Lagging Rochester 4bbl on 402 big block

[Chorner72]

Hey everyone,

This is my first post on here so forgive any errors. Recently hauled my 1972 Gmc Sierra Grande out of “ storage” after 15 years. After all the basics ( change engine oil, remove old fuel, blow out fuel lines, new battery, etc) it was wanting to quit on me once it reached operating temperature, seemed like it was flooding. So carb rebuild time. Stock 402 big block under the hood

Rebuilt the carb and it idles like a dream. One problem, upon acceleration it sounds like it just sucks air, almost to the point of killing the motor, then it will begin to fire normally if i ease off the gas.

Any ideas or thoughts would appreciated.

Thanks

[Steeveedee]

Does it only do it when under hard acceleration, or part throttle? Where did you buy the rebuild kit? First thing to check is if the accelerator pump is working properly. With the engine off, look into the primary throttle bore and open the throttle. You should see two solid streams of fuel squirting into the bores. If not, you either have a blockage in the accelerator pump circuit, or the float level is too low. If it works OK at part throttle, but fades when going to wide open throttle, check that the cam in the secondary air valves is lifting the secondary jets. You can do that with the engine off. Defeat the secondary lockout if needed (if they don't open manually, just look for the lever that is keeping them from opening- it keeps the secondary air valve from opening until the choke is all the way open), and manually open the secondary air valve.The jets are hung on them, and should lift as the valves are opened. If not, that cam is toast.

[Chorner72]

Originally Posted by Steeveedee

Does it only do it when under hard acceleration, or part throttle? Where did you buy the rebuild kit? First thing to check is if the accelerator pump is working properly. With the engine off, look into the primary throttle bore and open the throttle. You should see two solid streams of fuel squirting into the bores. If not, you either have a blockage in the accelerator pump circuit, or the float level is too low. If it works OK at part throttle, but fades when going to wide open throttle, check that the cam in the secondary air valves is lifting the secondary jets. You can do that with the engine off. Defeat the secondary lockout if needed (if they don't open manually, just look for the lever that is keeping them from opening- it keeps the secondary air valve from opening until the choke is all the way open), and manually open the secondary air valve.The jets are hung on them, and should lift as the valves are opened. If not, that cam is toast.

It does from idle and then as soon as i accelerate. I have to very gently hit the gas then it will go. I replaced the accelerator plunger on the rebuild. Would the secondaries open even not hammering the gas pedal?

[Steeveedee]

Originally Posted by Chorner72

It does from idle and then as soon as i accelerate. I have to very gently hit the gas then it will go. I replaced the accelerator plunger on the rebuild. Would the secondaries open even not hammering the gas pedal?

I suspect the accelerator pump circuit, or a vacuum leak, which assumes that all the electrical components are performing correctly. Check to see what kind of spark you have and get back to us. Should be a nice fat blue spark, not red or yellow. I made an early assumption that the electrical system was good. I doubt that the thing is flooding, given the conditions you are posting. Check around for broken vacuum line hoses, including on the transmission vacuum modulator, if you have an automatic. And no, you'll have to have a heavy foot on the throttle to get the secondaries to open and cause the problem I mentioned. Is there someone on the board that may be close by and could take a look? Direct eyes on makes a huge difference.

[Chorner72]

Originally Posted by Steeveedee

I suspect the accelerator pump circuit, or a vacuum leak, which assumes that all the electrical components are performing correctly. Check to see what kind of spark you have and get back to us. Should be a nice fat blue spark, not red or yellow. I made an early assumption that the electrical system was good. I doubt that the thing is flooding, given the conditions you are posting. Check around for broken vacuum line hoses, including on the transmission vacuum modulator, if you have an automatic. And no, you'll have to have a heavy foot on the throttle to get the secondaries to open and cause the problem I mentioned. Is there someone on the board that may be close by and could take a look? Direct eyes on makes a huge difference.

Accelerator pump circuit? First time thats been brought up. Is it common for them to fail.

I will check all vacuum lines, im thinkng it could be that as well. Thanks

[Steeveedee]

Originally Posted by Chorner72

Accelerator pump circuit? First time thats been brought up. Is it common for them to fail.

I will check all vacuum lines, im thinkng it could be that as well. Thanks

It is a common problem when there is a hesitation problem. A truck that sat that long can have some serious crud deposited, restricting orifices everywhere. I bought a '69 Skylark that ran horribly when I bought it. Once all the vacuum leaks were taken care of, it ran like a top. The guy had parts from 3 different carburetor kits in the trunk. He even put an HEI in it to get it to run better. But there were hoses laying everywhere, some of which connected to nothing.

[geezer#99]

A few things.
Big vacuum leak.
Fuel pump dieing.
Secondaries air door set too light.
No timing.

[Chorner72]

Originally Posted by geezer#99

A few things.
Big vacuum leak.
Fuel pump dieing.
Secondaries air door set too light.
No timing.

Had a vacuum gauge hooked to it to set the air fuel mixture, would that tell me? It takes awhile to start even if it sits for only a day, fuel pump? Ill check the secondaries. How would i check timing?

I should mention i changed the plugs as it was backfiring a bit prior to me changing them

[geezer#99]

Could be leaking well plugs. Fuel drains out overnight.
You check timing with a timing light.
Plug off the vac advance when you set it.
Try 12 degrees initial.

[Wrenchbender Ret]

I suspect the float level is too low. Match the carb. # with the instruction sheet to get the proper spec. There is a lot of variation in different applications.
First check the accel pump like Dee said.
George

[Chorner72]

Originally Posted by Wrenchbender Ret

I suspect the float level is too low. Match the carb. # with the instruction sheet to get the proper spec. There is a lot of variation in different applications.
First check the accel pump like Dee said.
George

I took care to set the float when i replaced it, so i’m thinking that isnt the problem but you never know.

I’ll check accelerator pump

[BCOWANWHEELS]

Accellorator pump

[Chorner72]

I will be replacing all the vacuum lines and readjusting the float and taking a look at the accelerator pump hopefully sometime this week. I will report back with the findings. Thanks to all for the suggestions and input.

[Steeveedee]

The pump piston should slide easily up and down in the bore. Check around in your area parts stores to find a piston that has a cup that is impervious to the alcohol that is added to the "cat pee" we call gasoline these days. This is a well-known problem, and I suspect that this is what is going on. Another possibility (really unlikely) is that the metal portion is too tight in the air horn. Easy to check when you get it apart- just see how the metal to metal friction is when the carburetor is apart. I buy my kits from Cliff Ruggles, but there are other vendors. You can buy just the accelerator pump at most shops that build carburetors.

Maybe someone else in the frozen north can point you to a shop that has the appropriate piston...if that is what the problem is.

Edit to add- you did not report back if you got a stream of fuel when opening the throttle, engine off. The diagnosis is still in limbo until we have that information.

[Chorner72]

I promised I would report back

So after removing the top half of the carb, I seen the bottom spring on the accelerator pump had binded somehow and hence my problem with it, got it all fixed.

I had forgot the ball check valve when rebuilding the carb so installed that as well.

Checked fuel pump to make sure it was working adequately, it is.

And I also checked the float level, it was too low so adjusted it.

Go to fire it up..... same thing.

Took it apart again, only thing I could come up with is the float had got stuck. Made sure it had room to move freely and reassembled. Crossed my fingers and turned the key.

Voila, runs like a top.

Now to see if there is fuel in it tomorrow when I go to start it.

Thanks all for your input

[Chorner72]

Originally Posted by Chorner72

I promised I would report back

So after removing the top half of the carb, I seen the bottom spring on the accelerator pump had blinded somehow and hence my problem with it, got it all fixed.

I had forgot the ball check valve when rebuilding the carb so installed that as well.

Checked fuel pump to make sure it was working adequately, it is.

And I also checked the float level, it was too low so adjusted it.

Go to fire it up..... same thing.

Took it apart again, only thing I could come up with is the float had got stuck. Made sure it had room to move freely and reassembled. Crossed my fingers and turned the key.

Voila, runs like a top.

Now to see if there is fuel in it tomorrow when I go to start it.

Thanks all for your input

Should also mention I have not changed any vacuum lines as of yet. Hopefully next week

[Steeveedee]

I've never much cared for the float pivot clip on the Q-Jets, they can be fussy when assembling. That's partly because of my big mitts.

[brown7373]

Where did the rebuild kit come from? As previously mentioned, the accelerator pump needs to be impervious to the crap in today's gas. I also buy my kits from Cliffs and have never had a problem. Previously, using parts store kits, sometimes the pump cup would distort and distort and consequently not give a shot of fuel. Sometimes they lasted days, sometimes weeks and sometimes months. I would bet some parts store kits have improved, but can you be sure?