h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Rochester's Quadrajet carburetors were a staple of General Motors V-8 powered vehicles from the late sixties until the switch to electronic fuel injection was finished in the late eighties. Capable of both good fuel economy (for a V-8) and performance they made a name for themselves, although it is sometimes used as a curse.

When introduced it was the most complicated carburetor of its time, incorporating four-barrels and many functions (i.e. fast idle, choke). It was a fearsome rebuilding task for most technicians who were used to tuning Holleys and Carters. The myriad of linkages, internal circuits, and easily lost tiny pieces were incomprehensible to some. With age the Quadrajet earned a following of techinicans who understood its design and recognized its potential.

The primary barrels of the carburetor are tiny compared to most four-barrel designs, but this is what gives the Quadrajet its gas milage edge. In contrast, the secondary barrels are huge, providing a performance edge. During normal driving the primary barrels are adequate for cruising speeds. The beast comes out when the pedal is depressed further. The secondaries open and there is the slightest amount of delay as the accelerator pump richens the mixture. A Quadrajet carbureted car is often distinguishable from other cars by the sound of the engine as the secondaries open. There is a moment of quiet followed by a large increase in exhaust volume, sometimes described as a 'booming' noise.

Most performance enthusiasts shun the Quadrajet as a stock carburetor laden with useless emissions controls. In reality the Quadrajet offers performance on par with most aftermarket carburetors while retaining good driveability and gas mileage. With a little modification most Quadrajets can easily reach 750CFM airflow.

There were many iterations of the Quadrajet, even including some electronic versions produced while General Motors was dragging their feet in changing to electronic fuel injection. The most desirable are the ones produced in the mid-seventies on big-block powered high-performance and luxury cars. These can flow up to 725CFM in the stock configuration.

I re-built one Rochester when I was a teenager(some 26 years ago)...When I finally got that monster back together again(believe it or not, it worked!),I swore I would NEVER attempt that again!LOL

The amazing thing was that I actually found all those itty-biddy pieces which fell on the floor as I took it apart! Our neighbor, at the time, was a GM carburator specialist. He used one of them in an experiment on a full size Chevy truck..350 cid. Took the truck from 10 mpg to near 40mpg with little, if any, loss of performance! That was back in '74!

Looks like a good Article you've got going there! I'll tell you a trick I learned a while back to save space in my Journal, though....I opened an Article page and labled it "Workshop". I do my initial writing in there, then "copy and paste" to a new Article page when I'm ready for criticism. Works out well. Saves space on my User Page, and keeps it out of site 'till I'm ready to take it public!

NM

Thanks for the tip, but I've been working it the other way. I write in my journal and then copy the text to a file on my harddrive to remember what I've written.

I've got an electronic quadrajet on my '81 Camaro. I rebuilt it with the help of one of the master techs I worked with when I was a Nissan tech. He had all the little gadgets to adjust the settings. The car runs great and I learned a lot. I also have one of the larger q-jets from a '76 Cad 500. The motor's rebuilt and waiting for the day when I install it in the Camaro and put the q-jet back on top.

A 500 Caddy in an '81 Camaro!!!OUCH!!! Let me know how it turns out!

As far as writing articles, I usually start off-line on "Word"....then transfer to "Workshop", then on to a "Release" page. Saves headaches in case AOL decides to kick me off mid-sentence!
NM

It'll be a long while before I get the 500 installed. Gotta get the RX-7 running first. Gotta get a house before I fix the RX-7.

Also, that's why I don't use AOL (that and all the advertisements). I forgot to log off one time and noticed two hours later I was still connected.

Lucky on the connection with AOL. It usually kicks me off if I stay on the same page for more than 10 minutes!

What exactly is wrong with the motor in the RX-7?.....Just curious.

NM

Low compression, the apex seals are shot. The side seals are still good, it doesn't smoke, but it barely runs. The motor has 173,000 miles on it. I've found a company that sells rebuilt rotaries for $1350, and they street port them for another $250. But that'll be after I get a house (I don't have a garage right now, and the apartment managers frown on rebuilding cars in the parking lot).

That's understandable....the city of Gardendale don't like it either, and I have a garage!LOL

HHmmm... it's got the Wankle in it.(the RX-7) If you don't know how to do it yourself, you might as well learn. Around here, if you hit a mechanic up about a rotary, they just tell you to buy a motor......none of them know anything about the format!

Mean engines, though! Whole lots of horsies and torque......small engine with very few moving parts. I've always wondered why they didn't catch on better.

NM

1) The parts alone to rebuild the engine would run about $1000.
2) I don't know enough to port it myself.
3) Everbody I know ruined the first rotary they tried to rebuild.
4) The company I'm going to buy from has a warranty.
I have a friend in a Mazda/Nissan dealership near Tulsa, they don't rebuild rotaries, they replace them.

I think those are some of the reasons the rotary hasn't caught on yet, but they're about to introduce some new cars with them. You might even see a rotary powered Ford soon (since they own Mazda now).

The main reason rotaries are having a hard time is they don't hold up if they aren't maintained. The oil and coolant MUST be changed regularly. Most people don't do any maintenance unless their car breaks down. With a rotary that can get very expensive.

You're right though, mean engines! I'd turn 7000rpm before shifting almost all the time. I ran that engine hard, but kept the oil changed. It would get a good second gear scratch with both rear tires (limited slip diff). And it lasted 173,000 miles doing it (I bought the car with 120,000 on the odometer).

Small, lightweight, high reving......with great performance! 173,000 miles ain't a shabby record for any engine!

Standard piston engines with that many miles are hard pressed to be rebuildable. If they are, the cost is about on line with replacement! It's my opinion that if the cost is near equal, replacement is the easiest way to go, anyway!

NM

Here's a trick for sometimes resurrecting rotaries with "low compression".... Sometimes, because of carbon deposits, the apex seals can stick back on their springs, and won't make compression.

Here's what you do (and if you do it and bugger things up, don't come back to me). Pull the leads off the ignition coils so your plugs don't fire. Be careful not to touch them while doing this, you could get a nasty, nasty shock.

Pull off the air cleaner cover.

Pour about three to four ounces of Automatic Transmission Fluid (ATF) down the bore of the carb. Give it some time to run down the intake.

Turn the key to turn over the motor good a few times. Don't overdo it, you'll kill the battery or your starter. Having a healthy well-charged battery or jumper cables is not a bad idea.

Let the motor sit overnight.

Do this two or three times, over two or three days.

After you have done this, hook up the ignition leads. Then start the car as normal. It will make a hideous enormous amount of stinky smoke for a few minutes, don't worry about it, it is just the ATF burning off.

If you are lucky, the ATF soak will allow the apex seals to spring back into place. If not, your motor is still as screwed up as it was, nothing much lost. This is also good every now and then for maintaining the health of good rotary motors as well....

Sounds a bit like refreashing lifters in a piston engine!........sounds like something good to try!

NM

I took it to a dealership I used to work at (they got rid of the Nissans and have Mazdas there now) and did a rotary compression test, it was low all across the board (no specific problems). They said they'd never seen one with such low readings run. Being that ALL the readings were extremely low, and that the mileage is sitting at 172,000, I think it's pretty much done for.
Thanks for the tip though. I've also heard the condition you describe as 'flooded' and 'the apex seals have wiped all the oil off the rotor housings'. There's an easier way to fix these problems. Push the car in 2nd gear at about 25mph for about 50 feet. Crank the starter with your foot to the floor. When it starts it'll smoke like crazy. I'd had problems starting my car after it sat for a few days if I didn't give it lots of choke and pump the gas about 5 times. If it failed to start this method ALWAYS started it quickly and easily (all I had to do was find a push car and another driver).

Sorry, but what I am describing is NOT flooding. That is when you dump too much gas in the car trying to start it, and rotaries can't clear the gas out like a piston motor. The gasoline won't seal the apexes, and the motor won't start. Push starting will often clear it. Pouring an ounce or two of ATF down the carb and then starting will let the apex seals do their job and start a flooded motor.

So listen up! Although similar, that is not what the procedure I am describing is for. What I described is for loosening carbon-encrusted apex seals and knocking carbon off the rotor housings. The fact that the guys at the local shop found NO compression in that rotor tells me that they are clueless, as that is not how rotaries work. You can't correctly check rotary compression with a standard compression guage anyway. Do yourself a favor and try what I described.

The only thing that you stand to lose is that your motor is still screwed. Go do what I described, then come back and let us know what happened. All it is going to take is $1 for a quart of ATF and some time. Wouldn't you rather have a running rotary if that's all it takes?

"AMEN"!

NM

Okay,okay. I'll try it. But read more carefully, the shop I took it to used a ROTARY COMPRESSION TESTER, the digital kind that is made FOR rotaries. I did most of the actual work of testing it, they gave their professional opinions on the condition of the motor (damn! it still runs?!) I didn't say it had no compression, it was low in every combustion chamber, which means that instead of an apex seal hanging up (which would cause low compression in two chambers) I have severe wear (all the apex seals are weak).
I do appreciate the advice, and I'll try it, but I'm skeptical of the benefit.

Check this FAQ - http://www.mazspeed.com/atf.htm

Just came back from the FAQ. That's interesting!

Flyboy.....I realise you have a high mileage engine, but it definitely looks like a "first try" proposition! May not work, but for the money, I'd have to give it a shot!

NM

I put a capful of ATF down each primary yesterday and spun the motor over - it sounded a bit better. Did the same today, but the motor still doesn't turn over very fast. The battery's far too weak, even when jumped with the Camaro's 800 CCA battery (dead batteries can suck a lot of juice). Tomorrow afternoon I think I'm gonna try starting it, after I temporarily replace the battery with the Camaro's battery. I hope it does run... of course I'll have to put it back on insurance (about $20 more a month).

He-he...I know the insurance routine!

If all else fails, run 2 batteries in series. If 24 volts won't spin it fast enough, you need a starter!
NM