Toyhead Auto Restoration Services

Rick's  Tuning Tips

AFTER MARKET CARBURETORS AND PLUG READING - Carburetors are one of my favorite modifications on stock vehicles because they respond so well over factory issue carburetors. Yet they are one of the most difficult modifications to tune and set up right without proper basic knowledge of how they actually work and function.

Most of the time you can not just bolt on an after market carb and expect it to work just fine. The carb will most likely have to be tuned and jetted to your engine application.  Jets are small brass screw that set inside of the carb and have a hole in the center of them. They have a number stamped on them indicating the calibrated size of the hole. Depending on the type of jet, they regulate the amount of fuel or air passing through the hole. Sometimes you can get lucky and the carbs will work fairly well, but you will never know the true performance potential of it until you try different jet settings to see if you can get it to run stronger throughout the rpm range. There are a number of publications out to tell you the basics of how to tune a particular carb and the basic functions of all of the working systems within the carb (See for available Weber carb publications).

Most Toyota carburetor vehicles are deprived of their peak performance because of the heavily smog-choked carburetors they came with. There are a number of carburetor choices to go with for these motors. Downdraft-style carburetors are configured like the original stock setup, sitting upright on top of a manifold that flows the mixture into the intake ports at about a ninety-degree turning angle from the carburetor.

Popular down draft carburetor selections are the Weber 32/36 DGV two-barrel series and Holly/Weber of the same configuration. The Weber 38/38 DGxS two-barrel series and Holly 350 and 450 cfm four-barrel carbs. Side-draft models, as the name implies, are mounted sideways on the intake side of the engine. They are usually run in pairs and the fuel mixture flows straight into the intake ports though a short manifold. Popular side draft models are Weber 40 and 45 DCOE , Mikuni 40 and 44PHH carbs. Other selections, although less popular, may be Solex and Dellorto carbs.

One of the keys to tuning a carburetor is learning how to read color on a spark plug. The spark plug will give you a basic idea of what kind of fuel/air mixture ratio is being inhaled into the motor. The part that you need to read on the plug is the insulator cone -- the white part (when new) that surrounds the center electrode (where the spark jumps from), not the outer metal ring or any other part for that matter. If the insulator is black and sooty, the air/fuel mixture is too rich (too much fuel). If the insulator is more white (clean) or ashy, the air/fuel mixture is too lean (not enough fuel). You want to get the plug to have a tint of color to it, a bit on the brownish side. Some fuel additives may make it tint a slightly different color, maybe more orange. But for the most part you want to get it somewhere between the two extremes. Also, make sure the car just ran at the rpm you want it to run best before you read the plug. Do not read the plug after sitting at idle speed for ten minutes or so, unless you are tuning for the idle.

The best way to get an accurate read at say four or five thousand rpm is to take the car out for a run. Get it to the rpm you want (to run strong) in the gear you want to be in. When you reach that point, quickly turn the ignition off and coast over to the side of the road. And no, you do not want to do this on a major busy highway - BE CAREFUL not to turn the ignition key so far back as to lock the steering wheel as you coast.

The plug will be very hot, so take gloves if necessary. Read the plug and take notes. Always make notes of all the adjustments you made and what the results were because the combinations are endless and you cannot remember everything. I personally use Autolite plugs non-resistor type as they last and read well. I stay away from platinum plugs because they actually reduce the voltage of the spark. Platinum plugs are for people who want to get the most mileage out of a plug between changes. They will last longer but they do not necessarily give peak performance for a high-energy racing ignition.

Written by Rick Dormoi

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