# The mystery vacuum leak



## Boomstick (Sep 13, 2021)

So back in December I had the 350 in my 69 "legoat" pulled and rear main replaced and headers installed. They did not do any exhaust, just open headers. On the way home from picking it up, it seemed like it had no power, wouldn't shift right and ran like crap. I got home started poking around and found they forgot to tighten the distributor bolt. No biggie, I adjusted the timing. It ran ok, seemed like it was choppy at idle but I figured it was just the headers. Strangely though, it only shifted above 3000rpm and shifted hard like it had a shift kit in it. Also it seemed like it had no top end power and after posting about it here and other places everyone seemed to agree the 600 summit carb was no longer enough carb since the engine can breathe better with the headers. I bought a 750 summit carb, got it on and really nothing changed. Still had good bottom end but no nuts at the top and idled rough and shifted weird. I screwed with the timing and nothing helped. Well yesterday I decided I was going to adjust the carb and timing using a vacuum gauge and timing light instead of "by feel" like I had been. Well I get it tuned to 20 inches of vacuum and it purrs like a kitten. I disconnect the vacuum gauge and hook the trans vacuum back up, start the car to take for a test drive and it runs like crap again. So I shut it off, disconnect the trans line and hook the vacuum gauge back up, start it and it runs great. I disconnect the vac gauge again and same thing, runs like crap. So I pinch the trans line while it's running and it smooths out. Let go, choppy idle again. I go looking around and the rubber hose from the metal trans line to the modulator is barely on the metal line from the engine.

I always thought it was a vacuum leak and checked every hose under the hood more than once...Dang I hate vacuum leaks you can't find...

Car runs great now, has much more power than it had before!


----------



## armyadarkness (Dec 7, 2020)

This is why persistence and determination are the two most important resto/ repair skills.


----------



## pontrc (Mar 18, 2020)

armyadarkness said:


> This is why persistence and determination are the two most important resto/ repair skill.


Amen to that


----------



## AlaGreyGoat (Jul 6, 2006)

The best way to find a vacuum leak around the carb is with starting fluid.
While idling, spray a small amount around the intake area. When you get near the leak, the idle rpm will go up.
Only spray in short bursts and have a little time between them. You don't want to create a fire hazard.

Larry


----------



## Limelight (Aug 19, 2017)

Ha, thanks for the tip. My old SD TA car has one and havent been able to locate. Will try this trick to trouble shoot.


----------



## Boomstick (Sep 13, 2021)

I use an unlit propane torch. Does the same thing with almost no fire hazard...


----------



## Boomstick (Sep 13, 2021)

Limelight said:


> Ha, thanks for the tip. My old SD TA car has one and havent been able to locate. Will try this trick to trouble shoot.


Check the trans modulator line, that's where mine was...


----------



## Limelight (Aug 19, 2017)

Will do, thanks for the follow-up.


----------



## geeteeohguy (Feb 2, 2008)

Late, hard shifts and rough running is classic modulator hose/line failure. Have seen many over the years. Also blown modulator diaphragms that let the engine suck ATF so the car looks like a crop-duster. Back in the '80's a co-worker picked up a cherry '73 Monte Carlo for $75 because it 'needed an engine'. He installed a new modulator valve for $10 and had a cherry car that no longer burned oil.


----------

