View Full Version : 77 CoupDeville hesitation with 425 engine

06-21-11, 06:35 PM
I have a 77 CoupDeville with the 425 engine in it. Everytime i step on the gas i get alot of hesitation. If i take a can of carb cleaner and spray inside of the carb to clean it out the hesitation goes away for a few days, then it comes back. But if i clean the carb again with carb cleaner the hesitation will go away for a few days again. I have just recently installed new plugs, (ac delco) wires, cap,rotor,and fuel filter. the car was doing the hesitation before i changed all of thoes parts so i am wondering what it could be. Other then that the car rides and runs like new money, any input,advice or suggestions would be a big help.

06-21-11, 06:49 PM
Sticking choke blade?

06-21-11, 07:05 PM
hello jayoldschool, what would you reccomend?

06-21-11, 07:19 PM
Sounds carb related to me. Stuck butterflys usually mean its about time for a rebuild. Primary jets may be a little clogged.

06-21-11, 07:20 PM
Does anyone think its time to replace or rebuild the carb?? The car has 122,000 miles on the odometer. This hesitation and having to spray carb cleaner every few days is getting real old real fast....

06-21-11, 07:22 PM
The butterfly is all the way open when it does it also, i just dont know what else to do!!! i am about ready to go buy a new carb. Anyone have any sugestions on a new carb??

06-21-11, 08:23 PM
^Is the butterfly open with the engine warm and running, or is the engine cold?

If you replace the carburettor, you want to get a re-manufactured Rochester. There are a few on eBay, that would be stock for your car:

1977-77-CADILLAC-OEM-4-BARREL-425-CID-CARBURETOR (http://cgi.ebay.com/ebaymotors/1977-77-CADILLAC-OEM-4-BARREL-425-CID-CARBURETOR-_W0QQcmdZViewItemQQhashZitem41563220d7QQitemZ28061 9000023QQptZVintageQ5fCarQ5fTruckQ5fPartsQ5fAccess ories)

CADILLAC-425ci-4BBL-CARBURETOR-17057234-1977 (http://cgi.ebay.com/ebaymotors/CADILLAC-425ci-4BBL-CARBURETOR-17057234-1977-_W0QQcmdZViewItemQQhashZitem4840d8c1beQQitemZ31032 5592510QQptZMotorsQ5fCarQ5fTruckQ5fPartsQ5fAccesso ries)

CADILLAC-425ci-- 4 bbl-CARBURETOR # 17057230--1977 (http://cgi.ebay.com/ebaymotors/CADILLAC-425ci-4-bbl-CARBURETOR-17057230-1977-_W0QQcmdZViewItemQQhashZitem2564bcbb21QQitemZ16060 3880225QQptZMotorsQ5fCarQ5fTruckQ5fPartsQ5fAccesso ries)

I would stay away from anything that doesn't specify "remanufactured" because a used carburettor would probably be no better than what you have now. I would also stay away from any aftermarket carbs because it would make the engine run improperly, and most don't have sufficient vacuum ports to operate your accessories.

06-21-11, 08:29 PM
Personally, I would do a little more work before replacing a carb. If the carb works after cleaning it, then you know that it actually CAN work ok. You just need to figure out what to do to get it to stay like that. Start by confirming correct choke operation when cold, warming up, and hot.

06-22-11, 12:59 PM
The carb needs a good cleaning and servicing. The 1977 factory service manual has a detailed explanation on how to do this. It sounds to me like the the choke is getting stuck and allowing the carb to flood.

06-23-11, 12:31 PM
My 79 Sedan deVille with 425 is behaving similarly, I've been told the carb needs to be rebuilt. I had wondered if using gas with ethanol was causing the problem, I've since switched to premium gas which I can find without ethanol, doesn't seem to make a difference. I'm having the carb work done as soon as my mechanic can get the car in, hopefully next week. I'm also having new plugs, wires, cap, rotor - etc. done at the same time and I will report back if it solves the problem. :)

06-23-11, 06:52 PM
Hello Bro-Mam, i tried using premium gas and it didnt work for me nither. I think its time for my carb to be rebuilt. I have 122,000 miles on the odometer so i guess its about time. Are you rebuilding or are you buying a new one? I am asking because i am trying to figure out what a reasonable price would be to rebuild or to replace. Thanks.

06-25-11, 09:02 AM
Is the vacuum advance and mechanical advance working in the distributor? I wonder if the accelerator pump is working too. Look down the carb and see if it squirts gas when you move the throttle lever. If not it could use a rebuild.

06-26-11, 09:30 AM
I've recently spoken with freind of mine who rebuilds and repairs engines. He claims that the fuel they are supplying us today is the main problem with older fuel delivery systems. The fuel has a trendencey to gum up and deteriorate over a short period of time. He claims that even with a stabilizer the fuel starts to get gunky within a short period of time ( 2 weeks ).
My cousin recently had a SBC high compressioned engine refreshed. He advised him to use Avgas with at least a 40/60 ( Avgas : Gasoline ) mix. Avgas hasn't been messed with as much as gasoline, has a higher octane rating and costs less than the comercial racing fuels.
Your problem could be caused by the change out of the seasonal fuel mix. With gasoline, the chemical mix of additives changes when we go from Winter into Summer. That combination of Winter gas to Summer gas may not be a compatable mixture and may cause the residues left on the walls of your fuel tank and your carb fuel reservoir to strip off and mess with your fuel delivery system.
Try running about a 40/60 mix of Avgas to your tank. It should help clean out your system so that it will be more stable throughout the season.

06-26-11, 11:15 AM
The problem that your friend is referring to is from ethanol. You can get additives that take care of the ethanol problem. Here, high test gas doesn't have ethanol, so that may be an option. Please note that if you are using avgas, MAKE SURE you do not get leaded.

06-27-11, 02:53 PM
I would send that Rochester out to get rebuilt.
I don't have the link at the moment, but there's a gentleman (I think in Florida) that is extremely good at this.

That Rochester is one fine piece of engineering... yet a PITA to keep running tip-top.

07-03-11, 10:37 AM
Tip in hesitation hot sounds like either the accelerator pump seals are shot or the linkage isn't moving the rod at the proper time. I've had 2 experiences with this. One was a bad rebuild I had to un-screw that someone had put the wrong accelerator pump into. It was too big and the seals would actually jam in the bore. The other was on a 76 CDV where the linkage that moved the pump lever was goofed up and had a lot of slack in it.

An easy check would be to turn off the engine, put on safety glasses, pull the air cleaner, look down into the primary (front) bores and depress the accelerator pump (or open the throttle) and see if fuel squirts down in there. If its weak that's your problem. Also make sure there's no slack in that linkage. As soon as you move the accelerator lever it should push down on the pump.

If that's not the problem it could be timing related, I've seen pick up coils start to go however usually the engine will cut off then jump back to life in those cases. Less frequently I've seen carbon on the intake valves build up to a point where the carbon will actually absorb that initial fuel charge on tip in as opposed to allowing it into the combustion chamber. A can of top-engine cleaner usually works in those cases.

Don't overlook the usual suspects either, double check for vacuum leaks, misrouted hoses, heck even the carb base gasket can leak. I think a few of the intake bolts are threaded into air passages too, maybe a/c compressor mounting IDK.

As far as the ethanol thing goes, I think it's more hype than reality. When I lived up north we used to get that crap on a seasonal basis and never had problems related to it. MTBE caused more issues than ethanol. Ethanol up to 10% shouldn't be that big an issue. Yes it will rot rubber hoses a little faster and could accelerate some underlying problems that were already there. The biggest problem with ethanol is that it's a moving target. If vehicle manufacturers KNEW that pump gas was 10% they could build cars to suit it and run REALLY good on it. We'd even see modifications packages in the aftermarket to retrofit cars to make them run well on it. But we have NO CLUE what's coming out of the pump on any given day! 4% 6% 10% 0%???

07-04-11, 09:18 AM
Your right about the ethanol and the problems it causes. The percentage of ethanol changes from manufacturer to manufacturer and lot to lot of manufacture, but there have been more changes to the molecular structure of the product in recent years that contribute to a more rapid break down of the fuel than peviously. Just smell it. It doesen't smell anything like what my memory banks recall as of 5 years ago.
Our cars don't have the computers and newer variable fuel delivery systems of the newer cars, so the mixtures can't be adjusted on the fly, and make changes and adjustment with each tank of fuel added. Our cars were manufactured at a time when fuel was more stable, and didn't change a lot from manufacturer to manufacturer. In the ninties I took a trip down South and I noted that a tank full on Citgo would give me better gas mileage than a tank of the other fuels. When it came to the fuel's stability, it was unquestionably more stable than what we get today. Let a tank of fuel set through the Winter with or without a stabilizer and take a look at it. It almost looks like mud after a few months. If you let 20 gallons set through the Winter before, you didn't have to change your filter after running the tank out.
Things have changed, my freinds, and for those of us with older cars, it's not for the better.

07-05-11, 03:55 PM
An update on my 79 deVille which seems to have similar issues - last week I finally had the car in the shop and it was determined that my distributor is shot - new one ordered and is scheduled to be installed this week, hopefully. I am going to fit the car with new plugs, wires, cap, rotor, etc, before I have the carb yanked off and rebuilt. Stay tuned... :)

07-05-11, 08:50 PM
Bro-Ham, I hope your distributor replacement solves your problems. Too much free play in your shaft can mess with your timing. I usually shim them I believe free play is supposed to be .012 to .018 of an inch. Runout is to be minimal. Your reluctor shouldn't come in contact with the yoke. Your centrifical should be free.

07-05-11, 10:33 PM
Bro-Ham, I hope your distributor replacement solves your problems. Too much free play in your shaft can mess with your timing. I usually shim them I believe free play is supposed to be .012 to .018 of an inch. Runout is to be minimal. Your reluctor shouldn't come in contact with the yoke. Your centrifical should be free.

I hope my mechanic knows as much about this as you do... :)

I will let you know what happens when the new distributor shows up. My car still drives just fine although this slight hesitation is annoying. One way or the other it will be cured. :)

07-07-11, 07:09 PM
Hello all, and thanks for all of your info and help. My 77 coup with 425 engine had hesitation on acceleration but out of nowhere i was speeding up about a month ago and while hesitating the car backfired VERY,VERY loud through the carb, then all of a sudden it runs perfect with no hesitation at all. It runs like a dream now and i am glad of that but that is weird how a big backfire through the carb and now its been running perfect....any ideas anybody????

07-08-11, 01:23 AM
The backfire may have burned off some impurity clogging the carb.

07-09-11, 08:00 AM
There is a chance you have timing chain iissues. Check your freeplay at the crank.

07-09-11, 07:25 PM
I agree with PRdivlle.. If you don't know how to rebuild a carb, find someone thats good at it and have yours rebuilt... your chances of getting a good running carb are best sticking with the stock one you have and not one that says it fits your car and has the right number...

07-10-11, 03:06 PM
Just a report on my 79, had the new distributor installed, what a difference! Car still has very minor hesitation, next week it will get new plugs, wires, etc. Car seems to run a bit rich. If it's not back to 100% the carb is coming off and will be rebuilt. :)

07-10-11, 03:11 PM
It's all these threads that make me happy for computers and fuel injection. ;)

07-10-11, 09:17 PM
It's all these threads that make me happy for computers and fuel injection. ;)

:) Jay, you should know better than to say that. :)

08-18-11, 09:38 PM
Hey man! Way to go on getting rid of most of your problem.

Just from my personal experience with my '78 Coupe, get the carburetor rebuilt. The day after I bought mine in 2004, it choked and died in major traffic. I thought I had bought a really well-polished lemon. It was just my ignorance though, not a bad engine. It only had 35,000 on it at the time as well! It hesitated less after carb cleaner was used. I always put only premium fuel in it as well. I finally got the carburetor rebuilt in 2006 and oh. My. God. After the rebuild I have NEVER had the car hesitate unless I didn't let the engine warm up enough before I put into drive.

08-26-11, 03:27 PM
Glad the distributor helped. My 78 fleetwood ran like this. The issue ended up being that the advance weights were freezing up in the distributor causing it to run like crap. Also there is what I think is called a reluctor that wears out on the HEI's. It has 2 wires on it and has teeth on it that are down by the distributor shaft. The wiring is very thin and becomes very brittle over time causing the ignition to be really weak which makes these motors run like crap. You could either replace it yourself which would require taking out the distributor shaft to replace them or just buy a rebuild. Personally your best bet is to go with a rebuild because the advance weights wont be frozen up and the aftermarket reluctors have a lot heavier duty wiring. That is usually the most common to make these 425's hesitate. The next step could be the EGR valve if the carb has already been rebuilt.