Best way to check for vacuum leaks?

[Fred]

I would consider the carb/manifold gasket as one possiblility.  One corner of my 70 340 intake and Holley carb has very little mating surface area.  Had to put 2 gaskets.

[74 challenge]

I think Cryco is onto something with the timing aspect he talked about.

If it weren't timing I would say the off idle hesitation could be the accelerator pump but ......

The pump works as soon as the throttle is moved, nice jets of fuel into the carb, also the power valve is correct for the application and the car does quit if I screw in the air fuel mixture screws all the way. It has never backfired through the carb. Carb has somewhere around 400 miles on it bought it brand new.

I know you can adjust the vacuum can on the distributor but I have no idea what it does? you can put an Allen key in there and twist it but what does it do? I have never touched it. The car world seems mixed if you should even bother adjusting it or just disconnect it.

[HP_Cuda]

I like Cryco's advice of disabling the vacuum advance.

Basically he states:

1) disconnect the vacuum line and plug the carb port
2) put an allen key into the vacuum line port on the distrib and turn it counter clockwise until it stops (don't force it - when it's done its done)
3) plug the distrib port
4) Set the initial timing to 16 degrees and hence with the distrib having 20 degrees in it you will end up with 36 total at the end.

If you do this you will at least rule out the vacuum advance as being one of the problems.

[Racer57]

I like Cryco's advice of disabling the vacuum advance.

Basically he states:

1) disconnect the vacuum line and plug the carb port
2) put an allen key into the vacuum line port on the distrib and turn it counter clockwise until it stops (don't force it - when it's done its done)
3) plug the distrib port
4) Set the initial timing to 16 degrees and hence with the distrib having 20 degrees in it you will end up with 36 total at the end.

If you do this you will at least rule out the vacuum advance as being one of the problems.

"Plug the distrib port"..... Why ? To keep out dust/dirt ? Mine isn't plugged and never gave it any thought.

[HP_Cuda]

Cleaner look and nothing will get in there.

 

[Chryco Psycho]

Just for info , the screw limits the travel of the diaphragm . So turning the screw clockwise increases the total travel , going the other way 1 full turn will reduce the advance by approx 1* it will go approx 15 turns end to end so the vacuum can add 15-16* of advance , when it backs out completely you will hear the screw click past the last thread , it will not fall out or do damage just turn it the other way & the threads will engage again .

[74 challenge]

Just for info , the screw limits the travel of the diaphragm . So turning the screw clockwise increases the total travel , going the other way 1 full turn will reduce the advance by approx 1* it will go approx 15 turns end to end so the vacuum can add 15-16* of advance , when it backs out completely you will hear the screw click past the last thread , it will not fall out or do damage just turn it the other way & the threads will engage again .

I will address everyone here but I just quoted Chryco,

I won't lie I haven't the slightest clue where mine is set. I wonder if it is causing any issues since it's set god knows where and hooked up? Is there an easy way to determine if it's causing me problems? Just disconnect it and go for a drive after plugging the port on the carb?

I was going to look for leaks last night but it was raining and I don't want to run the car in the garage while I am dousing it in propane. So stay tuned for that update. I should get around to it on Sunday after the cottage dock is in.

What size is the allen key?

[YellowThumper]

1) even though you stated your power brakes still work, there may still be a leak in the diaphragm. Disconnect it and plug it's manifold port.
2) confirm your pcv valve is properly functioning. If it is gunked up and staying open more air will get sucked in.
3) as far as spraying areas to find leaks. All it really needs to do is temporarily "fill" the gap. That fill will change your air/fuel ratio and rpm will change. It doesn't have to be something that burns. Almost anything will work. I have used sprays of water, wd40, dish soap, etc...
Never tried it but you probably could use argon if you have it for a welder. Inert gas that displaces air.
4) have you tried just putting your hand over the carb intake to see if it picks up rpm while limiting the air flow? If it just chokes out most likely you have little or no leaks.


Mike

[74 challenge]

1) even though you stated your power brakes still work, there may still be a leak in the diaphragm. Disconnect it and plug it's manifold port.
2) confirm your pcv valve is properly functioning. If it is gunked up and staying open more air will get sucked in.
3) as far as spraying areas to find leaks. All it really needs to do is temporarily "fill" the gap. That fill will change your air/fuel ratio and rpm will change. It doesn't have to be something that burns. Almost anything will work. I have used sprays of water, wd40, dish soap, etc...
Never tried it but you probably could use argon if you have it for a welder. Inert gas that displaces air.
4) have you tried just putting your hand over the carb intake to see if it picks up rpm while limiting the air flow? If it just chokes out most likely you have little or no leaks.

Mike

all of these are dually noted.

I have never tried to cover the carb no but when the choke was not adjusted properly the car did die when the choke flap shut too much.

I cleaned the pcv valve when I rebuilt the engine, it's clean and seems to work well.

[dougs bs23]

What size is the allen key?
[/quote]
3/32 I think

[Racer57]

3) as far as spraying areas to find leaks. All it really needs to do is temporarily "fill" the gap. That fill will change your air/fuel ratio and rpm will change. It doesn't have to be something that burns.
Mike

I have no idea if your referring to propane use or not, but just in case........ it does not ignite/burn. It only displaces the air. 

[YellowThumper]

I have no idea if your referring to propane use or not, but just in case........ it does not ignite/burn. It only displaces the air. 

Only reference I made was for argon. Inert gas "shield" for mostly tig welding. You can use just about anything that will temporarily stop or reduce the flow of air being sucked in.

[74 challenge]

Update time!

I took some propane to my intake, distributor, brake booster, carb base etc... Nothing. I even put some down the carb to know what type of change I was looking for.

Covered the carb with my hand and the engine stalled almost instantly. I have ruled out vac. leak so the issue is in the carb adjustment.

Also checked the pcv valve and it's working A-Okay.

I did take it for a ride yesterday and it really smells of raw gas. I think at this point I am going to jet down my primaries and start over with the carb adjustments.

I also re-checked the choke to verify it's proper operation but it has been so hot here lately I think I will need to adjust it again.

When you are re-jetting do you go down one at a time? or got down a couple sizes? I am contemplating buying a set vs just buying them individually. Holley based pro-form 650 vac secondary elec. choke.

[HP_Cuda]

Typically done one at a time and start with the primaries only.

[YellowThumper]

Resist the urge to make multiple changes.
1 step at a time and....     repeat.