Doing this month's CAT feature Last Dance (see page 22), it occurred to me that, like everyone who started spinning wrenches in the 1970s or earlier, I now have a wealth of useful knowledge....that's ...
Doing this month’s CAT feature Last Dance (see page 22), it occurred to me that, like everyone who started spinning wrenches in the 1970s or earlier, I now have a wealth of useful knowledge….that’s essentially useless. Useless in the context of modern computer controlled port fuel injection, that is. In the spirit of this month’s CAT, here are a few tips that will likely never again appear on the pages of an automotive trade magazine.
After the flood
Carb flooding the engine? If it’s serious, you’ll likely go straight to the needle valve. That’s good, but what about the float? If they leak or saturate, there’s no way the needle valve will close properly and you could chase mixture settings forever to try to get it right. If you lean the mixture down at idle to compensate, it’ll go really lean at mid to WOT, possibly with disastrous results on a performance engine. If the engine requires an unusually lean almost-to-the-stop idle mixture setting, something’s wrong. Open her up.
Brass floats can be soldered (get all the gas out first) and foam plastic ones can sometimes be coated with resin, epoxy or Seal-All. The additional weight may require resetting the float level. Check if the float uses a tang to positively lift the needle off the seat as the float falls…it’s sometimes possible to miss the lip and jam the needle valve down hard into the seat. If the carb top doesn’t go on easily, there’s something wrong…take it apart and check! If the float bowl has what looks like a useless piece of plastic in there, don’t leave it out. It’s a “carb stuffer” and it’s there to keep the fuel from sloshing over in a hard cornering/acceleration situation and dumping fuel into the metering tubes or pullover circuit. The engine will run fine, then the customer will come back complaining of a strange stumble during a turn or when braking hard.
To Holley or not to Holley?
Holley carbs don’t make more power. Period. Any properly sized and set up carb for an engine will work as well as any other. Wanna run eight Webers on a small-block Chevy? It’ll work. Holleys are popular because they’re simple, easy to tune (many let you set the float level externally, sometimes with the engine running) and have a huge number of aftermarket and OEM parts options. You can go fast with a Quadrajet or Thermo-Quad, but there are fewer tuning options and a smaller knowledge base. If you have the knowledge, however, they’re just as good, CFM for CFM.
Bigger is Better
Just like you wouldn’t put high-flow injectors on a stock engine, it’s dumb to put a big carb on a stock engine…only worse. In the port fuel injection example, the ECU may be able to shorten the pulse width to compensate for the extra fuel mass, but the bigger bores of a high-CFM carb mean a weaker vacuum signal for the critical transfer/part throttle crossover circuits to work with, plus poorer low speed atomization. Spread-bore and two-stage progressive secondary four-barrel carbs help, but too big is too big. The real solution if you’re switching from a two barrel would be a vacuum-operated secondary four-barrel with primaries a little smaller than the two-barrel’s and secondaries the same or a little larger. Every teenager wants to start with big carbs, but wait until the rest of the engine is ready, especially cams.
If you’re of a certain age, you know what I’m talking about here….if you don’t, you don’t have grey hair. That’s good. No one should ever have to learn what happens when you lose that tiny check ball out of a Quadrajet.
Do you agree? Disagree? Let us know! firstname.lastname@example.org
Have your say: