By Charley Rockwell
Chapman Report - May 1989

Now you have finally done it! You got your ported and polished big valve super high lift long duration forged piston fully counterweighted crankshaft dual tri-Y low restriction turbo muffler and ram air intake 7500 rpm iron and aluminum jewel together at last! The only hitch is that it has a flat spot around 2000 rpm, surges on the freeway at 3000 rpm and backfires at idle. It must be those damn carburetor jets!

What size jets do you need? The chances are you won't find somebody with the EXACT same engine as your to find out. Even if they had the EXACT same engine normal production tolerances can vary enough to substantially alter optimum jetting. Furthermore, even if they have the EXACT same engine built to the EXACT same tolerances, chances are that their engine runs worse than yours! This article will show the average enthusiast what sort of problems can be expected when jetting a car, and how to go about determining what size jets should be used.

Most people pay little attention to jet sizes if the car runs without backfiring or hesitating. If your car has problems with hesitation or backfiring then try richer jets. If that doesn't work, then try leaner jets. If that doesn't work THE PROBLEM IS NOT CAUSED BY INCORRECT JETS. This seems ridiculously simple, but many people fail to grasp this basic concept. A major stumbling block for most people is that they do not know which jet to change.

If you are dealing with a Weber DCOE you can change the main jet, the idle jet or the pump jet. I recommend that you simply change the main jet first, then try the idle jets, then try the pump jets, in that order. I can hear the howls of laughter right now. Yes, I have read all the books about jetting Weber carburetors and I can write pages about how to tell if you need a different pump jet versus a main jet. And I can quote several authors, who have been tuning Maserati 250Fs since Fangio first started racing, about how to properly set-up idle jets. However, I think it all boils down to simple trial and error. First, try the main jets. Second, try the idle jets. Third, try the pump jets. When desperate I will change all three at once. If you have changed all three jet circuits and the drivability problem persists THEN THE PROBLEM IS NOT CAUSED BY INCORRECT JETS. Vent your frustration off to a convenient source, and seek out an expert to help you. A solution is always available.

Assuming that the car runs with no big flat spots and you are not sending smoke signals to Thailand out your exhaust pipe, your car may still need a little fine tuning of the carburetor jets. If you are racing or autocrossing you may be able to squeeze as much as 5% more power out of the engine if you find the optimum jet sizes.

The best way to find the optimum size is to use a chassis dyno to measure your horsepower when you change jets AND to perform timed acceleration runs in each gear with something like a VERICOM car computer. A chassis dyno will often give better results than a engine dyno for carburetor jet changes because the induction hardware, exhaust hardware and environment are rarely exactly duplicated with an engine dyno. The dyno can be used to determine the optimum jet sizes for steady state power, but you also need to determine the optimum accelerator pump jets by actual driving. The latest car computers and/or the G Analyst make this task simpler and more scientific than before. I usually like to perform 30-70 mph runs in third gear late at night in industrial areas. Ideally you should perform similar tests in each gear through the entire effective power band of your engine, but I rarely have the time. Drag racers must make the time.

Improved horsepower is not the only reason that you want to optimize your carburetor jet sizes. Engine longevity is effected by carburetion. If you mixture is too lean you engine may overheat or detonate. If you mixture is too rich, then carbon can build up, robbing power and causing detonation. Rich mixtures can also cause the oil on the cylinder walls to wash out leading to very rapid loss in power at sustained speeds and premature cylinder bore wear.

This latter point can be graphically shown on a dyno. Run an engine too rich and the power will reach a maximum similar to a correctly jetted car. After a few second the power will drop off. If you rapidly shut the engine off and perform a compression test you will find that the engine has lost a lot of compression! This is because there is no oil to properly lubricate the cylinder walls, thereby not allowing the piston rings to seal the combustion chamber! The effect can be very dramatic and is important to Bonneville, sports car, rallye and endurance racers. Not to mention the street driver interested in getting 100 Kmiles from the piston rings!

Air cooled cars, turbocharged cars and cars with aluminum heads can be very sensitive to lean mixtures. If the mixture does not contain enough EXTRA gasoline to cool the combustion process, then exhaust gas temperatures may get so high that they crack or melt the exhaust valve, exhaust valve seat, the piston or the head itself!

By now the hapless auto enthusiast is probably too scared to even contemplate changing carburetor jets, or anything that may require changing the carburetor jets. In fact he is probably ready to throw in the towel and keep his stock '76 Toyota Tercel, forswearing automotive recreation forever. Such dire circumstances need not be sustained for reasonable assistance is available to keep the enthusiast from blowing the engine and the budget! But, first let's dispel two jetting myths.

The first myth is that the common enthusiast can correctly jet the carburetor by driving around and reading the color of the tailpipe. According to some medieval legend a black exhaust tip indicates an overly rich mixture, a tan or chocolate exhaust tip indicates a correct mixture, and a grey tip indicates a lean mixture. Allow me to explain how it REALLY works. If your exhaust tip is black then your car is Italian. If your exhaust tip is tan or chocolate then the Italian exhaust system is rusting on English car. If your exhaust tip is grey, then you splurged for a stainless muffler and exhaust tip! This system works every time, or at least as often as the medieval system predicts correct jet sizes. Enough said!

The second myth is that you can correctly jet a car by reading the plugs. To get a correct plug reading the myth says that thou shalt put the hammer to the metal for 30 seconds (where the hell can you do that?) AT EACH ENGINE SPEED OF INTEREST (now try finding a hill where you can floor it at 3000 rpm, 4000 rpm and 5000 rpm for 30 seconds) THEN, QUICK, SHUT THE ENGINE OFF AND DEPRESS THE CLUTCH. Right. I tried this just outside Santa Cruz in my Lotus Cortina. The Sheriff was not impressed and some insurance company (who probably invented this jetting method) got rich while my wallet got lean. I never could find the right combination of hills to determine whether my mixture was correct.

Since opening my dynamometer tuning business I have performed many plug readings under ideal conditions and I have learned that reading plugs will lead you to an incorrect mixture 9 times out of 10. Any given engine may yield perfectly tanned plugs when it is so lean it misfires and has lost six horsepower, or yield brazen virgin white plugs when it is so rich it has lost 15 horsepower after a ten second run at full throttle. The only thing that reading plug colors will reliably tell you about your jetting is what color your plugs are! The plug color myth is so persuasive that accomplished tuners will jet their cars for slower lap times because the plug color is incorrect. Many dyno operators and engine builders know from extensive tests that spark plug readings are not the way to jet a car. Many people, carefully comparing their plugs to colorful little pictures in their precious "How We Made Horsepower in 1953" tuning books are tuning themselves slower! However, the psychological edge is greatly enhanced by precision trackside readings taken with a illuminated and magnified plug reader and a twelve page binder of little color plug pictures hung where the competition can be conveniently awed.

Now that the two myths have been expelled let's examine two methods of jetting your car that actually work.

The first method is to purchase a portable exhaust gas analyzer like those sold by Gunson. The portable analyzers use a thermistor element to sense the carbon monoxide concentration in the exhaust. This is not a terribly accurate way to determine your mixture, but you can take the unit to your local garage and compare it to their infrared analyzer. Once you have calibrated your analyzer you must secure the probe in the exhaust pipe with safety wire and racer's tape. Now you can drive the car around with your analyzer on the passenger seat and look at your mixture while you drive.

The thermistor type sensor elements have a lag time of a few seconds so you need to hold a steady speed to get a good reading. Several manufacturers are selling lambda sensors that light an LED display on your dashboard, indicating a rich or lean mixture. There are two problems with these units. The first problem is that you need to drill a hole in your exhaust system and weld a threaded boss to mount the lambda sensor. The second problem is that the lambda sensors are only accurate near stoichiometric fuel/air ratios. If you own a 50s, 60s or 70s sports car, then you will probably need a MUCH richer mixture for reasonable performance and drivability. The lambda sensor will not operate with the fuel/air ratios that you need.

Now that you are driving around town with your exhaust gas analyzer and a bag of jets you may find a common problem. At 3000 rpm your car runs too lean and at 5000 rpm your car runs too rich! If you change to richer jets, you run a correct mixture at 3000 rpm and way too rich at 5000 rpm! In situations like these you are going to need to consult an expert about your carburetor. What you need to show your expert is a MIXTURE MAP of your engine and carburetor. A MIXTURE MAP looks like this:

MIXTURE MAP
   LOAD ENGINE RPM %CO
   1/8 Throttle 2000 rpm 2.5%
   1/8 Throttle 3000 rpm 1.5%
   Full Throttle 3000 rpm 0.9%
   Full Throttle 5000 rpm 6.0%

With a mixture map as shown above a tuner familiar with your engine and carburetion combination can make a fairly accurate guess about what changes your carburetor needs. For instance if the above mixture map was shown to me for a Fiat, Alfa Romeo, Lotus or Porsche with dual Weber carburetors, then I could accurately guess that the car needed smaller venturis or a different emulsion tube.

The reason for this is that at small throttle openings the car is operating off the idle jets and is receiving a reasonable mixture. At full throttle the engine is not receiving enough gas at 3000 rpm, but receives plenty of gas at 5000 rpm. Changing the main jet to a larger size would improve the mixture at 3000 rpm, but you would be far too rich at 5000 rpm. To obtain a richer mixture at 3000 rpm without making the mixture too rich at 5000 rpm you would need to change to a smaller venturi and a smaller main jet. With a Weber carburetor, changing the air corrector jet will usually effect the mixture everywhere, not just at high rpm. so air correction jets are best forgotten unless you are using a dyno. Likewise, playing with emulsion tubes will not prove very effective UNLESS you are changing to an emulsion commonly used on your car. For instance, F9 for Alfa Romeo, F11 for Lotus and f2 or F3 for Porsche. Remember, once you have a MIXTURE MAP, an expert or yourself can make very accurate guesses about what your carburetor needs.

The second method of correctly jetting your car is to go to a dynamometer tuner. If you purchased you engine from and engine builder with a dyno then you are all set. All you need to do is dial in your accelerator jets with a few timed runs. If you are fortunate enough to live in an area with a Chassis Dyno then you can have your engine tuned while it is still in the car. Dyno time is not inexpensive because most dyno tuners charge at least $50 an hour for their services. However, this method of tuning is very precise, and is the only way you are likely to optimize your carburetion for maximum horsepower. Furthermore, most dyno tuning installations will only charge you for the jets, venturis and emulsion tubes you end up with. You don't have to purchase five different sizes of jets! This can sometimes make a dyno tune cheaper than jetting the car yourself!

If you cannot find a chassis dyno, your engine may be fairly easy to hook up an engine dyno if a local dyno shop has the proper adapters. VW and Porsche owners have a fairly easy job getting their engines on engine dynos. Some Ford are also very simple. The idea here is check around your area for people who are willing and able to help you.

In closing I would like to say that getting your trick engine, headers and carburetors all hooked up is only 75% of the battle! You need to reserve another 25% of your time, money and tears (yes, you WILL cry) to getting your car correctly tuned so that it operates without flat spots, black clouds or flames. When you anticipate this at the start of a project you will be ready to tackle the inevitable problems as they arise and finish with a smooth running, FASTER car.