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Zama Carb tuning questions


Old 07-19-2003, 07:18 PM
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Default Zama Carb tuning questions

I just finished my boat w/ a 31cc mtd weedeater motor and the stock zama carb. I can't get it to run reliably.

I get it running right w/ the two mixture screws one day and the next it stalls or floods, not sure what's going on.

Question: which is the low speed and which is the high speed mixture screws, I think the low speed is closest to the carb mouth, and the high speed is closest to the crank case, not sure.

Are there problems, like vacuum leaks w/ these carbs that would cause them to run unpredictably?

Are there alternatives?

thanks for any help.
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Old 08-23-2003, 06:48 AM
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Default Zama Carb tuning questions

How To Adjust Your Carburetor?
A Bit of Theory
You may have spent a few hours trying to adjust or re-adjust your boats carburetor, and in most cases you might managed to get your boat running but really didn’t know how you did it… Or in some cases, you just can’t seem to get it right, either your boat idles fine, or runs well wide open, but just won’t do both.
Believe it or not, the carburetor on your boat is a highly technical well engineered machine. The carburetors that are used on Enforcer boat engines are very similar in design to those used on full size outboard marine engines, snow mobiles, and personnel watercraft.

Fuel Pump
Carburetors utilize a vacuum actuated fuel pump that is built in carburetors and is powered by the negative and positive pulse that is created when the engine is in operation. When the piston is on the upstroke cycle, the crank case pressure becomes negative, vacuum is created. On the down stroke a positive pressure is created. Each engine block has a passageway that allows this changing pulse to enter the intake manifold and carburetor through a specific hole, usually located on the backside of the carburetor, in turn this operates the pump in the carb. The fuel pump gives your engine more stability by providing a constant supply of fuel.

Note: If your are using a machined intake manifold with your "Big Bore" Carburetor, the small hole also must be facing upward as well as the Walbro logo on top of the carburetor. This gasket between the carburetor and the intake manifold must be installed with the tiny hole facing upwards. This small hole allows crankcase pressures to flow to the carburetors fuel pump.

Carburetor Adjustments
An engine will perform poorly if the fuel mixture adjustments are off allowing too much or too little fuel to be metered, the engine may still start, but may not idle well or reach the correct level of performance.
All of today’s carburetors have 3 screw adjustments. There is the high speed needle, the low speed needle, and the butterfly adjustment, or also refered to as the “ main idle adjustment screw”. First you must know the location of these screws. I suggest that you refer to your owners manual to familiarize yourself with the carburetor that you have. Shown here are some popular Enforcer carbs.
Normally the low speed needle is that closest to the engine. The high speed needle is commonly found to the right of the low speed needle, or referred to as the closest to the hull. The main idle adjust screw is on the top side of the carburetor and generally much larger in size.

In most cases manufacturers adjust their engines carbs to run on the rich side. This means allowing more fuel to enter the engine resulting in less performance, but also allowing the engine to be initially operated at less than 100 percent for the required break in time. Once the first few tanks of gas have been used, you are ready to get more performance out of your engine. Follow the next few steps and you will be a pro at carburetor adjusting in no time.

1– Close both low and high speed needle valves. Do not close tightly, just until you feel them seat.
2– Open the low speed needle 1 1/2 turns and the high speed needle the same.
3– Adjust the butterfly or main idle screw so when the screw comes in contact with the throttle assembly it opens it about 1/8 “. This will cause the engine to idle high, but necessary for proper adjustment.
4– Supply water to the engine if it is water cooled. It is best to allow the boat to draw from a remote source such as a bucket if your boat has a water pump. A simple piece of tubing can be plugged into the water inlet.
5– With your boat secure on the stand, carefully start the engine. Be extremely careful of the spinning propeller. Always stay to the side of the boat and away from the spinning prop. Never have loose clothing or tools present in the prop area.
6– Starting with the low speed needle turn it in “clockwise” and listen as the rpm’s go up. There will be a point that a “woowing” sound will be present, just before the engine stalls out. Now back the needle back to the 1 1/2 turn out starting point and do it again until you can distinctly hear that sound. The next time when you hear that “wooing” sound, turn the screw out about 1/4 turn from that point. The engine will now start to operate at high idle, but more smoothly.

What we just did is cause the engine to go to a lean idle state, (too little fuel) at the low end and quit. The “wooing” sound I am referring to is to give you a present indication of what a lean state in a two cycle engine will sound like. The sound is caused by too little fuel being drawn into the combustion chamber resulting in a situation where the engine can not sustain operation. The fuel is being burnt off at a rate greater than the combustion timing is calling for, and the engine starts to pulsate. By realizing this sound, you will now be able to identify when your engine is too lean at low end.

7– Once you have adjusted your low end needle you can now simple adjust the main idle screw to the point that the clutch disengages or is recommended by the engine manufacturer. On high performance engines with clutches I suggest that the idle be set with prop in the water, this will allow the RPM’s to be set at a higher level, which is normally necessary for engines that have been modified. If set too low a race prepared engine will tend to “load up” and stall.

8– The high speed needle is a bit trickier… The high speed needle MUST be set under load. When an engine is put in a loaded situation they tend to lean out, therefore it is more of a guess than a science if you were to try to adjust on land. The best way to properly adjust the high speed needle is to launch the boat, get it up to temperature, a couple of laps at high speed will do, and then return to shore making small 1/8 turn increments (clockwise) starting from 1 1/2 turns out, followed by another lap or two. Do this until your boat has reached its top speed, or until you lean out… Lean out? No problem just open it a hair to where it was just before the last adjustment. All kidding aside, a lean indication on top end is indicated by a couple of things. First it is best to throttle up quickly, if the engine _ _ it’s and gets but the boat slows down and falls off pace after a lap or two you are boarder line lean. Just back it off a smidg. If it bog’s or dies when you open throttle from slow to full you are way lean, again back the needle out and give it another go. Running the engine too lean at the top end can and will cause engine damage or severely shorten an engines life span if allowed to run in this state
A rich top end is opposite, the boat will have a raspy sound at top end and not go as fast as it may be designed to go. This is not a bad thing, you may fowl a plug or two each season, but generally no real damage will come from this. Some boaters actually prefer to operate their models on the rich side.
The best way to check and make sure your engine is operating correctly at top end is to “read the plug”, this is pretty easy to do. Remove the spark plug and clean the electrode with a wire brush, get it as shinny as possible. Make sure there is no wire pieces stuck in the plug when you reinstall it. Run your boat for a half a dozen laps or so at top speed. Return to shore and remove the plug when it cools down. Look at the electrode, is should be a medium brown in color. This means that all is well with both carburetor settings and choice of pre-mix oil. If the electrode is white or grayish in color, you are running too lean. Richen the high speed needle and perform the plug check once again. Do this until the plug reads the safe color.
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