Saturday, October 3, 2009

Beater How-To: Carburetor Rebuild Tips

Like oleopneumatic suspension and DIRAVI, carburetors also operate on the principle of luck and magic. Given the complex nature of carburetors and the great variance between different models, it may not be that beneficial to do a step-by-step rebuild procedure, so here are some general carburetor rebuilding tips that will work for a refresh of any model.

1. Make sure you have the right rebuild kit

Comparing the base or float bowl gasket is possibly the easiest way to identify if your kit is even remotely correct before tearing apart the carb or the often non-returnable kit packaging.

2. Get a bucket of carb soak

Carburetor cleaner spray is great for light duty cleaning but for a total rebuild, only a bucket of carb soak will do. This stuff is available at most auto parts stores. Although quite pricey at roughly $25 per 1/2 gallon, it will last for many, many rebuilds.

3. Keep track of adjustments

Before tearing into the carb, write down the initial adjustment settings. Check this by rotating adjustment screws in and counting the revolutions it takes to seat them. Although these knobs have most likely been turned to compensate for that sticking float or massive vacuum leak, it will give you a baseline for your future tuning.

4. Find a good work surface

If possible, use a work table with closed sides to keep parts from rolling off. Make sure the work surface is clean, then lay paper towels over to the table to keep your workbench from being dissolved by the cleaner and the parts from getting grit on them.

5. Get a set of appropriately sized screwdrivers

There is nothing more frustrating than not having the right screwdriver to get the main jet out of the venturi. Ok, well there are plenty or more frustrating things, but it makes the job a lot easier with the right tool. Having small pocket screwdrivers as well as thin, broad screwdrivers is extremely helpful in avoiding stripped brass.

6. Take lots of pictures

Having a digital camera for use specifically in the shop is incredibly handy, especially when tackling memory-intensive projects like this. Set the camera on the closeup setting (little tulip in most cases) and snap away. Important places to take pictures of are the choke, throttle, dashpot, and float linkages, as well as the locations of ball checks and stops. They're free so take as many as possible.

7. Don't put anything non-metallic in the carb soak, including your hands

I did this and it sucked. Whether it's stale fuel or your epidermis, carb cleaner knows no mercy. Although it's not really necessary to climb into a Tyvek suit to rebuild carbs, some basic precautions should be taken to preserve the outermost layer of your skin. Dunking your hands in the cleaner will make them itch as well look like when you used to put Elmer's glue on yourself in Kindergarden.

Also, non metallic parts such as O-rings, phenolec, and floats should not be put in carb cleaner.

8. Check for shaft play

Shaft play can often cause large vacuum leaks by allowing the motor to suck air around the normally sealed throttle butterfly valve shaft. If it is possible to find a new carburetor body, this is the preferred method. However, two alternative avenues of repair are commonly used: boring out the shaft hole and putting in a brass bushing the take up the play, or by machining an O-ring groove into either the throttle shaft or carb body.

9. Draw exploded diagrams of linkages

Often times digital pictures can not adequately detail how a linkage is assembled. Break out that moleskin diary of yours and draw out the linkage, bushings, and washers in the order they were disassembled as well as the orientation of the different linkage bars. This will save a huge amount of head scratching when it's time to start the reassembly.

10. Never tip a carburetor upside down

This holds especially true on reassembly. Ball checks are often only captured by gravity and floats linkages can sometimes jam or come apart when turned upside down or rotated in ways that the vehicle would ordinarily not be.

11. Check that surfaces are true

Use a known level surface to check that the mating surfaces of the carburetor are true. If the body or base of the carb is warped, it is possible to either mill or grind the surfaces true. Be sure to wash them of grit and blow them out with compressed air afterwards.

12. Check the float operation

After completing the rebuild but before installation, check the operation and level setting of the floats. Check that the needle valve is open when the floats are down by blowing gently into the fuel inlet, then fill the float bowl with gasoline and check that the needle valve is closed when the float is up. Since many kits don't come with new floats, make sure that your float lives up to its name.

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