79' - possible vacuum leak

[jvmagic]

Hi,

My 79' Trans Am with an all original motor, carb, etc seems to have a vacuum leak.  Purchased from the original owner in 15' with 37k miles, I took the q-jet carb (original) to get rebuilt just to be on the safe side to a custom hot-rod shop that was recommended.  The car ran similar but I was satisfied as it turns over and starts nicely (choke still works) and it idles smooth.  The caveat is that it sure sounds like there's a vacuum leak somewhere near the carb.  I tried the standard spray carb cleaner technique but not much idle difference was notices if any. 

I also shipped the dizzy to get recurved (shop owner had a sun machine).  total timing at 34 at 2800rpm.  The static timing is set at 20-21 (oem calls for 18).  Port vacuum to the vacuum advance moved from manifold per the dizzy rebuilder.

The car starts smoothly and the choke kicks in nicely. I do notice a strong fuel smell the first few minutes upon starting the car (I assume it's normal cause of the high timing etc).

My 79' is a CA car and it has all the smog components and also passes CA smog when I turn the static timing down.  I'm still bothered by the vacuum leak sound (the fuel smell upon starting not so much; hope that is ok). 

My buddy has an identical 79' 403 CA Trans Am and I took a video of both cars running (his motor has the red water hose running across the motor).  You can hear the difference.

Any and all advice welcome. 

https://youtu.be/kHGoTJSzgl4

https://youtu.be/A8_UbQNwynY

[Burd]

I’m not going to comment on your vac leak other than on my 78 TA had a vac leak since new, No one could figure it out and no one ever checked the sec shaft, it needed to be bushed   It was sloppy from the factory, my mechanic wanted to try a new throttle body  I found one and it was the problem.  Trying to diagnose a vac leg on a forum is like asking  a cardiologist what’s wrong with your heart on a forum. 

[Kenth]

Due to todays fuels with 10% ethanol your engine will run LEAN.
This will NOT be cured by setting the ignition out of the scale.

You will have to enlarge the idle tubes and idle down restrictions some in the carbs idle/lowspeed circuit to feed the engine with proper amounts of combustible A/F mixture.
There is NO need to alter the factory ignition setting with the proper modifications in carb.

Your engine will NEVER reach its full potential igniting a too lean mixture with timing out of the scale, add to that the risk for detonation and ping that will destroy engine bearings.

FWIW

[old cars]

Well I could not believe the original static timing was 18 degrees btdc, so I had to look it up. That is correct. Do you not know a reputable mechanic to look for a vacuum leak ?

[jvmagic]

Well I could not believe the original static timing was 18 degrees btdc, so I had to look it up. That is correct. Do you not know a reputable mechanic to look for a vacuum leak ?

Yeah, I had a Trans Am with a 400 and the static timing was something like 6 degrees.  Quite different. 

I don't really have a mechanic in mind, maybe the shop I took my carb to https://santaclaraautorepair.net/.

I'm in the bay area in case you know of a resource.  Thanks

[Cliff Ruggles]

"Port vacuum to the vacuum advance moved from manifold per the dizzy rebuilder."

+2 on Kenth's comments, moving the vacuum source to the distributor was not a good move there.  That engine makes PLENTY of vacuum at idle with 18 degrees initial timing.  It doesn't need the timing clear off the scale at idle speed.  This requires closing down the throttle angle even more and as you found out the exhaust can get pretty stinky.

Did the shop that "rebuilt" the carburetor install bronze bushings in the throttle shaft?  They are too loose from the factory for my liking and not uncommon to see them sucking some air around the primary throttle shaft if nothing is done to tighten things up there.....

[jvmagic]

"Port vacuum to the vacuum advance moved from manifold per the dizzy rebuilder."

+2 on Kenth's comments, moving the vacuum source to the distributor was not a good move there.  That engine makes PLENTY of vacuum at idle with 18 degrees initial timing.  It doesn't need the timing clear off the scale at idle speed.  This requires closing down the throttle angle even more and as you found out the exhaust can get pretty stinky.

Did the shop that "rebuilt" the carburetor install bronze bushings in the throttle shaft?  They are too loose from the factory for my liking and not uncommon to see them sucking some air around the primary throttle shaft if nothing is done to tighten things up there.....

Thanks for your response, I appreciate it.

So I assume it’s best if I put it back in manifold vacuum?

If the shop did not install the bronze bushings in the throttle shaft, what can I do myself? Is it a simple process or should I take it to the shop? Is there a way to confirm that this may be the culprit?

[tayto]

grab the bellcrank and see if there is play. i usually crack the throttle open and then do this.

[jvmagic]

grab the bellcrank and see if there is play. i usually crack the throttle open and then do this.

Is the attached photo what you are referring to?  Can I do this with the engine cold?
If this bell crank is loose or has play how does this cause hissing/vacuum leak. Sorry for the ignorance.

[jvmagic]

Q-jet bell crank ?

[tayto]

yeap thats connected to primary shaft which generally needs bushings

[Cliff Ruggles]

A complete and correct "rebuild" would have included installing bronze bushings in the baseplate for the primary shaft.   

The secondary shaft doesn't need bushings as the huge throttle plates seal tight in the bores.

Some shops install thin brass sleeves and other may use Teflon sleeves.  I much prefer installing self lubricating bronze bushings, it is a permanent repair for that issue.

I sell a kit to do it yourself, or you can send the baseplate here to be rebuilt......