Vacuum leak

[Jeffrey S.]

Hello all!
I just read a thread dealing with converting an L71 to full time vacuum advance. I didn't want to hijack the thread so I started new one. Some time ago I converted my '69 Quadrajet to full time advance by drilling a small hole just below the ported slot and into the vacuum advance cavity. I did not fill the old slot. I have always felt that there was a vacuum leak but never really tracked it down. Now that the engine has been redone, I am getting it dialed in and still feel there is a leak. I have eliminated all the vacuum systems as the source by plugging everything up at the manifold vacuum fitting and I think it is from either the carb or the intake manifold. Could not plugging up the slot make the engine act like there is a vacuum leak? Thanks for your advise.
Jeff

[Duke W.]

Re: Vacuum leak

When the throttle is closed there is a direct leak path between the former ported opening above the throttle valves and the new opening below the throttle valves, and the pressure differential is equal to manifold vacuum.

Since these passages are very small it may or may not affect driveability, but nonetheless, it's not correct.

Whenever converting from ported to full time the ported passage must be plugged or there will be a leak path.

Duke

[Stuart F.]

Re: Vacuum leak

I did the same modification to a Quadrajet back in 1972 with my Nova SS, but I did plug the port above the butterfly with Epoxy. We had EPA inspections in Illinois already back then and they would look for any physical modifications. I used a manual switch to control the vacuum switch (I believe that was called a TVS). It would open 7 seconds after the transmission shifted into high gear to allow full vacuum advance (something like that). I had also changed the weights and springs in the distributor and used performance metering rods in the carb, and it still passed their test with flying colors!

[Jeffrey S.]

Re: Vacuum leak

Duke,
In all of our communications I never thought to report that this slot was not sealed. Until now, I didn't think that it mattered. Thanks for the quick response. I will seal it up on Wed. and let you know how it works.
Jeff

[Jeffrey S.]

Re: Vacuum leak

Here's an update. Removed the carb this morning to fill the slot and was "disappointed" to find that the slot was filled nicely. I say disappointed because that meant that the leak was someplace else. I did find that all the screws underneath the throttle plate and most of the screws on the air horn were not tight. I tightened them and used washers on the front carb to manifold bolts so I could get a good tighten on them. Secured everything and, so far, it appears that the symptoms have disappeared. I was able to slow the engine down to near stall by putting my hand over the primaries. and could slow the idle speed to 550 in gear with smooth idle. Thanks for your help.
Jeff

[Patrick H.]

Re: Vacuum leak

I ...used washers on the front carb to manifold bolts so I could get a good tighten on them.
Jeff

Be careful not to overtighten. There is a torque spec for these bolts; check the AIM as I don't have the value in my head. Overtightening will bend the air horn of the carb and cause plenty of problems as well.

[Duke W.]

Re: Vacuum leak

The installed cam should allow the engine to idle smoothly and stabley in Drive at 450. Can you try that? I'd also like to know idle vacuum, in Drive, at 450, 500, 550, 600.

I've been trying for years to get manifold vacuum data idling in Drive on base engines with automatics, but no one ever responds, or they come back with idle vacuum in neutral, which is irrelevant because idling in neutral is not the normal idle condition for an automatic.

As always specify both vacuum and idle RPM since vacuum varies with idle speed changes.

Base engines with manual transmissions and full time vacuum advance, which yields total idle advance in the low to mid 20s pull about 18-19" Hg at 500. Emission controlled base engines with ported vacuum advance is usually a little less. Idling in neutral is the normal idle condition with a manual transmission.

Duke

[Jeffrey S.]

Re: Vacuum leak

Patrick- The torque value is 10-15 ft lbs. I torqued the bolts to this value. This is not all that tight but it made a difference. My feeling is that this is where the leak was because when I took them out they were little more than hand tight- my bad!
Duke- I will get these values next week. This week is Dream Cruise and we are busy driving Woodward every evening. I will get these numbers to you as soon as I get them.
Jeff

[Leif A.]

Re: Vacuum leak

Patrick- The torque value is 10-15 ft lbs. I torqued the bolts to this value. This is not all that tight but it made a difference. My feeling is that this is where the leak was because when I took them out they were little more than hand tight- my bad!
Duke- I will get these values next week. This week is Dream Cruise and we are busy driving Woodward every evening. I will get these numbers to you as soon as I get them.
Jeff

Believe it or not, Jeff, hand tightened carb base nuts is not as uncommon as you might think. Many folks have been traumitized into over tightening those bolts in fear of warping (legitimately) the base plate of the carb. 10-15 lb/ft is correct and barely tighter than finger tight. Check them occasionally for tightness. Glad to hear you found your vacuum leak.

[Duke W.]

Re: Vacuum leak

http://www.discounttiredirect.com/direct/infoTireMath.doDuke- I will get these values next week. This week is Dream Cruise and we are busy driving Woodward every evening. I will get these numbers to you as soon as I get them.
Jeff

Oh, good. Here's another test you can do during the Dream Cruise. With you foot on the brake and throttle, simultaneously let off the brake and floor the throttle. It should light up the tires.

Try the same by keeping the brake on hard and flooring the throttle. Within a second it'll be at converter stall speed. Then release the brake. Now that will light up the tires big time!

Either way - not bad for a "base engine" with TH400 and a 3.08 axle.

Duke

[Jeffrey S.]

Re: Vacuum leak

Oh, good. Here's another test you can do during the Dream Cruise. With you foot on the brake and throttle, simultaneously let off the brake and floor the throttle. It should light up the tires.

Try the same by keeping the brake on hard and flooring the throttle. Within a second it'll be at converter stall speed. Then release the brake. Now that will light up the tires big time!

Either way - not bad for a "base engine" with TH400 and a 3.08 axle.

Duke

If I did as you suggest, one of two things would happen: either I would run into one of the other 50,000 cruisers driving at 3 MPH or I would win an all expenses paid vacation in lovely downtown Royal Oak which would include room and board and 3 meals a day courtesy of the Royal Oak police. Other than that, great idea!!
Jeff