[iamadonut]

Hey everyone,

This weekend I finally got around to replacing my leaking PCV valve to carb hose. But, now that I've replaced it the car nearly stalls everytime I brake. It idles at around 900-1000rpm in park, but when I shift to Drive it drops down to around 700. When I hit the brakes on Drive it dips down a bit more to about 500-600.

Do I just need to adjust the carb, or is this the sign of another problem? If it's the carb, what do I adjust? (I've never played around with my carb before. Yes, I'm a newbie.) I might have other vaccuum leaks, but I'm not exactly sure.

Any help is much appreciated!
Ian

 

[ChezDaJez]

A small (200rpm) drop shifting into drive is normal.

It sounds like you have a vacuum leak somewhere assuming you have power brakes. If it is a manifold vacuum leak, you can find it easy enough by spraying some soapy water around the edge. It will bubble where the leak is. If you have a vacuum gauge, connect it to your vacuum tree and get a reading. It should be steady and around 20hg if I remeber right. If it drifts, then you jave a leak somewhere.

If the car ran fine before you replaced the hose, then the carb should be ok. You probably knocked a vacuum line loose somewhere.

This is just a guess but hopefully it is a starting point. Also, if you don't have any repair manuals, it might a good idea to pick up either a Chilton's or Haynes Mustang repair book. The Chilton's has a better troubleshooting guide but the Haynes has wiring and vacuum diagrams. You can find them at your local auto parts store or through one of the Mustang parts catalogs.

Good Luck.

Steve

http://chezdajez.mypicgallery.com/stevesmach1/my-72-mach-1.jpg
My 72 Mach 1 Q Code

 

[sts_5085]

Sounds like you have power brakes, is this correct? If so, I bet you have your distributor hooked up to manifold vacuum instead of ported vacuum. If your distributor is hooked up to manifold vacuum, then it is using the engine vacuum at the same time the booster is. You could hook the distributor to ported vacuum and it should solve the problem. If you do this, you will probably have to adjust the idle screw on your carb up a little.

 

[jmagilto]

Could also be a defective brake booster. Isolate the booseter. (remove all other vacuum items leaving only the booster). Make sure vacuum level does not change when the brake is applied. continue diagnosis from there 1 part at a time

 

[iamadonut]

Hey guys,

I do have power brakes. (Damn you guys are good! ) I had a leaking MC awhile back and it might have caused my PB to fill with brake fluid. I'll have to go pick up a vacvuum gauge to check the booster.

Can you tell me more about how to hook the distributor up to ported vaccuum? I believe mine is hooked up directly to the manifold right now.
Also, I assume the vaccuum tree is the 4-sided hose adapter thingy in the rear of the engine bay, near the PB? (Told you I'm a newbie ). When I hook up the vaccuum gauge, do I just unplug one of those hoses and attach the gauge to the tree?

Looks like another trip to the car store and some tinkering tonight!

 

[dailydriver]

If your old hose was leaking, replacing it will cause a difference in how the air/fuel mixture screws need to be adjusted. There are two components to setting the idle: first, there are the mixture screws (already mentioned), then there is the idle adjustment screw on the linkage.

The idle adjustment screw is key to a good idle. If it's set so the engine idles too fast, it will render the air/fuel mixture screws useless..!!! Reason being, there is a little slot that feeds the engine at idle. If the idle screw is set to open the butterflies in the carb, it starts to feed the engine around the butterflies, hence the idle circuit is bypassed. So to get the best idle, first adjust the mixture screws, then slow the engine down with the idle screw, then repeat, and adjust for proper idle speed.

OK, regarding the ported vacuum source. Remember the blurb above about bypassing the idle circuit if the idle screw is adjusted to open the butterfies too much? Well, there are often more than one vacuum ports on the carb. One of them operates full vacuum under the butterflies all the time. The other one operates above the butterflies, and only shows vacuum when the butterflies are opened. This is the ported source that should be used for your vacuum advance. If the idle screw is opened too much, your VA will be seeing vacuum at an idle, and will automatically adjust the timing accordingly.

While it may sound complicated, it's really pretty simple. Just file away the information in your head that if the idle adj screw on the linkage opens the butterflies too much, you're screwed on getting a good idle with a vacuum advance that works properly. Good Luck!

 

[iamadonut]

Well, I'll give it a try tonight. Thanks for the detailed info...it's VERY helpful.

Is it at all strange that the car ran fine when there was a vacuum leak in the PCV hose but now it idles too low without the leak?

Also, if I do have a faulty PB, is there a temporary fix I can use until I get a new unit?

THANK YOU!

 

[dailydriver]

Is it at all strange that the car ran fine when there was a vacuum leak in the PCV hose but now it idles too low without the leak?

Not strange at all. The carb had the idle circuit set to compensate for the LARGE vacuum leak that resulted from the leaking hose. Once you fixed that, it's bound to run like crap.

I wouldn't worry about the PB booster until you address some of the more obvious problems. It may be a non-issue.

 

[rickster]

Generally ported vac port is found on the carb body and manifold vacuum at the carb base. Best way is get a vacuum gauge and check the port. With engine running do a quick accel by hand on the carb linkage, if the vac gauge drops off low, it's manifold vacuum. If the vac gauge increases, it's ported vacuum.