QUADRAJET 102:

[eventhorizon66]

If you are pulling your plugs why don't you read them for us and let us know what they look like.  Are the insulators a nice caramel color, or are they blistered and light gray, or maybe charcoal black?  This could help the pros here diagnose your problem.  Check them all to tell if only one cylinder is off.

[VileZambonie]

Always use the highest quality ignition wires, caps, rotors etc. No need for MSD or anything unless you have a high perf ignition system but just stay away from the cheapies.

[Stewart G Griffin]

Air Cleaner/Air Cleaner housings Pt.2

NOTE: i'm going to address the ignition issues in a tic, but to continue with the discussion on aircleaner housings i (and yes i do work, but i came down with a very bad cold),

i was doing some reading and was somewhat shocked/suprised to discover that the stock air cleaner assemblies are actually designed to maintain the incoming air to the carb at 100F!

This isn't good, is it?  As far as performance, mpg, and general driveability?

i never knew what the circular thing on the snorkle was until yesterday.  Apparently, there is an air door inside the snorkle that regulates the amount of cold air vs. warm, underhood, air.

Would it be beneficial to disable this mechanism so as to provide only cool, outside air?  NOTE: i am emmisions  exempt.

[HAULIN IT]

Stewart, I don't know where you live or if you use your truck in cold weather, if not, then yes, you don't need it there. If you do drive it in the winter, simply remove the hose from the exhaust manifold in the warmer months. The way most of them work is a tube brings warm air from the exhaust manifold up to the air filter housing,aiding in early drivability.The valve on top (door in the snorkle) is pulled open from vacuum once the heated air triggers a heated vacuum switch inside the filter housing allowing outside temperature air to enter. The pieces you see on my set-up are from when the truck was driven year round, none of which work now. I put a freeze plug in the lower portion where the hose goes on & the pull rods,doors,ect. are gutted from inside the snorkles. Also many air filter housings have a shield/deflector right inside of the snorkle which also can be removed. Lorne

[Stewart G Griffin]

All this time i thought i was getting completely cool air (on the cars that had the ductwork intact), but no!  i know the factory did it for emmisions reasons, but what is the physics behind feeding the carb 100degree air as opposed to as cool as possible air?

It would seem like doing things such as raising compression, advancing timing, suppling coolest air etc. would help the engine run better thus lowering emissions, but apparently not?

[HAULIN IT]

Stewart, It is best to not try to think why or talk logic with anyone with basic knowledge of engines about what went on & why during the '70's-early '80's with the manufactures of vehicles. I don't have all of the answers, but know enough that the smart guys weren't making the decisions for themselves. There are allot of fators that go into vehicle rules & regulations,thus what & how it gets built. Also you have to look at the big picture, the manufactures have to make the vehicle operate for Jonny Q Public wherever he goes with his vehicle regardless of the temperature, ect. Where did you find the 100* air information? I'm not saying that is wrong, it's just not something I've ever heard or can't see how it is possable with say a stock air cleaner assembly on a '86 GM truck. There just isn't any "controller" for that to happen. Lorne

[VileZambonie]

The thermostatic air cleaner is designed to aid in EFE (early fuel evaporation)

Cold fuel in a wet intake manifold, be it carbureted or TBI will partially condense back into liquid fuel. Liquid fuel will not burn resulting in high HC emissions, fouled spark plugs, cold start stalling, washed cylinder walls, failed catalytic converters (especially in carburetted vehicles).

Once the engine warms up and the thermostatic vacuum switching valve turns off the inlet from the stove pipe the air is drawn in from the hose in front of the radiator support.

[Stewart G Griffin]

The 100degree air info is in the Doug Roe book, How to fix,tune and install Rochester carburetors.  i don't know the page # because the book is in my appartment.  There is also a book called "How to fix your Chevrolet" by Bill Tobolt that says the same thing.  Apparently, the air door will stay partially closed/open while the engine is running to maintain a 100degree intake temperature at all times!

Shocking isn't it? And apparently every Gm aircleaner from the 70s/80s with the circular thing on the snorkle does this.  The door is never completely shut unless the engine is not running or outside temps are 100 or above!

Roe's book states that a 100degree air intake temp causes a leaner mixture which reduces emissions.  i guess that makes sense;  Colder air equals denser air which, i guess, would bring in more fuel.

[HAULIN IT]

Ok, I'll do a little research on that. I have the Roe book. I guess part of my dismay to what you said simply is: What if it is 104* in Arizona? How does the hot engine keep the incoming air at 100*? Just making you think a little. Lorne

[VileZambonie]

It's designed to maintain between 85°F and 115°F. Again it's purpose is to maintain the best consistancy ensuring complete combsution, longevity of the engine, the oil, the catalytic converter and emissions standards. Their primary concern isn't if you can cram 10 more molecules of oxygen per gulp for tire spinning purposes.

That's why you rarely see any hotrodders running around with a functional thermostatic air cleaner.

Different emissions calibrations also had different specs...here are the specs from an 86 C20 with heavy duty emissions.

The TAC system is designed to improve carburetor operation and engine warm-up conditions. It achieves this by keeping the air entering the carburetor at a temperature of at least 100°F or more.
Carburetor air temperature is controlled by a pair of doors located in the air cleaner snorkel, which channel either preheated or under hood air to the carburetor.
Preheated air is obtained by passing under hood air through ducts surrounding the exhaust manifold, causing it to pick up heat from the manifold surface. The heated air is then drawn up through a pipe to the air cleaner snorkel.
Underhood air is picked up at the air cleaner snorkel in the conventional manner.
The two air mixing doors work together so that as one opens, the other closes and vice versa. When underhood temperature is below approximately 86°F., the cold air door closes, causing the hot air door to open. Hot air from the exhaust manifold stove is then drawn into the carburetor. As the underhood temperature increases, the cold air door begins to open until the temperature reaches approximately 115°to 130°F, at which time the cold air door is fully open and the hot air door is fully closed.
The doors are controlled by a vacuum motor mounted on the air cleaner snorkel. This motor, in turn, is controlled by a sensor inside the air cleaner which regulates the amount of vacuum present in the vacuum motor according to air cleaner temperature. Whenever manifold vacuum drops below 5---8 inches, depending on the unit, the diaphragm spring in the motor will open the cold air door wide in order to provide maximum air flow.
The vacuum motor and control door assembly in the left snorkel on outside air induction units does not have a sensor and is controlled only by manifold vacuum. This snorkel remains closed until full throttle is obtained. With manifold vacuum at 6-8 inches, the door will open, allowing maximum air flow.

[Stewart G Griffin]

What i'm wondering is how would disabling that mechanism affect mpg?  It seems to me that anything that helps the engine run better also, generally, helps mpg----like advancing the timing a little.

Then again, i heard that leaning out the carb as much as possible, but not to the point of damaging the engine, is best for mpg.

[HAULIN IT]

I just couldn't resist workin' over Stewart, nice guy & for some reason I like to bust his stones. I honestly didn't know those units (termostaic switch) were that sensitive. I allways knew when the vehicle warmed up some, it opened the door & the next time you looked when it was cold, it was closed. Thanks Stewart & Vile, You allways have room to learn! Lorne

[Stewart G Griffin]

Blows your mind, doesn't it?  All this time we thought, after the engine warmed up, that we were getting completely cold air from the outside.  Not so, Homer!  And this started in 71.

It's like we are all a bunch of suckers, and GM is evil.

Anyways, it's on pg. 160-163, Chapter 9 in the quadrajet book.  i found chapter 9 to be particularly fascinating.


But i'm still unclear about whether completely cold air vs. pre-heated air is ultimately better.  Specifically, my goal(s) is/are, maximum mpg without sacrificing engine durability/reliability.  Also note that i am emissions exempt.  While i do care greatly about the environment, one truck is not going to destroy it.

We know that in a racing/high-perfomance situation, cold air and, if possible, rammed cold air is the best.

But for street/Daily driving situations, i read that warmer air aids vaporization which helps fuel distribution for each cylinder in a 4barrel v-8 setup.  Also, warmer intake air=leaner mixture.

My theory is that for a multi-carbed setup, ideally 4 two barrel carbs, that completely cold air would ultimately be better for mpg.  While colder air=richer mixture, you simply would not have to give it as much throttle to maintain 65 mph?

[eventhorizon66]

Just out of curiosity.  If mpg is the ultimate goal, why aren't you considering EFI more seriously?  TBI kits are pretty cheap and they are going to offer better fuel economy potential than any carb setup.  They are definitely less $ than an exotic 4x2 setup.  Also keep in mind how popular late model engine swaps are with these trucks (tons of help available, no need to feel intimidated by this).  You can score a complete used 4.8L or 5.3L engine for real cheap nowadays (of course with this swap its all the little "conversion costs" that really get you).

On another note you seem like the type of guy that might be interested in "lean-burn" engines.  I read somewhere that GM experimented with this in the 70's (?), but one of these never made to production.  Honda makes some engines that run at about 22:1 A/F I've heard.  There's tons of info available with one quick google search.  Its a very interesting concept, I think.

[VileZambonie]

I believe you are referring to the PRE-combustion chamber 3 valve per cylinder design which all ended up with cracked heads, warped and blown head gaskets. Good in theory bad in real life. I do agree however Stew that you should consider a TBI swap. You can buy a bummed out suburban, blazer or pickup for $100-$200 bucks and just swap everything in.