Thank you for reading the long post and for your replies.

74/225/Holley 1945 (rebuilt - by me) - stock everything except fuel line and electric choke)- 56K on engine - my daily driver

What really has me stumped is that it does not do this all of the time. The problem went away for a while and now is back and worse.

Here goes my best description:

Start in moring (always starts rough) - allow to warm-up and then take kid to school - after 20 minute drive (that it runs great and idles perfectly at lights) I turn it off for about 10 minutes to wait for his school to open. When I restart it the can runs really rough/low idle speed/misses/surges. I get through the car line with lots of babying and funny looks and then drive to work. After about 10 minutes of freeway driving I get to a stop light where the car idles great. Problem gone!

It acts like a vacuum leak. All lines are new or checked and would a vacuum leak not always be present? Has a remflex manifold gasket. Manifolds were checked with straight edge and were true.

Could this be because the engine gets hotter when you turn it off and then cools back down as I drive it for a while? If so, where is my leak - in the manifold? How do I find it?


Frustrating problem for an otherwise well running engine.

Thanks for your help and patience.

Could be choke linkage is hanging up not allowing it to be in correct position for partial warm-up drivability.

Yes head will over heat after shut down, but with just ten minutes of driving this probably won't be causing a problem.

Many moons ago I had a Dart which suffered from worn choke linkage that caused it to hang up not fully closing only on damp days, yes damp days, which required linkage to be manipulated by wiggleing it by hand to fully close choke. I filed the worn spots enough to round the groves so it would not bind and solved the problem.

Next time your car acts up pull over, remove air cleaner and observe the choke's position. Gently manipulate the linkage by hand to feel for any binding, and determine if choke stove (the bimetal spring that forces choke open when warmed housed within a small pocket in manifold) is pushing choke open or weak allowing butterfly to flap around uncontrolled. When choke stove is new there is a way to adjust bimetal spring tension to correct this problem, but old corroded spring may not afford any adjustment. Fix would be to install an aftermarket electric choke.

Also at next over night cold start before cranking her up remove air cleaner, by hand rotate open throttle plate and observe choke's action. Dose it snap tightly closed with authority, and once closed and throttle is rotated open say half throttle by hand, dose the choke butterfly open a lot more than 1/4"?

What I want to find out is, once choke has been set, and during partial warm up condition, will actuating throttle force choke butterfly open to prevent over choking or rich condition under acceleration. You may have to repeat this test several times for your eye to catch all the action of the moving parts. To repeat cold start test, just open throttle and at the same time hold choke butterfly open while releasing throttle; this will reset choke as if the car had been fulyl warmed up and allowed to sit over night.

If it looks as if choke is working correctly, than there may be a problem with float level in bowl for what ever reason, set too high or low, or has it become fuel logged and sunk causing high bowl level.

One other device to inspect is the heat riser valve, the little flapper just between exhaust manifold and exhaust pipe controlled by a bimetal spring that directs hot gasses to intake at bottom of carburetor when cold, and retracts flaper to direct hot gasses down exhaust pipe once engine is up to temperature. This flapper can be come stuck closed, open or part way, and or the bi metal spring could be missing or broken disallowing operation of said valve. If stuck there is a special Mopar solvent to free the flapper which I don't recall its part number; Dan may chime in with said part number, or you could search the site for it.

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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

Thank you for the reply

I have an electric choke on it - I will check as you suggested and make sure it is still working as it should.

Heat riser valve - thank you for the suggestion. I will check it this weekend.

Also never thought about float. Very common sense answer - sitting for a bit would give it a chance to fill up or hang or show that it is poorly adjusted.

Going to try to go out tomorrow and see if I can make it do this and get under the hood and see if I can figure it out.

It runs well until it sets for 10 minutes and then runs well on the highway.

I was also thinking these would be the symptoms of a small crack in the manifold - only showing when hot.

Thanks again

If it's the stock 1974 Carb that came with the car then there are a few things that can cause issues here....

1) FYI the stock 1974 Holley that year came a bit lean in the mains bumping up to a #61 jet like the later Holley's had can help with some drveability issues.

2) Make sure all the vacuum fittings are not cracked and all the hoses are intact... given the EGR/Amplifier/OSAC/ CCEGR pollution control items a leak in one can cause and idle problem... Also making sure everything is plugged into the right component can also cause issues... I had bought my 1974 and it ran great, but over time I noticed that I only got better power and economy after the engine had warmed up... after goign through all the hoses I found the previous owner had plugged the distributor vacuum advance into the CCEGR switch on the radiator, and back to the EGR port on the 1945.... suddenly everything worked great after I put the hoses where they were supposed to be...

3) One item that gets over looked often is the vacuum amplifier, after about 40 years these give up the ghost and since they are plumbed into the vacuum tree on runner #6, they can rupture, cause a full vacuum leank and lean out #6 and cause an idle problem... sometimes you can look at your plugs and see if #6 is lighter than the other ones... even unplugging the amplifier hose and putting a rubber plug over the tree and taking it for a drive and see if things get better.

4) As mentioned before, hopefully you checked the float when reassembling the carb for freedom of movement, the 1945's dual 'foam' float has a tendency to hang up on of the bowl walls or the other sometimes.

5) Checking Choke settings and function are also a good idea.

6) you can use a non flammable carb cleaning spray to squirt specific points near the carb base and manifold to see if a change in idle occurs, if it changes, something is loose.... Sometimes if the carb's screws didn't get tightened well you get a phantom issue on idle quality.


Food for thought, good luck.

Heat riser intact (including plate inside the manifold) and working correctly -- yep, that needs checked and verified. Also the thermostatic air cleaner system needs to be intact and functioning all the way correctly as described here. If your flex duct is missing, replace it (parts store item). If your air cleaner thermostatic sensor doesn't work any more, which is common, give me a shout; I have new ones.

Make sure your EGR valve is not opening when it shouldn't -- this will include making sure all the vacuum lines in the EGR system are routed correctly or the EGR is permanently disabled by removing the valve, flipping it over (180°) and reinstalling it with a new gasket.

Also, this is not directly related to your question, but bypass your OSAC valve (widget on the air cleaner housing with one hose to the carb and one hose to the distributor -- instead, run a hose directly from carb port to distributor port) and your car's driveability will improve quite a bit.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Vacuum amplifier - unplugged it and got an immediete improvovement. Still stumps me as to why it only does it after i turn it off when it is hot. I guess that it is when it gives up.

Thank you for your help

My question - what funcntion does this serve and do I need one?

More concerned with drivabilty than originality

Also discovered my choke was hitting the lip on my breather preventing it from closing all the way. Cold start gretly improved.

Vacuum amplifier- acts as kind of a 'reservoir/plenum' for vacuum from the runner, so that it can provide vacuum along more hoses/'circuits' for the EGR and emissions to function correctly. If there is not a visual check for this emissions stuff in your area you can cap the vacuum tree/carb ports and remove the amplifier and hoses... and per Dan get an EGR gasket and flip the EGR to plug it.

Glad to hear things are moving in the correct direction.

Err...no, it's actually just a vacuum relay. Just like an electrical relay is an electrically-operated switch (small electric current operates the switch, which makes or breaks a high-current circuit) the vacuum "amplifier" sends strong manifold vacuum to the EGR valve in response to a weak vacuum signal from the carburetor venturi tap. That venturi vacuum is a good indicator of when it is and isn't appropriate for the EGR valve to be open, but it's too weak to operate the valve on its own -- hence the vacuum "amplifier" (relay).

If disconnecting it improved matters, the EGR system is in need of attention and repair (or removal). It's showing up as a temperature-sensitive problem because this is a CCEGR ("Coolant controlled exhaust gas recirculation") system that uses engine coolant temperature as a parameter for when to activate and deactivate the EGR system as a whole.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Perhaps I have found the problem. There is no electricity or wire going to the device.

Will do some more looking and see what I can figure out. After 40+ years lots of things get messed with, even on cars as original as mine.

This also makes perfect sense as to why it only shows up at high temperatures.

Any negative effects of flipping the egr valve and capping the vacuum ports and disabling this whole system?

No where to plug it in??

Car does strt and run well.

The choke should have power to it, and the choke assist module/timer bolted to the head should have light blue wire to it from the harness at the firewall. The Vacuum amplifier has nothing electrical on it.

There is no down side to flipping the EGR over and disabling it.

Going to which device?
On a car such as yours, no, not really.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Dan - Sorry - I thought I read in your reply that the vacuum amplifier somehow dealt with electrical.


Duster - Electric choke works great - once I figured out it was catching in the breather and it closes as it should.

Thank you to everyone for your responses and I apprecite the wealth of knowlegde and all of the support.

As a Chevy person (my other vehicle is a 70 chevy 4x4 with a 350 that I am restoring) I have enjoyed learning about my car and appreciate all of the help and knowlegde that folks are willing to share. I just bought it as a cheap daily driver that I could work on myself. (2k for a 50k car with air and totally original, looks brand new - original everything - took the factory belts and hoses off it) Have pulled the head myself and pretty much done most all of the maintenance myself with a factory manual and this board.

I really apprecite everyone's help and these are really neat little engines that I can work on myself! And no, I not a mechanic - but I do have common sense and am not afraid to get dirty!

Took a long drive today and it runs great.

Thanks again