Hey everybody. This is my first post. I wish I could of been here longer before needing your all expertise but just found this while troubleshooting...so here's whats up: I went out to start my car and it started up normally as it does with a couple cranks...i set my manual choke on it and let it warm up as I went upstairs to get some stuff ready. I come back down like 20 mins later to readjust the choke and idle to normal settings and the engine started to run rough then shut off. I tried to crank it again but it would not turn over. So i thought i flooded the carb to i let it set then came back and tried again with no avail. its not a fuel delievery problem as far as I can tell. I havent been able to test the electrical with a spark test because i didnt have anyone there to start the car as I watched for a spark....any ideas? thanks in advance.

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1968 plymouth valiant signet. 2 dr post. 170ci Slant 6

Sounds like a vacuum leak.


Here is how i worked around this problem:

1. Take one spark plug wire out and attack a spark plug in it. Have a friend crank the car as you watch for a spark.. If the spark fires then you should have electricity. (If you have old spark plugs, change them) This test only tells you that the distributor has electricity.

2. Take the air cleaner housing off and pull the throttle lever. If you see two streams of fuel (On a carter BBD) If you don't have fuel then this is your possible problems:

-No fuel in the fuel tank
-Faulty fuel pump
-blocked fuel line
-Carburetor stuck float bowls.
-Carburetor blocked jets.

3. Pull the choke back on and see if it runs better.. If it runs worse: Too rich, lean out the carburetor
If it runs a bit better: Vacuum leak.



To test a vacuum leak, i tried this method:
Take a can of carburetor cleaner and spray it on the carb with the air filter housing away.. This way you know what kind of change the RPM's get when the cleaner goes trough the carb..

Now spray the intake manifolds near the head. If the rpm's drop then it's a vacuum leak on the exhaust/intake gasket.

Spray some on the carburetor base to check if the base gasket is leaking..

Next to check is the throttle shaft on the side.. Spray it for 4 seconds to make sure if it has a tiny leak.

Now go trough all the vacuum lines going to the carburetor and check those..


The last check to do is to put the air filter housing back on and try to spray the base to check if the base gasket is leaking..



My carburetor wouldn't idle at all but worked at high rpm's.
The problem was a worn throttle shaft.

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OK update...I finally got my wife down to the garage to help me do the spark plug test. It has spark. So I felt some relief. So I got behind the wheel and tried starting it again. I had a couple more times where it almost turned over. Then about to give up I tried one more time and it fired up. Literally lol. It started but was idling real rough. I had the choke all the way closed and I tried to keep it going and gave it more gas and it backfired up through the carb. I'm sure that was bad so I let it set for a few and tried again and got it running again and adjusted the idle to keep it running steady til it warmed up. I done this with the little lever on the throttle arm with different notches. The idle of the engine was mostly steady. There were times where the idle would lope a bit then speed up. After the engine warmed up I opened the choke all the way and nothing really changed. The I adjusted the idle to what I guess would be normal idle and it ran really rough.

Sorry about the long and drawn out story. I will get some carb cleaner and test for vacuum leaks and I'm also gonna change the plugs. This carb has all kind if vacuum lines that I plugged when I put it on. So I don't know if I have the carb set up right. I don't have a clue about them. Kinda learning this stuff on the go. Anymore advice is welcomed and I appreciated ur response Jsp

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1968 plymouth valiant signet. 2 dr post. 170ci Slant 6

Okay it really sounds like you need to adjust the valves and check for stuck valves.. Backfire trough the carb means that the intake valve is slighlty open when it combusts. This is very dangerous as it can ignite a leaky carb and then feed the flame though the fuel pump..

If it ran well with the plugged ports before then you should be okay.
It's not like it suddenly can't run with plugged ports.

Also i'd check if the PCV valve works as it's a really cheap part and it can cause weird things to happen like this..

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Firing back up through carburetor when opening throttle is also a sign of a vacuum leak.

Usual vacuum leak suspects were listed above except checking for loose nuts holding carburetor to intake manifold.

Does this car have power brakes? If so booster diaphragm and associated vacuum plumbing to it can be source of vacuum leak. Disconnect booster vacuum line from manifold, cap off its manifold tap, and see idle improves.

Could also be some crud in idle circuit restricting proper fuel flow resulting in lean condition.

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

I put some thread locktite on the carb bolts and they haven't rattled loose afterwards.. Cheap investment but a good investment.

The power brake booster hisses at you if it's leaking. (From my experience)
I'd just go crazy with the carburetor cleaner or starter fluid and go around the enginebay checking for leaks.. Post results and tell us what started leaking.

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Hey sorry it's been forever. Work and school and family have been keeping me away lol. I'll get some carb cleaner and try that. It does have power brakes so I will also check that. I have noticed a hissing sound but never tried to pinpoint it. I'll go out and see.

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1968 plymouth valiant signet. 2 dr post. 170ci Slant 6

OK I was thinking the booster was the master cylinder. So I'm not really sure where the booster is. I looked it up to see what it looked like and didn't really see it. But is the hose suppose to connect to the top of the intake runner closest to the firewall? I found a hose there that connects to something else that has 2 other hoses that go down by the distributor. Sorry bout being so vague

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1968 plymouth valiant signet. 2 dr post. 170ci Slant 6

A picture would help. The hose routing diagram should be on a decal someplace under the hood. All hoses are suspect; I've had hoses that looked new leak.

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If it was easy, everybody would be doing it.

Power brake booster is an 8" or so round metal disk about 3 inches deep, think cake plate cover, between fire wall and master cylinder. There will be a vacuum line running between #6 leg of intake manifold or from a vacuum tap in intake manifold centered below carburetor running to a "T" or fitting mounted in surface of booster. Within the booster there are rubber parts that degrade and crack with age which cause a big vacuum leak.

To bypass the booster for a test, disconnect its vacuum line from manifold, and cap manifold's vacuum tap. If engine runs better, you just found your vacuum leak.

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

ok i guess mine doesnt have power brakes. it doesnt have that behind the master cylinder.

How do you post picks on here? do i have to load it up to a site like photo bucket and then use the Img function?

I tried search the net for other slant 6s that have the vacuum tree setup but no luck. So ill try to take some pics of it when i get home

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1968 plymouth valiant signet. 2 dr post. 170ci Slant 6

lol forgot i had a car domain page for mine long time ago



dunno if that will work. heres the url for the photo gallery

http://www.cardomain.com/ride/858133/19 ... o-gallery/

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1968 plymouth valiant signet. 2 dr post. 170ci Slant 6