Quadrajet - Bogging out

[swissarmychainsaw]

So this just started happening. Under load (like making a dump run) my 1975 GMC 350/350 Quadrajet would bog out. As in you give it some gas and it sounds like its drowning. (this truck is new to me and I'm slowly working through a bunch of issues on it)

So it will work OK with LESS throttle or even with MORE but there is a zone it does not like.

It's a bit weird, because (now unloaded) it does not bog out all the time.

I believe the carb is original and likely could use a rebuild, but I would kind of like to know "what specifically" is going on before I just buy Cliff's book and rebuild kit and go at it.

I have NOT done a real tune up on this thing, and I will as a first step.

I am kind of wondering if it is getting too much fuel or not enough.

Cliff's book comes tomorrow. I'll start posting some pics as I go.

One thing: The PO put he air cleaner lid on upside down (so you can see the air cleaning with the lid on). When I put this on correctly, the truck would bog out when you gave it gas (like, engaged the secondaries). I was not sure what to make of that, but returned the air cleaner lid to the upside-down position and things were back to "fine" for a while...

Nick

[Dead Parrot]

My 75's OEM y-pipe on the exhaust was a double wall pipe. Inner wall collapsed on one side, forcing one side of exhaust through the intake crossover. This way overheated the carb and led to many issues. If yours has or had this problem while this carb was on the truck, the carb may be toast.

Assuming the exhaust issues have been fixed, the first thing is to replace the air filter. Then replace the in-carb fuel filter. Clean the fittings with carb cleaner first, then apply a good dose of penetrating oil. Twisting the fuel pipe is not uncommon.

Also, check the hose from the fuel tank. Both at the fuel pump and tank itself. If it is rotting out, you can suck air along with the fuel.

Don't overlook the possibility you got a bad batch of gas.

[mrolds88]

Your air cleaner should have a valve in the snorkle. Since it sat for a bit before you got it, somebody may have set up house in there. When you flipped the lid, it restricted the airflow. Also check your choke. If it started sticking that may be your bog.

[ron350]

Also check the secondary air valve and see if it has enough spring tension. If some one adjusted the spring too loose it will cause a nasty lean bog.

Remember to take lots of pictures so you can see how every thing was put together.

[swissarmychainsaw]

In the daylight I'll have a look at this stuff and take some photos.

The Carb book should be in the mail when I get home too!

Nick

[swissarmychainsaw]

Here's some more info.

When I FLOOR it (i.e. activate the secondaries) it works, meaning no bog.

When I give it 1/2 gas (I'm guessing) is when it tends to sputter.

I put it in the driveway, opened the hood and air cleaner and let it warm up to operating temp (or pretty close to it).

I then gave it some pretty good amount of gas, and watched the carb.

The Choke flap was not open all the way and I could see it flutter a small amount. (18 seconds of the vid)
On one occasion I saw a puff of what I'm assuming is FUEL out the top of the carb.
Lucky for me I caught it on video (at 8 secs) , but it's a bit hard to see, and admittedly is not all that exciting as far as youtube videos go.

Quadrajet - YouTube

[motornut]

hows your vac advance?

[naphtali5725]

sounds like a bad accelerator pump...
be sure to check timing and vac advance

[mpierce9]

When you say vac advance what do you mean?

[naphtali5725]

Originally Posted by mpierce9

When you say vac advance what do you mean?

the vacuum advance unit on the distributor... it should have a vacuum hose attached to it...

will look similar to this

the long flat part is what is inside the distributor

[geezer#99]

I watched your video Swiss!
The little puff of vapour is a lean misfire caused by retarded timing.
Bump your initial timing up to 12* and see if that helps.
Most suspected carb problems are timing related.
Timing first, carb second!!

[swissarmychainsaw]

Thanks guys!

I'm getting alignment done today (long story) and this weekend I'll have a look at the timing.

This truck is new to me and I have not even done a tuneup yet.

It's my understanding that Dwell has to do with points, and since this is a '75 does it have points or Electronic Iggy?

thanks again all!

--Nick

[Dead Parrot]

A 75 should have HEI, which looks like this: http://www.superchevy.com/technical/..._distributors/ That site also give a brief description of the various HEI parts.

As someone else mentioned, the accelerator pump is a likely suspect if the bog is momentary then it recovers. If at part throttle, it keeps bogging/misfiring, check timing timing first. Factory spec ranges from 4btc to 8btc depending on how it was ordered. Most 350s work well with 10~14 btc if everything else is working properly. Be sure to measure timing with the vacuum advance disconnected.

What is the status of emission equipment on this one?