Looking for best MPG on 454

[akdale]

Hi all,
Cliff's book should be in my hand this afternoon so I will have that to refer to, but there is no substitute for real time input.
I have been lurking and reading this site for a while, and have not quite located the info that I an seeking.

I have a 1984 31' class A Pace Arrow Motorhome.
I estimate that the weight is about 14,000 lbs.
It is on a P30 chevy truck frame.
The engine is a 454 that appears to be untouched although I am not sure if the factory put in a light cam for the motorhome application.
The tranny is a 3 speed turbo 400 which I would like to change to a 4 speed. (That is not in the cards yet until I can find a reasonably priced stand alone computer to manage it.)

When I bought the coach, (about 1500 miles ago) I found that I was getting about 4.5 MPG.
Looked at the carb and found that someone had tried to improve the economy by removing the choke plate..  They also Jammed the secondary air closed where they could not function so I suspect that when the manual secondaries opened, it was sucking fuel through the jets like a straw and just running super rich without the proper air mix.
There were so many vacuum leaks that the exhaust was not black.

I put a kit the carb re-using the jets and metering rods that were in it.
I also attempted to use JB weld to patch the leaky bowl.  Maybe worked for 5 minutes.
Going to tap and plug them this time.
Engine is not running bad.
No Bog
Secondaries working great.
With the dog house open, I can see them open all the way.
(I have to be careful when they open all the way---The duelies break loose and she starts getting sideways..... 
Idle slightly rough with a intermittent miss.
Timing is at 6 BTDC  Supposed to be at 4 BTDC. (Vacuum disconnected and plugged from Dist.)
Mechanical and Ported vacuum Dist.
13-14hg Vacuum at 800rpm idle
Smog still hooked up.


Carb number= 17084502 2373HNX
Jets=73
Primary rods=49M
Secondary rods=DH
I do not have the Bleed and passage info.

The question that I have is,
What is the recipe that will get the most MPG from this setup.
(Driving like Granny) I am rarely into the secondaries.
Thanks,
Dale

[429bbf]

dale i built a carb for a buddy of mine for a ford motorhome about the same specs as you have. i used a 17085212.according to your no.170845## yours is a california model. heres how we set this one up.idle tubes .034  idle down channel restrictors .056 upper air bleed .068 lower .062 idle bypass air .075 mixture screws .060  jets are .075 needles are 58s   single main air bleed carb in air horn .052 acc.discharge .026 holes for 4 barrel wells .040 secondary needles EF i adjusted the throttle plate stops to 90 degrees and bent the linkage so he could run on the primaries untill the very last amount of throttle pedal  then the secondaries will start to open. it used to have a holly at 4-5 mpg now hes running 8-9 the APT is set about 4 turns up and the idle screws are out about 8 turns. runs like a dream.hope this helps dean

[akdale]

Thanks Dean.
Quite an improvement.
Looks like you have less fuel at cruise/mid range than I do now?
Dale

[429bbf]

dale i dont know if your carb has the top limit on the APT.i had his set about 3 turns down and he said it seemed lean just before the 4 barrel was opening . so i had him raise it 1 turn and he says now he very seldom uses the secondaries. hope this helps.when i got done adjusting things the primaries were about 90 percent open before the sec.started to open.

[akdale]

I have the adjustable lower limit.
I think that the only way for me to adjust the rich limit is to bend the hanger.

[akdale]

Just finished reading Cliffs book.   Nice!
Not sure if the specs for the ford carb will be the same for the chevy.
So, It looks like the recipe 1 is the one for my engine and application?
and since my carb uses the hot air choke, no Idle bypass?
Anyone have a choke plate and screws kicking around?  Someone removed the one from my carb before I got it.  Choke is a good thing in Alaska.
Thanks,
Dale

[akdale]

Cliff--
Is recipe 1 the one that I need to use?  Do I need to change anything?

I have a 1984 31' class A Pace Arrow Motorhome.
I estimate that the weight is about 14,000 lbs.
It is on a P30 chevy truck frame.
The engine is a 454 that appears to be untouched although I am not sure if the factory put in a light cam for the motorhome application.
Timing is at 6 BTDC  Supposed to be at 4 BTDC. (Vacuum disconnected and plugged from Dist.)
Mechanical and Ported vacuum Dist.
13-14hg Vacuum at 800rpm idle
Smog still hooked up.

Carb number= 17084502 2373HNX
Jets=73
Primary rods=49M
Secondary rods=DH
I do not have the Bleed and passage info.
My bits should be here tomorrow.

The tranny is a 3 speed turbo 400 which I would like to change to a 4 speed. (That is not in the cards yet until I can find a reasonably priced stand alone computer to manage it.)

Thanks in advance
Dale

[akdale]

I can't edit the above post to insert the following info.
I destroyed an Idle tube removing it and will need a pair.
Not sure about the jets and primary rod sizes.  They were working great.
Will reducing the size of the primary metering rods overly richen the mix and use more fuel, or will it make the engine use the fuel more efficiently?

Carb number= 17084502 2373HNX

Idle tube=.042
Idle down channel=.042
Upper Idle Bleed=.067
Lower Idle Bleed=.060
Idle Screw Holes .090
Jets=73
Primary rods=49M
Secondary rods=DH

[Cliff Ruggles]

The best course of action for a stock carburetor on a stock application is to completely and correctly rebuild it with good parts using stock settings.  Then test the carburetor and go back and address any areas of concern.

Leave the plug out over the APT screw, or tap the castings for a screw in plug, so you can raise and lower the primary metering rods without taking the top off of the carburetor.

Set the idle mixture screwsd for the best idle quality at the leanest settings, and APT screw back to the stock setting.  If the stock setting is not known, or tapering is suspected, set it at 3.5 turns up for seated for initial testing.  Using our "tip-in" procedure from the book to fine tune the APT will also get you very close to where it needs to be.

I would also inspect the distributor to make sure the mechanical and vacuum advance is working correctly.  Timing is equally as important as fuel in the search for best engine efficiency and fuel consumption.....Cliff

[akdale]

Thanks for the reply Cliff.
Does the above configuration look like it may be the stock configuration for the application?
The carb had been opened before I bought the motorhome.
Dale

[Cliff Ruggles]

It has a lot of idle fuel, otherwise about right for jets and metering rods.....Cliff

[akdale]

Idle tube and idle down channel need to be smaller?
What would be the recommended size?
Thanks

[akdale]

Went with .035 on the idle tube.
Drilled out idle screw hole to .090 and cut springs so idle screws can seat.
Set idle screws to 2 turns out.
Engine fired and ran as soon as gas hit the float bowl.
Set idle screws to get best manifold vacuum.  about 4 1/2 turns open.
Set idle to 700 rpm.
Timing is 6* BTDC with no vacuum at the Dist.
Rechecked idle screws--no change
Used Cliffs tip in procedure for setting primary power piston.
Engine is running perfectly.
Smooth transition from idle to mid to Secondaries.
Secondaries start to open at about 5-7HG vacuum and completely open at WOT.
Smooth transition from Idle with WOT all the way to cruise at 55 mph
Cruise at 50 mph is 2900RPM and 13HG Vacuum.
Now to see what the MPGs will be.....

[429bbf]

ak sounds like you hit it on the head . let us know the milage you get the ford doubled .but it had a holley on to start with . had to use an adapter to install the q jet. dean

[TommyK]

I would strongly recommend getting the vacuum advance hooked up and set properly if the distributor has one. It will get your cruise vacuum up and increase fuel mileage.