Carb too big?

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Brown8404

Brown8404

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Hey gents,

I am working on my new to me 74 duster with the slant 6. I have had it about a week now. I am working on the carb and it has a 750 CFM edelbrock manual choke 1407 model. currently the vehicle is not running, but I did have progress. I had a really large back fire out of the carb. any help would be awesome.

I am not sure if it is the distributor that is causing the backfire, but really learning as i go.
 
Is this a street vehicle? Do you have some sort of wild cam and very, very, very heavy head mods? Otherwise, the 750 sounds way out....

The backfires can generally be from cam/valve issues or ignition. Lots more info needed.
- General condition of car
- Any known history of car?
- Any other mods?
- Trans and such
- Expected use
 
I have owned this bad boy for about a week and I have done nothing but put parts back together. The previous owner said nothing about engine work like that.
 
I know the car has a 2 super 40 flow masters on it but the condition of the car is good. there is no good history to the car it was bought from a auto body shop because the owner could not pay the bill. it has the automatic 3 speed and the rear end is the 7 1/2 i believe, but I know its not the 8.25
 
Well, the 750's flow is 3 to 4 times bigger than the stock carb if that gives you an idea of how far out it is.

So is the engine in any sort of good condition? Clean? How did the oil look? Have you ahd the valve cover off? Has it been sitting for a long time or what?
 
There was no oil in it when I bought it, but I have taken the valve cover off and it looked clean. It has been sitting for about 6 months before I bought it.
 
OK, good sign in the inside being clean. /6's can get the valves to stick easily if they get gum in the guides.

Since you have not idea on condition, back up and start with a set of compression readings on the 6 cylinders. Take out all plugs, prop the throttle wide open, and then take compression readings. You can read up on how to use a compression gauge and get one as a loaner tool form many box auto parts stores.
 
I feel like a Holley 390 would be a better option if nothing is going on with the engine modification-wise. I'm not familiar with edelbrocks but I'd assume there's a way to get the flow down through "jetting" or whatever they use. Are you able to check timing at all while the car is cranking?
 
It is highly questionable if the secondaries of the 1407 will ever open at all on that 225....and I would guess that the idle and transition circuits will be outside of their normal range of calibration.
 
I have a 600 Edelbrock that I will trade you.....but that's still a little big. Within tuning range, at least though.
 
The math works out such that a 225 cubic inch naturally-aspirated engine at 5000 RPM is lucky to be capable of breathing about 350CFM, assuming it has the right cam to let it even breathe somewhat efficiently at that RPM (think 225+ deg @ .050 lift) in the first place.

Whoever had that car previously must've subscribed to the mind-set that bigger carbs magically increase an engine's displacement and RPM band.
 
BTW, when you make the compression readings, do them twice and write the numbers down and let us know what you measured on each cylinder.
 
69DartDave said:
The math works out such that a 225 cubic inch naturally-aspirated engine at 5000 RPM is lucky to be capable of breathing about 350CFM, assuming it has the right cam to let it even breathe somewhat efficiently at that RPM (think 225+ deg @ .050 lift) in the first place.

Whoever had that car previously must've subscribed to the mind-set that bigger carbs magically increase an engine's displacement and RPM band.
Click to expand...

I agree 100%.

It's funny though, whenever I throw that formula up here, I get railroaded. lol
 
Here are the pictures that were requested.
 

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Disconnect the secondary linkage and wire them shut. You could probably tune the primary to run decent until you find a smaller 4 bbl.
 
I wouldn't even put that carb on my 65 Newport w/ 383. Not only is it too large for a 225, it has mechanical secondaries that will open as you floor it, so the gas would just dribble in at the slant's air flow. The only big carb's I will try on my 383 are spread-bores - I have a Thermoquad and a Quadrajet. They have very small primaries for good mileage and drive-ability, but still much too big for a slant (some TQ's flow 1000 cfm with secondaries open). But, FI is even better (excl. the Holley Pro-jection 2D I have used for 20 yrs).

Many use those Edelbrocks. Their saving grace is simplicity and easy adjustment, but they aren't as fuel efficient as a Holley w/ vacuum secondaries. Now that one company sells a nice fuel injection TBI kit for $1000, I see those Edelbrocks declining.
 
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