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If you still have any small thought that slapping a quick kit into anold carb will be the end of it, you are deep in denial. For thatmatter, if you think that you can grab a new carb off the shelf andmake it work right, you are equally uninformed.

This is important in that when you build that carbureted engine, eitheryou or your customer will have to face the harsh reality that thingshave changed. If not, you could be plagued with complaints about poorrunning, poor performance or even damage resulting from impropermixture or functions. Even if you never touch a carb yourself, you needto be aware of the situation, know what to do, or advise your customerwhat to do, and protect yourself along with your shop reputation. Youknow I’m right, and you’ve probably already found some charactertrashing your reputation and threatening legal action because HIS carbended up damaging YOUR workmanship.

While there’s certainly nowhere near the volumes required to fullycover this now, we can at least be aware of the principle issues thatcan trip you or your customer up and that will prevent most of yourtroubles.

1) If you change the engine, you will likely have tochange the carb. More cam, more cubes, improved flow from heads ormanifolds, dual exhaust instead of single, and others will all changethe performance level and therefore potentially mean you have toreplace or recalibrate the carb. It just takes more fuel to make morepower – right?

2) Carbs wear out. There’s probably no better example than theQuadraJet. A fine carb, thoroughly adjustable, efficient and durable,it has two consistent problems. One is that the primary throttle boreswear out and need to be re-bushed. Fail to and you get vacuum leaks,lean mixtures, poor running and, potentially, mechanical damage. Theother is leaks from the fuel bowl body plugs. These need to be preppedand epoxyed or they never stop. If someone doesn’t fix the carb so itfunctions as it was designed, you are risking an unhappy customer.

3) New carbs are not calibrated for your specific engine. Theyare roughly calibrated, so over a wide range they will be close enoughto run the engine and get you to a starting point. As a policy, carbmakers will jet the carbs so they are over-rich (prevents lean-outdamage and angry customers with lawyers) and generally the secondariescome in late. When we dyno an engine, the carb is first checked andcalibrated, because unless it is functioning right we can’t get peakperformance, or worse – a valve or piston could end up fried. If youmake sure your customer understands this and is prepared to make theneeded changes, it is no longer your responsibility and you’ve donethem an honest turn.

4) Fuels have changed. Modern fuels are no longer designed forcarbs, but for EFI. It makes a big difference. The most importantchange I know of is the lowering of the evaporation point. Simply put,fuel now has a lower boiling point than it used to. This results infloat bowls drying out and harder starting. It causes accelerator pumpsto fail when the fuel and additives and wet/dry cycles destroy the pumpcups. It causes vapor locking, overheating and detonation from leanmixtures. You may be surprised to learn that the engine you arebuilding FAILED because of these things and now you are going to putthe same accessory parts and carb on without curing the problems! Youshould be able to see what’s coming.

5) Alcohol. There’s a big push to add alcohol to fuels. We allknow the reasons that this is happening and, frankly, I’m not going todebate it one way or the other. What I do know is that alcohol changesmixtures in carbureted engines. It takes almost twice as much alcoholas gasoline to produce the same amount of power. That means that tofeed your engine, if alcohol is added, it will require more fuel and aricher mixture. You either have to de-tune the engine or re-jet thecarb. Sometimes jets, siphon tubes, internal passage sizes, powervalves, and more need attention. If you send out an engine that alreadyruns lean and the carb is not recalibrated to accommodate alcoholfuels, you could end up with performance complaints or damaged internalcomponents. EFI works differently because these systems look for proper mixtures bysniffing the exhaust and listening for knock and sensing heat. Carbscan’t and don’t make adjustments like EFI does. In my daily driver, ifI use straight non-alcohol regular I can get 20 MPG all day long. If Iput the alcohol fuel in the tank this drops down to between 14 and 16mpg. A 10-20 percent drop in fuel mileage is not uncommon. However, EFIwill accommodate this change in fuel quality and simply add more fuelto normalize conditions. With a carb you just cause running problemsand engine damage if you don’t recalibrate

6) Over-carburetion. We still suffer from "bigger is alwaysbetter" syndrome. I don’t know how many times I have to say this, butit’s NOT. Too often people will take their fresh new engine, slap itinto the hole, and then add about twice what the engine can ever hopeto use for carburetion. They think they are improving performance, butusually they are making problems. Too much carb will cause hesitation,stumbling, decreased performance, poor fuel mileage, and lots ofdisappointment.

On’s  forum, you will see a section withcalculators (this is a free site). Plug in the engine data and you’llget a cfm capacity for that engine. It will surprise you when you findout just how often people use way more carb than the engine can handle!For example, a 350 cube engine, with a red-line of 6,500 rpm andoperating at 100 percent volumetric efficiency (few engines do this)needs just 658 cfm. Throwing that 850 cfm unit on this engine will killperformance where it can be used for the sake of theoreticalperformance above where the engine will ever be run. Am I the only onewho sees how dumb that is?

The bottom line here is that you can do a perfect, high-quality job inyour shop and send out an engine that should run well and long only tofind that because the carb wasn’t rebuilt, repaired, recalibrated, orsimply the wrong one you see it come back with failures and complaints.I’m suggesting that at the very least you educate yourself and yourcustomer to prevent this.

Talk with your customer and fill him in about this. Make it clear thatit is an essential and even critical part of making that engine runright and last. Make it clear who has the responsibility and who willpay if this is not done. Frankly, I think it needs to be part of thepaperwork; something the customer reads and signs so the importance andresponsibility are clearly stated and understood. You may find that bythe simple effort of communicating this kind of thing clearly you willbe not only protecting your finances and reputation, but doing a realcustomer service as well.ok, so you have a high-quality rebuild, perhaps even a custom-built performance engine you are going to charge serious money for. is this what you want to see hosed off and slapped on that engine? do you think that maybe it could be the cause of failures and complaints? protect yourself!”These

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Doc Frohmader

Doc Frohmader

Founder and President at
Doc Frohmader is the founder and president of, a website dedicated to hot rods and vintage engines.
Doc Frohmader

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