I Installed headers and my mileage is worse!!!!! I haven't changed anything from the original setup. My EGR is stilled hooked up to nothing I suppose. The hose is in place. Should I adjust my carburetor? There is no choke hooked up. What is going on here?

You don't want the EGR opening with headers. Plug the hose. The EGR is a fuel saving device, but I don't think it should make a huge difference.
How much worse?

I know after I make modifications, I can't help the temptation to 'test' them a little bit more than normal.

Stomping the gas after every stop sign is a lot of fun, but seems to hurt my mileage.

Is it possible that the free flowing headers have thrown something like your carb adjustment or timing curve out of balance. Re-sealing the manifold gaskets might also have fixed a leak which requires the carb to now be adjusted.

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1980 Aspen 225 super six

I used to get 16.5 MPG and now I get 14.9 MPG.

Erm...the EGR has a few different functions, but it is not a fuel-saving device, in theory or in practice.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Did you remove the intake manifold to install the headers? also did you change any of the downstream exhaust system? Headers won't do much for flow if you leave the rest of the exhaust system unchanged. Also I suspect a change in the intake/ carburetor. Remeber headers give better exhaust flow but do nothing for the intake. Head flow, carb and intake are the other half of the system. What type of headers did you buy? Most headers for the slant don't have a choke system which could adversely effect cold starts.

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Sweet Emotion

New Clifford headers, fabricated Y-pipe into FlowMaster Series 50SUV muffler with 2 1/2" inlet and 2 - 2 1/4" outlets. I checked all the hoses and found many in the wrong place. Most wrong connections could have been made when intake was pulled to mount headers. I intend to put a manual choke cable on to correct the cold starting problem. Maybe it is now running too rich. I don't know if just installing new headers could cause a rich condition.

Using headers will cause your engine to run lean. Richen up the mixture (if you have a Holley go a 2 or 3 steps up with the jet sizes) some and see if that dosn't help. Lean mixtures can also cause poor milage.

This is the first step in my rebuild project. I have only installed the headers/muffler part of the rebuild. I am currently running my original Holley 1945 1-barrell on my stock 221,000 mile D-150 truck. I hope to rebuild for more performance.

I may be missunderstanding, but it seems to me that the EGR's function is to add inert gasses to the fuel/air mixture which allows the motor to run with less fuel/leaner mixtures.

What other functions am I missing?

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1980 Aspen 225 super six

the 1945 is one big fart from holley. get a 1920 or a 1 bl carter and see if that fixes your MPG #'s. Also, I'm with eric W about the lean mixture causing poor mileage. Haven't heard that fron anybody in a while but's true as it can be. If your headers don't have an intake manifold heating tricky fab, your mileage will be worse.

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Juan Ignacio Caino

Please use e-mail button istead of PM'ing. I do log in sometimes but I'll be answering quicker thru e-mail.

The EGR is to reduce peak combustion temperatures to reduce the formation of NOx emissions.....

A properly setup EGR should neither increase nor decrease mileage.

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

This part's entirely correct.
This part's not.

The mixture strength doesn't change because of EGR, all that changes is the amount of combustible mixture in the intake charge.
The reason to admit inert gas to the intake charge is to reduce peak combustion chamber temperatures (smaller amount of combustible mixture = less combustion heat), thereby reducing the formation of Nitrogen Oxides (NOx), which are toxic in their own right and are also a precursor to photochemical smog.

In the late '80s and '90s, with the advent of truly adaptive engine management systems and more effective reduction catalysts, EGR transitioned from a crude "hang it on and pray!" emission control device to a means of counterbalancing the tendency of aggressive spark advance curves and higher compression ratios to cause preignition. By using the engine management system to finely control the EGR (rather than the more or less "all on or all off" EGR control of the vacuum-based systems we have on our cars), it became possible to go to the higher compression ratios and greater spark advance levels that had disappeared in the early '70s when NOx emissions became a concern.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

H'mmm. I'm not trying to pick on you, but this was a little (a lot) backwards. Headers probably weren't the best choice of exhaust system upgrades in the first place (Dutra Duals are better), but putting headers on a tired pony is...kinda backwards. There are a lot more "bang for buck" improvements to be made on a post-'74 slant-6 truck before headers even appear in the distance as a potentially desireable modification. As you proceed with your upgrades, you may want to ask in this and other forums for guidance on how to prioritize your purchases and modifications.

Furthermore, your system sounds way too big. A 2-1/2" muffler inlet and dual 2-1/4" outlets is practically guaranteed to kill low-end torque, which causes you to push harder on the accelerator, which worsens your mileage.

Be aware that even with a system of properly-sized pipe based on volume flow rate calculations, headers do not necessarily improve performance or mileage. They can improve either or both, but it is not guaranteed and it is not universal. Their benefit is mostly at the kind of high-RPM engine operation most normally-driven trucks don't see; they often give poorer driveability and mileage at lower engine and vehicle speeds. What's more, just as careful tuning can radically improve the exhaust system's scavenging of the cylinders, careless tuning (or none at all) can actually increase exhaust backpressure at certain engine speeds. Both effects are influenced by the management of standing waves and pressure pulses within the exhaust system. These, in turn, are a function of primary and collector diameter and length, as well as diameter and length of the remainder of the exhaust system. It's really not as simple as bolting on a set of headers and *presto* getting better performance and mileage.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

The reason why the headers were first is so that I could drive the thing while I collect the parts I needed. Then having the parts I could take it apart in the garage, put the new stuff in it and be able to drive it within my neighborhood. I am totally new to this subject. I am trying to go slowly and pick the right stuff. The current dilema is carburetor/cam combination. I don't think that even after this engine is rebuilt it will see anything above 3000. Right now it looks like the CompCams 252S is the proper choice. But maybe the MP P4120243 is the better one.