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Author Topic: 425 engine techs please.  (Read 5481 times)

R Schroeder

  • Guest
425 engine techs please.
« on: August 21, 2010, 04:40:09 PM »
I have a 1978 Cadillac with the 425.
Car has 15,000 miles on it.
 It has a slight flat spot when you step on the gas. It drives me nuts. This is the first car I have ever owned that has had this sort of a problem, that I couldn't get rid of it. If engine is running with the air clearer off,and I razz the throttle it picks up right away. Jets of gas are right there. Only happens when I drive the car.

 I replaced ;
 Cap. rotor ,wires and plugs.
 EGR valve
 All emission controls on the air cleaner.
 All vacuum lines
 Rebuilt the carburetor, new dash pots
 New vacuum advance
 New PCV valve
 New thermostat
 New radiator cap
  All vacuum ports are working at the right time.
 Engine pulls 21 inches of vacuum at an idle, steady needle.
 The only things I have not done anything to are the fuel pump and distributor.
 Car takes off like a rocket after the flat spot.
 Roy

  GO TO THE END IF YOU HAVE READ THIS BEFORE
« Last Edit: November 21, 2010, 07:09:22 PM by R Schroeder »

Offline bill henry

  • Posts: 192
Re: 425 engine techs please.
« Reply #1 on: August 22, 2010, 07:33:05 AM »
when I bought my 78 it did the same thing and also seemed to lack power. believe it or not it cleared up after I took the advice of my old mechanic and drove it like I stole it for a few weeks.
Bill Henry

R Schroeder

  • Guest
Re: 425 engine techs please.
« Reply #2 on: August 22, 2010, 09:53:25 AM »
I have put 3000 miles on it since this work was done.
Dont think that will help..............ha

Offline bill henry

  • Posts: 192
Re: 425 engine techs please.
« Reply #3 on: August 22, 2010, 10:19:33 AM »
sounds carb related not electrical. Are you sure the secondary air door is not hanging up this will cause a flat spot until the vaccuum is high enough to pull it open.
Bill Henry

Offline TJ Hopland

  • Posts: 10193
Re: 425 engine techs please.
« Reply #4 on: August 22, 2010, 10:20:09 AM »
Is the mechanical advance working in the distributor?   Also its worth checking the pick up coil wires in the dizzy.  That low of miles you would not expect them to be broken but who knows.   Its under the rotor and has the white and green wires that go into the module which is also under the rotor.  Sometimes the wires crack internally so when the advance moves it looses the connection which causes the vacuum signal to change and they re connect.   Just unplug it and connect an ohm meter while flexing the wires.  Reading should stay steady.   You could also try running with the vacuum advance unhooked.  

Have you tried it with the EGR unhooked?  Its possible that maybe you got a part that is not quite right and its opening too soon or not closing fast enough.  

That low of miles you would not expect the carb to be worn or abused so that should have been a very straight forward rebuild.

Was there any little black chunks in the carb?  On my 78 something rotted out in the charcoal canister and was letting charcoal bits get sucked into the carb.  Those little bits got down in the jets and would just float around and mess things up when they felt like it.  At first I thought they were just bits of old fuel line but the second time after I had replaced all the lines (including back at the tank) I started looking for other causes.  Should have known that if it was hoses it would not have got past the filter.

Did you set the timing properly?   I dont remember the details but it seems like its something like 16* at 1800 rpm.
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

R Schroeder

  • Guest
Re: 425 engine techs please.
« Reply #5 on: August 22, 2010, 11:46:48 AM »
BILL - secondary is good. I just cant believe its in the carb. It works so dam nice once your past the flat spot. This is taking off from a dead stop now.

TJ - I wonder about the advance , because it seems the return springs are weak. I did lube them with a light oil. They work ,but seem weak.
 I'll have to check the wires under the dist. I found in the repair section about checking different components with the ohm meter.
 Yes I plugged line to the EGR and a lot of other things and drove it. It still was there.

 Carb wasn't dirty or worn when I rebuilt it. I was just going through everything to make sure it was good. I even did the front bushings on it, just because I had it out.

 Timing. Here is another question. On the plaque on the car it says 22 degrees before TDC, at 1400 rpms. Vacuum plugged.  The repair manuals say 18 degrees, in all the books I have . What is right ? these are all Cadillac manuals I have too.
 Roy
« Last Edit: August 22, 2010, 05:14:53 PM by R Schroeder »

Offline Otto Skorzeny

  • Posts: 3853
  • 1956 Coupe de Ville aka Bismarck
Re: 425 engine techs please.
« Reply #6 on: August 22, 2010, 12:32:24 PM »
Did the flat spot exist prior to the carb rebuild?
fward

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R Schroeder

  • Guest
Re: 425 engine techs please.
« Reply #7 on: August 22, 2010, 05:09:57 PM »
Just got home.
 Yes Otto, it was always there.
Its not like you have to hit the gas twice to get it going. Its just a 1/2 second hesitation, and then its fine.
Just enough to bug you, when you know it shouldnt be there.

Offline Otto Skorzeny

  • Posts: 3853
  • 1956 Coupe de Ville aka Bismarck
Re: 425 engine techs please.
« Reply #8 on: August 22, 2010, 05:23:28 PM »
That leads me to believe that the accelerator pump is defective.

Check also for low fuel pump pressure or low volume.

If it is an ignition problem it could be fouled plugs or plugs gapped incorrectly. A weak ignition coil could also cause the symptoms described.

I'm leaning toward the accelerator pump, however.

fward

Ask not what your country can do for you, but what you can do for YOURSELF

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R Schroeder

  • Guest
Re: 425 engine techs please.
« Reply #9 on: August 23, 2010, 08:11:52 AM »
Well Otto, I just cant say its the carb. It squirts gas in it right away when you open throttle. Everything is new in it too.
 I never checked the fuel pump, because it was running well. I still don't know why low fuel pressure would cause this sort of a problem , because it does keep gas in the carb. I know the books always say low pressure will cause the problem, but I'm not sold on that just yet.
 I feel its something in the distributor.
 I am still working on my rear fender extensions, so I'll jump into this when those are done. They should be back from body shop tomorrow.
 Weather is finally getting back to normal for me.   90 and muggy isn't my cup of tea. I can work on a car all day when its 60 or 70.

 

Offline boper39

  • Posts: 16
  • Name: Gaylord G. Goclan
Re: 425 engine techs please.
« Reply #10 on: August 23, 2010, 08:46:11 AM »
If you are having this on a fairly heavy hit of the throttle, try increasing the spring tension of the secondary air door. There is a locking allen head that is at the top rear passenger side of the carb top cover and a small flat head screw in the side. The allen faces down, and the screw can be seen through the hole in the linkage. As soon as you loosen the allen the adjusting screw will unwind so use two hands and have a screw driver of the tension screw. Tighten clock wise to increase (try 1/4 turn). Hope this helps.  GG
Gaylord G Goclan

R Schroeder

  • Guest
Re: 425 engine techs please.
« Reply #11 on: August 23, 2010, 09:15:04 AM »
Well, I'll go back and check this out. A couple of guys were hitting on the secondary door here already. Anything is possible.
Thanks.
 Roy

Art Woody

  • Guest
Re: 425 engine techs please.
« Reply #12 on: August 23, 2010, 02:33:40 PM »
No matter how few miles, your parts are old. Otto is telling you straight, accerator pump dried out or dirty and/or something in carb gummed up. Classic, off idle flat spot caused by accelerator pump if you are sure of timing. It does't matter how smooth it idles or runs past flat spot, you have no load sitting in park.

R Schroeder

  • Guest
Re: 425 engine techs please.
« Reply #13 on: August 23, 2010, 05:40:17 PM »
ART - The carb was completely rebuilt, to include the pump.  I have a list in my first posting of all the things that were done to the engine area.
 This secondary spring though is something I have to look at. Waiting for cool weather.
« Last Edit: August 23, 2010, 05:45:03 PM by R Schroeder »

Art Woody

  • Guest
Re: 425 engine techs please.
« Reply #14 on: August 23, 2010, 08:28:46 PM »
Seeing as you have the engine running reasonably well, and are confident in your carb rebuild and certain the float level is absolutely correct, I would set the carb and the timing with a vacuum gauge. But since you have a timing light, record current timing and advance a couple of degrees first and test drive. If you have lessened the flat spot, then advance again. Good luck, Art Woody

R Schroeder

  • Guest
Re: 425 engine techs please.
« Reply #15 on: August 23, 2010, 10:20:52 PM »
Will keep that in mind. Athough its pulling 21 inches steady now.
 Going to be trying a few things here in the next week or so.

Offline TJ Hopland

  • Posts: 10193
Re: 425 engine techs please.
« Reply #16 on: August 23, 2010, 10:23:35 PM »
You could try running with the vac advance unhooked.   If it does not stumble then you may be looking at a timing issue.  If it does then you are back looking at fuel.    You may have to bump the idle slightly because the 78 runs off manifold vac so you wont have any advance at idle.  

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

Re: 425 engine techs please.
« Reply #17 on: August 23, 2010, 10:41:34 PM »
Had the same problem on my '73 Eldo -- turned out to be the choke "pull-off" which
is a vacuum diaphram set up on the carb.  They were problematic.

Don't know if this applies to your engine, but since you've checked everything else
look into it!

Mike
1955 Cadillac Eldorado
1973 Cadillac Eldorado
1995 Cadillac Seville
2004 Escalade
1997 GMC Suburban 4X4, 454 engine, 3/4 ton
custom built by Santa Fe in Evansville, IN
2011 Buick Lucerne CX
-------------------------------------
CLCMRC Museum Benefactor #38
Past: VP International Affiliates, Museum Board Director, President / Director Pittsburgh Region

R Schroeder

  • Guest
Re: 425 engine techs please.
« Reply #18 on: August 23, 2010, 10:59:43 PM »
TJ- that is one of the things I plan on trying this week. One more muggy day, and I'll get to the car again. I have had the idle set up faster and back down. That didnt seem to help any, if that helps.

 MIKE - the pull offs are new too. I went whole hog on the rebuild.

 I have read some interesting things here, and I plan on doing all of them.
 Thanks for all the tips. Keep them coming if you have any. I'm sure one of them will hit on the problem.

 Roy

Offline TJ Hopland

  • Posts: 10193
Re: 425 engine techs please.
« Reply #19 on: August 23, 2010, 11:29:08 PM »
Is the whole country muggy or are you in the midwest?   I dont ever remember having this many days with this sort of heat index.  Working on my car today I sweated enough all over the engine to clean it a bit.  Glad I had the hood to give me partial shade.
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

 

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