Quadrajet cock-up

[antijam]

When I first bought my small block '71 it was fitted with a non-original quadrajet and had severe bogging and popping and banging on the overrun. after much fruitless fiddling I replaced the carb with a correct, professionally rebuilt quad and after upping the jets three sizes above standard to compensate for the better breathing afforded by headers and a free flow exhaust, bogging and banging were cured. It ran well like this for some years although every time the plugs were checked they were pretty sooty suggesting the car was running rich. Anyway, having recently corrected an annoying and persistent vacuum leak I wondered if I could come down a jet size to compensate. Accordingly I replaced the jets with one size smaller and the car ran fine except that the popping and banging on the overrun had returned! At this point it dawned on me that since the new carb ran fine on fitting, other than the jet upsizing I had made no adjustments to the other settings. Since the popping and banging were effectively occurring at idle, had I neglected to check, and if necessary adjust, the hot idle mixture settings?
OK, better late than never, I give it a go. The quad has two mixture screws in the base of the body - one is arrowed yellow in this pic.....



....and they need to be adjusted to achieve maximum manifold vacuum at hot idle. So after warming the engine up until the choke is fully off I remove the PCV tube (arrowed blue) connection at the carb and attach my vacuum gauge instead.



I restart the engine and carefully tweak the mixture adjusters one at a time to achieve maximum vacuum; this stabilises at just over 15 inches of Hg.



I noticed that both adjusters were several turns off the optimum setting.This done I adjust the idle to a stable minimum which turns out to be 500 rpm. The car is a manual so this idle is fine; an auto would probably need around 700 to 800 rpm.
With the PCV line reconnected I restart the car and we're off for a test drive. Popping and banging have disappeared and the car seems to be pulling better than ever! I fill the tank and put in around 150 miles over the next few days before pulling and checking the plugs. All are now the textbook light tan colour indicating proper combustion. If I'd done what I should have done when originally fitting the carb the problem would have been sorted correctly years earlier. Oh well, even at 82 I'm never too old to learn.
Incidentally after the few days running I decided to check my fuel consumption. I don't normally do this on any classic car since good fuel consumption is never one of the criteria for purchase. Anyway, consumption turns out to be a shade over 13 mpg. Not very impressive but I have to admit I do tend to give the secondaries a fair amount of action - primarily for the noise! Hey ho - it's only money.....

 

[Roscobbc]

You can see why people favour Holleys can't you? - not saying that Quadrajet isn't a good carb......just that few people seem to know how to get the best out of them........BTW you fuel economy @ 13 mpg is abysmal - is it a higher hp engine with auto?

 

[antijam]

You can see why people favour Holleys can't you? - not saying that Quadrajet isn't a good carb......just that few people seem to know how to get the best out of them........BTW you fuel economy @ 13 mpg is abysmal - is it a higher hp engine with auto?

The Quadrajet is an excellent carb but as you say Rosco, it pays to know what you're doing before fiddling with it. I should have done a little more research before making assumptions about mine.
Engine is a base 350/270 manual with headers being the only performance mod as far as I know. It does go very well but there is always a faint smell of fuel when driving. I can find no signs of any leakage and the plug condition now suggests it's not running particularly rich. Consumption is based on one fill up with no correction for non-stock tyre size or any refill level error but those are not going to make a great difference. I need to investigate further.

 

[Vetman]

Why not add an O2 sensor and gauge?

 

[Oneball]

Hate to tell you but youve made a bit of a mistake, you shouldn’t have set it up with the PCV disconnected. It is in essence a small vacuum leak and needs to be connected when setting the carb up. Connect your gauge to somewhere else, servo line is good if you can’t find another port. I’d also expect a bit more vacuum at idle, what is your initial timing set at?

 

[antijam]

Hate to tell you but youve made a bit of a mistake, you shouldn’t have set it up with the PCV disconnected. It is in essence a small vacuum leak and needs to be connected when setting the carb up. Connect your gauge to somewhere else, servo line is good if you can’t find another port. I’d also expect a bit more vacuum at idle, what is your initial timing set at?

Thanks for that Tim. I'll repeat the setting with the vacuum takeoff in the servo line. I thought the idle vacuum was rather low. Initial timing is 8° BTDC ; I don't think I quite saw total advance of 36° all in.

 

[Oneball]

I’d see if it likes some more initial timing too.

 

[Roscobbc]

ADVANCE CURVE KIT, NON HEI

Advance Curve Kit. Fits: All 1957-'75 GM/Delco Non-HEI

www.moroso.com

ADVANCE CURVE KIT, GM HEI

HEI Advance Curve Kit, special design centerplate eliminates over-centering of the centrifugal weights. Designed by Nelson Crozier. Fits: All GM HEI Distributors

www.moroso.com

For a mere £20 or so (and depending on model year and whether HEI or earlier ignition/distributor) this is potentially the bigest 'bang' you'll ever, ever get for your buck!
Simply replace existing bobweights in your distributor with the replacements and a pair of new springs to make the ignition advance come 'in' earlier and hopefully making the performance 'snappier'.
Use with discretion.....so a heavier pair of springs on a small block with automatic transmission and high numerical rear axle........lighter springs on an earlier manual small block and perhaps lightest springs on an early manual big block engine to get that adavance 'in' a little earlier and 'perk' things up a little.
But remember not too much so as the engine perhaps 'pinks' under load!

 

[johng]

I've used one of those advance curve kits and it made a noticeable difference, I would suggest you send an email to Lars Grimsrud at V8FastCars@msn.com and ask him for a copy of his latest timing paper. it gives a great description of what you need to do. You could also ask him for his Q-jet tuning paper, again lots of useful info there. With my standard L48 engine I've got over 20 inches of Hg at tickover.

 

[Roscobbc]

I've used one of those advance curve kits and it made a noticeable difference, I would suggest you send an email to Lars Grimsrud at V8FastCars@msn.com and ask him for a copy of his latest timing paper. it gives a great description of what you need to do. You could also ask him for his Q-jet tuning paper, again lots of useful info there. With my standard L48 engine I've got over 20 inches of Hg at tickover.

In the 'States and on Chevy and Vette forums Lars is only second to God in terms of knowledge, understandi g a advice relating to carbs and stuff......a well rated person!

 

[antijam]

I've used one of those advance curve kits and it made a noticeable difference, I would suggest you send an email to Lars Grimsrud at V8FastCars@msn.com and ask him for a copy of his latest timing paper. it gives a great description of what you need to do. You could also ask him for his Q-jet tuning paper, again lots of useful info there. With my standard L48 engine I've got over 20 inches of Hg at tickover.

Thanks for that John. I have Lars' Quadrajet tuning paper (that I should have paid more attention to) but not his timing paper; I'll certainly ask him for it. His Quadrajet knowledge is unsurpassed and he expresses it very clearly.