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77 carb match up

Started by Matt #12861, June 11, 2008, 02:32:10 PM

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Matt #12861

I have a rebuilt carb from a 472 motor that I am trying to put on a 425 ('77 I think) motor.  The older carb's choke has a coil that bolts to a recessed spot in the manifold that is not on the newer motor.  Is there a way to put a newer automatic choke on the older carb?  It looks like the newer carb uses a "dash-pot"?  from what the owners manual says.  Where can I find the parts?  Does anyone have a breakdown with numbers?  Or do I need to hunt a new carb? 
thanks!
Matt

"Cadillac Kid" Greg Surfas 15364

Matt,
If you are not real familiar with Quadrajets I believe the modifications your might face in not only installing an electric choke (easy) but jetting and tuning the older carb might prove to be a challenge.  You should have no problem getting the later carb at one of the Cadillac (specialty) "salvage" yards.
Unless you are going for some other modifications, it would be better to remain original.
Greg
Cadillac Kid-Greg Surfas
Director Modified Chapter CLC
CLC #15364
66 Coupe deVille (now gone to the UK)
72 Eldo Cpe  (now cruising the sands in Quatar)
73 Coupe deVille
75 Coupe deElegance
76 Coupe deVille
79 Coupe de ville with "Paris" (pick up) option and 472 motor
514 inch motor now in '73-

TJ Hopland

The choke thing is much easier said than done, there used to be kits but when they come up on ebay they seem to go for a lot ($100 IIRC).    If you are not into endless tweaking I would just rebuild the original carb.   If you are looking for an easy and cheap performance boost do some tweaking on the distributor.  Next best thing you can do is a new cam.  I have a 78 and tore apart the carb 3x over the last 6 months chasing various problems that mostly turned out to be distributor problems.  As long as I was at it I did some grinding on the weights and is now like a whole new car, nothing like my freshly rebuilt EFI 500 but it can get out of its own way now.

Just incase you dont already know that vintage Cadillac motor came from the factory with a plastic covered cam gear that did not tend to hold up well with age and mileage.  Good part is you can see enough of it to get an idea of its condition by removing the distributor and looking down the hole.  If the motor is real dirty it can be hard to tell but IMHO is well worth the trouble just to know if it is falling apart or has been replaced.
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

Matt #12861

IF I was going to put an after market carb on- what size (CFM) would be equivalent?  I'm not trying to increase horse power.  I saw a formula
CFM = RPM * Cu IN / 3456 * efficiency,   4400 * 425 / 3456 * .8  = 432CFM  Does this sound right?  The 80% efficiency is a guess, I got the 4400rpm out of the fast idel setting speed.   I didn't see the rochester CFM rating in the owners manual.  What would a 750 CFM carb do?  Overkill?  Is it better err on the small or big side?

Thanks again

Ted in Olympia WA

For some reason the 472 was a 750cfm carb and most 425 I have seen are 800cfm.  A 750 would work just fine and maybe smaller might be better.  But I would agree with the rest to just get yours rebuilt or buy one made for it that is.  When the Q-jet works it works great and will give you the best mileage.

TED
Selling used Eldorado Parts from 1971-1978.  Member Number 25659.

matt 12861

Ok, so I found out the motor was a '79 425 and I got a '79 carb rebuilt.  Now,  the manual says the electric choke is hooked up to a wire from the oil pressure switch so it only works when the engine is on.  It also talks about resistors inside of the choke that work at different temperatures but doesn't give their values.  There isn't an actual wiring diagram in the manual that I can see.  Here are a few more questions:
  Do I fuse the wire?  What gage wire would I use?  Since I don't have an oil pressure switch (I replaced it with a gauge), can I just run the wire from the ignition when the ignition is on?
Thanks,
Matt

TJ Hopland

What manual do you have that does not have wiring diagrams?  The FSM's have nice fold out ones in color.   You can run the choke off of a key on source, the down side is if you leave the key on it will start heating the choke and if you didnt start the engine right away it would be hard to start.    Fuse or not depends on what else is hooked where you hook it and what thats fused at.  I have never had a choke short, only wire harnesses melted into manifolds and such.  Im not sure what you are talking about resistors, if its inside the choke sounds like something you should not have to worry about.  On some cars I have added electric chokes to I have used a ballast resistor (typically 2ohms) to increase the life of the chokes, aftermarket chokes dont seem to like working at full system voltage.
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason