holley vacuum issue
Thread Starter
Racer
Joined: May 2009
Posts: 316
Likes: 1
holley vacuum issue
I have been trying to track down a vacuum problem on my 68'. I have used the smoke canister method and have been able to locate smoke exiting around the carbs primary throttle shaft (throttle lever side). Its tough to see that area to determine if its exiting just below the shaft at the carb manifold gasket or if its the shaft itself. Does anyone know if the shaft has a bushing or something (exploded diagrams aren't clear) that prevents a vacuum leak or is this an area that doesn't effect vacuum. I have tried applying propane in this area but no change in RPMs was noticed.
Instructor
Joined: Jul 2016
Posts: 195
Likes: 37
From: Poway CA
As far as I know, the throttle shaft is bushed, but it won't stop 100% of air being sucked into that clearance. I wouldn't think you should see smoke exiting, unless you are talking about exiting from the shaft INTO the butterflies.
You may want to post more info such as your vacuum gage readings, steady or not, how many inches at idle, etc. A vacuum leak is usually indicated by a lower than normal steady reading.
Did you ever find your rich condition problem? I suggested to look at your power valve for the 4165.
Kirk
You may want to post more info such as your vacuum gage readings, steady or not, how many inches at idle, etc. A vacuum leak is usually indicated by a lower than normal steady reading.
Did you ever find your rich condition problem? I suggested to look at your power valve for the 4165.
Kirk
Race Director
Joined: May 2006
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holley shafts have a slight depression or groove where a nylon strip acts as a bushing. There will be a very slight leak there unless severely worn.
have you sprayed carburetor cleaner around the base of the carburetor and noticed a change in idle?
have you sprayed carburetor cleaner around the base of the carburetor and noticed a change in idle?
Race Director
Joined: Oct 2008
Posts: 19,224
Likes: 1,672
From: Long Island
If you are trying to track down a vacuum issue, I would recommend disconnecting the vacuum system for the headlights and wiper door at the carb / source and plug that feed.
It may make life easier.
Had one yesterday , a friend had issues - everything operated ok. It turned out to be the canister for the wiper door with a slight leak, not a constant leak either. The rear seal was soft / bad.
It may make life easier.
Had one yesterday , a friend had issues - everything operated ok. It turned out to be the canister for the wiper door with a slight leak, not a constant leak either. The rear seal was soft / bad.
Thread Starter
Racer
Joined: May 2009
Posts: 316
Likes: 1
My vacuum hose for wipers and lights attaches to my manifold rather then the carburetor. I disconnect that hose which takes those accessory items out of the equation and attach my vacuum gauge there. Contacting Holley they confirmed there will be some normal minor primary shaft vacuum leakage they also didnt think based on symptoms that it was a power valve issue. At present my vacuum has been steady at 13 on my gauge at 750 rpm. talking with the original owner of the car he mentioned that the cars vacuum was never at the 20 range that is "normal". He attributes that to the stock cam on the 327/350 motor. So perhaps I am hunting for a leak thats nonexistent and the carburetor may be the culprit.
If you are trying to track down a vacuum issue, I would recommend disconnecting the vacuum system for the headlights and wiper door at the carb / source and plug that feed.
It may make life easier.
Had one yesterday , a friend had issues - everything operated ok. It turned out to be the canister for the wiper door with a slight leak, not a constant leak either. The rear seal was soft / bad.
It may make life easier.
Had one yesterday , a friend had issues - everything operated ok. It turned out to be the canister for the wiper door with a slight leak, not a constant leak either. The rear seal was soft / bad.
Last edited by greggome; Feb 20, 2017 at 11:19 PM.
Thread Starter
Racer
Joined: May 2009
Posts: 316
Likes: 1
At his point I have the car timed to 6 degrees BTDC with vacuum line disconnected. The cars service manual specifies 4 BTDC. Yet the glove box owners manual indicates 8 BTDC for one model # distributor while another model # gets 4 BTDC. Mine is the 8 BTDC model but since the shop manual dosent get that specific I compromised at 6@ BTDC.
vacuum drops from 13 to about 11 as rpm drops off a bit when distributor vacuum line is disconnected.
vacuum drops from 13 to about 11 as rpm drops off a bit when distributor vacuum line is disconnected.
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Safety Car
Joined: Oct 2007
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From: Austin Texas
Corvette of the Year Winner 2017
2016 C3 of Year Finalist
isolate all vacuum lines from engine, 16" is good vacuum.
doubt you will ever see 20"
13" is typical for non-stock cams.
overall vacuum is dependent upon timing, and valve condition.
I think your chasing nothing. if your initial problem is low vacuum.
take a baseline with everything disconnected,
add on components 1 by 1 until vacuum drops. then chase that leak.
doubt you will ever see 20"
13" is typical for non-stock cams.
overall vacuum is dependent upon timing, and valve condition.
I think your chasing nothing. if your initial problem is low vacuum.
take a baseline with everything disconnected,
add on components 1 by 1 until vacuum drops. then chase that leak.
Safety Car
Joined: May 2001
Posts: 4,759
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From: Fairfax Virginia
When I bought my 1968 BB Convertible it had sat for a while. The vacuum accessories did not open or close very quickly and the system lost vacuum pretty fast. Everything worked but just sluggishly so I replaced the entire Vacuum hose with a kit from Dr. Rebuild.
After re-plumbing the entire vacuum system with bright, well marked, soft and flexible fresh hoses my vacuum accessories all "snap" open and closed. The car could sit for weeks and still hold vacuum. It takes some work to install the hoses in their original routes but the outcome was worth it. The best part is I then had some idea of where everything is located after finishing.
If I ever buy another C3 the first thing I would do is replace the entire vacuum hose set before I went chasing after the gremlins in the system. Dr. Rebuild also sold me a beautiful laminated chart showing the hoses and routing in living color, this is a lifesaver for any 1968 C3 owner as our cars are just a bit "different".
After re-plumbing the entire vacuum system with bright, well marked, soft and flexible fresh hoses my vacuum accessories all "snap" open and closed. The car could sit for weeks and still hold vacuum. It takes some work to install the hoses in their original routes but the outcome was worth it. The best part is I then had some idea of where everything is located after finishing.
If I ever buy another C3 the first thing I would do is replace the entire vacuum hose set before I went chasing after the gremlins in the system. Dr. Rebuild also sold me a beautiful laminated chart showing the hoses and routing in living color, this is a lifesaver for any 1968 C3 owner as our cars are just a bit "different".
Instructor
Joined: May 2016
Posts: 178
Likes: 22
From: Vermont
There are 2 ports where you can measure vacuum on a Holley. The one ( I think ) that comes out of the jet block is "timed " and will read lower than directly at the base of the carb. Holley has done this to regulate distributor vacuum timing.
Hope I remembered all this correctly. I kept changing Power Valves with no vacuum change until I discovered my mistake. DUH !!
I agree that you will probably read about 13-15" with the 327/350 motor. Kinda hot cam.
Hope I remembered all this correctly. I kept changing Power Valves with no vacuum change until I discovered my mistake. DUH !!
I agree that you will probably read about 13-15" with the 327/350 motor. Kinda hot cam.
Thread Starter
Racer
Joined: May 2009
Posts: 316
Likes: 1
Thanks for the info. I have been checking vacuum at the manifold where all my accessories attach. By pulling that hose I eliminate all the accessory items which leaves me with only the carb and engine to hunt down a leak. I am assuming now that its probably a carb issue. symptoms dont really support the power valve theory but I might try having a look at it anyway. I was considering doing away with the spreadbore Holley and using a manifold adapter to fit a Holley square bore instead but Ive heard this will sacrifice power. I'm getting to the point where I may have the original Q jet rebuilt and give that a try.
There are 2 ports where you can measure vacuum on a Holley. The one ( I think ) that comes out of the jet block is "timed " and will read lower than directly at the base of the carb. Holley has done this to regulate distributor vacuum timing.
Hope I remembered all this correctly. I kept changing Power Valves with no vacuum change until I discovered my mistake. DUH !!
I agree that you will probably read about 13-15" with the 327/350 motor. Kinda hot cam.
Hope I remembered all this correctly. I kept changing Power Valves with no vacuum change until I discovered my mistake. DUH !!
I agree that you will probably read about 13-15" with the 327/350 motor. Kinda hot cam.
Thread Starter
Racer
Joined: May 2009
Posts: 316
Likes: 1
The motor was rebuilt many years ago sticking to stock parameters. looking at the vacuum gauge it holds steady with no flutter. As I understand it, Flutter or occasinal drops on the vacuum could mean valves need adjustment. Since I have not noticed this I have not messed with valve adjustments.