Secondary Vacuum Diaphragm Issue
Thread Starter
Racer
Joined: Oct 2010
Posts: 259
Likes: 3
From: Plano TX
Secondary Vacuum Diaphragm Issue
Test question: What's wrong with this picture? :-)
As it relates to this one.
From the 1st set of dyno runs onward, there seemed to be a problem with A/F from the start of the pull through the midrange. The chart after what you see ends up at a 12.5 at 5800 rpm, and isn't my focus and so I've left it out.
Looking at it, both the dyno guy and my 'vette shop guy felt that there must be jetting issues, unless there was a leak in the vacuum system (which there was not). During a pull, I could stand on the right side of the car and see the secondaries opening all the way. A couple of jetting changes were made, and they simply seemed to move the lines as you see them up and down the scale. The red dashes mark a 12.5 A/F mix.
Notice how, at about 3800, the mix goes "lean" and then drops dramatically to "rich". It may be that all carb'd cars do this, but repeated pulls just didn't look right.
We also noticed that the Secondaries seemed to start opening much later than Holley's spring chart would indicate with a mod built 454 motor (about 350 rwhp), no matter which spring we tried.
So, after the last set of pulls and when I got the car home, I pulled the spring cap on the secondaries (it's a Street Avenger), and noticed the crease in the rubber that's hanging over the metal bushing (?); don't know exactly what to call that part. Lower right in the picture. Taking my finger and pushing up on the diaphragm rod, I could feel resistance initially which appeared to be related to the crease pushing back on the metal bushing.
I removed the entire secondary unit, grabbed my postal scale, pushed down on the scale with the end of the rod to see how much pressure was generated to push the diaphragm up. The maximum, which was a the start of the movement, varied between 4 and 5 ounces. Looking at it another way, before vacuum in the secondary system was even starting to overcome the spring pressure, it had to overcome 4 to 5 ounces of resistance caused by the crease in the diaphragm.
I put a new diaphragm in (petroleum jelly really does keep it "stuck" to the housing so you can get a clean install), and when I re-measured, the resistance averaged about 1 ounce.
Putting the thing back together, there was a noticeable impact on sub 3500 rpm performance under acceleration, and I'm looking forward to my next set of dyno runs. Unfortunately, everything I have to date regarding A/F is corrupted because of this problem, but now I think I should have some "clean" data to work on.
Everyone else may know this, but I thought I'd pass it along anyway. If you're using vacuum secondaries, check out your diaphragm.
As it relates to this one.
From the 1st set of dyno runs onward, there seemed to be a problem with A/F from the start of the pull through the midrange. The chart after what you see ends up at a 12.5 at 5800 rpm, and isn't my focus and so I've left it out.
Looking at it, both the dyno guy and my 'vette shop guy felt that there must be jetting issues, unless there was a leak in the vacuum system (which there was not). During a pull, I could stand on the right side of the car and see the secondaries opening all the way. A couple of jetting changes were made, and they simply seemed to move the lines as you see them up and down the scale. The red dashes mark a 12.5 A/F mix.
Notice how, at about 3800, the mix goes "lean" and then drops dramatically to "rich". It may be that all carb'd cars do this, but repeated pulls just didn't look right.
We also noticed that the Secondaries seemed to start opening much later than Holley's spring chart would indicate with a mod built 454 motor (about 350 rwhp), no matter which spring we tried.
So, after the last set of pulls and when I got the car home, I pulled the spring cap on the secondaries (it's a Street Avenger), and noticed the crease in the rubber that's hanging over the metal bushing (?); don't know exactly what to call that part. Lower right in the picture. Taking my finger and pushing up on the diaphragm rod, I could feel resistance initially which appeared to be related to the crease pushing back on the metal bushing.
I removed the entire secondary unit, grabbed my postal scale, pushed down on the scale with the end of the rod to see how much pressure was generated to push the diaphragm up. The maximum, which was a the start of the movement, varied between 4 and 5 ounces. Looking at it another way, before vacuum in the secondary system was even starting to overcome the spring pressure, it had to overcome 4 to 5 ounces of resistance caused by the crease in the diaphragm.
I put a new diaphragm in (petroleum jelly really does keep it "stuck" to the housing so you can get a clean install), and when I re-measured, the resistance averaged about 1 ounce.
Putting the thing back together, there was a noticeable impact on sub 3500 rpm performance under acceleration, and I'm looking forward to my next set of dyno runs. Unfortunately, everything I have to date regarding A/F is corrupted because of this problem, but now I think I should have some "clean" data to work on.
Everyone else may know this, but I thought I'd pass it along anyway. If you're using vacuum secondaries, check out your diaphragm.
Melting Slicks
Joined: Oct 2002
Posts: 2,317
Likes: 111
From: St Louis MO
Answer: Diaphragm needs replacement.
This is a great post on the importance of maintaining the rubber based parts. I'd like to add that one should also check their power valves every couple of years to ensure that the rubber diaphragm therein hasn't hardened and is working correctly. I tossed several PVs in my carb parts box that hadn't even been used which had simply gotten old and didn't make the numbers any more!
How to do this?... Use the 1" diameter suction disk which comes with a mini-vac set to pull a vacuum over the large end of the PV and watch for movement at the small end. When they get old, the vacuum required increases before the PV closes. This will make the car run rich at part throttle and will kill economy and responsiveness. (Sorry for the digression.)
This is a great post on the importance of maintaining the rubber based parts. I'd like to add that one should also check their power valves every couple of years to ensure that the rubber diaphragm therein hasn't hardened and is working correctly. I tossed several PVs in my carb parts box that hadn't even been used which had simply gotten old and didn't make the numbers any more!
How to do this?... Use the 1" diameter suction disk which comes with a mini-vac set to pull a vacuum over the large end of the PV and watch for movement at the small end. When they get old, the vacuum required increases before the PV closes. This will make the car run rich at part throttle and will kill economy and responsiveness. (Sorry for the digression.)
