Rebuilt 350 - Abysmal Fuel Econ

[rich weyand]

Some of that could be inconsistency in when the pump shuts off (you did go back to the same pump, right?). These gas tanks are weird in terms of when the pump shuts off. Very variable.

Having said that, not having the choke wired up is where your mileage is going. It is an electric choke, but the electricity does not enable the choke, IT TURNS IT OFF. So you are running full-choke rich all the time. Connect the choke and report back.

Move the vacuum advance to the driver's side port. As for advance, the more the merrier. No sense having the advance any less than it can be. It's like a horsepower adjustment. Having the fire get to the piston later is less power and less mileage. The advance at idle with the vacuum (8 degrees) all in should be about 25 BTDC, and the advance at 3000 rpm with the mechanical (20 degrees) all in should be 36-38. At cruise, total advance (base +vacuum +mechanical) should be between 45 and 50 BTDC.

 

[Nalanthi]

There has to be some rhyme or reason to it.

I'll go through the suggested obvious things that I should have done anyway, most importantly backing down on the idle and re-timing initial to around 17 BTDC without the vac advance hooked up.

It's become a bother considering that this engine has been totally refreshed, and should be easily a 10-12mpg machine, maybe more considering altitude and carb calibration.

 

[Skweegle89]

Some of that could be inconsistency in when the pump shuts off (you did go back to the same pump, right?). These gas tanks are weird in terms of when the pump shuts off. Very variable.

Having said that, not having the choke wired up is where your mileage is going. It is an electric choke, but the electricity does not enable the choke, IT TURNS IT OFF. So you are running full-choke rich all the time. Connect the choke and report back.

Not to sidetrack anybody, but my choke was unhooked when I bought the truck a month ago. green wire was just zip tied out of the way. When I plug it back in the choke light on the dash comes on. I drove it around for a few days like this but didn't notice any difference, so I just unplugged it again. I have adjusted the mixture screws 100 times to get rid of the rich smell before this with no dice. guess this might be my problem.

 

[rich weyand]

Easy to tell. Remove the air cleaner. Start the engine and let it idle for about fifteen minutes. The choke light should go out (if you have one; I don't) and the choke plate should open up at around five minutes. It also should come off the high idle cam if you give it a shot of the throttle to lift the fast idle screw to release the fast idle cam.

If the choke plate does not open, yeah, you are gonna run rich as hell, and mileage will be about half what it should be. Then you need to check all the wiring between the fuse block and the choke, and the choke itself. Possible problems include: blown fuse; bad oil pressure switch; bad choke heater spring; broken wires and bad connections.

Basically, what the choke is is a spring doodad that holds the choke plate closed and the fast idle cam in place when it is cold. The choke is often wired through an oil pressure switch and a light on the dash; mine is wired directly to ignition. The power to the choke runs through the spring doodad, heating it up. It expands and through levers and such opens the choke plate and rotates the fast idle cam out of the way. It will stay warm, and continue to hold the choke plate open until you stop the truck and it cools down.

There is an adjustment that involves loosening three screws and rotating the black cover the spring is in. Since the center of the spiral spring is connected to the can, this changes the time the choke plate will remain closed after the engine is started.

 

[Skweegle89]

It doesn't fast idle at all, even when I start it in the mornings. I talked to a couple guys about the choke and they said they just unhook theirs and don't worry about them, however their trucks are strictly used for mud runs. Im just tired of smelling like gas every time I get out of the truck, and the crappy mileage.

 

[rich weyand]

There's a way to disconnect the choke wire and then adjust it so it doesn't hold the choke plate closed, but it makes it miserable trying to get the engine started and idling when it's cold out. If they are doing mud runs in nice weather all the time, they wouldn't care. But if you disconnected the choke and didn't adjust it to stay open, the plate will stay closed all the time and 4-5 mpg would be expected.

 

[Skweegle89]

I will check it out when I get off work and see whats going on. Thanks for the tips.

 

[Nalanthi]

...which is probably a lot of the problem in both of our situations.
I thought that engine vac played a role in the choke's assisted opening - I take it that the electric must be hooked up and the choke adjusted in order for it to move?
Hell - my choke is open, but barely. maybe a quarter to a half an inch.

I think I had a n00b moment. lol

 

[Skweegle89]

I just checked mine and its open.

 

[Nalanthi]

I just checked mine and its open.

I'll keep you in the loop on what I find - lemme know what work you end up doing and how it helps.

 

[Skweegle89]

Will do.


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[MadOgre]

For a stock motor an initial of 17 BTDC is way to much, if you have a cam ok. Stick between 8-12 BTDC.

You may have 1000rpm as idle if your choke is not opening. The front choke plate above the primary bores must be fully opened in order to set you idle. Idle should be 550-600 rpm.

In warm weather or once your engine is fully warm an electric choke can open fully by itself, as it is still just a wound spring that expands with heat, electric, being heat is supplied by current running through the wound spring thus basically your choke will open fully after a certain amount of time, regardless of engine temp, as it is electronically controlled not thermostatically controlled by the engine.

If your wound spring becomes hot whether by the under hood temperature or by the electric current it will open the choke plate regardless of the heat source.

So on a nice hot summer day you will probably not notice an electric choke that is disconnected, however on a nice winter day of say minus 20 your going to notice that the choke never comes off and it idles at 1500 rpm all day long

The effects will be noticed more or less by how tight the choke spring is wound which is determined by where the choke cover is turned to. Qaudrajets have one stock setting but Edelbrocks have an adjustable set up. The further it is turned clockwise the tighter the spring is wound and the more heat is required to expand it to release the choke plate and vise versa.

So once your choke is not an issue set your idle, engine fully warmed, choke plate fully opened to 550 rpm. On both quadrajets and Edelbrocks the Idle setting screw ( not to be confused with idle mixture screws ) is on the drivers side just in front of the throttle linkage. it is a slotted screw on both carbs. turn clockwise for more idle and counter clockwise for less idle. Also make sure that the throttle return spring is doing its job in returning the throttle all the ways back to closed. Sometimes the throttle shaft itself will bind on old carbs and cause a problem with returning to idle.

 

[rich weyand]

MadOgre and I disagree on ignition timing. The street rod guys all tell me to advance it until it knocks under load, like punching it from stop up a steep hill, then back off two degrees. I was running 16-18* BTDC on the completely stock engine in my truck before I changed the engine out and it loved it. Chevy V8s love advance. Running less advance than you can limits horsepower.

"A typical advance curve for an engine built with a mild camshaft and having a compression ratio correctly matched to the cam will look something like this:

14-18 degrees initial advance
18-22 degrees centrifugal
10-12 degrees vacuum advance
Mechanical advance all in by =/< 3000 RPM"

http://www.crankshaftcoalition.com/...typical.22_performance_ignition_advance_curve

 

[rich weyand]

...which is probably a lot of the problem in both of our situations.
I thought that engine vac played a role in the choke's assisted opening - I take it that the electric must be hooked up and the choke adjusted in order for it to move?
Hell - my choke is open, but barely. maybe a quarter to a half an inch.

I think I had a n00b moment. lol

Quarter to a half an inch is "closed". The choke never goes to completely shut. When the choke is open it will stand pretty much straight up.

Choke on the 1406 is completely electric. No vacuum involvement. The choke has to be hooked up and adjusted for proper operation.

 

[rich weyand]

I just checked mine and its open.

Open, straight-up-and-down open, or open, kinda-sorta-ajar open?