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Quadrajet talk

TommyD11730

TommyD11730

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So this winter I noticed no matter how much I backed of the primary vacuum pulloff the choke opened too much on startup. So I would get a start... stall and restart was fine.
Took notice last night that the secondary pulloff was actually the problem. I was able to back it off some and now the choke doesn't open as much on start up.
That got me thinking... I thought the secondarys pulloffs job was to strictly keep the secondary closed on cold engine operation. When the engine warms up the thermal switch kills vacuum to it, and now the secondarys function ok.
Can someone school me on this?
 
There is a bar on the secondary shaft that keeps the throttle plate from opening while the choke is active, that part is normal

What in particular did you want info on?
 
How
Bent77 said:
There is a bar on the secondary shaft that keeps the throttle plate from opening while the choke is active, that part is normal

What in particular did you want info on?
Click to expand...
Operation of secondary pulloff.
 
You’re gonna make me actually look aren’t you?

I pulled all that stuff off mine... lol
 
dyeager535 said:
Believe there is a "dogleg" mechanical piece that engages the secondary throttle shaft(?). Been a long time since I looked at one of these things.
Click to expand...
Yeah once the coil on the intake pulls back, the choke opens and the dog leg frees the secondary shaft
 
I just don't "get" how the secondary is opening the choke butterfly. I thought that was the primary job???
 
a pic is worth a thousand words...a video a million!

 
Also believe that the secondary pulloff isn't just to keep them shut. They are mechanically locked out on cold engine operation (why the secondary throttle shaft is not a solid connection to the primary throttle shaft) the vacuum helps control the speed at which the secondaries open and also keep the secondary throttle valve (the upper plates) from "fluttering" under certain conditions.

Obviously through the years GM made hundreds of modifications to the carb both in components, adjustments, and how each operated...this is one of the reasons that when people talk about EFI being complex I chuckle. There is nothing simple about getting a Q-jet to run properly. Once setup, sure, they tend to stay good, but no different than EFI in that regard. Sure, other carbs are simpler, but the tradeoff is that the adjustment just isn't there, and GM had to worry about emissions, which the aftermarket does and did not. Startup/cold engine operation was a huge focus as time went on and why the Q-jet got increasingly complex.
 
Hey guys, while we're having a meeting of the minds, here's another choke (kinda sorta) question.
I converted my hot air choke to electric. I still have the heat riser on the engine. Is there any reason to still have it hooked up? Oh, just to make sure... vacuum pulls it closed... no?
 
Yes,the vacuum closes it--that is a fail safe setup ,so if the diaphragm failed ,the valve stays open and wont cause an exhaust restriction or over heat the intake under the carb and boil the fuel in it..

I would say leave the EFE (heat riser) valve intact,if possible,you'll be better off if your in a climate where it is foggy,cool,or damp out very often,that is when you'll need the intake heated up under the carb to prevent carb icing and stalling during warm up,it also helps the fuel vaporize better and allows it to run leaner with lower emissions..
The heat riser doesn't reduce exhaust flow much when its open,so there is not much to gain by deleting it..I tried ditching them on a few of my trucks are regretted it,the carbs would frost up and make it stall or idle like crap until the engine was up to operating temperature for a good 15+ minutes...
 
Tend to agree with you regarding the heat riser. Don't see any RPM change open vs closed so on this constipated motor it's just fine. Here in NY it sure can't hurt.
Welp, choke works a-ok now. Just have 1 lingering problem with the throttle not returning back to completely closed under very light throttle . My return springs are just fine so I wonder if I have a little carbon under the carb. (Even though it's not the original one and definitely was rebuilt at one time). It's not the throttle or TV cable, and it's not popping back on high speed cam... just sticking a little off idle.
Don't recall there being an adjustment to set tension on throttle plates to return closed.... thoughts??
 
No adjustment on the primary throttle butterflies as far as I know...
There is an adjustment for the air valve above the secondary butterflies,but that wont affect idle,just the rate of opening on the air valve butterfly..

Several things can keep it from returning fully to idle,including the return spring,if its not the dual spring stock setup especially..if you have added one that wasn't factory you may need to hook it up to the throttle cable "ball" itself or try a different location on that plate where the ball is attached ..too much tension can egg out the throttle shaft bushings..

It may have worn throttle shaft bushings,and the butterflies bind up at the idle position...could be dragging them on the base gasket too,or carbon build up like you suspected too,or maybe the carb was over tightened and warped at some point..sometimes the PCV valve fitting area gets all gunked up with goop and carbon.some may get on the throttle plates..

Often the secondary throttle plates stick open a bit,and don't close fully, and cause a fast idle and or failure to return to slow idle fully--the stepped cam for fast idle can fail to drop all the way to its lowest step if the choke fails to fully release or it gets gummed up and or rusty,but you said you checked for that already..

I have had a Q-jet that failed to return to idle as fast as it should due to the accelerator pump linkage binding up,it also has a spring internally on the pump cup to return it after you open the throttle,in rare cases the cup or that spring may cause it to stick open past idle or return slow to idle..usually the return spring compensates for that though..
 
Cntymnty77 said:
I fixed mine by getting a holley. Runs great now!
Click to expand...

I gotta say, this is always my philosophy as well. Quadrajets are great carbs when they are tuned correctly and running well but when they go south I just yank it and replace it. There are very few left that can build one correctly, remans suck hardcore and the guys that can build them want more to do it than a new Holley costs. Waaaaay too many moving parts and extra junk on the quadrajets. Holley is simple, straight forward and easy to adjust and understand. Just my .02....
 
With me,it's exactly the opposite..

I've had nothing but grief from Holley carbs with 2 exceptions,and have put Q-jets I bought at swap meets for $10 on in their place and had the engine run much better..
Granted the Q-jets are rather "involved" as far as rebuilding one,but after I did a few ,it was no big deal to me..

Holley carbs in my experience are fussy as far as weather conditions,and they have a bunch of different things like accelerator pump cams and jets to swap in order to get them dialed in,they never seemed to hold an adjustment for me either..
I never found them "simple" to work on,the ones I had drove me insane--one I had caught fire when the float stuck or sank,almost burnt my truck to the ground,another blew out power valves often,one small spit back if you opened the choke a bit too soon and poof,it was junk...only 2 "good" ones I ever had were spread bore bolt on replacements for a Q-jet and a few "Economaster" new ones that are nearly identical to the original carbs they replaced..

I had good luck with Carter AFB's and Edelbrock carbs too,the only gripe I had with those was they flooded easily if you got your truck off camber or climbed a steep grade...
 
Well it's Qjet 1 Tommy 0. Pulled her off right quick and really couldn't duplicate the problem on the bench. Might have a touch of slop in the primary throttle shaft... maybe.
No crust on the plates... looking through the bottom (light down the throat) just a touch of light shined through the circumference of the throttle plates.
Replaced the fuel filter and mounted her up. Fired her off and with just the return spring installed you can feel the throttle drag some. Should have got a new base gasket as I wonder if the plates are dragging on the gasket? Maybe the return spring pulling on the shaft is enough to cattycorner it and then it binds?

Don't know but I'll keep at it. Oh and just so you guys know we're talking it only hangs up a few hundred RPM.

Thanks for your suggestions, good stuff!

20180207_210017.jpg
 
Could be the gasket, had that before.

What rpm is it sticking at and dropping to?
 
I don't have a tach...
Best guess sticks 1k-1200 vs base idle 6-800 RPM
 
Between the return spring and the throttle shaft, it’s probably pushing the bushing in the throttle shaft enough one of the mechanical links for the choke is barely catching

Had one do that once, but I’ve slept since then. And can’t remember what it was or the fix

With engine off see if you can get it to catch in the spot. Then work your way around the carb poking and tapping.
 
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