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59 cdv Rochester carb issue

Started by Paul, August 24, 2019, 09:21:35 AM

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Paul

Wonder if anyone knows why my Rochester won’t kick down to slow idle. It sits on the low shoulder of the fast idle cam and should switch to slow idle where the fast idle screw is off the fast idle cam after the engine is fully warm. Can’t seem to get it to kick down? Any thoughts? Linkages are not jammed. Would some grease be appropriate? Thanks for your thoughts.
59 Caddy, Seminole Red with Dover White top

Dave Shepherd

59 4gc carb?  If so check if the secondaries are fully closing.

Big Fins

Does the 4GC have the thermostatic choke coil? It may need to be looked at for proper adjustment. It could be set a tad too rich keeping the fast idle cam and the choke from fully opening. Just a thought...
Current:
1976 Eldorado Convertible in Crystal Blue FireMist with white interior and top. (Misty Blue)
1969 Fleetwood Brougham in Chalice Gold FireMist with matching interior and top. (The Old Man) SOLD!

Past and much missed:
1977 Brougham de Elegance
1976 Eldorado Convertible
1972 Fleetwood Brougham
1971 Sedan de Ville
1970 de Ville Convertible
1969 Sedan de Ville
1959 Sedan deVille

fishnjim

I don't recommend grease.   If anything, a dry lube.   Dirt builds up then you got a bigger stickier mess.
This is one of those, needs looked at by someone knowledgeable, issues.   No way to know what condition it's in to even guess or condition of the vehicle.
Here's the manual, may help in your diagnosing.   

gkhashem

#4
I am no expert on carbs but before I would play with the carb check this first.

On my Oldsmobile 1959 with a Rochester carb I had this issue, check the thermostat choke spring coil. There is a tube that comes from the intake manifold and supplies heat to the spring which allows the butterfly (or choke plate) to open fully when warm. (on passenger side of carb has a black cover on it with an adjustment screw) If the carb  butterfly (or choke plate) does not fully open it will not kick down to low idle. I have seen all kinds of playing with the linkages in an effort to fix this from people who did not know what they were doing or were taking a short cut fix.

See the attached photo of my 1959 Cadillac CDV with a Rochester. That tube should be so hot when the car is warmed up you cannot grab it without burning your hand.

If it is not you need to remove the spring cover (make sure you adjust it back to the proper adjustment) and check for carbon build up.

If you have a lot of build up your tube buried in the intake manifold that this upper tube connects to is corroded or broken inside and needs to be replaced. If it is broken vacuum will suck up all kinds of exhaust flumes when it should be pulling only hot air.

Now you may need to clean out that upper tube too along with the choke spring housing. The upper tube should be able to be blown out with some compressed air, if real bad you may need to get it boiled out. But cleaning it out will only start the clogging up again since if it is clogged your tube in the intake has a hole in it.

Also check to make sure the thing is getting vacuum, this is what pulls the hot air up to the spring.
1959 Oldsmobile 98 Holiday Sports Sedan
1960 Cadillac Coupe Deville (CLC Sr #72)
1964 Oldsmobile 98 Town Sedan (OCA 1st)
1970 GMC C1500
1977 Oldsmobile 98 Regency Coupe
1978 Cadillac Coupe Deville (CLC Sr Crown #959)*
1992 Oldsmobile 98 (OCA 1st)
1996 Oldsmobile 98
*CLC Past President's Preservation

Past Cadillacs
1959 Coupe Deville
1966 Coupe Deville (Sr #861)*
1991 Eldorado Biarritz (Sr #838)

Paul

Turns out it was,indeed, the choke tube. It was cold when the engine was hot. Flipped to electric choke and problem solved. Thanks for all the suggestions!
59 Caddy, Seminole Red with Dover White top

gkhashem

Yes, switching to the electric choke bypasses the hot air set up and will work.

Since I am a compulsive original purist, I replaced the tube in the intake manifold and cleaned out the exposed tube and the spring housing. Then made sure I had good vacuum to the spring to pull the hot air.

But again you bypassed that. However if any exhaust gas is pulled into the carburetor it will carbon that all up too eventually. Unless it is totally blocked. So you may still have a problem again soon. Especially if your carb was very dirty and you just cleaned it.
1959 Oldsmobile 98 Holiday Sports Sedan
1960 Cadillac Coupe Deville (CLC Sr #72)
1964 Oldsmobile 98 Town Sedan (OCA 1st)
1970 GMC C1500
1977 Oldsmobile 98 Regency Coupe
1978 Cadillac Coupe Deville (CLC Sr Crown #959)*
1992 Oldsmobile 98 (OCA 1st)
1996 Oldsmobile 98
*CLC Past President's Preservation

Past Cadillacs
1959 Coupe Deville
1966 Coupe Deville (Sr #861)*
1991 Eldorado Biarritz (Sr #838)

Paul

I, too, am a stickler for originality. The electric choke is a temp solution.
59 Caddy, Seminole Red with Dover White top