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Rochester adjustments

Started by Patrick Fant, December 25, 2020, 05:41:16 PM

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Patrick Fant

1956 Series 62 - I know carb adjustments are finnicky - but it's worth asking for some input here I believe.  New problem: when car idles too long in traffic or parking situation, sometimes starts to chug chug (not very technical) and rpm slows way down and eventually it just stops.  Not good. Does not happen all the time makes it more tricky.  Carb has been professionally rebuilt a year ago (has performed beautifully) Engine has had complete R&R.  Solution:  guess is adjust for little richer mixture and/or a few more RPMs at idle.  Any thoughts on which way to go. pat fant
Patrick Fant
Houston
CLC #31723

Lexi

You could be experiencing a bit of vapour lock, which if so, carb adjusting I don't think will help if the vapour formation is occurring before the carburetor. I used to experience vapour lock in my old '56 CDV a lot, but my 1956 Series 75 has yet to develop full blown vapour lock. That said, the engine was rebuilt & tanked and cleaned out of crud, (saw it in the shop all cleaned out). Water pump rebuilt, rad in good shape and flushed just in case. New hoses everywhere. Have a 6 bladed fan on the engine which also helps. Car has ran the coolest of all my '56s. With a 180 degree thermostat, she is usually at 180 or 190 or so even in the summer. 200 is about it on a real nasty day which for these cars is great. Prior to rebuild it was nothing for it to hit up to 220 on the hottest of days, (which I am told can be normal for this year of Cadillac especially if equipped with AC). I have installed a new mechanical gage so these numbers are accurate.

I recently had my carburetor serviced and it was when I was installing the rebuilt Rochester on my engine that I noticed something else that may also be a factor in combating vapour lock. I noticed that my intake manifold which has that trough to allow the flow of hot exhaust gases under the carburetor for a faster warm up cycle, was totally PLUGGED with carbon. I don't think that has happened since the rebuild. Perhaps the rebuilder thought it was normal, and left it. It is not normal.

I was going to scrape it all out and make it spotlessly clean again. The carb rebuilder had provided me a new base gasket MINUS that cut out and he also gave me a pair of threaded blind grub screws to plug those 2 ports on the intake manifold that are connected to that pre-heating trough area, as a means to combat vapour lock. I got to thinking that because that part of my intake manifold is no longer carrying exhaust gasses to the base of my carburetor, that may yet be another factor which has helped my current ride avoid vapour lock. I installed the rebuilt carb and left the carbon plugged zone in the intake manifold alone. I reasoned if it ain't broke don't fix it.

I have heard people talk about different ways of combating vapour lock on this Forum, including blocking off exhaust gas to the carb base, but it looks like my car did basically the same thing on its own. A few years ago Lou Commisso, Administrator for the now defunct Mid-Century Cadillac Forum, ran a number of tests on his newly rebuilt '56 Caddy 365 engine with vapour lock in mind. Using an electronic thermometer even his all rebuilt engine was running into early vapour lock signs at around 20 minutes or so, of idle.

I think I would first explore that aspect of this situation, then proceed elsewhere should early signs of vapour lock issues be ruled out. I say to do this first as this problem is a common one. I am assuming your car in other respects, is a reasonably well running vehicle. As you are still breaking your engine in I also presume, your car is going through a "teething" process where all sorts of things can come loose or undone. You may want to inspect all for tightness especially where there may be the possibility of a vacuum leak. After my rebuild I had to retighten a lot of fasteners, and correct a couple of minor vacuum leaks. Clay/Lexi

cadillacmike68

You can run a return line to the fuel tank. That's what Cadillac did in the mid-late 60s to combat vapor lock on Climate Control equipped cars. I've never had vapor lock on my 1968 and that temp gauge has gone pretty far over at times here in FL. Not today though, it's like 42 outside.
Regards,
"Cadillac" Mike

Omarine

This might be the time to test the fuel pump. Ensure it produces the pressure psi, and volume that the manual states. Probably 3.5-5.5 psi and fill a bottle with fuel while car idles.

A pump can get very weak at idle if the rod is worn. These things happened to mine, and i put a 6 volt electric fuel pump and bypassed the mech fuel pump. Solved heat and weak pump issues.

Cheers
1949 Harley Davidson FL
1952 Cadillac Coupe de Ville
1961 Chevy Impala 2 dr hardtop

fishnjim

Sounds more like a fuel problem than carb to me.   Not delivering enough at idle.   It could be timing, etc. but without a thorough engine check, impossible to guess correctly over internet.   I'd say this is more common as these cars age, fuels and parts changed. 
Easy things to check; fuel filter, float condition/adjustment, idle mixture, fuel lines, and the screens in the carb inlet, any opportunity for restriction/debris. 
You can test the FP but most don't have tools or technique.   Might benefit from the electric FP solution.   But "carburatus unfamiliarius" is more common.
There weren't that many cars in '56 so sitting in traffic would have been infrequent.   

Lexi

It does sound more like a fuel delivery problem. The '56 Cadillac engine ran hot when new, and there were notes in the Cadillac Serviceman publication that year on this topic. One was the secret warranty of installing a 6 bladed fan for customers who complained about the car running hot (if theirs was equipped with AC). A General Service Letter even provided a part number for this fan, a number that I have never seen in any of the MPLs I have checked. This was apparently a stop gap measure. In '57 things changed and a different part number (and a slightly modified fan), appeared. Increasing the recommended RPMs was another procedure discussed. Of course, a hot running engine can produce vapour lock.

My personal experience is that these cars can be made to run at a reasonably cool temperature, but all has to be in tip top shape. As fishnjim noted these problems become more common as these cars age and fuel changes, etc. Tough to isolate over the net, but fuel delivery would be my first area to investigate, hence my earlier comments about vapour lock and its early signs. I have notes on the testing of the mechanical fuel pump for this year of Caddy and can post if you wish. Clay/Lexi

Patrick Fant

Thanks I appreciate the good responses very much.  Lots to think about.  Since the weather is cool in Houston now I am inclined to believe that fuel delivery is the issue more so than vapor lock right now -- in the 40s and she is running quite cool overall.  I have a little Edelbrock in-line pump under there but disconnected it and went back to manual after the carb was rebuilt.  I think I will reconnect and see.  thanks much, pat fant
Patrick Fant
Houston
CLC #31723

Daryl Chesterman

Pat, is there any chance that the carburetor is running rich and "loads up" when idling, causing the "chug, chug"?
You can check this by looking at the exhaust to see if black smoke is coming out of the exhaust pipe when it starts the chug, chug.  If there is black smoke, the next thing to look at is if the choke is completely openâ€"the choke butterfly/blade should be completely vertical.

Daryl Chesterman

russ austin

Check your shop manual to see if the carb has a hot idle compensator. It may be stuck closed.
R.Austin

Patrick Fant

Thanks for the great ideas here -- Hot Idle Compensator and rich mixture possible -- checking.  pat fant
Patrick Fant
Houston
CLC #31723