next step -->>> high speed, or main jet adjustment. Newsletter