Setting Carburetors for Peak Operation

My previous carburetor adventures with Cork Groth of the Tractor Trauma Center have led me to the point of mounting the newly rebuilt apparatus on the engine, hooking up the linkage and gas line, and giving it a try. 

Groth took me outside to his pet tractor, an unexciting Model B Deere that he lovingly calls Wimpy B. He employs the tractor to adjust newly rebuilt carburetors prior to sending them to customers. His unpainted and bedraggled-looking mechanical friend provides a ready platform to demonstrate a few points about fine-tuning a carburetor.

First get ’er started

He begins the process of adjusting the rebuilt carburetor by setting the throttle just one click up from idle. Next, he rolls the flywheel backward two turns and then, with a grin, looks me in the eye and says, “You don’t have to twist the flywheel off a well-tuned engine to get ’er started.”

Groth slowly rolls the flywheel forward until it reaches compression on the Deere’s first cylinder. The magneto clicks, followed by an immediate engine start. Now, that quick-and-easy start occurred on Groth’s test tractor. The first time you start an engine with a rebuilt carb, it will likely produce different results. Adjustments are in order, starting with turning the needle valves down to the bottom of their seats.

Initial valve settings

Next, turn the load valve (refer to your operator’s manual to identify the load needle) out 1½ turns and then turn the idle valve out two turns.

Turn on the gas to the carburetor and set it for a slight amount of choke. Position the throttle up from its idle stop. Turn the engine over a few times. It may surprise you and fire right away, but often the engine will take a little coaxing.

The engine may run rough after starting. Regardless, allow it to first warm up a bit.

If the engine is sputtering and backfiring, turn out the idle screw ¼ turn to increase the amount of fuel to the venturi. You may have to turn the idle out another ¼ turn to smooth out engine operation.

Back off when smoking

If you see black smoke coming from the exhaust, the fuel mixture is too rich. In this case, turn the idle valve back in ¼ turn.

If these techniques don’t clear up engine backfiring and erratic operation, the float level may need to be adjusted. To do this, shut off the fuel, remove the float bowl, and very gently bend the float upward while securely holding its brass arm. This adjustment allows a little higher fuel level in the float bowl, and the intention is that it will fix the too-lean condition that contributes to engine backfiring. Replace the bowl and start the engine.

Setting the load valve

Once you have smoothed out engine operation via idle valve and float bowl adjustment, bring the engine up to full throttle. Turn the load screw one way or the other until this adjustment smooths out the engine’s running speed. Turn the load screw out until you see black smoke coming out of the exhaust. Turn the load valve back until the smoke disappears.

Bring the engine to idle again and adjust the idle screw out until you see smoke in the exhaust. Turn the idle valve down until the smoke clears.

Again, bring the engine back up to full throttle and turn the load screw out until you can see smoke from the exhaust. Turn the valve in until the smoke clears up.

With this series of adjustments, your carburetor should be set for ideal operation. But, as Groth points out, it also takes good compression and a good ignition system to keep an engine operating at peak performance.

By Ron Van Zee

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