Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor

Too Rich Idle,Cadillac 500

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Jim Elliott:
My guy out here on the "left coast" changed mine out to 74s from 73s (Grrrrr) but it runs fine, One thing I would do is stick an "Indexed vacuum gauge" somewhere and read your vacuum on slight to medium hill climbs....

Most of the times the vacuum is below 6" and those power rods are dumping a ton of fuel that is not needed so I've got/found some 6" springs and run it that way....

Don't know what the 4 springs from Cliff are vacuum wise but that change is perfect for a motor home that will languish below 11" of vacuum with a slight head wind blast.

I'm getting lazy'r by the day and still need to trim the idle screws in a dab just to clean it up, MPG averages out to around 9 mpg with a (3120# HHR in tow).

Jim

Cliff Ruggles:
Fuel consumption increases with engine load as throttle angle increases and engine vacuum falls off.

The greater the differential in vacuum/pressure above and below the throttle plates, the greater the signal to the jets. 

The power piston is designed to raise the metering rods for situations where the engine is seeing heavy load, or full power is needed.

Using metering rods allows for much leaner fuel delivery when the engine load is light, then adequate fuel delivery under heavy load.

It's difficult to tune specifically for "in between" conditions, as seldom does the engine duplicate the same vacuum reading under various driving conditions.

Two other items to consider in that deal are the centrifical advance curve, and vacuum advance unit.

When engine vacuum falls off, so does timing from the vacuum unit.

At 6" or less, most vacuum units are not adding any timing.  This is probably a good thing as detonation would be a possibility with a motor home application.

Over the years I've done considerable testing with towing applications.  I've never once found improved total fuel economy trying to keep the power piston down for "in between" conditions......Cliff

Jim Elliott:
 Cliff the timing is sick on a stock 454 motor home....Base 4º and mechanical 16º it's no wonder the carb is dumping fuel even a a slight hill with zero vacuum...
Setting up the base to 10º and extending the mechanical to 22º+ will aid the power and fuel mileage greatly BUT one must cut back the amount of vacuum advance to & around 10º

With full manifold vacuum my engine will advance right around 400 RPM more so I can cut back the idle speed and that's and with a GM stock 10º vacuum advance (# 1973577) my total advance is 10º base, 22º mechanical and 10º V.A. which hands out a 42º advance lite footing it down the road....

Never have seen a lean running 454 motor home in my life but mine is very close and just needs a slight dial in on the idle circuit, Also by running the full manifold to the V.A. trimming down the idle speed helps the fuel consumption ( not by much) but knowing everything adds up and I'll take it...

Now that"s the reason I'm looking real hard at those 4 springs cuz with one RV which only weighed 10,200 pounds (454) after the ignition properly setup it would pound up the test hill with a manifold vacuum of 8" while mine lugs around slightly over 18,000 pounds.

Jim

Jim

Cliff Ruggles:
It's a "hat trick" to keep the vacuum advance timing added and the power piston down when the engine sees a load. 

Doesn't really matter where you hook up the source for the advance, the manifold vacuum sourve only adds timing at idle and when coasting compared to the correct ported source.

The problem with trying to tune for any particular loaded situation, is that the load is seldom exactly the same from grade to grade, hill to hill, headwind added in or not, etc, etc.

The best tuning methods I've been able to come up with, is to find the best jet size first.  This is done assuming enough load is applied to have the PP up and the vacuum timing out.  Once you have determined the very best jet size for your application, then tuning via the APT, PP spring, and MVA can be done with good success.

Early in my testing with towing applications, I used to think that "lean" settings would improve fuel economy.  This only happens if you drive like a little old lady and pretend there is an egg under the gas pedal and you don't want to break it!

In the big scheme of things, you just can't get around the basic laws of  physics.  It's going to take X amount of BTU's to accomplish any given task.

When one tries for lean settings and keeping the PP down/vacuum timing on when the engine sees load, etc, more times than not, the operator has to continue to increase throttle angle to keep the vehicle at speed.

My testing involved towing on many occassions over the years with gasoline powered trucks.  Trips across the little mountains of WVA quickly taught me that best fuel economy was found tuning for best power, to keep up vehicle speed on steep grades, and to set the APT and vacuum unit for best efficiency at very light load.  The "in between" stuff sorts itself out in the long haul.......Cliff

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