Throttle Body vs Carb Spacer

[Foxtrot]

I don't think I understand the basic difference between a throttle and carburetor. Isn't the throttle just a part within the carburetor-the valve itself that actually controls fuel flow while the carb is responsible more for the airflow and mixing the fuel?

I've been doing research on improving the economy of my truck. One suggestion is to add a throttle body spacer. I started looking into these for my model, and found there were none, but was something called a carburetor spacer instead. Did further research, and I'm just kinda stumped. What's the difference between the two, and for my MKIV ('79) Big Block, which do I actually need?

Assuming a carb spacer is what I'm after... From what I have learned, the carb spacer has two purposes: keeping the carb cooler and allowing more air volume/atomization/less resistance. As such, a plastic 2" carb spacer would be better at doing its job than an aluminum 1" spacer? More volume and less heat conductivity?

Again, for reference, I have a 1979 C20 Camper Special (Silverado) with a 454--all original.

[geezer#99]

Throttle body spacer is for a tbi (throttle body injection)on a newer motor.
Promotes better flow into the intake.
Not applicable for your carbed motor.
A carb spacer is used primarilly to reduce heat to your carb to reduce or eliminate the fuel boiling in the carb. It might help flow if you have a poor flowing intake.

A throttle is part of a carb. It's the primary plate. It controls the amount of fuel vapor that flows into the intake and on into the cylinder to be burned. The fuel is vaporized in the top of the carb.

You want better economy then you need to optimize your timing and possibly adjust the APT (adjustable part throttle) in your carb.
What's your initial timing?

[Foxtrot]

Thanks for your reply.

Sounds like a carb spacer would be pointless for my goal. I don't seem to have any intake issues, the engine runs exceptionally well and while we've only had it during cold weather, the climate here in the Sound is mild (if the heat difference could cause boiling in less mild climates).

Originally Posted by geezer#99

You want better economy then you need to optimize your timing and possibly adjust the APT (adjustable part throttle) in your carb.
What's your initial timing?

I'll be honest in saying I don't know exactly what you're saying.
When you ask for the initial timing, do you mean what's listed as a spec in the service manual or do you want to know the current timing as read with a timing gun?

I'm mechanically inclined and have never had issues doing anything in terms of replacing/maintaining parts. Getting into tuning is something new to me, but I learned everything else, so time to start doing some research. Overall, sounds like I'm a bit ahead of myself trying to do things like change parts without making sure its tuned well first.

[Foxtrot]

Did some research. Adjusting timing is easier than I'd anticipated. Correct me if I'm wrong, but essentially get a timing gun, check the timing, and adjust the rotor as necessary to get the timing back to what's listed in the service manual, 8°

[geezer#99]

Originally Posted by Foxtrot

Did some research. Adjusting timing is easier than I'd anticipated. Correct me if I'm wrong, but essentially get a timing gun, check the timing, and adjust the rotor as necessary to get the timing back to what's listed in the service manual, 8°

That's close!!
You don't adjust the rotor exactly. You rotate the distributor body clockwise for more advance, counterclockwise for less advance.
8* might be stock but you should be able to advance that up closer to 12 to 14. I live at the same elevation and weather as you do and run all my stuff at 14 initial. Block off the vacuum to the vac pot on the distributor when you check and set timing. Also increased initial timing will likely increase your idle slightly. Just turn the idle back down on the carb if needed.
Here's a link with some good info on timing.
http://www.badasscars.com/index.cfm/...d=76/prd76.htm

I did mention adjusting your apt but get your timing good first and then we'll decide if tearing the carb down for some mods is worthwile.

Could you also post a pick of your engine bay.

[MikeB]

Originally Posted by geezer#99

That's close!!
You don't adjust the rotor exactly. You rotate the distributor body clockwise for more advance, counterclockwise for less advance.

I've worked on only one BBC, but I think it's CCW for more initial advance, CW for less, just like a small block. However, I recently worked on a 400 Pontiac, which is just the opposite, because the distributor is driven by the other side of the cam.

I agree that 12-14 degrees will probably be the sweet spot, maybe even 16. The symptoms of too much advance are:

-- pre-ignition (knocking) on acceleration, especially when engine coolant is up to temp
-- engine turns over slowly when you're trying to start it

Also, make sure there are no vacuum leaks anywhere. Check for cracked hoses, and then suck on the hoses connected to the vacuum advance canister, transmission modulator, any thermal vacuum control valves, EGR valve, etc. to make sure there is resistance. Especially important is the large hose running from the back of the carb or intake manifold to the brake booster.

[geezer#99]

Originally Posted by MikeB

I've worked on only one BBC, but I think it's CCW for more initial advance, CW for less, just like a small block.

I.

Oooops!!
You are correct!

[Dead Parrot]

Here is a thread on timing: http://67-72chevytrucks.com/vboard/s...d.php?t=274806

Is your truck still covered under any kind of emissions testing/inspections?
Verify that the following still work:
Exhaust heat riser valve
THERMAC system
EGR(if equipped)
PVC system, PVC valve and filter are cheap
Both mechanical and vacuum advance

If they are several years old or age unknown, consider new cap, rotor, plug wires and plugs. While the cap is off, repair or replace any broken advance parts as needed.
Replace any cracked or hard vacuum hoses.

Sounds like a lot but if you do your "how to" research first, then buy or borrow any needed tools should take less then a day to fully check things out.

Trying to tune an engine when these systems are not working properly is an exercise in frustration.

[fleetsidelarry]

Originally Posted by Foxtrot

Again, for reference, I have a 1979 C20 Camper Special (Silverado) with a 454--all original.

Originally Posted by Foxtrot

..the engine runs exceptionally well..

my first thoughts are: a '79 camper 454 isn't going to be getting much in the way of gas mileage and "if it ain't broke, don't fix it". but that last part isn't very appealing to most of us

aside from that, knowing what your current initial timing is is always a good thing.

your vacuum advance may be operating off a different vacuum source than geezer's which will make a difference.

and Parrot's comments about emissions control stuff are pertinent 'cause all that mess works against fuel mileage/performance

[Foxtrot]

Thank you all for the great responses, lots of good knowledge around here. Makes sense of and lines up with this 1100 page service manual from GM.

Slow to respond, busy weeks and spent time on the truck doing an oil change and working on some electrical (the stuff I'm really good at). Need to get a timing gun still before I can check all that out. Turns out neither autozone or o'reilly have any to loan out.

That aside, a couple more follow up questions.

Originally Posted by geezer#99

8* might be stock but you should be able to advance that up closer to 12 to 14. I live at the same elevation and weather as you do and run all my stuff at 14 initial.

Originally Posted by MikeB

I agree that 12-14 degrees will probably be the sweet spot, maybe even 16. The symptoms of too much advance are:

-- pre-ignition (knocking) on acceleration, especially when engine coolant is up to temp
-- engine turns over slowly when you're trying to start it

How exactly do I tell I've advanced it far enough though? I understand the physics/chemistry/thermodynamics behind the timing and such, but how do I tell where between 8* and 16* is best? Or is there no way short of a dynamometer?

Originally Posted by geezer#99

Could you also post a pick of your engine bay.

The fat lady is certainly not pretty, but she sings beautifully. I've attached a few shots under the hood.

Originally Posted by Dead Parrot

Is your truck still covered under any kind of emissions testing/inspections?
Verify that the following still work:
Exhaust heat riser valve
THERMAC system
EGR(if equipped)
PVC system, PVC valve and filter are cheap
Both mechanical and vacuum advance

If they are several years old or age unknown, consider new cap, rotor, plug wires and plugs. While the cap is off, repair or replace any broken advance parts as needed.
Replace any cracked or hard vacuum hoses.

Sounds like a lot but if you do your "how to" research first, then buy or borrow any needed tools should take less then a day to fully check things out.

Trying to tune an engine when these systems are not working properly is an exercise in frustration.

About that... that A.I.R. pump may or may not be working, but for a couple weeks it was making all sorts of noise (sounded like something rotating was striking something stationary on each revolution).
I'm free from emissions checks here, so I'm contemplating removing the A.I.R. pump in lieu of replacing it. I posted a different thread here about the issue, but it didn't get any responses. Everything else you've mentioned is in condition worth leaving for now. A lot of maintenance was done not long before we acquired the truck--new alternator (I guess my all original claim is a bit false), new radiator hoses, fresh battery, fresh spark plugs. The plug wires/rotor/cap aren't brand new, but they aren't original either.

In case anyone is curious about the full state of it all, there are a handful of small things I'm working on with the truck too. A small coolant leak, some excessive/poorly routed wiring to be reassessed, new windshield washer system needs to be purchased, rear quarter fender that came w/ needs to be installed (we'll do that after the engine/electrical are tip-top), horn's not working (something after the relay I think), cruise isn't working (though I think I fixed it last night and just didn't know how to activate it, ...trim-down used to be a thing apparently), and the cold air intake snorkel--if you can call it that--is AWOL.

[prostreetC-10]

Originally Posted by Foxtrot

How exactly do I tell I've advanced it far enough though? I understand the physics/chemistry/thermodynamics behind the timing and such, but how do I tell where between 8* and 16* is best? Or is there no way short of a dynamometer?

How it will run will be your answer. You are going to need to find the timing line on your balancer and you will probably need to clean it off. There will be a timing tab on your chain cover that will have marks on it. Best thing to do is to look at one of the 10,000 you tube videos on setting timing.

A stock 454 will love a lot of initial timing. 18* has always been the sweet spot for me. 8* will be out of the question. Since these make all there power down low, you want that timing coming in fairly quickly. I would guess 14*-18* will be nice and snappy on the throttle. As you advance, your idle speed will certainly come up and you trim that down with your throttle position (idle) screw on the carburetor.

Having properly set timing can be one of the most incredible wake-ups to a motor......especially if someone last set it with the old Chilton's recommended 6*-8* or so.

[Dead Parrot]

If you have no emissions worries, remove the AIR pump. One less thing in the engine bay and that one does rob some power and causes more exhaust manifold heat. Its main purpose is to squirt air into the exhaust to burn off unburned fuel. Sometimes the timing was deliberately retarded a bit from normal to have more unburned fuel to improve the efficiency of the exhaust manifold burn. So you have the drag from the pump plus the loss of power and efficiency from the smaller timing advance.

You will need a plan to plug the AIR ports in the manifold. Chances of getting all 8 out after 38 years is pretty slim.