Engine, intake manifold and carb mixture on a 1973 Q-code - which will work?


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Jul 10, 2011
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Germany, Southwest, Black Forest
My Car
1973 Ford Mustang Mach 1 T5 Q-Code 4-Speed
Hi guys,

I am in the engine-thing at the moment with a very experienced buddy of mine... He restored a 1969 Mach 1 from the ground, a few other cars and he is at a 1973 T5 convertible at the moment. So I think I am in well hands! But now the story:

First, my mustang did not run very well, too high idle and rough. I changed the spark plugs, the plug leads and all the liquid stuff. After that we checked all and came to the conclusion, that the aftermarket Edelbrock 600 cfm was done. The ignition was bad adjusted to compensate that. There is not the original intake manifold on the block, it is a Offenhauser on it, we do not know which at the moment.

So we yesterday tried a restored original 1970 Motorcraft 4300 carb with 600 cfm from my buddy on it and adjusted the ignition by distributor and the carb itself. After that it runs a lot better but sometimes it feels like he could a lot more if he is allowed to run better because of something we have to find out these days. Sometimes he turns up like a beast as he should and sometimes he runs like an oldtimer with a half of his power. When he is in good times he runs in the middle rpm perfect. In the lower it is a bit rough from time to time. And at high rpm over 4.000 it feels like he has a handbrake on it. We have the distributor and the original breaker contact ignition to check more on our list among other things these days...

I do not know, if I have the original Q-code CJ under the hood - I assume so, because of it was said, that this is the original engine and I get another stripped engine which is a 1970/71ies 351 4bbl from the casting numbers. There is also a 1970/71 intake manifold, it is not the extreme square bore like in 1973 with the big worm route but a more slightly square bore, almost similar 4 holes on it, it is the same as this


Therefore I have a suitable 1971 original 4300 Motorcraft carb (D1OF-AAA) which my buddy restores at the moment.

My questions at the moment are:

Where can I have a look on the engine if this is the original CJ Q-code?

We will try at the next step my 1971 4300 carb on the Offenhauser manifold. If it does not bring better results and we had checked the distributor and ignition things I could turn out that the Offenhauser is not the right thing. Thoughts?

I think that the combination of the 1970/71 intake manifold with the 1971 4300 carb could work good on that engine block - other opinions?

With this combination would we need a spacer between carb and intake manifold for any reason or was this combination in 1970-1972 without a spacer right out the box originally in our mustangs anyway?

The generic 1973 Q-code had a spacer with the EGR-system on it - I do not have the original intake manifold and EGR-spacer and the things around and I would run without it.

I heard some stories that an additional spacer brings more power - is this right? Is it necessary? And if all so and we need one where to buy and not to steel one? On the other hand I heard that the ram air guys cannot use spacers with their carbs because of the systems height. With an original 1973 EGR it could cause problems... All nonsense or right?

Questions after questions and a long story :D But I would very appreciate thoughts and opinions about all that, thank you!



Sep 5, 2011
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My Car
73 Q code Mach 1
If it has 4 barrel heads, I would run an edelbrock performer intake with a 600-750 cfm vacuum secondary holley carb, unless you wanted a more performance oriented setup.

If it does not have 4 barrel heads on it, and they are running a 4v head intake on it, then that could be the reason why it's running rough.. It wouldn't seal to the smaller port heads.