I have a 750 Demon double pumper that I want to lend to a friend who has a ’73 convertible with the 351 CJ engine. His engine is bone stock, and I’m not sure but I think he has the spreadbore intake manifold. Wheren’t they all spreadbore intakes?. Anyway, I want to put on the 750 Demon for giggles for a week since he needs a new carb – his autolite is leaking gas and falling apart. Will the Demon fit o.k. on a spreadbore intake? I figure it should. Anyone know for sure? Thank you.
Will 750 Demon carb fit on stock 351 Cleveland intake?
- Thread starter '69Stang
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I dont see why it wouldnt fit.
However, that 750 demon is WAY too big of a carb for that engine. Let us know how it performs though.
However, that 750 demon is WAY too big of a carb for that engine. Let us know how it performs though.
SadbutTrue
Founding Member
it'll fit, but you will soon see what overcarbing means. That demon is probably twice the carb of whatever came on a stock 73.
My notes show a Motorcraft 4V carb to be original equipment on the '72-'73 351 Cobra Jet. Stock Sizing was 470 cfm, however the carb may not be way too large for this engine to handle afterall for trial purposes. A vacuum secondary would probably be a better setup. Jetting may be necessary with the Demon, but the 351 CJ still had a few goodies left even in '73, my notes show the Cleveland engine still had 2.19/1.71 valve sizes, 9.0:1 compression, and a .481/.490 lift camshaft, so if he has done any other performance mods, (which the big restriction on the 351CJ was exhaust and intake), than that will only help the engine to breathe better, and bolting on a 750 carb may not be that big of a stretch for that Cleveland to handle. A healthy ignition to light the fires is a plus, so I'd check the points and such to make sure they are in good shape if still equipped with OE ignition.
But ultimately, a few performance mods would need to be done to really madate a 750 DP carb like your Demon. For example the 330 horse, solid cammed, 11.7:1 '71 Boss 351 was fitted with a 750 cfm carb.
But ultimately, a few performance mods would need to be done to really madate a 750 DP carb like your Demon. For example the 330 horse, solid cammed, 11.7:1 '71 Boss 351 was fitted with a 750 cfm carb.
The factory intake is a spead-bore. The square-bore Holley will bolt on to the carb studs and the primaries will line up OK but the secondaries will not line up with the holes in the intake. In fact the secondary butterflies will probably not open all the way because they will hit the intake.
I used to run a square-bore Holley on a spread-bore intake. I had to "open up" the secondary holes in the intake to get it to work.
I think that you will find the 750 DP to be too much carb for a stock 351CJ motor.
I used to run a square-bore Holley on a spread-bore intake. I had to "open up" the secondary holes in the intake to get it to work.
I think that you will find the 750 DP to be too much carb for a stock 351CJ motor.
I totally agree with you guys that this 750 Demon is too much for the stock 351 CJ. I would recommend a Holley 650 – 700 CFM Holley with vacuum secondaries on a stock application. Though I have to mention that I ran a 750 Holley double pumper on a 351C 2v engine and it loved it. After talking to him a little more he did remember that 10 years ago he had the engine re-done and that they did put a bigger cam in it, and when I heard it and drove in it the other day the engine did have a bit more grumble then I thought a stock engine should. Other than that the engine is completely stock, exhaust manifolds, everything. We will be putting in a Petronix 2 ignition in at the same time.
In any event, so what I am hearing is that the Holley won’t line up with the spreadbore intake? Did all the 351CJ’s have a spread bore intake? Will I have to grind away the intake manifold, or will a spacer fix the problem?
In any event, so what I am hearing is that the Holley won’t line up with the spreadbore intake? Did all the 351CJ’s have a spread bore intake? Will I have to grind away the intake manifold, or will a spacer fix the problem?
pabear89
Active Member
Demon 
But I am running a 750 holley on mine and love it.
When it had the 600 holey it would struggle at getting above 5k,
Now it will push past 6k without any trouble and wants to go more.
Not all 351-4v are speadbore's.
PB
But I am running a 750 holley on mine and love it.When it had the 600 holey it would struggle at getting above 5k,
Now it will push past 6k without any trouble and wants to go more.
Not all 351-4v are speadbore's.
PB
Sometime in 1971 Ford changed from the "M" code 351 4V (square bore carb) to the "Q" code 351 4V (spread-bore carb). All Q-codes were spreadbores (as was the R-code Boss 351). However the cast iron M-code intake is very common and cheap and actually flows pretty well. If you want to run a square-bore carb and you don't want to buy a new aftermarket intake I would get an M-code intake instead of fooling around with some kind of adapter.
Well, I went down to the local speed shop and my buddy bought an Edelbrock ¾” spacer made for spreadbore applications. The spacer is nicely contoured and will allow the use of the square bore Holley on the stock manifold for the time being. But the next modification will probably be a new intake, a Blue Thunder, though it is kind of expensive. There isn’t another good dual plane intake out there for a 4V Cleveland is there?
pabear89 said:Demon
When it had the 600 holey it would struggle at getting above 5k,
Now it will push past 6k without any trouble and wants to go more.
Not all 351-4v are speadbore's.
PB
That's good to here, I kind of suspected it could be all right. Does yours stumble or hesitate under acceleration? The “Demon” 750 is just a trick Holley carb. Did you jet down the carb to make it run well or did you just slap it on right out of the box? Again, I had a ’71 Mach 1 with a 351C 2v and I ran an iron 4v intake and a 759 Holley double pumper and it liked it. Was it over carbed? Sure, a little. But it ran a lot faster than the Motrocraft carb.
pabear89
Active Member
The Carb was an Ebay buy, fresh out of the box new with a broken choke cover.
After replacing the cover I bolted it on an only did some minor mixture adjustments, it ran fine.
The was some slight stumbling from off idle, But a few minor adjustments to the accel pump and they were gone.
Have not touched it sence, Went to tuson az and back avg 15.87 mpg
at a 75mph avg speed.
Right now it sits atop a 351cj stocker with a 1in spacer and accel duel point dist and a open type air cleaner as its only Mods so far.
More to come in the near future.
PB
After replacing the cover I bolted it on an only did some minor mixture adjustments, it ran fine.
The was some slight stumbling from off idle, But a few minor adjustments to the accel pump and they were gone.
Have not touched it sence, Went to tuson az and back avg 15.87 mpg
at a 75mph avg speed.
Right now it sits atop a 351cj stocker with a 1in spacer and accel duel point dist and a open type air cleaner as its only Mods so far.
More to come in the near future.
PB
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