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Old 07-25-2020, 06:16 PM   #1
Rollo Grande
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New Q-Jet will not settle down to idle, it's driving me %$#%$#%$

1970 C/20 350 4v

My old carb backfired, spit flames, and sucked in every way...but it idled just fine.

Bought a brand new Quadrajet from a carb warehouse and it solved all of the big problems, but it will consistently not come down off high idle. I have to drive around blipping the throttle constantly to get it to calm down. It hangs up at 2500rpm, 1500, 1200, 1900, wherever.

I just tuned both screws using a vacuum gauge to 19 inches and set the idle speed around 800. Nothing changed.

Also this new carb is giving me 5mpg city, just like the old one did.

I suspect it might be missing a piece, that rich/lean clockspring thing, but they sold it to me as a complete plug and play "divorced choke" or whatever. I don't know that much about these things.

Any idea what to do?
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Old 07-25-2020, 06:56 PM   #2
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Re: New Q-Jet will not settle down to idle, it's driving me %$#%$#%$

Can you post a pic or two of the right (passenger) side of the carburetor? That would allow us to see the linkage, which is probably where your problem is.
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Old 07-25-2020, 09:06 PM   #3
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Re: New Q-Jet will not settle down to idle, it's driving me %$#%$#%$

^Yup. Pics will help a lot!
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Old 07-25-2020, 09:39 PM   #4
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Re: New Q-Jet will not settle down to idle, it's driving me %$#%$#%$

Choke sticking on carb, return spring g on throttle not strong enough, or the weights in the distributor stocking.
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Old 07-26-2020, 10:05 AM   #5
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Re: New Q-Jet will not settle down to idle, it's driving me %$#%$#%$

There is an old hot rodding adage that "90% of carburetor problems are ignition". Before spending a lot of time on the carb, I would suggest a complete study of the ignition system. My '70 GMC big block truck regularly got 14 mpg. If you were getting 5 before and after carb swaps I think your ignition needs some work.
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Old 07-26-2020, 10:19 AM   #6
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Re: New Q-Jet will not settle down to idle, it's driving me %$#%$#%$

A couple things come to mind.
Choke thermostat not opening the choke from either not adjusted or blocked heat passage under the manifold.

Worn out throttle bushings which leak air causing high idle and the same wear not allowing consistent closing of the throttle blades.
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Old 07-26-2020, 10:21 AM   #7
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Re: New Q-Jet will not settle down to idle, it's driving me %$#%$#%$

To test the choke staying open you can place a piece of cardboard between the lever and the carb body on the passenger side to hold the chokemopen. If the idle stays up an blipping brings it down then the throttle shaft bushings are worm and need replaced.
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Old 07-26-2020, 01:30 PM   #8
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Re: New Q-Jet will not settle down to idle, it's driving me %$#%$#

I doubt you bought a brand new QJet , reconditioned ones are available. If the throttle shaft is worn I would send it back for a refund l
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Old 07-26-2020, 03:23 PM   #9
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Re: New Q-Jet will not settle down to idle, it's driving me %$#%$#%$

Since things changed after you changed things my personal motto is "Always look at the last thing I screwed with FIRST"
First thing is a careful inspection of the linkages on the carburetor. There are at least a half a dozen ways they could be screwed up. For some reference see the link below it has a lot of photos and information on the Qjet I have on my Burban. It's likely you have a similar one.
Scroll to post 590 to start then post 618 and there is more after that.

https://67-72chevytrucks.com/vboard/...698377&page=24
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Old 07-26-2020, 04:26 PM   #10
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Re: New Q-Jet will not settle down to idle, it's driving me %$#%$#%$

Those old choke pots suck. I battled one religiously until I just gave in and got an edelbrock. Its been on there 10 years now.

Manual Chokes are nice.

So I have not given up on my old Quadrajet, yet I am under the influence they are more difficult to get set up. Once set up properly they are great!

Look up a guy by the name of Cliff Ruggles, he does some very good work.
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Old 07-28-2020, 12:08 PM   #11
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Re: New Q-Jet will not settle down to idle, it's driving me %$#%$#%$

Ah sorry guys! I was working over the weekend and did not get email notifications for replies to this thread. Pics attached.

I think you are on to something with ignition. The engine seems to run okay but it may be on less than 8. I took first real highway trip to Concord and it really struggles.

It also may be part of my other problem which is: in 4th gear I am turning 4000 rpm at 50 mph. I assume that's partly gearing and partly a little bit of tach error, the speedo is dead-on balls accurate according to GPS. I dont know why the axle would be geared THAT low even in 1970. Especially in an AC/tach truck.

@HO455 I read your Burbon thread and felt a bit like Forrest Gump (I'm not a smart maaaan) but i think I get the gist of what you are saying. Your carbs big vacuum port goes to some kind of choke attachment and mine was going to the air cleaner. Capped in the photos and i don't know what that little one does.

Anyway I have decided to call in a lifeline and go see my Corvair mechanic who is an old-school Chevy factory guy and have him look it over. Which i probably should have done a long time ago, it will be good to see him again.

I know how to make the truck look good, but getting it to run good, (at least for a camping trip next week) is beyond me at the moment.

Thanks for the help and I will reply with what I learn.
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Old 07-28-2020, 12:18 PM   #12
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Re: New Q-Jet will not settle down to idle, it's driving me %$#%$#%$

Is that piece of tape supposed to block off that vacuum port ? I would get a proper rubber nipple for any port you are not using. ..
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Old 07-28-2020, 01:00 PM   #13
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Re: New Q-Jet will not settle down to idle, it's driving me %$#%$#%$

The tachometer has to be
wrong . A 5.12:1 gear set ,27 inch
tall tire , at 50mph in 1:1 drive
would be turning 3271 rpm’s
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Old 07-28-2020, 01:08 PM   #14
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Re: New Q-Jet will not settle down to idle, it's driving me %$#%$#%$

What is the number stamped on the carb? Driver's side by the throttle linkage. Without the choke stove what are you using to hold the choke open with? I second getting some rubber or vinyl vacuum caps for those plugged off ports. The small port by the choke linkage should go to the air cleaner, not sure what the large one in front would be for- thats where the stamped number on the carb helps to make sure its correct for your application.
Are you sure you have a V8 tach and not a 6 cylinder tach? What rear end gears do you have?
Where are the black hoses running to on the driver's side, one from carb and one from the intake?
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Old 07-28-2020, 01:20 PM   #15
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Re: New Q-Jet will not settle down to idle, it's driving me %$#%$#%$

54blackhornet had same thought at first glance I did. Masking tape is poor at holding a vacuum. A piece of the correct size rubber hose with a screw in the end works as good as anything to stop a vacuum leak.

All or part of your problem may be because you don't have the choke stove installed. The stove hooks to the piece the green arrow is pointing to. That piece in turn is supposed to control the fast idle cam. (Blue arrow) If the piece the green arrow is not secured in the choke open position it flops around and causes the fast idle cam to engage intermittently.
It is easy to lock the fast idle cam in the down position with a zip tie around it and the bracket. Take the truck for a drive and see if helps.

Your factory tachometer appears to be as accurate as mine. Mine reads about 8-900 RPMs high when the engine is turning 2500.
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Old 07-29-2020, 10:37 AM   #16
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Re: New Q-Jet will not settle down to idle, it's driving me %$#%$#%$

The correct carb for your application should be stamped 70405XY
70= Rochester Carburetor
4= 1970-1975
0= 1970
5= California emissions
X= 0,1 or 2= Chevrolet
Y= transmission application Odd #'s 1,3,5,7,9 for manual trans, even #'s 0,2,4,6,8 for auto trans

Quadrajet Power LLC has some nice photos of a 7040502 that you can compare yours to.
https://quadrajetpower.com/chevrolet...rajet-7040502/
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Old 07-29-2020, 11:25 AM   #17
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Re: New Q-Jet will not settle down to idle, it's driving me %$#%$#%$

I've dealt with those re-pop Q-jets before. They're fine. Follow this procedure for your choke. I copied this, I don't want to type a bunch of stuff. I think it's a Lars procedure, anyhow do this first. FYI, Lars has written lots of good info on the Q-jet.

Of course, get the missing(and needed parts) for your carb. It's not a divorced thing or whatever, as you stated. It needs to be hooked up and adjusted before you do anything else IMHO.



Divorced Choke Systems (1967-1974)
*Disconnect the divorced choke rod from the lever on the passenger side of the carb. Leave it attached to the choke coil box on the manifold.
*Open the throttle slightly and fully close the choke by pushing on the lever arm that the disconnected rod normally attaches to.
*Push the choke rod all the way down into the choke coil until it hits the stop. If the engine is dead cold, it may already be bottomed out.
*At this position, the top of the choke rod should be level with the bottom edge of the choke rod hole in the lever on the carb.
*Bend the rod to obtain this relationship.
*Once complete, hook the rod back up to the lever.
*With the rod hooked up, push the choke rod back down to the seated position once again. This should fully close the choke blade. If the choke blade is not fully closed in this position, bend the choke intermediate rod that comes up through the body of the carb and attaches to the choke blade lever. Bend the rod so that the choke blade is fully closed.
*Remove the short piece of vacuum hose attaching the choke pulloff to the vacuum nipple on the carb. Attach a long (about 2’) vacuum hose to the pulloff.
*Crack the throttle slightly and push down on the divorced choke rod to close the choke fully. Release the throttle. Keep light finger pressure on the choke rod to maintain light closing pressure on the choke.
*Suck on the vacuum hose to retract the choke pulloff. If the pulloff does not retract, it must be replaced.
*With the pulloff fully retracted and light finger pressure on the choke rod, use your other hand to lightly push down on the forward lower edge of the choke blade to simulate to force of the air across the blade. This will open the choke slightly. At this point, measure the distance between the forward lower edge of the choke blade and the forward wall of the airhorn. This distance should be ¼”. You can use a ¼” drill bit as a simple gauge to check it. To adjust, bend the tang on the choke linkage where it contacts the choke pulloff rod.
*Re-attach the vacuum hose to the pulloff and the carb.

Now see if it comes off fast idle!
Good luck,

j
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Old 07-29-2020, 11:55 AM   #18
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Re: New Q-Jet will not settle down to idle, it's driving me %$#%$#%$

Jeff great writing on proper choke setting ! Thanks for the information..Jack
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Old 08-01-2020, 01:31 AM   #19
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Re: New Q-Jet will not settle down to idle, it's driving me %$#%$#%$

Alright here's the update, sorry been crazy this week, thank you for all your great suggestions!

Sometimes it pays to go and see the wizard. I took it to my retired Corvair mechanic, a 57-year Chevy veteran and admitted, "I can make it look good but I can't make it run right. Can you do your magic?" Don't know why I didn't do that months ago. We talked story and dicked around under the hood for a couple hours and he taught me things. Old mechanics rule.

#1 Incurable high idle? Throttle return spring was on backwards. I never would have noticed that.

#2 As several of you pointed out, the choke stove is missing. That solves the cold start problem. O'Reilly's said "Derp?" but I'll find one. Thanks Jeff, MJN and HO455 for the adjustment instructions.

#3 What I thought was dump truck gearing keeping me at 45mph was actually timing. When I got the truck it was at +4 and I set it at +8. Louis said, these engine rebuild kits come with some really random cams, you might not be able to tune it by the book. We have to guess what's in there but I bet if we go extreme on the timing it will have a lot more power. He set it to +16/18 and WHOLE. NEW. TRUCK. Tons of power at WOT, no backfires, and all the downstream problems sorted themselves out. Clutch no longer judders and the rear axle stopped binding up, two things that I thought were major mechanical issues. It's still geared low but 50-60 is easy, the tach reads in the 30s and probably still off by a good margin like you say. (Pretty sure its the original tach, although its not on the SPID.)

When I bought it on eBay last year the ad said "drives like it came off the assembly line in 1970" (with no exhaust, lol) but now it actually does. A week ago I hated this truck and now I'm really excited about it. What a difference.

Last edited by Rollo Grande; 08-01-2020 at 01:39 AM.
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Old 08-01-2020, 09:07 AM   #20
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Re: New Q-Jet will not settle down to idle, it's driving me %$#%$#%$

#1. Spring on backwards!
Your pics show it’s correct.
It doesn’t belong on the front if that’s where he put it. You’ll wear out the throttle bushing faster with spring in the front.

Timing is a bit high. 14 would be better. You might have a little detonation that you won’t recognize and break a ring land or hole a piston.

Choke stove parts found here.


https://quadrajetparts.com/
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Old 08-01-2020, 04:36 PM   #21
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Re: New Q-Jet will not settle down to idle, it's driving me %$#%$#%$

If you can get the silicon type of vac port plugs. The rubber ones only last about 6 months.

4000 rpm at 50 mph? I would stop running the motor till you get it figured out. At least running it on the road.
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