383 Rough Idle Stops WITH Vacuum Leak. Huh?

RogueOne

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My 67 Polara 383 w AC (not working) has what appears to be a vacuum leak and consequently its idling, accelerating (bogging) and driving rough. In checking all the vacuum hoses, and spraying brake cleaner around the carb and intake manifold, I find that the engine smooths out when I take the vacuum hose OFF the Brake Booster and let it suck in atmosphere. So, in looking stop a leak to smooth out the rough idle I find out that CAUSING a leak fixes the problem. The rough idle returns when I simply plug the brake booster vacuum hose. Argh. Help.
 

Davea Lux

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My 67 Polara 383 w AC (not working) has what appears to be a vacuum leak and consequently its idling, accelerating (bogging) and driving rough. In checking all the vacuum hoses, and spraying brake cleaner around the carb and intake manifold, I find that the engine smooths out when I take the vacuum hose OFF the Brake Booster and let it suck in atmosphere. So, in looking stop a leak to smooth out the rough idle I find out that CAUSING a leak fixes the problem. The rough idle returns when I simply plug the brake booster vacuum hose. Argh. Help.
It is probably running too rich. Check the idle adjustments and the carb float level.

Dave
 

RogueOne

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Brakes running normally. I adjusted the carb float with no change. Adjusting your idle mixtures screws also had no effect. Setting the timing with a vacuum gauge DID help to smooth the idle significantly. Trouble is now it’s running VERY hot. Am going to go back to idle mixture screws to lean it out more and also check the thermostat. I have a feeling it may have been deleted. It ran very cool when timing was advanced to 12.5 degrees (per the book) but it sputtered and bounced around. The timing mark and dampener May have migrated over the years. I appreciate all the help though. It’s getting me there.
 
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Davea Lux

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Brakes running normally. I adjusted the carb float with non change. Adjusting your idle mixtures screws also had no effect. Setting the timing with a vacuum
gauge DID help to smooth the idle significantly. Trouble is now it’s running VERY hot. Am going to go back to idle mixture screws to lean it out more and also check the thermostat. I have a feeling it may have been deleted. It ran very cool when timing was advanced to 12.5 degrees (per the book) but it sputtered and bounced around. The timing mark and dampener May have migrated over the years. I appreciate all the help though. It’s getting me there.
You would also want to check the timing chain. Do this by rotating the crank by hand until you are at TDC, easily done with a socket on the damper bolt. Remove the distributor cap, now have a helper rock the crank back and forth and note how many degrees of rotation it takes to get the distributor shaft to move. 15 degrees or more and you need to replace the timing chain.

Dave
 

RogueOne

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You would also want to check the timing chain. Do this by rotating the crank by hand until you are at TDC, easily done with a socket on the damper bolt. Remove the distributor cap, now have a helper rock the crank back and forth and note how many degrees of rotation it takes to get the distributor shaft to move. 15 degrees or more and you need to replace the timing chain.

Dave
Excellent. I’ll put it on my to-do list. Thanks you
 

RogueOne

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Retarted timing = heat, tough sounding idle.
It was running cool when it was timed per specs (advanced 12.5 degrees) but it was idling rough at that timing. Now the timing is retarded and it idles much better but it now overheats. Am going to try a few tweaks. What am I missing?
 

1970FuryConv

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It was running cool when it was timed per specs (advanced 12.5 degrees) but it was idling rough at that timing. Now the timing is retarded and it idles much better but it now overheats. Am going to try a few tweaks. What am I missing?
Cooling system:
Fan clutch OK? Significant resistance when turned manually?
Fan shroud to help air draw at idle?
Fan positioned with blades half way out of shroud?
Thermostat could also be stuck. Missing thermostat does not make the car overheat. I have removed thermostat for flushing, figuring out cooling problems, etc. Overheating not an issue.
 

RogueOne

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Cooling system:
Fan clutch OK? Significant resistance when turned manually?
Fan shroud to help air draw at idle?
Fan positioned with blades half way out of shroud?
Thermostat could also be stuck. Missing thermostat does not make the car overheat. I have removed thermostat for flushing, figuring out cooling problems, etc. Overheating not an issue.
The fan clutch is definitely NOT doing it's job. It has small amount of resistance but turns with almost no effort. Shroud is in place and the blades are positioned as you recommended. Will investigate/replace fan clutch. Thanks for the head's up on that!
 

furious70

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Unfortunately you can't test a clutch fan like that, unless maybe it has absolutely no resistance. There is no bench test for them.
Your idle screws making no difference is a indicating a hidden problem yet. What carb? What were your vacuum readings with different timing settings?
 

RogueOne

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It’s a Carter 2 Barrel BBD-4297S. I just rebuilt it and put in a new gas tank, fuel uptake unit and fuel lines. Vacuum was around 22 in.Hg when timed to 12.5 degrees advanced. About 20 in.Hg when timed to zero degrees. I’m starting to think it’s not getting enough air somehow. I’ll be damned if I know where it would get it other than the carb which seems to be working fine. I take the vacuum line off the brake booster and it smooths right out. Put it back on or plug it with my finger and it’s back to non-rhythmic shuddering and stumbling. It’s like a vacuum leak in reverse. I put in new plugs and wires and distributor cap hoping there was an intermittent short I was missing but same rough idle. Will keep digging.
 

Loadrunner

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I take the vacuum line off the brake booster and it smooths right out.

A vacuum leak leans out the mixture the same as pulling the choke out a little fattens it.

My inner Spock tells me you have a rich mixture at idle.

Float level too high?

Should be square with the carb top hanging upside down or equivalent check.

That has to be 100% squared away - involuntary pun - before anything else.

Moving to the 1st circuit, the idle.

The needles need to be in good shape, look at them.

If they're worn out, replace them.

They could probably be cleaned up if replacements couldn't be found.

Seat the needles in fully several times, but gently, preferably by hand and not with a screwdriver but if with a screwdriver do not force the needle seat shut excessively hard.

Turn the needles back out a couple turns, start car and adjust for decent idle, but more importantly re-adjust when hot.

This adjustment has to be made, again and as final adjustment hot so that the choke is fully open and engine up to to operating temp.

Anytime on the car, with engine off, you can pull both needles - if you're not familiar with the carb, run the needles in gently till they seat and record the exact number of turns - do not mix them, blow the seats out with air and reseat is you suspect crud could be interfering. It can happen, especially with the float needle/seat.*

Probably not so much with a newly rebuilt carb, if you did the work yourself. If it's a reman, it's probably full of beadblasting sand, or there could be a dead mouse holding the float up. Extreme sarcasm emoji.

I usually run the right side in til it - go slow - until it just starts to get too lean - sounds like crap, faltering - bring it back out to a nice smooth point, and maybe just a teensy bit further for a better idle with engine cold, and power.

Lean = backfiring through carb with cold engine and does not make good power or keep engine cool. The latent heat of fuel cools the engine as it goes through so it's got to be right.

Besides the idle circuit, remove the float and make sure the jets are seated correctly, with proper fitting screwdriver.

This is all assuming your choke is opening all way.


*If you suddenly have crud in the carb needle seat flooding the carb out, rap or tap the side of the carburetor with a wooden hammer handle - non marring - it works wonders, it will dislodge the particle and the float level will normalize.



In these parts, the PNW, everybody always deemed Mopars to be "cold blooded", that' what I heard growing up.

Translation, you don't know how to properly set a choke to work as intended, but in many cases the heat crossover in the manifold is plugged with carbon, for better or worse. I'd have to say the Dodges were just climate ready as it has warmed up a lot, we may not need heat crossovers much longer, and I loathe the idea for pure performance.
 
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1970FuryConv

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The fan clutch is definitely NOT doing it's job. It has small amount of resistance but turns with almost no effort. Shroud is in place and the blades are positioned as you recommended. Will investigate/replace fan clutch. Thanks for the head's up on that!
Another check for fan clutch is to watch the fan while you rev the engine with the throttle linkage. If it does not pickup much speed while you accelerate, clutch is a problem.
Oil on the fan clutch is a sign of leakage.
You can also use a fan spacer the same size as the clutch and see if you have a difference in performance.
Or just replace the fan clutch and see how it works. Cheap insurance against a cooling problem.
 

RogueOne

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A vacuum leak leans out the mixture the same as pulling the choke out a little fattens it.

My inner Spock tells me you have a rich mixture at idle.

Float level too high?

Should be square with the carb top hanging upside down or equivalent check.

That has to be 100% squared away - involuntary pun - before anything else.

Moving to the 1st circuit, the idle.

The needles need to be in good shape, look at them.

If they're worn out, replace them.

They could probably be cleaned up if replacements couldn't be found.

Seat the needles in fully several times, but gently, preferably by hand and not with a screwdriver but if with a screwdriver do not force the needle seat shut excessively hard.

Turn the needles back out a couple turns, start car and adjust for decent idle, but more importantly re-adjust when hot.

This adjustment has to be made, again and as final adjustment hot so that the choke is fully open and engine up to to operating temp.

Anytime on the car, with engine off, you can pull both needles - if you're not familiar with the carb, run the needles in gently till they seat and record the exact number of turns - do not mix them, blow the seats out with air and reseat is you suspect crud could be interfering. It can happen, especially with the float needle/seat.*

Probably not so much with a newly rebuilt carb, if you did the work yourself. If it's a reman, it's probably full of beadblasting sand, or there could be a dead mouse holding the float up. Extreme sarcasm emoji.

I usually run the right side in til it - go slow - until it just starts to get too lean - sounds like crap, faltering - bring it back out to a nice smooth point, and maybe just a teensy bit further for a better idle with engine cold, and power.

Lean = backfiring through carb with cold engine and does not make good power or keep engine cool. The latent heat of fuel cools the engine as it goes through so it's got to be right.

Besides the idle circuit, remove the float and make sure the jets are seated correctly, with proper fitting screwdriver.

This is all assuming your choke is opening all way.


*If you suddenly have crud in the carb needle seat flooding the carb out, rap or tap the side of the carburetor with a wooden hammer handle - non marring - it works wonders, it will dislodge the particle and the float level will normalize.



In these parts, the PNW, everybody always deemed Mopars to be "cold blooded", that' what I heard growing up.

Translation, you don't know how to properly set a choke to work as intended, but in many cases the heat crossover in the manifold is plugged with carbon, for better or worse. I'd have to say the Dodges were just climate ready as it has warmed up a lot, we may not need heat crossovers much longer, and I loathe the idea for pure performance.
Lots to of good stuff to try here thanks. I did rebuild the carb myself a few dozen miles ago. Im not an expert on carb tuning so there's a lot I may have messed up. I read the instructions as closely as I could but nothing beats and expert doing the job. I just took the top off a few days a go to reset/lean the floats as I thought the bowl might have been overfilling. Perhaps I switched the idle screws in disassembly/assembly. Will take a look at those. I remember focusing on the jets as one broke when I was reinstalling. I got OE replacements and was extra careful not to over torque or damage the flat-head driver slots on them. Lots to go on here thanks
 

HWYCRZR

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What does your timing light do with vacuum advance hooked up and then pinch it off. You rebuilt the carb. Is your vacuum advance to the right port. Removal of your booster vacuum may be reducing your vacuum advance, which at idle should not come in to play. If too high of idle (butterflies open to far) you will get too much vacuum advance to distributor. If hooked to the wrong vacuum port you will also get too much vacuum increasing your advance. Set initial timing with brake booster hooked up and vacuum advance line crimped or plugged. If when you re-attach vacuum advance and it changes your condition you are getting vacuum advance too early.
 

RogueOne

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What does your timing light do with vacuum advance hooked up and then pinch it off. You rebuilt the carb. Is your vacuum advance to the right port. Removal of your booster vacuum may be reducing your vacuum advance, which at idle should not come in to play. If too high of idle (butterflies open to far) you will get too much vacuum advance to distributor. If hooked to the wrong vacuum port you will also get too much vacuum increasing your advance. Set initial timing with brake booster hooked up and vacuum advance line crimped or plugged. If when you re-attach vacuum advance and it changes your condition you are getting vacuum advance too early.
Excellent advice! There are only two ports on the Carter 2 barrel BBD. One to vacuum advance and one to vacuum choke assembly. I am sure they are both properly routed. I will do as you suggest re: clamping off vacuum advance and setting the timing. You're definitely on to something here. I keep coming back to the change in idle when the brake booster vacuum line is off. It's trying to tell me something for sure. Will report back.
 

Loadrunner

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I switched the idle screws

They're the same, but you'd prefer to keep them in order.

I remember focusing on the jets as one broke when I was reinstalling.

As long as the jets are seated properly you're fine.

Typically, the vacuum port for the choke pulloff is very near it, a very short hose, and the distributor feed is in the case of a BBD the only other port, so not really an easy mixup. Choke pulloff as indicated gets full vacuum from the instant the engine starts - to pull off the choke - while the other gets gradual vacuum because of a passage typically near the throttle - or butterfly - plate.

Hooking them up backwards would cause a lot of advance at idle, for better or worse, and a somewhat confused choke pulloff.
 
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