vacume advance~ timing~question
Thread Starter
Instructor
Joined: Mar 2006
Posts: 246
Likes: 1
From: west melbourne florida
vacume advance~ timing~question
i changed my vacume advance to a crane cam vacume advance adjustable.. when i start my vett with no vacume advance hooked up she idles at 800 rpm and has 13 inital timing...when i hook the vacume advance up she goes up to 1000 rpm and a timing setting of 38.. im using a electronic dial back timing light...is my timing coming in too early..and if so what do i need to change to fix it.....also when i installed this it made my vette run cooler before she would hit 220 fast
Le Mans Master
Joined: Apr 2004
Posts: 7,106
Likes: 38
From: Newark DE
first of all I would recommend getting the correct vacuum advance for your motor, not an adjustable one, but regardless.......
plugging in the vacuum advance and seeing an increase of idle speed means that the vacuum advance is connected to a full manifold vacuum source so that's good. After setting your timing correctly and reconnecting the vacuum advance, simple lower the idle speed down to the correct level.
if your initial timing is jumping from 13 to 38 degrees with the vac can connected than the can is adding in 25 degrees of advance which is a lot, you are usually looking for about 14-16 degrees advance from the can. Since it's an adjustable can adjust it with the adjustment screw to reduce the amount of advance it gives you. you want the times set so with the can DISCONNECTED your total timing is at about 36º with timing all in between 2800-3000 rpm and than with the vac can reconnected the total timing with vacuum advance doesn't exceed 50º-52º.
seeing the operating temp pf your motor reduced by adding the vacuum advance and having it connected to full manifold vacuum source is one of the advantages of the system. Be glad it's there and you have it connected correctly and you are seeing the benefit.
now you just need to get it set correctly
plugging in the vacuum advance and seeing an increase of idle speed means that the vacuum advance is connected to a full manifold vacuum source so that's good. After setting your timing correctly and reconnecting the vacuum advance, simple lower the idle speed down to the correct level.
if your initial timing is jumping from 13 to 38 degrees with the vac can connected than the can is adding in 25 degrees of advance which is a lot, you are usually looking for about 14-16 degrees advance from the can. Since it's an adjustable can adjust it with the adjustment screw to reduce the amount of advance it gives you. you want the times set so with the can DISCONNECTED your total timing is at about 36º with timing all in between 2800-3000 rpm and than with the vac can reconnected the total timing with vacuum advance doesn't exceed 50º-52º.
seeing the operating temp pf your motor reduced by adding the vacuum advance and having it connected to full manifold vacuum source is one of the advantages of the system. Be glad it's there and you have it connected correctly and you are seeing the benefit.
now you just need to get it set correctly
Thread Starter
Instructor
Joined: Mar 2006
Posts: 246
Likes: 1
From: west melbourne florida
thank you i might get a diffrent vacume advance. dont care for the adjustable one.like to have the right part for the job. might put the one that came with my msd distributor back on as it was originaly hook up to the carb ported side. not to manifold vacum...save the wave
Race Director
Joined: Mar 2006
Posts: 14,112
Likes: 28
From: Florida
The operation of the adjustable cans is greatly misunderstood and that is probably why a lot of people don't like them. They are made by the same manufacturers as the fixed cans btw and the spring and diaphram work identically in both.
The adjustment screw only sets at what vacuum the rod pulls in at not the degrees of advance.
The lockout plate is what adjusts the amount of degrees of advance it will pull in.
Here are some detailed instructions and a pic of the lockout plate.
http://www.73-87.com/7387garage/drivetrain/hei.htm
The adjustment screw only sets at what vacuum the rod pulls in at not the degrees of advance.
The lockout plate is what adjusts the amount of degrees of advance it will pull in.
Here are some detailed instructions and a pic of the lockout plate.
http://www.73-87.com/7387garage/drivetrain/hei.htm
Le Mans Master
Joined: Oct 2005
Posts: 6,247
Likes: 471
From: Glen Allen, VA
I would get in touch with Lars, he is the authority here. I just did this last week on mine, I changed my springs on my HEI distributor's centrifugal advance to get 36 degrees by the recommended 2500-2800 rpm. I used the 2 gold springs in Mr Gasked kit #929. My car has never run better. It starts easier, has more vacumn and most importantly, has never pulled as hard at wide open. The only downside is my cam doesn't sound as aggressive as before.
