Q&A for How to Adjust an Air Fuel Mixture Screw

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How do I check the air-fuel mixture if my vehicle doesn't have a carburetor?

Hovig Manouchekian
Auto Repair & Design Specialist
Hovig Manouchekian is an Auto Repair and Design Specialist and the Manager of Funk Brothers Auto, a family-owned business operated since 1925. With over 30 years of experience in the automotive industry, Hovig specializes in the process of auto repair and maintenance. He is also very knowledgeable in common automotive issues and needs including engine repair, battery replacement, and windshield accessory and maintenance. Hovig's knowledge and hard work have contributed to Funk Brothers Auto winning Angie's List Super Service Award for five consecutive years.

Hovig Manouchekian

Auto Repair & Design Specialist

Expert Answer

On a modern-day vehicle, air-fuel mixture adjustment is controlled by the computer of the vehicle. Use a scanner to get an air-fuel mixture reading, and you can also use a five-gas analyzer, which is used in emission testing.

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Question

What's the problem when I have to pull the outboard so many times before it starts?

Eddmoore99

Community Answer

Usually this indicates a lack of fuel. Priming an engine is necessary if it has been sitting in storage for any length of time. The fuel evaporates and this causes the engine to not start when compression is achieved.

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Question

Two-stroke engine fires once, then won't fire. how many turns out should the mixer screw be out?

Eddmoore99

Community Answer

On the majority of engines that are normally aspirated, a common rule of thumb is to tighten the adjusting screw all the way in and then back it out two full turns. This usually get you to the center of the adjustment range. From there it is much easier to make the fine tuning once the engine has started and been warmed to operating temperature.

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Question

There are no gold screws but it has two screws on the right of the carburetor. Right side (from the back) so I don't know which one is the idle speed screw and idle mixture screw

Eddmoore99

Community Answer

That depends on the type of carburetor you are working with. If it has only a single throat or as many as four, this makes a big difference. On single barrels, there is only one adjusting screw, but on multi-barreled units ,there is usually an adjuster for each set of two.The easiest way to find out is to screw the adjuster all the way in. Then back it out two complete turns. This usually puts the adjuster in the center of its range. From there it is fairly easy to get things going.

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Question

Which screw is it, there is one on bottom and 2 on top?

Eddmoore99

Community Answer

On a single barrel carburetor, the lower adjuster is what you are after. Anything above the base plate and butterfly are not for adjustments.

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Question

Should I want to set it on how the bike sounds smooth?

Eddmoore99

Community Answer

Yes, you want the engine to be smooth and stable at idle, so the mixture is important for proper operation. The timing is also affected by improper fuel mixture on many newer engines.

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Question

If the mixture screw is turned clockwise is the fuel lean or rich?

Eddmoore99

Community Answer

When the adjuster is turned clockwise, then it reduces the flow of fuel and makes the engine run lean. If you remember tightening the screw stops fuel flow, loosening the screw increases it.

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Question

Too much fuel with mixture screw all the way in. The engine runs, but spits excess gas onto the underside of the air cleaner cover. 16 horsepower Onan 2 cylinder horizontal shaft with Nikki carburetor.

Eddmoore99

Community Answer

This is a sign that the accelerator pump is hanging up. If you have no flow at mixture the only way the engine can feed is from a source that is not normal. The carburetor has feed jets to increase fuel when you advance the throttle as well as butterfly valves to increase the amount of air. If the jets are leaking fuel the accelerator pump is open and the vacuum created by the engine is sucking fuel through them.

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Question

Is it ok that the fuel mixture screw has no tensionsion spring?

Eddmoore99

Community Answer

The spring is important because it gives the adjuster tension to stay in place. Without it the screw can vibrate loose and cause a bad mixture, or worse yet fall out!

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