Cold Weather Quadrajet Adjustments

[kpurser]

I have a '78 K20 Scottsdale with a 400 and Quadrajet. This carb has a divorced choke.  I installed the carb this last spring and everything runs fine in the warm weather. Since the weather has gotten colder I see the truck really needs to warm up for a while before I can drive. This is my first and only vehicle with a carb instead of FI so I'm not sure how much time I should expect it to need to warm up. For example, last night it was 25deg outside. I started the truck, truck started right up. Before cranking the engine over I press the pedal to floor once and then don't touch it while I'm cranking. After the truck is started, if I touch the pedal the truck will die. I have to wait for it to idle about 25 min before I can drive. What would you expect to be an average time for a truck to warm up to drive? Should I try adjusting the choke or fast idle?

[zieg85]

a properly adjusted choke should drive just like the EFI's do.  The only difference is 1 pump of the go pedal to set the choke.  Sounds like the spring needs tightened or replaced.  Mine I can take right off after a cold start, no problem

[Stewart G Griffin]

Do you have a tach?

X2 that it should operate as good as fuel injection IF adjusted properly.

i'm very sorry that i'm very busy and can't really get into it.  But watch these videos etc.   i think they can help:


View the jpeg sequences in this thread:
http://www.301garage.com/forum/index.php?topic=6098.0

Fast idle cam at high step = throttle opened the most and highest rpm
Fast idle cam at high mid step = throttle opened a little less and a little less rpm
Fast idle cam at high low step = throttle opened even a little less and even less rpm
Fast idle cam at low step/curb idle = throttle at normal/warmed up rpm


https://www.youtube.com/watch?v=qmFCh8LhRrU&feature=youtu.be


But really, the first thing that needs to be checked is the choke pull off and actual choke adjustment even though you state the engine idles ok once you start it:
https://www.chevelles.com/techref/Adjusting_Automatic_Chokes.htm

Sorry, can't really get into more detail as not only am i too busy to get anything done, on top of that i got some health issues that need to be fixed as well......... 

[kpurser]

those are some good links. I ran through them real quick. hopefully this weekend I'll have time to go through it and make some adjustments. thanks for the help

[philo_beddoe]

Exactly, when cold out, one full pump and it should start, idle and after about 1-2 min. a slight kiss of the pedal and will kick down to a quiet purr. When super cold out like below 20, then maybe 2 or 3 pumps. Although i have had the luxery of VZ adjust and fine tune my carb,,,so..    I have read it is better to start and drive off slowly than to sit and idle to operating temp.  The good news is, once the choke and carb is set right, they stay that way for many many years.  The qjet that is.

[Henry]

Hi kpurser:
My 76 has a 4MV qjet and it warms up within 2 minutes in warm temps(greater than 70F) and as much as 5 minutes in temps lower than 45F from a overnight cold start.  When adjusted properly, there is no special adjustment for winter vs summer conditions. My service manual has cold start routine as pedal to the floor once and then hold half way for cranking and when it starts then release the pedal. When very cold (like 25F), the routine is pedal to the floor twice and then hold half way for cranking. My truck starts with no problem with this routine. If truck has been driven previous day, it only takes 2 seconds of cranking to start...8 seconds if it has not been driven in a week.

Your qjet should have cold start idle modes that proper adjustment will allow the carb to go into by itself initially. The highest idle mode is when it is really cold and mid-idle for not so cold temps. When in high idle, you should be able to quickly press and release the pedal half way to force the carb down to mid-idle from high idle to go ahead and drive away while it is still warming up. When it is real cold (less than 35F), I cannot kick my carb down to mid-idle unless it has had at least 5 minutes of high idle warm up. After a little bit of driving in mid-idle, the carb should drop itself down to low idle when sufficiently warm.

I didnt look at videos that others have posted but one thing to check is the adjustment of the divorced choke rod for your model year (.220 or .290 inch for 4MV). Adjust your fast idle cam and then also your vacuum break for vacuum hold and spec adjustment.

Regards,
Henry

 

[philo_beddoe]

Ditto Henry! (If adjusted properly)

[JohnnyPopper]

Does your air cleaner have this hardware?

https://www.ebay.com/itm/Chevy-1-Ton-Van-400-CI-SBC-V8-1978-Air-Cleaner-Assembly-w-K-N-Filter/202589095536?hash=item2f2b404670:g:mo8AAOSw8KJb4JoQ:rk:1:pf:1&frcectupt=true

Connected to positive vacuum, when cold,  it closes a damper in the horn.

The tube is run down to a adapter plate at the Right exhaust manifold where it picks up hot air right outside cylinders 4 & 6.

When the green AC sensor rises to operating temperature, it closes the vacuum off and the damper is wide open. 

I know it is a smog device, but in cold weather it would be worth it. You can always deactivate when thing warm up.