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78 Caprice 350 4bbl won't run

2K views 24 replies 8 participants last post by  xuno_gil 
#1 ·
Hello all, I acquired this car from my mother (God rest her soul) but I have not been able to get it to run right and now she won't even run.
Initially, when I picked it up it started and idled just fine but it would not run right. From a stop if I floored the gas pedal the car would barely roll forward but then once she got up to speed she was fine. I was able to drive the car about 15 miles on the highway at highway speeds. The next morning I had to get the car to my house so I again drove it another 15 miles. She again started right up and drove ok except for the very slow to respond from a stop. This seamed to be getting worse as the drive went on. So bad that if I was stopped at a light at a slight incline it would be almost impossible to get the car going from a stop. Anyways it got worse and worse but made it home, but as I pulled into the garage I could tell it was stumbling a bit.
Anyways, next day started fine but the issue was so bad that it barely ran and after backing it out of the garage and then putting it in drive it would just not go and eventually died.
Sorry about the long story but this is where I am now. I can only start the car really if i floor the gas pedal. I can only really keep it running if I keep the pedal down between 75-100% travel.
The carb is getting fuel. I can see it squirt inside the throat when i play with throttle linkage. For as best I can see the main vacuum lines are fine.
I undid all the plugs and they were just caked in with soot and oil, I cleaned them pretty good and reinstalled them to see if it would make any difference at all and it didn't. Initially I was thinking the issue was fuel but I have ruled that out and now am left with only spark.
When I have the engine running, with the pedal to the floor, the engine is shaking all over the place and I think some of the plugs are missfiring sometimes. I think the timing has to be ok since the vehicle ran fine (or at least ran) with the timing as it is now and the distributor is not lose.
What do you all think is my issue, part of me says just new plugs but the other says that if it were just the plugs I would have noticed some change after cleaning all the plugs out, but that did not happen.
 
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#3 ·
Condensed version

If you don't want to read all that (which I would not blame you) her is a condensed version.

1978 Caprice all stock with original 350 & rochester quadrajet

Initially the car would run but it would be very sluggish from a stop, but I would eventually get up to speed and be able to cruise comfortably and smoothly on the highway an normal highway speeds.

Sluggishness then got worse to the point where now it will only start if I floor the gas pedal and only stay running if I have the gas pedal almost all the way to the floor.

The car is getting fuel, verified this at the throat of the carb, took the air horn off on the carb and everything seemed to be fine, vacuum lines for the most part are fine, timing has not changed from when the car was running fine and the distributor is tight.

Now when engine is running, it runs very rough with guides me even more so to an ignition issue, maybe my plugs are just bad? I inspected them all and they were very dirty (black soot and oil) but I cleaned them up and I was hoping to notice some difference afterward to tell me ok it is the plugs but nothing at all changed after cleaning them.

Any thoughts?
 
#4 ·
REWNUTS,

Initially I was thinking it had to be fuel related and that maybe fuel was just not getting to the intake, but I then take the car out and separated the airhorn and didn't notice anything obvious. But now that you mentioned that part of me wants to agree with you, but allow me to play devils advocate if I may.

Are the needles that come up when the secondary flaps open only for the secondary barrels? so if those where clogged shouldn't the car at least idle?

When I removed the air horn I also removed the jets to inspect them and they were clean and free of obstructions, is this not the main fuel feed for the primaries?
 
#7 ·
Sounds like a carb problem to me also, either flat level/needle and seat problem or choke not opening....resulting in a too rich condition.

Generally, cleaning plugs doesn't help much, if they are fuel or oil fouled the fouling is melted right to the inner porcelain and cannot be wire brushed off or sandblasted off....so it is best to get a new set in there so you know they are good.

Ignition problems are generally spark or no spark as far as distributor internals, but a corroded cap and rotor contacts and old plug wires can be a problem.
Any idea how old or how many miles are on the cap, rotor, and plug wires??
 
#14 ·
Sounds like a carb problem to me choke not opening....resulting in a too rich condition.

. Start there... one step at a time... was the choke flapper opening fully as the engine warmed up? When the engine is cold, the vacuum pulloff opens the choke about 1/8"-1/4" (adjustable) the instant the engine starts to give some running air...


. The old plugs are prolly permanently shorted out by now... wire brushing puts a steel smear on the electrode that further shorts them out also... get a new set of Champion RV15YC's... USA made again now... only about $1 or so each... the 15's are a little hotter and work better in an old oil-burning engine...


. The sparkplugs didn't match, says there was some problems with this engine...


. Protect the new sparkplugs by driving a couple miles and stopping to make sure the choke is open... the electrical plug on the choke stove corrodes over time and needs to be unplugged/plugged a few times to clean it off... it gets 12 volt power from the oil pressure switch next to the distributor when the engine is running...
.
 
#8 · (Edited)
Replace the distributor rotor and distributor cap, spark plug wires and spark plugs. Grind down to bare metal on the firewall, the frame and the engine block, punch a hole with an awl and install new ground straps with sheet metal screws so that they're all grounded to each other. Do not remove any grounds that are already on the car. You cannot have too many grounds. Once you get the ignition resurrected, you can start on the fuel system. Replace any fuel filters you find between the fuel tank and the fuel pump, then either learn how to rebuild the carburetor yourself or purchase a rebuilt one from a reliable source like Summit Racing. If you opt for a rebuilt carburetor, buy one with the same style of choke that is on the old carburetor.
 
#9 ·
No, not sure when the last tune up was done. When inspecting the plugs they did seam like they were older but the electrodes and insulators did not look worn just very black. Also, I think there were 6-7 Bosch plugs and 1-2 champion plugs, the lettering and such did look kind of retro though... I might just replace them but I like knowing for sure what the issue is before I start replacing things, not that new plugs would hurt anything.
I was also wondering about the plug wires, they all look good but I would like to test resistance through them but have been unable to fine any info on what the specs should be. Not even the Haynes manual says anything about ohms values.

I also think it has to be carb related but like I said everything looked fine inside, BTW using starter fluid or pouring a little fuel down the carb throat makes no difference.

I am going to inspect the cap and wires tomorrow to the best of my ability but I don't expect to fine any major issues there.

When I was checking for spark I had one spark plug laying on a bracket to, well, check for spark. And the plug seemed to fire as it should but then (I cannot be 100% sure) it then it seemed like it was missing 1 of 10 sparks. I just checked one wire and plug so, I would have to run more test to see if that means anything but it was just curious.
 
#12 ·
Hello fellow B-Body owner, yes definitely start with a tune up, cap rotor, wires, plugs. I hope those wires dont say delco on them with a date code of 77 or 78 lol, Anyway just like everyone else said, id do the tune up and have the carb rebuilt, quadrajets are easy for me but for a first timer, i probably wouldnt tackle that job myself with out some help. To me it just sounds fuel and ignition related and hopefully nothing worse like cam lobes.
 
#15 ·
Hey GM B-Body, thanks.
The car is all original and in fantastic shape, but I do not think it is the original plug wires. In a way it would have been pretty cool if they were.

Buzzlol - I am going to be doing a tune up today but I would like to know more about what you are mentioning about the choke because that was one of the areas I was looking good and hard at while I was trying to figure this out. Do you know of a video or something that would give me an idea of how the choke flap is suppose to operate?

First off when the car is not running: I can open the choke flap manually, but as soon as I touch the throttle linkage the flap completely closes.

When the car is running, to me it seamed like the flap would still not open, but I tried starting it with the flap opened (by hand before start) and by using something to hold it open. Now when I was using something to hold it (in this case a fuel filter) the car would start and of course rev up and idle pretty high, if I removed the obstruction then it would close the flap completely and start to bog and then die. Initially I was thinking the car was starving for air. But then I tried it by having the flap opened just ever slightly with something holding it like that but I could still not get it to idle. So then i started to leave the starving for air idea alone.

Does this make any sense?
 
#16 ·
When the engine is cold and not running if you blip the throttle the choke should close completely. When you start the engine there is some sort of vacuum diaphragm that will open the choke about 1/4" or so. It should within a few minutes be open completely. I would not replace any plugs until you are sure the choke is functional as you will just foul the new ones. If the choke is not opening it is probably binding on something or the stove that controls it has failed somehow. It is a pretty simple mechanism and should be easy to fix.
The reason it ran fast when you held the choke open is there is also fast idle cam that is tied to the choke but the way the linkage is configured the choke can open while the fast idle cam is still engaged. If you open the choke by hand and blip the accelerator the idle will come down to normal
 
#17 ·
yikes, to late with the plugs

alright well I am starting to agree with t-bucket and the other who have been mentioning the choke, I do not think the choke is operating right. It was mentioned that the flap should open all the way, well I have never seen it do this at all. it stays closed except when the throttle linkage is pulled almost all the way then it opens a little. When the car is running I can tell it is running very rich.
And I think I am putting the pieces together, when the engine is cold the mixture would be a little rich right? so the choke blade should be more closed but then when warm or when running the blade should open to allow the air in?
Anyways, I am back to my original though that it is starving for air.
Plus, unfortunately, I just got done doing the complete tune up and it did not really change anything.
So next goal, is to either make sure the choke is working properly and/or fix it so that it is working properly.
This stove or diaphragm that controls the choke flap, which is it and as simple as checking to see if it hold vacuum?
 
#19 ·
ok so here are a few pics of the carb.
This time I got the choke flap to stay opened without having to prop it open, and I made sure to stay off the throttle as to not flip it closed again, I started the car and got her going and she ran ok. I let it warm up so that the choke would not close and then played with the throttle a little and she seemed ok. Put her in reverse and move it a little and then put it back in drive to drive it back to see if the hesitation or very slow from a stop was still there and yes it was. I then did it again and basically floored the gas to move forward and it very slowly moved forward, not as if it was going to die just very lazily. When I let go of the gas the car died. I checked the choke plate and it was still open. Anyways, i started her up again and she idled ok, I got her back to the parking spot and turned the car off. Then I went back to the carb and could not get the choke plate to close again, maybe it is because the car is now warm but still, to me it felt like there was some mechanical stop on it where, besides for the play in it, the blade would not move at all.
If the car is off, regardless of whether the car is warm or cold, should the choke plate close as soon as you blimp the throttle?
And then maybe open if the heated element in the choke opens it?
To say again, I could not close the plate at all, I don't mean spring tension would then open it again or I could close it if i really pushed, none of that. It just felt like there was a stop on it.

Anyways, here are the pics.
Auto part Engine Vehicle Fuel line Car


Auto part Engine Vehicle Fuel line Car


Auto part Engine Vehicle Car
 
#20 ·
. OK, you scared me when you said you opened the choke flapper and it stayed open... you damaged something or a linkage fell off or something... need to investigate closely and see what...


. Not sure on this older Q-Jet, but on newer ones the vacuum pulloff opens the choke flapper slightly for running air the instant the engine starts... yours may not do that?


. I'm sorry, I thought you had an electric choke... yours is heat operated by some exhaust in the intake manifold being diverted through the round black bakelite plastic heat stove... inside there is a bimetal strip spring... two different kinds of metal riveted together... different expansion rates of the two metals with different temperatures causes the spring to open or close the choke flapper... the spring can eventually break... or the tiny heat passageway can become blocked/clogged...


. The choke stove also looks adjusted too rich, keeping the choke flapper closed... it should just barely close the choke flapper when engine is off and cold... loosen 3 screws to turn/adjust it...
.
 
#22 ·
I think it is fixed?

Well, I think I might have fixed it, there might still be some fine tuning left but the car seems to start just fine and have acceleration more in par with what I expect it should have.

I took the cover off of the choke coil cover and reinstalled it right, (not sure if it was right initially or not but I'm thinking it might have not been) set it so that the flap is open about 1/4 of an inch when the flap is closed and now she opens up when warm and it seams to drive fine.
 
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