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Rochester Quadrajet...with picture

C

Christopher S.

Member
Good Morning Gentlemen, attached is a picture of the first New pickup truck my Grandfather ever bought. It was restored around the year 2000 and then was put into storage for a while. It was state inspected in 2015 but only had about and additional 50 miles put on it since then. We would like to get it back on the road again but ran into a carb issue I would like to get your opinion on.

The truck is a 1970 C10 with a 350 ci motor and a 4 barrel rochester quadrajet. When we dug into it last week, the float was stuck on the carb. We disassembled, cleaned and put it back together and it will now run pretty well. The issue I'm having now is when you stab the throttle it will kill the motor. If you feather your foot into it, its fine. At highway speed you can put your foot to the floor and it is also fine. The problem seems to be only at low speed when trying to pull out. I'm leaning towards a bad / worn accelerator pump but would like conformation of this. We have ran an entire tank of fuel through the truck and fresh fuel didn't seem to change the situation.

Also, on the back of the carb, there appears to be a small nipple that would allow a vacuum line to be attached, was this connected to the factory air cleaner and should now be capped off?

Let me know what you all think.


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Sounds like accelerator pump but there is no way to "confirm."

A friend of mine has two yellow and white Chevrolet trucks of similar vintage, 72s, I believe.

One is a 2500 Series 4X4 with 400 CI small block engine AC and all the bells and whistles. The other, also a loaded 2500 Series, is 454 powered and has been converted to a dually. He also has a matching yellow and white Blazer of similar vintage.

I see them regularly at local car shows.

Dean
 
The quadrajet on the 1969 Chevrolet Caprice we bought new had a bad habit of the metering rods sticking in their bore and causing lean out when trying to accelerate fast. They finally replaced the carburetor and the problem went away . The vacuum line to the air cleaner probably operated the thermal temperature door for cold startup.
 
Yes.

perhaps the carb needs service beyond what you have done.

there are other items that can effect drivability
does the accelerator pump squirt?
a relatively simple visual inspection may indicate

there are a number of areas that can become effected that a quality rebuild can address

is the vacuum advance connected and operating properly?

points and timing set and correct?

Choke operation?

appropriate jetting

emulsion tubes

metering rods

are the layers of the carb still tight?
vale adjustment?

manifold leaks?

internal filter?

It no doubt had a warmed air intake that has been omitted. That can cause stumbles.

How many kinked inlet lines I've seen, but that usually exhibits under load
 
That vacuum connection needs to be plugged if it is drilled. Check for vacuum on it when the engine is running. Some ports like that were for accessories which were not drilled on vehicles not using the accessory (like vacuum powered cruise control).
 
LOL we used to call those carbs Quadra junk Always something with them most just put a Holley on and never had a problem.
 
With the engine off look down in the throat of the carb and open up the throttle and you should see two pretty good squirts of fuel steaming down into the throat. If not more than likely you have a bad accelerator pump. Probably dried out over the years. As far as the nipple on the back put your finger on it while idling and see if you can feel any vacuum. Then bring the rpm's up to about 2000 and see if any vacuum on it then. If you have vacuum at any time on that port you can just plug it off. I believe it was used for the original choke system but that looks to be updated to electric choke now. Also may sure the dwell on the points is right at 28 to 30.
 
Sure sounds like the accelerator pump is not working. You should be able to look at it squirt as you open the throttle with the engine off. It should squirt as soon as you move the throttle lever. Perhaps it or it's linkage did not get put back on correctly ? good chance from sitting the rubber cup on it is bad.
I forget where that spot on the carb. that nipple goes to ? For sure if there is any sucking of vacuum at that port it needs blocked off.
 
Classic accel pump issue and easy fix without taking the carb off the engine. Either stuck, out of adjustment, or just dried out and hard. Just uses a little umbrella shaped seal with a spring inside.

15 minute fix.

You can easily check just but looking down into the carb when moving the throttle. It should squirt raw gas like a little water-pistol.

Quadrajets were good carbs with proper tuning.

One other issue that is sometimes found on 1970 small-blocks when it comes to poor throttle response is the lack of manifold heat. Sometimes the factory put little tin block-off plates in the runners that go under the carb for emissions purposes. This made poor throttle response until completely warmed up - but once hot - the engine would work fine.
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remove air cleaner and look in carb to see if acc. pump is working. while opening throttle by hand u will see 2 squirts of fuel going to each venturie. yes plug that hose.
 
-Yes it is common for the accelerator pump to shrink after sitting dry for an extended period and it can cause the symptoms you describe.

Other common things with similar symptoms;

-Bad diaphragm in vacuum advance
-Sticking/rusty advance weights and springs in distributor
-Rod that controls accelerator pump in wrong hole or arm bent causing a lag in the timing of it spraying in the fuel on hard acceleration.
-Vacuum leaks; carb, intake, accessories, hose to modulator valve on transmission (if it has an automatic) often gets cracked or knocked off
-Mouse nest in exhaust system
-Not running on all 8 cylinders, electrical, valves, comression etc
 
Hole on the rear of the air cleaner shroud is where the PCV valve was connected. On engines using a little oil, the air cleaner would have an oil smudge at it's location on the outside.

The Quadrajet has a metallic fuel filter at the fuel inlet connection...where the steel hose connects from the fuel pump. Changed that??????

The Distributor has mechanical advance and may be frozen. No MA with acceleration floods the engine as it can't setup the explosion fast enough to occur before the piston position is ready for it......exactly your mentioned problem! Pop the points mounting plate and the arms are right there....as I recall...1971 400 cu in big block, Chevy station wagon. I had to have that thing for some reason. Went to Steakley Chev. in Dallas to buy one. Tried 3 and all had the same problem....small inlet in the air intake shroud supposedly feeding air to the QJ. Would have done a lot better with a Suburban but they weren't all that great back then with only one rear door...a 3 door, not 4 like the station wagon....and hindsight is 20-20.

QJs need air to run right. Big problem with OEMS was that stupid air filter shroud with the tiny little hole where air was supposed to enter. Flipping the lid upside down on the shroud made a tremendous difference in acceleration ability but was noisy when you "got on it"......reason Detroit designed the stupid AC shroud.....opinion.
 
I can't help much either, but nothing beats the sound of a quadrajet when all 4
barrels open up!

Ben
 
Bingo, 730.

The Rochester Quadrajet was an excellent carburetor.

Incorporating sophisticated fuel metering with small primary venturis, it was very economical, yet offered high air flow on demand and was suitable for use of engines across a wide range of displacement.

Inexpensive to manufacture as compared to other HP alternatives, it obsoleted prior 3X2 systems in all but rare niche markets. It, along with the 4GC, were two of the best carburetors ever produced in high volume, and GM sold millions of each.

Dean
 
I'd put my money on a accelerator pump. If it's been sitting for so long then I'd do a rebuild just to get it all clean out. Then you'd have a new accelerator pump and gaskets and no crud in it. My old ford did the same thing when I got it, new accelerator pump and clean carb solved that problem.
 
As others have said accelerator pump is bad. Easiest way to fix that is get a full rebuild kit and rebuild it.

I have 3 trucks very much like that one. I have 3 1969 Chevy pickups. I'd love to build one up as a mini semi but just have not gotten around to it
 
The quadrajet is a fine carburetor and has given me less problems over the years than those freakin Holleys on my Internationals. That open vacuum port is a possibility. It needs to be connected to something or else plugged. The missing heat stove is no big deal. It can sometimes cause carburetor icing if running without but that usually disappears once the engine heats up. Mine has been missing since 1986 and I have no problem flooring the gas on the old Sierra Classic and hearing the sweet sounds of the secondaries pulling air and producing horsepower. With no hesitation.
 
The little nipple visible in your third photo originally supplied air that had passed through the air filter to the "choke stove" in the exhaust crossover passage in the intake manifold.

Since you now have a carb with an electric choke, the nipple isn't needed or used and can be capped off.

In addition to possible accelerator pump issues that have already been mentioned, there's a little "dogleg" latch lever on the RH side of the carb between the choke mechanism and the carb that is supposed to latch the secondary throttle plates closed until the choke is fully off and the engine (hopefully) is warmed up a bit.

If this little lever is stuck, missing, or misadjusted and the secondaries can open with a cold engine, it will act EXACTLY as you describe.

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Also, the (rather hidden) secondary "airvalve spring windup" setting on the torsion spring at the RH end of the secondary air valve is CRITICAL in getting the carburetor to operate properly.

If too tight, the secondaries won't open as they should, if too loose, the secondaries will open to0 early and too much causing a "bog" or "stumble".
Click to expand...
 
Poster also mentioned having opened up & cleaned carb & if I remember correctly (many moons since I have been in one) there is a small ~1/8" steel ball that acts as a check valve that is associated with the accelerator pump & if it fell out during cleaning...............
 
(quoted from post at 09:16:56 09/24/19) Poster also mentioned having opened up & cleaned carb & if I remember correctly (many moons since I have been in one) there is a small ~1/8" steel ball that acts as a check valve that is associated with the accelerator pump & if it fell out during cleaning...............
Click to expand...

"there is a small ~1/8" steel ball that acts as a check valve that is associated with the accelerator pump & if it fell out"

Jesse, I considered mentioning that, as well, but after thinking about it, ruled that out as the issue here.

There's a ball, a tiny coil spring, and a "T"-shaped retainer that gets staked in place to keep the spring and ball where they're supposed to be.

Checkball is on the discharge side of the accelerator pump.

Purpose is to prevent fuel from being pulled through accelerator pump circuit when not needed, causing poor mileage at cruise.

If missing, I would not expect that to cause the stumble off-idle, IMHO.
 
This just adds some info to what was already mentioned, plus one other item important for 1970s. Your truck originally had a heat-riser valve and I suspect it was eliminated when those headers were put on? It used to force heat under the carb to warm in and that made it drive better in cold weather. Also, for emissions reasons, if the intake manifold had been off - often the new intake-manifold gaskets have little tin plates blocking heat to the underneath of the carb. If that was my truck - even with no heat-riser - if it was an all-year driver I would want those tin-plate removed - assuming you have an OEM intake manifold.

Also - if that carb had been apart? As the last poster mentioned, there is a small steel check ball in there. If it got lost - the accel pump cannot work and it will bog whenever you stomp on the gas. Same if the pump umbrella seal is hard.


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Whut Bob said...

Secondary adjustment is crucial... A quick google " how to adjust secondaries on a quadrajet" I did not watch it all maybe there is a better video...



https://www.youtube.com/watch?v=xXRl6rcT1no

The accelerator pump should be obvious it will be a beach to start cold... To eliminate the secondary tie it off with a wire...
 
Nice pickup, my quad on a 472 is capped off on the front side and has remained after a rebuild that also included the acc pump. The car has never run better.
 
Looks like it's about all been covered.

The solenoid valve behind the carb, does it stop vacuum to the vacuum advance under certain conditions, as in only when in high gear? Don't know what they were thinking, but if so that is a performance/mileage killer.

Bypass it, take the vacuum straight off the carb, either ported or manifold, which ever runs best. If the distributor hasn't been gone through lately, take it out, disassemble it, inspect the weights, the pivots, the springs... All that stuff requires maintenance to stay right. You can buy weight kits, springs, bushings, adjustable vacuum advance, play around with it, it will make a lot of diference when it's right.

Q jet is the finest carb Detroit ever made! Fully tuneable, easy to understand. If it hasn't been adjusted for the headers, it's too lean. some thinner metering rods, bigger jets, different rod spring, lots of things can be changed to fine tune it.

Whatever you do, DON'T let anyone talk you into replacing it with a Holly! Gas leaking garbage out of the box!!!

If you are going to make this a reliable truck, something that is going to start after sitting a few months, look into one of the throttle body injection systems. They are great, easily tuned, better mileage, no choke problems or cold natured start up, much more compatible with ethanol gas than any carb.
 

That electric choke is not OEM I would like to know where it came from are a better pix of it... I need a carb for one like yours are a throttle plate if anyone has one.
 
I think the other posters have got you going in the correct direction, so I'll just add that any unused vacuum ports should be plugged.


Completely unrelated: as a kid growing up we would sometimes visit friends in Butler and go swimming at Silversides. Great memories.
 
Number 29 on that parts list, power piston, stuck down. Vacuum holds it down, when accelerated vac. drops and a spring raises the power piston with primary metering rods attached. With engine stopped you should be able to see a little bar down that 1/4 inch pipe on the top front of carb., with no vac. the piston should be up under spring pressure. With a small screw driver that bar should press down and spring back up. Have seen a bunch of them stuck. I like those Q-Jets after you go through the adjustments.
 
i have disassembled hundreds of QUADRAJUNKS years ago and adjusted and dialed them in, usually wasn't worth the effort they were still junk....
a Holley was the answer, the unfortunate thing with Holley was if it set for very long periods the gaskets would dry up and shrink and when you finally got around to running the motor they would leak like a sieve my Holleys were in constant service so no problems there..
eventually i started using Edelbrock carbs a copy of the AVS carb which was an improvement of the earlier AFB carb over the years i have installed roughly 5 in different vehicles i have owned dependable, and very reliable.
you can still buy the AVS but i do not believe the Quadrajunk is still made [that should make every one sleep better]
 
Agreed, Steve.

Holley four barrel carburetors, though the "go to" carb of hot rodders in the muscle car era, were crude devices.

Though Holley built models with high CFM ratings, fuel metering was poor and such devices were not suitable for OEM applications in all but niche markets.

Yes, they were easy to work on.

Yes, they often leaked.

Dean
 
Helped a young man build an engine for his Chevy PU some years back.

It was a fresh rebuild on a 350, lots of extras, cam, headers, aftermarket heads, intake, a real strong well tuned engine in a really nice truck.

But he was a lead-foot, even though it would light up the tires, he wanted more, more, more...

His all knowing, nothing doing buddies talked him into putting on a Holley. All that tuning to make the Q Jet scream, get great mileage, down the drain.

Within one week, he stood and watched his dream truck go up in flames on the side of the road!

May have saved his life though...
 
Like the Quadrajet, the AFB/AVS was a good carburetor.

Holleys were quite cimple and crude, but well suited to racing. Not well suited to high volume OEM applications as history has proven.

Dean
 
I believe I have a Quadrajet around here somewhere but do not know what it was originally used on, nor do I know what it's worth.

Dean
 
For fuel economy,street manners,and even performance,a well tuned quadrajet works fine. On my drag car,I use a Holley,easier to jet,and it's wide open all of the time when making a pass,idle quality is not too important.Pic is the quadrajet on my all original 70 454 Chevelle,great to just cruise around in.
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Check the vacuum advance and see if its working .Take the distributor cap off and take the vacuum hose off the carb going to the distributor. If you dont have a vacuum pump just suck on the hose and see if the advance in the distributor is working
 
I would expect vacuum advance not working, accelerator pump bad or power jets stuck down.

Quadrajets are super simple.

Pull the hose off the vacuum advance, engine should sound noticeably blah and should be vacuum on hose.
You should see a good squirt into the lower part of carburetor front venturi's when you pump it and feel the resistance.
You can take a small screwdriver down into the small tube on top and test the power valve, spring should hold it up and able to press down.
 
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