Too Rich Idle,Cadillac 500

[78GMCMH]

Thank You for the great book and excellent rebuild kit. Having a problem with my GMC motorhome. Swapped out the 403 for a .020 over 507 Cadillac. When I try to adjust the idle air screws it idles poorly and will not stall with them all the way in. Afr reads very rich when all the way in, 12.5 to 1. Idle worsens the father out they are. Meter confirms that it gets even richer. I,m lean at part throttle and it stumbles when I give it more throttle, but I'm working on that. More worried about the idle. Since its a MH and we do spend time in campgrounds where people are asleep when we're leaving and its early,,,good idle is important. I'm using a 1984 carb of a P30 454 MH. #17080212, second line,1444CPW.
Cam.  Comp    202 int.    212 ex. on a 110 ctr. line (should have orderd on a 112 or 114)
Idle tube   .037
Idle down channel   .046
Lower idle air bleed   .056
Idle bypass air   .110
Mixture screw holes   .064
Main air bleeds   .052
Idle air bleeds   .067-.068
Main jets   72
I have in stock hangers,,D,,K,,M,,O,,P and rods,,CH,,CV,,CG,,DR,,DP,,CP,,DD although some of those are really fat.
 I spoke to you on the phone about this build along time ago and the carb # and date was unusual. It was off a 1984 454 P30 33 foot motorhome.Thank you for your time,,,,PL

[78GMCMH]

Opps,,,primary rods are .026 tip,,,41B.....thanks,,PL

[78GMCMH]

Again,,,sorry. Was looking at the 72 Caddy rods. The ones I'm using are "54V". Hard to tell if the V is a Y or not though,,,,but the tip is .036,,,,PL

[Cliff Ruggles]

Those are the motorhome metering rods more than likely. They have a long taper for the upper section.  You will need larger jets to effectively use them in that carburetor.  72's will be pretty lean.....Cliff

[78GMCMH]

Thanks Cliff,,,I'll go up on the 72's to 73 or 74's. But how do I get the idle to improve???

[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