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The Gentleman's Express: Tech-Tips from the JIOC

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Beasties and Things that Go Bump in the Carb

Author: Phil Deushane

Spotting likely sources of problems can begin while you are dealing with the basics. In performing the initial diagnosis you will probably have the air cleaner canister off to examine various other components. This is the time to keep an eye open for likely trouble spots.

While you are looking in on the fuel filter and running your fuel filter check, take a quick look at the overall condition of the fuel hoses and pipes. Look at the rubber hoses for unusual swelling or distortion. Hoses sometimes swell internally as well as externally, causing a restriction. Flex the hoses and look for areas that may be fractured in such a way as to remain sealed when the hose is undisturbed, but leak when the hose is stressed by engine movement or vibration.

Observe the metal fuel lines for crimping that might restrict the fuel flow. And watch for lines that may have become detached or misplaced and have fallen close enough to an exhaust component to create vapor lock problems. Your observation of the general condition of the fuel supply system will give a good indication of what you might expect to find inside the carburetor as well.

As you perform your linkage checks grab the throttle lever itself and give it a tug from side to side (not in and out) across its centerline. The amount of play of the shaft inside the carburetor body will again indicate the overall condition of the carb. Ideally, play will be almost non-existent, but don’t expect it very often. Some manufacturers allow quite a bit of play at this point so it is sometimes difficult to draw any conclusions. But in general the less play there is, the better the condition of the carb in terms of wear.

If at this juncture you are suspicious of throttle shaft wear, you can confirm the extent of the problem with a simple test. If the engine is still together enough to start, fire it up and let it idle. Apply a solution of soap and water, or other non-flammable liquid around the shaft at the carburetor body. If the shaft is adequately sealed, there will be little or no change in engine speed. If the result is a big jump in rpm, then you know that the carb is badly worn and in need of attention. Further, you can conclude that a bench rebuild may be unsuccessful because re-sealing the shaft is not usually covered in a rebuild kit. It may be time for a replacement.

While you’ve got the engine running, have a look down the throat of the carb and observe any uncontrolled fuel spillage. At idle there should be no fuel feed from above the throttle plate. If there is, further checking is in order. If fuel is discharging from the accelerator pump shooter, you should look for a stuck accelerator pump check valve. If fuel is discharging from the auxiliary venturi, that’s the bomb-sight-like guy that sits in the middle of the throat, then it’s a good bet that the throttle is adjusted too far open.

The condition can be further checked by adjusting the idle volume (mixture) screw. If the engine does not respond to adjustment, then ,a throttle position problem can be suspected. To check further, close the idle adjustment screw (gently) all the way. If the engine continues to run, you know you’ve got a problem, and that problem may not be with the carb to begin with. All too often a throttle gets cranked open in order to compensate for some other problem, ie: a burnt valve, and that prevents the engine from idling properly.

If the engine does respond to adjustment, or you can stall the engine by closing the volume screw, and yet fuel still pours forth from the A.V., you should have strong suspicions of a fuel pressure or float level problem. That, or you have a carb that defies the laws of gravity.

Have a good look around the carb with an eye toward pieces that may be loose or missing. I have seen Solex carbs with the auxiliary venturi dropped down inside the barrel. I also expect to find the secondary vacuum capsule on some carbs hanging-out somewhere down by the gearbox. Things like that are not at all uncommon. Investigate the nooks and crannies. That grease filled hole may once have been home to an idle jet holder or an adjustment screw. The electric idle solenoid mounted at a 45 degree angle probably wasn’t meant to be that way. Chances are it’s stripped, or loose and falling out.

If you proceed to check the float level, note whether the top of the carb fits properly down on the body. Is it tight? It’s pretty hard for the carb to control fuel flow if the top is rattling around like a BB in a box car. Look at the gasket. Is it imprinted with the outline of the mating surface all the way around? If not, you may have a problem with a warped top or body. If you are in doubt, use a straightedge to sight across the suspect piece. A Warped component will show daylight between the straightedge and the component’s surface. A warped component may again indicate the need for a replacement.

Going back to your analysis, consider the condition of the float itself. If it’s a brass or a hollow plastic float, it could possibly leak. Shake it and listen. Do you hear or feel any sloshing? If you are still in doubt, warm up a pot of water to just below boiling. Toss in the float and watch for any bubbles to issue forth. Bubbles signify a leaker. If the float is of the solid plastic variety, weight it against the factory spec given in the repair kit instructions or the service manual.

Check the automatic choke unit for solid attachment and take a look at the position of the flap or butterfly. A loose choke unit can wreak havoc with overall operation, and the butterfly position may tell you that the unit is sticking at an inopportune point. Either condition can be contributing to the owner’s complaints. Pull the throttle linkage through its range of operation and observe the accelerator pump discharge shooter. A strong stream of fuel will confirm the state of the pump. Lack of same may explain a throttle response problem, especially one that occurs at moderate to high speed.

Yes, the carburetor really isn’t such a simple device. There are many goblins that can lurk within it’s confines. However, a successful exorcism comes with a systematic trouble-shooting procedure.

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Contents copyright 2008, 2009 - Jody F. Kerr

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