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Carburetion
Carburetion, Page 2
Carburetion, Page 3
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Carburetion, Page 5
Carburetion, Page 6
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A great story to insert here. Once upon a time I decided to really science out the jetting dilemma on the Max Wedge manifold. I had a manifold off the car, carbs bolted on, tops off and fuel in the bowls to a proper level. I then raised the front of the manifold to simulate the lift of the car and altered the jet pattern to observe the fuel spill through the boosters as the level moved away from the jet in one bowl and toward the jet in the other bowl. Long story short, the jetting was bizarre!

Off to the track we went where we performed the typical A-B-A test and guess what? No difference! I was so distraught, I jerked the tops back off and REMOVED the back jets completely and you guessed it….no difference. Don’t do this at home. There could have very well been something else wrong. But the point is, CONCENTRATE ON DRIVEABILITY.

Follow the secondary scheme provided and you will be happy.

Most of the jetting described here is available OTC from Edelbrock (also available from dvorakmachine.com), but for the experimenters, we have blank jets with .020” starter holes and a nice approach. Now I heard the same BS you have about the drilled jet actually running leaner than the factory jet but don’t you believe it! We have done back to back to back tests on carburetors/manifolds where jetting really does matter and drilled jets work every bit as good as factory jets… so there!

Some things you should notice so far: I always use the same metering rods, a .071”/.047” (Edelbrock P/N 7147) because:  a) it comes in all the Edelbrock carburetors so availability is easy;  b) I love the splits of .024” differential between the economy step and the power step;  c) if we ever do change a jet we will maintain the same splits which will keep our all important drivability intact and;  d) when we get to a later chapter on all out competition, we will be discarding the metering rods entirely and running on pure jet only!

While we are in the area of the metering rods, not enough can be said about the smooth and free operation of the piston/rod/spring assembly when you go from high vacuum to Wide Open Throttle, (WOT) in an instant. That is what separates the men from the boys and what separated the smiles from the frowns. First things first, the small spring clip that holds the piston to the rod? Well, we just unhook that and throw it away. If you look closely, you will see that it cocks the piston downward such that it drags the piston against the walls of the carb top as the rod is pulled out of the jet, slowing the rod down and scuffing the piston... Carburetors that have been run always have to be polished in the piston bores and the pistons. They work fine without the spring clips. Get inside the wells and polish the heck out of the walls and pistons, turn the pistons upside down on the rods and run them up and down with light oil and check for burrs.  

Step up rod springs. Rarely in our world have we ever used any other springs than red (orange) or pink. Sometimes they need to be modified a bit to get you to perfection and again we are looking for UD, ultimate drivability. Before we make that final adjustment, a bunch of other considerations must be addressed. We should default to orange (red) to begin and install the retaining plate such that it doesn’t cover the top of the piston completely and you can observe the action of the piston while the engine is running, all four pistons should be visible and half the opening is open and the screw is tight.

Things that don’t seem important, but are important. We don’t want to get too far down into the engine but we need to assume that at some level, this manifold/carburetion system is a relatively new combination so we need to be certain of a few things like the alignment and fit of the manifold gaskets; that they fit the ports nicely and the bolts screw into the holes easily, (NEVER REUSE MANIFOLD GASKETS) and that the bolts have washers on them that take up most of the opening of the bolt hole and that no one wallowed out the bolt holes to make the manifold ‘fit.’ That’s important. Vacuum signal will be weak and even lost if this fit is done poorly. Different thickness manifold gaskets are available from us or many other places if you get in trouble.
More gasket match…

All these manifolds, especially original equipment, are porous and suck oil and air out of EVERYWHERE! I sealed one up and pressured it up to 15 pounds and sprayed soap water on it and it looked like soap-bubble-boy out of every pore, under the manifold, on top, the bolts, studs, everyplace. The only fix is to PAINT the underside of the manifold with a quality oil and heat resistant EPOXY paint, many coats, paint brush will do, and the top with an epoxy primer before you color paint it. ALL STUDS, 4 inside carb studs, bell crank stud, and throttle hold down bolts all get silicone.

The gasket should match the manifold. If you have a small hole manifold, don’t use a one hole gasket or a four big hole gasket. Use a gasket with four matching holes to the manifold and of course they will also match the carburetor! NEVER REUSE THE CARBURETOR GASKET! They leave imprints of the circuits in the base and they will never install exactly the same as they did the first time. Go ahead! Grab an old carb gasket. Look at it closely; it will look like 3-D. You will lose signal and idle quality. When you find the right gaskets, buy a handful. Keep them fresh! Call me!

DRIVABILITY: we’re back to that again. What we are looking for is to first have the carburetors synchronized, that is, having each passing equal quantities of air through the air horn at idle. This is the only slightly complicated portion of the tuning.



 

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