Low vacuum
Thread Starter
2nd Gear
Joined: Sep 2021
Posts: 2
Likes: 0
From: Cudjoe Key, FL
Low vacuum
I have recently purchased a 1970 coupe with the L46 engine. The headlights and wiper door were slow to operate.
So I replaced all the hoses and the headlight parts. That seemed to help but didn't cure it completely.
When I checked the vacuum at an Idle, it red between 10 and 12 inches. The needle wasn't steady.
All the cylinders had over 200 lbs compression. This is an 11 to 1 engine.
I'm also having trouble finding the correct spark plug. One sight said it take an AC R43.
I would appreciate any input, thanks.
So I replaced all the hoses and the headlight parts. That seemed to help but didn't cure it completely.
When I checked the vacuum at an Idle, it red between 10 and 12 inches. The needle wasn't steady.
All the cylinders had over 200 lbs compression. This is an 11 to 1 engine.
I'm also having trouble finding the correct spark plug. One sight said it take an AC R43.
I would appreciate any input, thanks.
Safety Car
Joined: Sep 2005
Posts: 4,213
Likes: 643
From: Orange County Ca
Vac is low for a stock engine.
A good tune can really change your vacuum, so look at that.
Also look for vac leaks. Pinch off any vac lines coming off the intake manifold to try to isolate which system it is.
Spray carb cleaner, etc., around the intake manifold and the carb base to see if idle speed increases.
Unfortunately, if you changed all the parts of the headlight system, you probably have some modern china crap repop valves. Many are real crap and leak excessively even when new.
It make take some time and trial and error to get it to work right.
Good luck.
A good tune can really change your vacuum, so look at that.
Also look for vac leaks. Pinch off any vac lines coming off the intake manifold to try to isolate which system it is.
Spray carb cleaner, etc., around the intake manifold and the carb base to see if idle speed increases.
Unfortunately, if you changed all the parts of the headlight system, you probably have some modern china crap repop valves. Many are real crap and leak excessively even when new.
It make take some time and trial and error to get it to work right.
Good luck.
Race Director
Joined: May 2006
Posts: 18,665
Likes: 2,564
What is your idle speed?
Do you have the vacuum advance connected to full manifold vacuum?
What is your initial timing set to with vacuum advance disconnected?
Have you adjusted the mixture screws for best vacuum at idle?
Do you have the vacuum advance connected to full manifold vacuum?
What is your initial timing set to with vacuum advance disconnected?
Have you adjusted the mixture screws for best vacuum at idle?
Burning Brakes
Joined: Sep 2018
Posts: 1,002
Likes: 410
Typically a R43 is too cold for the street -- have you read your plugs?
If the color taken on by the insulator is not a tan color what color is it?
If it appears darker then try a R44
IF you distributor vacuum can is stock I would swap to a B26 or B28.
If the color taken on by the insulator is not a tan color what color is it?
If it appears darker then try a R44
IF you distributor vacuum can is stock I would swap to a B26 or B28.
Thread Starter
2nd Gear
Joined: Sep 2021
Posts: 2
Likes: 0
From: Cudjoe Key, FL
My initial timing was set at 8 and the idle is at 700.
I'll be sure I have new plugs before I try to set the mixture at idle.
I'll also use the carb cleaner around the carb to se if the gasket may have a leak.
Thanks for the help, more to follow.
I'll be sure I have new plugs before I try to set the mixture at idle.
I'll also use the carb cleaner around the carb to se if the gasket may have a leak.
Thanks for the help, more to follow.
Race Director
Joined: May 2006
Posts: 18,665
Likes: 2,564
The factory idle speed was 750 and that should probably be raised to 800.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,114
From: Crossville TN
You have some vacuum leaks in your system. You should be getting 15" Hg or so. Lots of possible areas for leakage to happen in that system. But, it is relatively easy to diagnose leakage issues if you have a vacuum gauge and a couple of squeeze clamps to pinch off vacuum lines. Your first point of reference should be to determine how much vacuum your engine COULD produce by removing and blocking all vacuum lines and by verifying no leakages at the intake manifold 'seams' and around the carburetor.
Once you know your engine's 'potential' vacuum, you can hook up one hose at a time until you identify the hose with significant leakage. Then it is just diagnose leakages in THAT HOSE path and fix them till it doesn't show significant leakage. Continue that process till the entire system is reconnected and system vacuum is no less than 1-2" Hg lower than the 'potential' vacuum level for the engine. You can do this in one morning (or afternoon). Simple techniques.....persistent and deliberate action.
Once you know your engine's 'potential' vacuum, you can hook up one hose at a time until you identify the hose with significant leakage. Then it is just diagnose leakages in THAT HOSE path and fix them till it doesn't show significant leakage. Continue that process till the entire system is reconnected and system vacuum is no less than 1-2" Hg lower than the 'potential' vacuum level for the engine. You can do this in one morning (or afternoon). Simple techniques.....persistent and deliberate action.
Last edited by 7T1vette; Sep 27, 2021 at 12:14 PM.
Drifting
Joined: Sep 1999
Posts: 1,970
Likes: 91
I have recently purchased a 1970 coupe with the L46 engine. The headlights and wiper door were slow to operate.
So I replaced all the hoses and the headlight parts. That seemed to help but didn't cure it completely.
When I checked the vacuum at an Idle, it red between 10 and 12 inches. The needle wasn't steady.
All the cylinders had over 200 lbs compression. This is an 11 to 1 engine.
I'm also having trouble finding the correct spark plug. One sight said it take an AC R43.
I would appreciate any input, thanks.
So I replaced all the hoses and the headlight parts. That seemed to help but didn't cure it completely.
When I checked the vacuum at an Idle, it red between 10 and 12 inches. The needle wasn't steady.
All the cylinders had over 200 lbs compression. This is an 11 to 1 engine.
I'm also having trouble finding the correct spark plug. One sight said it take an AC R43.
I would appreciate any input, thanks.
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Cruise-In VIII Veteran
So far everyone is convinced you have a vacuum leak. But no mention of which Cam you have in this engine. Is this a factory cam shaft? If not do you know what is in there? If you have a larger, lumpier cam, your timing would need to be adjusted to compensate.
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,809
From: Cool Northern Michigan
wdbecker,
I would recommend Googling:
Vacuum Gauge Diagnostics. There, you will find several drawings of several issues commonly found with a non-steady gauge needle.
Compare your Vac gauge action with the charts. Then and only then will "The Needle Wasn't Steady" stated in post one, make any sense.
Usually that is a sticky valve issue.
I would recommend Googling:
Vacuum Gauge Diagnostics. There, you will find several drawings of several issues commonly found with a non-steady gauge needle.
Compare your Vac gauge action with the charts. Then and only then will "The Needle Wasn't Steady" stated in post one, make any sense.
Usually that is a sticky valve issue.