Big vacuum drop/lean spike when shifting into gear, cause?

MericaMopar

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I am having an issue with my '64 Newport, 361, 727 torqueflite auto. Everything is stock except for the 4 barrel Holley street warrior 600 and intake manifold. The car drives fine when it is in gear, idles fine 14AFR, has 21-22 inHg at idle (neutral), in steady state has 18inHg idle in gear.

The issue is whether the car is cold or hot, there is a big lean spike (16-18+AFR) and corresponding vacuum drop (down to 10 or less sometimes). When it is cold, the car is more likely to die and not recover. When it is hot, it is still there and pronounced almost all the time. Ideas for what could be causing this?


Things I have checked:
Timing is at 16 at idle (good strong idle and vacuum reading there, it seems happy)
Vac advance is disconnected for now
transfer slots are square on the carb
idle mixture is adjusted to be 14-14.5 in gear at idle (odd thing about the mixture screws, they need to be only a turn or slightly less out to attain the correct idle mix

Here is a video showing the issue. I change into gear once the cooling fan turns off. I go into and out of gear 3 times, first time it does it bad, second time not at all, and third time it does it but not as bad as the first time.



Thanks for your time.
https://youtu.be/pC94fHT5nAA
 

Davea Lux

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One of the things that happens when you shift into gear is that your foot will be on the brake pedal. What you describe is a classic symptom of a leaking power brake diaphragm. Try unhooking the line to the power brake unit and plug it to see if you still get the vacuum drop on shifting into gear. Be sure that the park brake is set and a wheel is chocked as you will not have power brakes running this test. You might be able to get the same response by pushing down on the brake pedal and not shifting in to gear although the lean out will be less pronounced due to a no load situation on the engine. If the engine misfires with the brake pedal depressed, the diaphragm is leaking.

Dave
 

MericaMopar

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One of the things that happens when you shift into gear is that your foot will be on the brake pedal. What you describe is a classic symptom of a leaking power brake diaphragm. Try unhooking the line to the power brake unit and plug it to see if you still get the vacuum drop on shifting into gear. Be sure that the park brake is set and a wheel is chocked as you will not have power brakes running this test. You might be able to get the same response by pushing down on the brake pedal and not shifting in to gear although the lean out will be less pronounced due to a no load situation on the engine. If the engine misfires with the brake pedal depressed, the diaphragm is leaking.

Dave
Thanks, I will try that. In the video my foot is pressed down on the brake the entire time. One thing I do notice is the engine makes a suttle noise change (bad description, but I can't really find a good way to describe it) when I let off the brake pedal in neutral. I don't notice anything once in gear and driving/stopping/starting when pessing/releasing the brake.
 

CBODY67

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Idle mixture afr should be checked/adjusted with the trans in (the equivalent of) "Park" or "Neutral" with the parking brake on firmly. NOT in gear. IF the vac level drops enough with the trans in gear, it can activate the power circuit in the carb, causing mixture enrichment.

When you put the trans into a gear, it loads the engine and will lower the intake manifold vacuum.

On one of my cars, I installed an emission-spec Holley 4160 with a two-stage power valve. With the idle mixture at "lean best idle", I applied the parking brake and put the shift lever into "D". The engine slowed appropriately and the manifold vac dropped to 10.5" Hg. The first stage of the two-stage power valve opens at 10" Hg.

My one experience with a failing power brake vacuum booster was than applying the foot brake would cause the engine to miss. Letting off of the foot brake would close the internal valves and the engine would not miss.

Even on a good engine and booster, sometimes if you continually and quickly pump the brakes in "N" or "P" at idle, the engine will start to miss, returning to normal as the manifold vac level gets back to normal. I've discovered than on a few cars I've owned.

Just my experiences,
CBODY67
 

MericaMopar

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One of the things that happens when you shift into gear is that your foot will be on the brake pedal. What you describe is a classic symptom of a leaking power brake diaphragm. Try unhooking the line to the power brake unit and plug it to see if you still get the vacuum drop on shifting into gear. Be sure that the park brake is set and a wheel is chocked as you will not have power brakes running this test. You might be able to get the same response by pushing down on the brake pedal and not shifting in to gear although the lean out will be less pronounced due to a no load situation on the engine. If the engine misfires with the brake pedal depressed, the diaphragm is leaking.

Dave
I think you may have been right. There wasn’t a gasket between the master cylinder and brake booster. I put a thin bit of gasket maker between them as my booster is supposed to be sealed there.

I also plugged the vacuum fitting going to the cabin’s vacuum system for the HVAC. So it could be that to.

But, after sealing the booster and plugging the HVAC port, the issue went away. Thanks
 
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