Rochester Carburetor Repair Manual: Tuning, Repair, Adjustments, Overhaul, Modifications - by Haynes

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Rochester Carburetor Repair Manual
Rochester Carburetor Repair Manual
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Item Description

Carburetors are precision instruments, and although to many DIY mechanics they represent a system that should not be tampered with, all that means is that they end up paying someone else for repair or modifications.

There's no need to spend a considerable amount of money on carburetor repairs when, with the right information, you can do the work yourself.

This manual will show you, with words and photos, how to correct common problems associated with Rochester carbs, overhaul your carb, perform repairs, and extract the maximum performance from your engine.

Although Rochester carburetors are primarily found on GM vehicles (Rochester is owned by General Motors), they have been used by other manufacturers, too.

The Rochester Carburetor Repair Manual by Haynes, will teach you how to maintain and fix your carb, and realize its full potential.

This guide covers:

  • 1M (One-barrel)
  • 1MV (One-barrel)
  • 1ME (One-barrel)
  • 1MEF (One-barrel)
  • 2G (Two-barrel)
  • 2GC (Two-barrel)
  • 2GV (Two-barrel)
  • 4MC (Quadrajet four-barrel)
  • 4MV (Quadrajet four-barrel)
  • M4MC (Quadrajet four-barrel)
  • M4MCA (Quadrajet four-barrel)
  • M4MEA (Quadrajet four-barrel)
  • M4MED (Quadrajet four-barrel)
  • M4MEF (Quadrajet four-barrel)
  • E4MC (Quadrajet four-barrel)
  • E4ME (Quadrajet four-barrel)
  • M2MC (Dualjet two-barrel)
  • M2ME (Dualjet two-barrel)
  • E2MC (Dualjet two-barrel)
  • E2ME (Dualjet two-barrel)
  • 2SE (Varajet two-barrel)
  • E2SE (Varajet two-barrel)

Book Excerpt: Rochester Carburetor Repair Manual: Tuning, Repair, Adjustments, Overhaul, Modifications

CARBURETOR FUNDAMENTALS

ACCELERATOR PUMP CIRCUIT

Fuel is quite heavy compared to air. Remember that the only thing that keeps it well vaporized at idle is the high manifold vacuum produced by the closed throttle plate.

When you open the throttle abruptly to accelerate from idle, manifold vacuum drops to zero. Also recall that the only thing that keeps fuel well vaporized at cruising speeds is the high venturi vacuum produced by the speed of the air rushing through the carburetor. But when you open the throttle quickly to accelerate from cruising speeds, airspeed through the carb - and therefore through the venturi vacuum - lags behind engine speed momentarily.

In either case, some of the bigger fuel droplets in the vaporized air/fuel mixture condense back into liquid form ( an especially thorny problem on big-port and large-plenum manifolds since there's more surface area for fuel to condense onto).

A throttle bore that's suddenly opened needs a significant increase in fuel to replace the fuel that falls out of suspension. But the throttle plate is already too far past the idle and idle transfer ports for them to help out.

And, without a strong vacuum signal in the venturi area, the main circuit can't flow a drop. The result? Air rushes through the carburetor, but fuel flow is momentarily halted, so the mixture sucked into the cylinders is ultra lean.

A moment later, some pressure difference returns to activate the main circuit, but by then it's too late - the engine hesitates or stumbles on the lean mixture.

In other words, when the throttle is moved quickly, there's a time lag between demand (throttle opening) and main system activation. Part of the reason for this phenomenon is engine load.

Put the transmission in neutral, rev out the engine and what happens? The engine accelerates smoothly. Why? Because when the throttle is opened in the absence of a load, engine speed can build smoothly and quickly.

So manifold vacuum returns to its normal 14 to 17 in-Hg rather quickly. But, put the transmission in gear and do the same thing. Now the engine may stumble badly if the accelerator pump isn't working properly.

Why? Because when the engine is put under a load, airflow and vacuum stay low for a lot longer than when the engine is unloaded. During this period, the carburetor flows little fuel because without airflow and vacuum, the regular circuits can't do much.

The accelerator pump system makes up for the deficiency by squirting a stream of raw fuel into the carburetor right above the venturi. The mechanically-activated accelerator pump (see illustration) is linked to the throttle lever to provide a squirt of fuel each time the throttle is opened quickly.

When you open the throttle, the pump linkage pushes down on a plunger and piston inside a pump well. This raises the fuel pressure in the pump well, which (on mid-60"s Rochester's) forces the pump in-let check ball inside the plunger head onto its seat so fuel won't escape from the pump well.

Some models have an inlet check ball at the bottom of the pump well, at the mouth of the passage from the float bowl to the pump well, instead of in the plunger head. The rise in the fuel pressure also raises the discharge check ball off its seat, allowing fuel to be injected or discharged through a pump jet or "shooter" into the venturi.

Some shooters are aimed at the throttle plate or against the bore. But on carbs with venturi boosters, shooters are usually aimed right at the booster. The shot is pulled toward the trailing edge of the booster by air streaming into the carb, breaking up the fuel for better vaporization.


SELECTION AND MODIFICATION
VALVE SPRING COIL BIND

Whenever a camshaft with higher lift than stock is installed, the springs should be checked for coil bind. Due to the increased travel, the valve spring coils may hit together (bind) causing considerable damage.

Perform this check with a new camshaft and lifters in place and the valve covers off. The cylinder heads, rocker arms and push rods must be in place and adjusted. Using a socket and breaker bar on the front camshaft bolt inside the lower pulley, carefully turn the crankshaft through at least two complete revolutions (720-degrees).

When the valve is completely open (spring compressed), try to slip a 0.010 inch feeler gauge between each of the coils (see illustration).

It should slip through at least two or three of the coils. If any spring binds, stop immediately and back up slightly. Then correct the problem before continuing. Usually, the valve springs must be replaced with special ones compatible with the camshaft.


SPRING RETAINER-TO-VALVE GUIDE CLEARANCE

Sometime high lift camshafts will cause the valve spring retainers to hit the valve guide. To check for this, rotate the crankshaft as described above and check for guide-to-retainer interference (see illustration).

There should be at least 1/16 inch clearance.


Subject: Transportation: Automotive: Intake System: Rochester carburetor overhaul, repair, adjustment and tuning manual. ISBN-10: 1563920689 | ISBN-13: 9781563920684 | Haynes 10230

Specifications
  • Publisher: Haynes Manuals
  • Pages: 188 - Hundreds of b&w photos and illustrations
  • Binding: Paperback - 8.5 x 11 inches
  • ISBN: 978-1-56392-068-4
Table of Contents
TABLE of CONTENTS:
  • Introduction
  • Tools and Equipment
  • Carburetor Fundamentals
  • Carburetor Identification
  • Troubleshooting
  • Carburetor Removal and Installation
  • Overhaul and Adjustments
  • Overhaul Tools and Preparation
  • One-Barrel Models
  • Two-Barrel Models
  • Quadrajet Four-Barrel Models
  • Dualjet Two-Barrel Models
  • Varajet Two-Barrel Models
  • Selection and Modification
  • Conversion Factors
  • Glossary
  • Index
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