Author Topic: Best way to check for vacuum leaks?  (Read 7069 times)

Offline 74 challenge

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Best way to check for vacuum leaks?
« on: May 25, 2016 - 10:31:16 AM »
I'm at it again trying to figure out WTF is wrong with my car.

I have checked the Engine by spraying it with parts cleaner but I can't find any issues. I also just had the engine repainted so I don't want to ruin it my engine bay by blasting it with chemicals, it finally looks acceptable. 

Is there a more accurate way to figure this out? I don't think there is a vacuum leak but I can't get my vacuum pressure to be a consistent so I'm starting over for the 5th or 6th time.
1-8-4-3-6-5-7-2




Offline HP_Cuda

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Re: Best way to check for vacuum leaks?
« Reply #1 on: May 25, 2016 - 05:57:50 PM »

It might be easier to tell us what you changed recently so we could narrow it down.

Intake
Carb
Carb spacer
Vacuum line to booster
Vacuum line to distrib
etc

B
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1970 Cuda 383 4 speed - yellow - SOLD

Offline tman

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Re: Best way to check for vacuum leaks?
« Reply #2 on: May 25, 2016 - 06:21:16 PM »
What is your vacuum readings?  Engine stock or modified with an aggressive cam?  Cam will affect vacuum.  If you have a vacuum booster, does it leak within the unit?  Does it idle ok?  info you provided is vague right now.  If you posted on another thread, some may not be aware of those details like me.

Offline 74 challenge

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Re: Best way to check for vacuum leaks?
« Reply #3 on: May 25, 2016 - 09:07:26 PM »
Sorry I was posting at work between tasks.

I am trying to tune my carb but I cannot get consistent vacuum it always fluctuates by 3 psi.

Brake booster seems to work without issues.

The entire engine is new to the car, I installed the intake and carb, vacuum lines etc...

In the process of trying to tune my carb I decided to verify if there are any vacuum leaks and started spraying the base of the carb, intake sealing areas, lines etc...with parts cleaner but it always leaves a residue that I so t want on my newly painted block.

So I am wondering if there is a better way to search out for vacuum leaks without dousing the engine on chemicals that would ruin my newly painted block.

I'm going back to vacuum leaks because I get the carb running good but from day to day it changes. Idles great and the next day it's sparatic. It all comes down to trying to tune the car but the basics are not there. I can't achieve consistent vacuum.

I want to rule out a leak once and for all. no matter how I adjust the screws on the carb a fluctuation by 3 psi is the best I can do. From 15-18 at 800 idle.

The car has a lunati cam very mild. Part number 10200701

Good compression in every cyl.
1-8-4-3-6-5-7-2

Offline 74 challenge

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Re: Best way to check for vacuum leaks?
« Reply #4 on: May 25, 2016 - 09:12:37 PM »
Forgot to add that it is running the vacuum line to the distributor and the idle doesn't change when I spray it.
1-8-4-3-6-5-7-2

Offline Racer57

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Re: Best way to check for vacuum leaks?
« Reply #5 on: May 25, 2016 - 09:28:25 PM »
Something I've done on motorcycle engines and works great when checking for leaks and you don't want to mess everything up. Get a propane torch. Don't lite it !!  Just turn it on all the way and run it all around everywhere possible. If there is a leak the propane will change the rpm. Bad leak on motorcycle will kill an idling engine. And no, there wasn't any explosions.  :D

Offline Chryco Psycho

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Re: Best way to check for vacuum leaks?
« Reply #6 on: May 25, 2016 - 09:53:49 PM »
what is the carb ?
 What is the vacuum at idle in gear ?  recheck torque on intake bolts & carb bolts , make sure the carb gasket is the right one as some will not seal to various intakes .
 I assume the idle speed is unstable also so it could be vacuum advance or the advance weights in the dist messing with it at idle .
 I was tuning a six pac one day & it would idle at 500 rpm , if you brought it up at all it would idle @ 1200 , nothing between finally after trying everything with the carb found the problem in the advance weights , they would move substantially at 510 rpm & change everythng

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Offline dodj

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Re: Best way to check for vacuum leaks?
« Reply #7 on: May 25, 2016 - 11:17:11 PM »
Something I've done on motorcycle engines and works great when checking for leaks and you don't want to mess everything up. Get a propane torch. Don't lite it !!  Just turn it on all the way and run it all around everywhere possible. If there is a leak the propane will change the rpm. Bad leak on motorcycle will kill an idling engine. And no, there wasn't any explosions.  :D
The only method I have used. Works well. Admittedly, doesn't seem safe, but I'm still alive.  :grinyes:
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Offline AARTA340

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Re: Best way to check for vacuum leaks?
« Reply #8 on: May 25, 2016 - 11:52:20 PM »
You say the motor is new to the car. Is the motor new? I have seen bad valve jobs make the vacuum fluctuate.  :2cents:

Offline 74 challenge

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Re: Best way to check for vacuum leaks?
« Reply #9 on: May 26, 2016 - 09:14:56 AM »
what is the carb ?
 What is the vacuum at idle in gear ?  recheck torque on intake bolts & carb bolts , make sure the carb gasket is the right one as some will not seal to various intakes .
 I assume the idle speed is unstable also so it could be vacuum advance or the advance weights in the dist messing with it at idle .
 I was tuning a six pac one day & it would idle at 500 rpm , if you brought it up at all it would idle @ 1200 , nothing between finally after trying everything with the carb found the problem in the advance weights , they would move substantially at 510 rpm & change everythng

Chryco it's a profrom 650 vacuum secondary electric choke unit, you were trying to help me Tune it in another post. I was following the directions in that post but I'm back to questioning my vacuum because the trouble shooting section on the holley site seems to put that as the common "issue" I'm trying to eliminate issues one by one. Fix what I know and can easily tell if it's good or not.

I Have never messed with Idle weights before, I'd be lying if I even knew how they worked other than they control advance/ retard by centrifugal force, but no idea how to adjust those or even check if they are working correctly?

Vacuum is between 15-18 PSI but in park at idle.

the carb gasket is a brand new square (meaning one big hole not the 4 individual round ones, or 2 holes that separate the primaries from the secondary's) rubber unit from fel-pro I believe, with no spacer. Profrom carb is an aluminum base.

for AARTA340, the motor is new to me, it's a 72 360 with a rebuilt by a good engine shop a few years ago. It sat unused for the last year as the owner of it died and his son sold it to me.

the propane torch scares me not going to lie but I will give it a go.

1-8-4-3-6-5-7-2

Offline Racer57

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Re: Best way to check for vacuum leaks?
« Reply #10 on: May 26, 2016 - 10:10:46 AM »
When using the propane torch, just use common sense. Put the nozzle right up against the areas your testing. If the fumes get heavy from extended use, then stop and let it air out.

Offline Fred

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Re: Best way to check for vacuum leaks?
« Reply #11 on: May 26, 2016 - 11:35:14 AM »
Does is hesitate upon a quick throttle?

Offline HP_Cuda

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Re: Best way to check for vacuum leaks?
« Reply #12 on: May 26, 2016 - 11:35:54 AM »
Does your proform carb sit level on the intake with no binding?

This could also create a vacuum leak. Make sure all linkages do not interfere with the intake at any point.
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Offline 74 challenge

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Re: Best way to check for vacuum leaks?
« Reply #13 on: May 26, 2016 - 11:48:09 AM »
Does your proform carb sit level on the intake with no binding?

This could also create a vacuum leak. Make sure all linkages do not interfere with the intake at any point.

I will answer both you and FRED,

the carb sits flush against the intake and I assume fairly level? I have never put a level on it though.

and for FRED I do get a hesitation on quick throttle especially going from idle to acceleration. I can overcome it by tapping the gas just a quick on and off then giving it throttle like you would drive any other automatic car.
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Offline HP_Cuda

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Re: Best way to check for vacuum leaks?
« Reply #14 on: May 26, 2016 - 11:51:46 AM »

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 ......
1970 Cuda Clone 440 4 speed - sublime green
1970 Cuda 383 4 speed - yellow - SOLD