I know this is an Oldsmobile forum, but I'm searching for help on Quadrajet 4MV carb. Any and all help is greatly appreciated!


I have a '72 GMC P/U with 350, A/T, Headers and Qudrajet 4MV...

Choke is working fine. But...I have to pump accelerator pedal 7-8 times in the morning, then it might start. If not, I pump it about 1 and a half times more then it starts.

The original 'cold start' instructions on the back of driver's side sunvisor says to press pedal all the way to the floor, and slowly release. Then start engine. If engine does not start, repeat procedure.

How do I get this to work right? I don't remember truck ever starting that easy, but would be great if it would.

Thanks,
Keith

 

Is the fuel leaking or evaporating from the float bowl after sitting overnite? It may be cranking so long because the pump is filling the bowls back up.

 

When you say choke is working have you popped the air cleaner off in the morning before touching anything? The choke flap should be wide open. Then work throttle as per instructions and choke should go closed. You also need to check that accel pump in carb is squirting every time throttle is worked. And yes it is common for the bowl casting plugs to leak causing the bowls to go dry and it takes time for the fuel pump to refill while cranking. If this is happening overnite the leak is pretty good.

 

Not sure...is there any way to check without removing carb, and disassembling?

 

If you have a fuel return line, you might want to make sure it's connected and functional (this line was used to combat vapor lock).

 

yes...I've removed air cleaner and checked choke. Fully open, then closes when accelerator is depressed. After engine does start, choke opens about 1/16-1/8th engine until engine warms up...

'not' getting a good defined squirt of gas when throttle is worked...no filter in carb, just inline filter on fuel line going to carb...

 

no fuel return line...that I know of...just single line going from pump to carb

 

Accelerator plunger could be starting to fail...usually though you will notice a almost stall condition when trying to accelerate from a stop. It is likely bowl is evaporating or going dry due to a leak. Do you have a thick gasket under the carb.... this would help with not evaporating the fuel.

 

actually when I depress throttle, there is no squirt, more like a faint dribble of fuel...just enough that when I stick my finger in there, it dampens my finger with fuel...

have not noticed any stall problem when accelerating from a stop unless I hit it hard for a fast start...then it doesn't affect it all that much...

purchased this carb a few months ago [remanufactured]...is it possible that the ports the gas comes out of could be clogged somehow...if so, is it possible to clear clog...

it has a reasonably thick base gasket...I'd say about 3/8-1/2 thick

 

More than likely, it's the well plugs leaking!
Only way to fix it is to epoxy the plugs, which will require removeing the base of the carb, a O/H kit, and some epoxy!
Around $30 and 2-3 hours.
Be careful, if you can smell gas in your oil!!

 

You could try a test in the am. Get a small squirt bottle with gas and put a few shot glasses of gas into the carb thru the top vent tube. Then try the accelerator pump to see if it squirts better and engine fires quicker. This will confirm a dry bowl issue. Be sure you dont dribble any gas into the carb throat as it will effect the test results.

 

just checked, and no smell of gas in oil...

can base of carb be removed without disassemblying whole carb?

 

I'll try this in the morning...if accelerator pump squirts better, and engine starts better...does this mean that it is a dry bowl issue...not quite sure about how accelerator pump works...does accelerator pump pick up gas left in bowl when depressed which provides the squirt...I thought it got gas from fuel pump through gas line to carb and this is where ther 'squirt' came from

 

Yeah, if you've done one before, but I'd let the carb sit at least 4 hrs upside down, first!
You'd have some linkage to deal with, but it shouldn't be too hard.
Might check with a yard, first, as some will sell a used one for $25 - $50.
I got one once w/30k miles at a swap meet for $25 - worked great!

To check; Crank the motor without touching the gas [cold] for about 15 seconds.
Stop - hit the gas 1/2 way to floor and crank again.
If it starts right up, it's the well plugs!
Good thing is, you're getting oil pressure before starting - no 'dry start'.

 

Thanks guys for all your help!
As of now, truck is cranking after 3 or 4 pumps of the gas pedal. I'm gonna live with that until it gets worse. There is no smoke, or gas in the oil, so we'll see how it goes.
Haven't rebuilt but [1] Quadrajet in my life and although it turned out ok, I'm not in any hurry to do another.
Guess now I'll concentrate on the 'hard start when hot' problem, and fixing driver's side window that is hard to roll up and down. Oh...and also changing ignition wire to the HEI distributor that I put in it years ago. [was told original wire is a resistor wire, and needs to be changed to 12volt wire]
Thanks again,
Keith