This is a complete copy a Warehouse Hobbies bulletin about installing and adjusting carburetors on Homelite engines. I am quite convinced that Mr. Tony Castronovo knows what he is talking about. For that reason, except for unintentional typographical errors, I didn't alter a single coma on his writing. Although focused on Walbro carbs and Homelite engines, the bulletin should be read by anyone who intends to learn how to adjust a Carb. As the bulletin is available trough the Internet, I believe that Mr. Castronovo won't mind seeing it in this page, complete credit for this document is given to Mr. Castronovo.

This seems to be the most frequently asked question on our tech line. Though it seems complicated it is really quite easy once you know how the carburetor functions on a gasoline type engine. First, unlike carburetors on nitro engines which require a constant fuel pressure to operate properly, the gasoline carburetor has a built in fuel pump which operates once the engine is started. This pump is a diaphragm type and normally requires no attention and lasts years if operated with gasoline that has no additives, pump or post added. This means if you use pump gas not exceeding 93 octane your carburetor will operate trouble free.

The most frequent problem I hear is, "the engine idles fine but as soon as I increase the throttle it stalls". This applies to customers who install our aftermarket or "Big Bore Carburetor". As I stated above the carburetor has a fuel pump, it is powered by a negative and positive pulse from the crankcase. If you look closely at the intake manifold [whether stock or our modified version] you will see a tiny hole either above or bellow the main air/fuel intake hole. Now look at the back side of the carburetor, you will notice the same, a small hole, sometimes depending on the manufacture, one above and one bellow the main fuel/air intake hole. Remember these carbs were originally designed to be easily modified for many different engine manufactures. In all cases only one of the small holes is ported into the pump chamber. If you look close you can see the opening. If you are using an Enforcer CA5003 Big Bore Carb, it has been designed to be installed with "Walbro" facing up. With this in mind; if the carb is installed correctly but the intake gasket is upside down, where the hole is covered by the gasket, the pump will not function, so make sure that you correctly install the gasket as well as the carburetor to avoid this problem. When hooking up the fuel lines on our aftermarket carburetor it is important to properly attach the right one to the feed. On the Enforcer fuel tank, the line closest to the cap is the feed, this line attaches to the fitting on the top of the carburetor (silver in color). There is no brass colored fitting, a lot of people mistake the throttle assembly arm for a fitting. However, you can attach the other line to that shaft with a small tie strap and use it for a plug, it is not necessary for a vent as the gas cap has a vent built into it. If you are racing your boat and wish to have more air intake in the tank, you can use a brass thru-hull fitting and attach the vent tube to it. Keep in mind that if your boat flips over with an exposed vent, water can seep into the tank.

Carbs of this type may look different but most of them meter fuel in the same manner. For instruction purposes we will number each adjustment starting with the top screw #1.

Screw #1 sets the carburetor butterfly or the plate located inside the carb that allows air to enter, this is the screw that allows you to adjust the engines idle. Start by turning the screw until it moves the sliver plate open about 1/8". This is actually a little too much but it will make starting and adjusting much easier.
Screw #2 sets the low end fuel mixture, this needle valve controls the internal fuel flow from zero to mid range. Open this screw 1 & 1/2 turns, be sure when you close a needle valve that you just turn it in until it just seats, it can be damaged as well as the seat area if it is tightened too much.
Screw #3 is the high speed needle, this needle valves makes the transition from mid range to top end. Start by opening #3 1 and 1 1/4 turn open. The settings given are on the rich side, this is a good point to start, it is easier to lean an engine to it's proper fuel mixture than richen. Before I go on, here is a little theory on combustion that will help you better understand a properly adjusted engine. We all know that in our case the gasoline is the catalyst and the oxygen sustains the burn. But what most don't know is that there is only between 14 and 23% oxygen in our atmosphere, this changes as the humidity increases or decreases, but at best it isn't much. Gasoline on the other hand will burn and or explode. When put under pressure gasoline will vaporize resulting in a much better burn or controlled explosion. Too much pressure can cause what is known as pre-detonation, (I will go into depth on this subject in the future), our engines (25-30cc) really can't produce more than 160 pounds per square inch in the cylinder, and detonation occurs at a higher compression rate. It is this reason why race fuels such as those used for auto racing or aircraft should not be used, this fuel is designed for high compression engines to detonate at higher pressure. Using it in our engines usually causes higher operating temperatures and an insignificant performance gain, not to mention its' potential to damage the carburetor diaphragm and internal rubber components. Since we know about the air or oxygen and the fuel, we now need to learn on how to mix it properly, this will be done by both ear as well as operating your boat.
Note: It is a good idea to have an external water source if you will be setting your carburetor on land. Water-cooled engines should not be allowed to run more than 30 seconds without water, damage can occur.
1)- If your carburetor is equipped with a choke close it at this time
2)- If your carburetor is equipped with a squeeze bulb (clear plastic bubble) push and release until bulb fills.
Note: The Enforcer CA5003 Big Bore Carb has been designed for high performance use, therefore it is not equipped with either priming device. To prime the Big Bore Carb, simply cover the Velocity stack (if applies) with you finger or hand while pulling the rope 5 or 6 times, or until you hear the engine bark or fire.
3)- Open the throttle and continue to pull the rope until the engine starts, immediately reduce the throttle and allow the engine to idle at a medium rate.
4)- Set needle #2 by turning slowly clockwise while listening to the engine climb. When the engine starts to make a "wooing" sound it is an indication that the low end is being leaned or turned in too far, back the needle at that point about 1/8 turn. This adjustment is very important and can affect the high end adjustment, so it is important to take your time and learn what sound you are listening for. Remember rich sounds are raspy and lean sounds tend to make a "whooing" sound. The objective is to set screw # 1 to where the engine idles between 2400 and 3200 rpm's depending on the amount of engine modifications to your engine. Stock engines will idle at 2400 whereas our Pro-Mod idles properly at about 3200 rpm's.
5)- Needle # 3 must be set with the boat under load (running in the water). Any engine tends to lean out when put under a loaded condition, therefore it is very difficult to properly set the high speed needle on land. The setting stated above, should allow the boat to run on the rich side and is good for engine break in. You will have to look, learn and listen to what the engine is saying. When you increase the throttle quickly to full and the engine stalls it is telling you that it is not getting enough fuel for the required demand at top end. At this point you must back needle # 3 to enrich the mixture. If the boat runs great for a couple of laps and then slows down, it is close to the right setting but still a little lean, do not allow the boat to run in this state it will cause engine damage, again slightly back the needle out until the engine has a nice crisp sound.

There are many ways that the engine can tell you what how it is running, another good way is "reading the spark plug". Yep, just like a book, after making a few laps, remove the plug and look at the electrode. If the engine is rich or the oil you are using is not burning clean it will look black, wet or both. Hopefully you are using Amsoil as we are very familiar with its characteristics which allows a plug to be read with ease. If the plug electrode is white or gray, (this is bad!) too lean. But if the plug looks light to medium brown, by George I think you got it! Just do what the engine is telling you by means of the carburetor screws and you will enjoy years of trouble free gasoline model boating, Enforcer style...