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Old 09-18-2019, 05:28 AM   #1
Chris_oz
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1970 350 quadrajet questions.

Hi all,
My 70 C20, 350, 4speed has recently developed a great love for unleaded. When I first got it, on highway trips keeping it under 60mph I was getting 15mpg with ease.
I have had her parked for a little while and just started driving it again, fresh tank of fuel doing my normal commute, which is a 80-20 split country roads/light urban.
Even driving easy I can literally watch the fuel gauge move.
Last 3 days which is 175 miles has used 21 gallons. It is so rich it makes my eyes hurt.
I pulled the too off the carb and the float was under the fuel level (or just level with the top of it. I weighed the float, it is 14 grams, which is twice as much as the manuals say it should be.

Could it be this simple, would a dead float cause it to be so bad?
It still runs great, plenty of power etc.

I have checked the jets and needles, they are 75 and 39/26 which as the should be.

There is fuel in all the chambers around the float bowl, at 650rpm there is fuel running out of the boosters. Have I just got to replace the float and set the level?

I have a manual but have not played with a quadrajet before.
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Old 09-18-2019, 05:40 AM   #2
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Re: 1970 350 quadrajet questions.

How long was the truck sitting before you started to drive it again. Ethenal fuel absorbs water while sitting in storage.The modern ethenal fuel will attack the gaskets and rubber parts of an old carb. If has not been rebuilt lately with new gaskets and accelerator pump you might consider rebuilding it.
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Old 09-18-2019, 06:11 AM   #3
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Re: 1970 350 quadrajet questions.

I sounds like the float may have a pinhole leak and you are correct in assuming a float replacement should be the remedy. I wonder if ethanol plays havoc with the metal in floats, not that a float filling with fuel is anything new since ethanol. Is ethanol in use in Australia?
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Old 09-18-2019, 06:45 AM   #4
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Re: 1970 350 quadrajet questions.

Hi,
Pretty much all our fuels have some ethanol in them.
I have ordered a rebuild kit, new float, new needles, (mine are very worn) new jets, and a few other small bits so I can bring it back to new.
The site I got the parts from had some really good info on how to restore and set it up. So fingers crossed it will all be good.

I will not completely strip it until I have the parts so I don't forget where it all goes.
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Old 09-18-2019, 08:27 AM   #5
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Re: 1970 350 quadrajet questions.

That sounds like a good plan. My '72 truck w/350 QuadraJet sat from the first week of January until about the first week of August. I have to revamp the fuel gauge wiring (3 tanks) so I have no working gauge. I keep a tablet in the glovebox for date/miles/gals. When I took it to fill the first time, straight away once running, it had 100 miles on it and took 15 gallons!! I wondered if I had someone stealing gas or did I have some carb problems. No smell of fuel when near the truck. The next time I got fuel it came out right back at it's normal thrifty 10mpg
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Old 09-18-2019, 11:23 AM   #6
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Re: 1970 350 quadrajet questions.

The current blends of gasolines are way different from even 10 years ago.
I am a member on a Pontiac web site. Cliff Ruggles is Pontiac enthusiast. he posts there a lot.
He sells all kinds of parts, has several books (one is for Quadrajets).
He doesn't work on carbs a lot right now.
Still sells parts and helps with advice ( IF, IF ou buy parts from HIM!!!)
Cliffshighperformance.com

Your float appears dead. whether it's plastic or metal. Cliff will not use a brass float anymore in his builds. The current manufactured brass floats seem to work OK for a short period of time, then leak. 1/4 inch is a safe float level...sounds like you just need a float...
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Old 09-18-2019, 12:56 PM   #7
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Re: 1970 350 quadrajet questions.

Your bad float is definitely causing the rich condition. When the float sinks it raises the fuel level in the bowl & causes a rich mixture. These plastic floats did this before we had ethanal in the gas, so I wouldn't blame the gas. They still are better than replaceing with brass floats.

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Old 09-18-2019, 03:01 PM   #8
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Re: 1970 350 quadrajet questions.

And test fit a new float. Some can hang up on the power piston tower.
And compare the float length from pivot to the end.
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Old 09-18-2019, 03:17 PM   #9
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Re: 1970 350 quadrajet questions.

Quote:
Originally Posted by geezer#99 View Post
And test fit a new float. Some can hang up on the power piston tower.
And compare the float length from pivot to the end.
Yes!!! IF, IF,you install that tiny clip on the needle, and DON'T know how to install it correctly, it will be a problem. If in doubt leave that clip OFF!!!
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Old 09-18-2019, 03:37 PM   #10
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Re: 1970 350 quadrajet questions.

Clip install in here.
https://www.crankshaftcoalition.com/...4MV_carburetor

Leaving it off would be like leaving out a metering rod because it won’t go into the jet.
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Old 09-18-2019, 03:45 PM   #11
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Re: 1970 350 quadrajet questions.

The clip I am referring to was not in any of the pictures in your link. It attaches to the top of the needle (that goes into the seat) . You have to hang it around one of the 2 holes in the float. It is made to keep the needle from possible sticking shut hwn sitting for a long time....
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Old 09-18-2019, 03:52 PM   #12
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Re: 1970 350 quadrajet questions.

I've had that problem in the past with the non brass floats. They can absorb fuel, and cause the float level to rise causing all the problems you mention. Replace the float. Don't over think it.
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Old 09-18-2019, 03:55 PM   #13
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Re: 1970 350 quadrajet questions.

Quote:
Originally Posted by 68Gold/white View Post
The clip I am referring to was not in any of the pictures in your link. It attaches to the top of the needle (that goes into the seat) . You have to hang it around one of the 2 holes in the float. It is made to keep the needle from possible sticking shut hwn sitting for a long time....
Yes it is. Right after choke and linkage.
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Old 09-19-2019, 09:10 PM   #14
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Re: 1970 350 quadrajet questions.

Hi again,

I have a high quality rebuild kit, new float, powervalve retaining clip, power valve springs all on the way over.

I looked at the primary needles again last night, they are very worn, the power tip which should be 0.026 is 0.023 at its thickest and is tapered.
The low speed section that should be 0.039 is under 0.036 in places.

I would like to replace them. Which leads to my question.
What is the stock primary jetting (& needles) for my truck.
1970 C20, 350 4 speed manual?

I have several charts which have it listed as anywhere from
0.068 jets and 0.043/0.026 needles up to
0.076 jets and 0.039/0.026 needles.

I currently have 0.075 and 0.039/0.026

If someone can show me a chart, or a link to a chart which is correct, or someone who has jetted their truck give me a starting point.
I know it all changes now we have unleaded fuels.

Thanks for the replys

Last edited by Chris_oz; 09-19-2019 at 09:52 PM.
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Old 09-19-2019, 09:32 PM   #15
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Re: 1970 350 quadrajet questions.

Find your carb number in here and that info might be there.

https://www.digitalcorvettes.com/for...ad.php?t=88376
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Old 09-19-2019, 09:32 PM   #16
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Re: 1970 350 quadrajet questions.

The ones you have, if you can read the numbers, will probably be correct. The range you are seeing is because of calibration for engine size, cam, compression and altitude, just to name a few variables. You could try contacting one of the carburetor gurus about the proper jets and metering rods.
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Old 09-19-2019, 10:23 PM   #17
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Re: 1970 350 quadrajet questions.

Quote:
Originally Posted by geezer#99 View Post
Find your carb number in here and that info might be there.

https://www.digitalcorvettes.com/for...ad.php?t=88376
Hi,

I had seen that right up before, but it didn't have the listing attached. thanks.

I also noticed that for 1970 there is a CALI spec for jetting.
My truck was built, sold and lived in Cali

Cali spec is 0.076 with 0.044/26
Fed spec is 0.075 with 0.039/26

I have the fed spec, in a cali carb, which is somewhere to start with some new needles which are in spec.
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Old 09-19-2019, 10:37 PM   #18
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Re: 1970 350 quadrajet questions.

In that link I posted be sure to check out the section on the adjustable part throttle. It’s a handy tuning tool to have on a quadrajet.
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Old 09-20-2019, 04:46 AM   #19
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Re: 1970 350 quadrajet questions.

Quote:
Originally Posted by geezer#99 View Post
In that link I posted be sure to check out the section on the adjustable part throttle. Itís a handy tuning tool to have on a quadrajet.
I have read that, I took the plug out for the front adjustment screw, the screw even moved, so I can get a small amount of adjustment.

I can read the rods, they ar stamped 39 but measure 37/23
The jets are 75.

The rest of the carby is in fantastic condition, I stripped it down and cleaned it up tonight, just waiting on my parts to arrive.

I lso found a new elelbrock manifold I forgot I had.
Should I fit it up now or wait until I get the carb sorted?
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Old 09-20-2019, 09:53 AM   #20
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Re: 1970 350 quadrajet questions.

I’d leave the intake on the shelf until you get the carb done.

And then after it’s running good find someone to trade that air gap to for a regular performer.
Then you’ll have a working choke and a heated plenum with no tuning issues.

Last edited by geezer#99; 09-20-2019 at 01:13 PM.
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Old 09-20-2019, 10:17 AM   #21
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Re: 1970 350 quadrajet questions.

Agree. One change at a time.

What is the part number on that intake? Is that an air gap? Does it have provisions for the choke coil on the pass side?

The Edelbrock 2101 should be a direct swap. The air gap I'm pretty sure does not have provisions for the choke coil, so it needs to be converted to electric choke.

Good document to have on the quadrajets:
http://www.capitalcorvetteclub.ca/at...ng%20Paper.pdf

If you really want to get into tuning, a wideband air fuel gauge is awesome. No more guessing, see your exact air fuel ratio. Narrowband is OK for tuning idle and cruise, but doesn't really have the range for wide open.
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Old 09-20-2019, 04:30 PM   #22
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Re: 1970 350 quadrajet questions.

Quote:
Originally Posted by rpmerf View Post
Agree. One change at a time.

What is the part number on that intake? Is that an air gap? Does it have provisions for the choke coil on the pass side?

The Edelbrock 2101 should be a direct swap. The air gap I'm pretty sure does not have provisions for the choke coil, so it needs to be converted to electric choke.

Good document to have on the quadrajets:
http://www.capitalcorvetteclub.ca/at...ng%20Paper.pdf


If you really want to get into tuning, a wideband air fuel gauge is awesome. No more guessing, see your exact air fuel ratio. Narrowband is OK for tuning idle and cruise, but doesn't really have the range for wide open.
Hi again.

The manifold is the air gap, 7601 I think.
I read lots and watched plenty of videos and reviews on the air gap vs regular model before I bought it.
Most were aimed at low temp issues in cold areas.
I live on the east coast of Australia, I don't have a choke on any of my cars, it is not required. Summer here is 3 months of 110 deg f temps and heat is a big issue. I hope the air gap will help with carb boil and such, if not, I will swap it out.

I bought a new wide band O2 sensor too, it has data log and such if I want, runs the Bosch heated sensor. I should have it here soon.

I am going to test myself, I plan on tuning it without the sensor, get it to where I think it's close and see how I go.

One thing I did notice was the cylinders being fed from the drivers side plenum were much richer then the ones being fed from the pass side.

I pulled the plugs to see how bad they were and was surprised that 4 were pretty good kother than the rich low speed) but the other 4 were darker.
With the carb off, inside the manifold is the same, one side is black and sooty, wet with fuel, the other is pretty clean.

Maybe just one barrel was getting all the extra fuel.
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Old 09-20-2019, 04:39 PM   #23
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Re: 1970 350 quadrajet questions.

Looking at the carb number, it's the correct year, but a federal carb, not California, it is also from an auto, mine is manual, so looks like it's been swapped at some point.
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Old 09-20-2019, 06:18 PM   #24
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Re: 1970 350 quadrajet questions.

hi guys, that q-jet looks like a commercially rebuilt carb. I noticed the screw in the drivers side bypass air channel. and all the plateing is removed. not good, those q-jets have to be recalibrated a lot. some of the rod jet combos listed are way!!! rich. just my opinion, good luck, BROWN 70
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Old 09-22-2019, 09:52 AM   #25
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Re: 1970 350 quadrajet questions.

Just an observation with personal experience. Ive had more than one old float fall victim to age, ethanol gas, or a combo of both. Most recently the float in my 86 K10 took on fuel and had almost the same problems. Due to it being about 10 degrees outside, I didnt pull the carb, just removed the top plate, and swapped(calibrated) the float and its been running great since. If you liked your combo of hard parts (jets, metering rods, etc.) before, and the float was the issue, you'll be happy with the new float and maybe a rebuild too, with out breaking the bank. ONE other small observation, aluminum intakes are great (i have use many over the years) but unless your having hot fuel problems, your probably not going to notice a lot of seat of your pants improvement over stock in most dd truck applications. I do however like the cool factor when opening the hood though as well ....
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