Problems fixed. Took her out for a “spin”

Kind of, but we’ll get to that. A quick before and after. Before, I had a surging idle (from 660 up to 940 rpm at the worst of times) with random stalls and weak vacuum (~ 14 inHg). Now, I have rock-steady idle (850 +/-10 rpm) and good strong vacuum (18 inHg). Here’s a video showing the typical readings I get, both ‘before’ and ‘after’ the fix. Vacuum gauge on the left, RPM on the multimeter on the right.

The in between was a lot of tweaking with the carb and timing. I had thought the carb-to-manifold gasket was leaking so I replaced this but nothing changed. I also rebuilt the whole carburettor in desperation. Nothing could steady the idle. Well, saying that, setting the initial timing at 27 degrees sorted it, but that was just silly. After a lot of frustration I decided to swap the QJet and the Edelbrock manifold back to the stock 2bbl cast iron and 2bbl carb. This immediately smoothed out the idle and brought up the vacuum. This also let me hear something… the 2bbl and cast iron setup sounds so much better! So much grunt and growl! So a side note… I am now looking for a cast iron 4bbl intake to try out with the QuadraJet. If anyone knows where I might find one in the UK, please send a comment or email!


With the problem localised to either the manifold or carb, and ignoring for now the possibility that the manifold is leaking (cracked/warped) I focused on the carb. After again playing with the mixtures, I found I could level out the idle some. Previously, to calibrate the mixtures I’d followed normal procedure… Lean out the mixture all the way on one side (turning screws in), and then richen it up (turn out) until the vacuum reaches a maximum, side after side. Now, previously, with a mixture screw all the way in the engine would want to die. Backing out a single turn would save it and raise vacuum to about 13 inHg, then another turn would raise it to 14/15 inHg, turning out 6 full turns still gave 14/15 inHg, so I never went past this. Out on a limb I turned the screws out 10 whole turns on both sides, and miraculously the vacuum now sat at, but fluctuated around, 17 inHg and the idle speed smoothed out considerably. This was unexpected but welcome! After speaking with Cliff Ruggles (and this link I do endorse! And unpaid. Cliff was very helpful) it was decided that there was too much restriction in the stock idle circuit of this carb, for this engine.


So, to remedy this, the idle down channel restriction cups were drilled out from something less than a milimetre diameter, to 1.25mm diameter. Straight away after reassembling the carb and reinstalling I can have the idle mixture screws about 7 turns outs with vacuum at around 18 inHg and the idle speed steadier than I’d ever hoped.



A worthy note, at least for me: If you do a quick search for initial calibrations for QuadraJet mixture screws, multiple forum posts will probably mention 3 or 3.5 turns out as a baseline, and you should be around this mark. This is for ’65 – ’78 carbs. I have a ’79 and what I didn’t realise is that starting in ’79 the mixture screws were changed to a metric thread with a finer pitch. This obviously meant that the intial ‘baseline’ is different – and it’s not uncommon for the screws to be around 7 turns out. If I’d known before! A quick and rough way to check mixtures are correct is to cover the choke slightly with the engine fully warmed up. If the revs pick up by around 20-50 the mixture is probably fine. If the revs stay the same or even drop lower then it’s too rich. If they pick up more, say around 100, then it’s too lean.


And so! I took the car for a drive! On the drive. And only on the drive sadly. Unfortunately the car’s still not safe enough, and definitely not legal enough to go onto any road. This video shows braking and steering working. The TCI TH350 transmission is in and works. Mechanically, the car is heading towards completion. There’s still a few issues to work out. The parking brake when fully applied just about holds the car on a slope when in neutral, but when in gear the car rolls. I still need to mount the petrol tank and run the lines. I need to get some new rockers and polylocks and probably pushrods. Nothing too taxing (here’s hoping). So soon, I’ll be able to start work on the cosmetic side of things.


Mark Kelly | 12th October 2011 at 6:32 pm

Looking and sounding good. I am restoring a 67 Firebird, it is striped out and on a roll over jig so I can start the metal work repairs. Your website has been a great help, I think my progress will be a lot slower than yours as a new baby and house extention has put the car on the back burner. I will look forward to your good progress.

danny avalos | 30th December 2015 at 10:08 am

iam so glad you got it tuned correct it sounds great


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