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The ladybird guide to Rochester Quadra-jet to Holley carb conversion

I suppose I should start by saying why I went this way....rather than keeping the original Rochester Quadrajet. Well, my 1980 C3 L48 350 ci has only 43,000 miles which I know is true, but even with these low miles the old carb had two problems.
1/ The top part of the carb (the air horn) was warped which can not be cured by machining.
2/ The throttle shaft housing bushes had lots of play, which means air can enter the carb via the spindles and this in turn corrupts the idle operation and setting of the carb. The Rochester Q-jet uses engine vacuum to work correctly and if is not well then it just won't play ball. Causes a lumpy idle, which you can't cure with the mixture screws.

Sure, you can get a rebuilt carb but the carb will only end with the same problems as it had before, they are around 300 +vat and the quality can be patchy.
Performance wise the Q-jet probably has slightly better low speed pick-up when 'new' as the primaries are smaller giving a higher air speed at low rpms. The Holley has the same size primaries and secondaries but you would need a stopwatch to see the difference in performance between it and the Q-jet and a new Holley is far far better than a worn out Q-junk!

NB:- Primaries are the first two barrels (venturies, bores, chokes) that open with the thottle action and the secondaries, are well, the second set!

The Q-jet, because it has two pairs of different sized barrels or bores, is called a spread bore carb, the Holley is called a squarebore cos all the bores are the same and equi-spaced, makes sense!

To fit the Holley to the original inlet manifold you need an adaptor so the Holley can be fitted. The adaptor fits only way but the correct way looks wrong! The flat face of the adaptor must butt up to the bottom of the carb??.these adaptors are sometimes slagged off wrongly because they have been fitted upside down, I know as I have 3 other yanks that the adaptor fitted the wrong way around!

So, what do you need?
I bought all I needed from Real Steel, a Holley 4160 universal 600 cfm vacuum secondary carb with squarebore adaptor, throttle cable bracket and fuel union for £228.00.
To this you will need to add a new air filter if you have the low-line bonnet which I think are 1980 onwards? I bought an Edlebrock from Tom @ Claremont for £35 and it fits real sweetly. Gaskets come with the carb, but you will need some fuel line, don't use metric fuel line.... it does fit but its slightly larger than is correct,..that GRP bonnet makes a great BBQ if you have a fuel leak! Instead use imperial fuse hose?can?t remember the size but take the fuel union to the parts shop, while you are there buy some new hose clips. The brake servo pipe is the same size as the fuel line and it has the right properties as well.

I fitted an in-line fuel filter whilst I had the opportunity. UK fuel is real clean but the US have a far lower standard for cleanliness and unless you have cleaned your tank since it has been in the UK the tank may have some muck from it?s US days and to a limited extent its UK time, so it?s worth the couple of quid they cost!

OK here we go then?.


Remove your Q-jet and keep for an anchor

Clean up your manifold faces

Remove the EGR valve (Exhaust Gas Recirculation) the large valve at the rear of the inlet manifold and next to the distributor. You need to plug the holes where it fitted, if you don't exhaust comes out of one hole and air goes into the other hole, not good. I used a steel gasket I made from sheet steel with Hylomar and the original steel EGR plate to secure everything, you will to cut 5mm from the bolts as they are too long standard. I feel sure that this blank can be bought over the counter but haven’t asked for one? Don’t loose sleep over the lack of EGR, a better running engine without EGR is far cleaner than a poorly running engine with EGR!

Fit the carb adaptor with gasket both sides and Hylomor gasket cement to ensure a good seal. The instructions with the adaptor are good and easy to follow for fitting the various nuts and bolts leave all the bolts and nuts finger tight for now....leave the driver's side rear nut off until last as you need to fit the throttle under this nut

Fit your Holley carb and keep the cover on it at the moment? We don’t wish to drop anything down the carb at this point and embarrass the neighbours with the cussing!

Fit the new throttle cable bracket and 2 return springs..ensure these go on correctly, it will be a drama if you cant close the throttle! Torque up all the bolts..I think 35 lbin?

The throttle cable needs fitting to the carb you cant use the old fittings on the Q-jet as they are welded on, but an elegant solution is to use a 5 mm bolt thru the eye of the cable and the eye of the Holley throttle, space with washers either side and secure with a locknut, mines works real smoothly. The same goes for the kick down cable fittings. Adjustment of the kick down cable is made by removing the mounting fitting and twisting it around the cable actual. I have around 10mm of slack in my cable (far less than the Q-jet, but the Holleys throttle arm movement is different) and this works well. Too tight and you run the risk of the gearbox burning its self out as it tries to shift all the time. Work the throttle by hand a number of times to ensure that all works smoothly.

Check you have wide open throttle flaps when the gas pedal in fully depressed? Very easy to miss you think you have full throttle when the carb is only 80% open..I have seen this many times over.

Fit the brake servo pipe work and secure with hose clips ..then double check. There is a servo take off on the Holley.

Fit the vacuum advance pipe for the distributor, there is a take-off at the bottom of the Holley

Fit the fuel pipe ….hmm should be easy but you couldn’t get the old fitting out the Q-jet could you, and the pipe had to be cut….they always weld themselves up so you aren’t alone here! I cut into my fuel pipe along the top horizontal run at the front of the engine so it gave me more room to fit my inline fuel filter, when you cut thru the pipe, wipe some grease over the hacksaw blade and the cut as you go to help stop filings getting everything where.

Take off the feed to the HEI distributor as you want to turn the engine over now, but you don’t want it to start and any sparks just in case there is a fuel leak somewhere.
Crank the engine over until the fuel filter is full, you should be checking all the fuel unions for leaks, my Holley leaked where the fuel union went on, a real small jet of fuel under pressure that would of shot over to the exhaust manifold the Holley had a slight mark where the union mated to it causing this leak., a little dressing with wet ‘n dry soon cured this issue

You may now want to hook up a choke cable now, I didn’t bother. I know that 3 pumps on the gas pedal is all you need unless its winter and I didn’t want a manual choke pull ruining the look of the interior. You can buy an electronic choke for the Holley and you have the old feed for it if you have a later Q-jet….nearly forgot, if you aren’t using the electric choke feed off the old carb tuck it away somewhere safe…it a switched 12volt source.

Set the air screws to 1.5 turns out and all should be OK for a standard to slightly modded, say 250bhp 350 ci, anything else needs tuning. Visit the Holley website it’s the best site I have even seen from a carb manufacturer, it tells you everything you will need to need to know about the carbs, how to jet/tune, how to select, ID codes, everything, brilliant site! These Holley?s are a breeze to work on if you have a vacuum gauge, very simple but very efficient. Not bad for a 1960?s design.

Fit the air filter and go for a start, even the idle should be about right as all the Holley’s are 'wet tested' at the factory, if it isn’t, set when hot and running but don’t remove the air filter when you do this if the engine spits back you will get a fireball in you face!

My C3 now drives like a different car, 1st time start with 3 pumps, smoooooth idle, instant pick-up, better fuel consumption and best of all it doesn’t make your eyes water when you stand behind it 8)

I think that’s about it? I feel sure I have missed something so please comment.