Corvette Forum : DigitalCorvettes.com Corvette Forums banner

1 - 8 of 8 Posts

·
Registered
Joined
·
41 Posts
Discussion Starter · #1 ·
Ok, a few questions that are really going to set me apart from the smart folks here.......

Can adjusting the carb fuel mixture cause the brakes to fail on a 75? I had a guy at work adjust the mixture and the idle and the next morning my brake peddle sank to the floor. Now they have no power assist at all. I decided to richen up the mixture, back to where it was and found that there is no screw in the back hole on the passenger side of the carb. There should be a front screw and a rear screw on the pass side right? I have been fixated on the vacuum power booster but I just refuse to believe that it just died the same day I messed with the carb. Also the power assist unit is holding a vacuum and when I have the car in gear and press the brake peddle the engine dies....... I'm stumped. Any ideas would be appreciated. Thanks.
Dave
 

·
Registered
Joined
·
2,948 Posts
The only way your engine operation would affect your power brakes would be if you lost engine vacuum almost completely.

Disconnect the vacuum line from the brake booster to the vacuum tree. Cap it, and run the engine. How does it run? Pressing on the brake will no longer kill the engine.

Then, connect a vacuum gauge to the line that would feed the booster, and check your vacuum at idle, and while blipping the throttle.

I would guess you'd have sufficient vacuum to get power brakes.

I suspect your booster is bad. It's ruptured and when you press the pedal down you're allowing a massive vacuum leak which might be what's killing the engine.

I know it seems odd that the carb tuning and brake failure all happened at the same time, but it does happen and it's probably just a wild coincidence.

Let us know your findings.
 

·
Registered
Joined
·
41 Posts
Discussion Starter · #3 ·
Ok that does make way more sense, as far as losing vacuum when pressing the brake peddle. I checked the check valve, vacuum line going to the booster, which is nice and strong and I also shut off the engine then pulled the hose out if the booster and got the rush of air you would expect. That made me think the booster was good. I'll be messing with it tomorrow and get my vacuum gauges out and see what's sucking what :) thanks for the reply.
 

·
Registered
Joined
·
41 Posts
Discussion Starter · #4 ·
Also, what about the screw possibly missing from the rear pass side of the carb? There should be two screws on the pass side right?
 

·
Registered
Joined
·
41 Posts
Discussion Starter · #5 ·
more info

This morning I got the vacuum gauges out and did some fine tuning on the carb and got it running perfect, car runs great now. With the gauge attached to the line that goes to the brake booster I was pulling 35cm/hg with the car in drive. Next I attached a "T" at the booster and put the gauge in-line with the booster, in effect monitoring the pressure inside the booster. With the motor on I was still pulling 35cm/hg, before when I hit the brake it killed the engine, now it runs fine and the pressure drops to 25cm/hg and holds and goes right back up to 35 when the pedal is released. So it looks like the booster is holding pressure and not causing a complete loss in pressure. shouldn't 25 cm/hg be enough to assist the brakes? Oh and the best part......I still have no power assist. Any thoughts?
 

·
Registered
Joined
·
2,948 Posts
What happens when you turn on the headlights? Get some vacuum readings on their operation as well.

Booster still sounds like the problem.
 

·
Registered
Joined
·
41 Posts
Discussion Starter · #7 ·
Crap!

Oh yea.... I didn't think to check the operation of other vacuum components... Oh well, doesn't matter now anyway.

Ya know the best way to see if it the booster is the problem? REPLACE IT! so, I threw all my cards on the table and took the damn thing off the vette and its now siting on my workbench. After reading all the horror stories on here about taking it off I was a little scared to take on the project but it only took me about 30 minutes to get it off!! Dash in place and with the seat in too. I'm a skinny guy so even with my bad back I was able to get in there and get it off of there. I used an extension and a knuckle and put it on my cordless drill after it broke them free and spun them right off lickity-split! going to order a new one and put it on and see if that fixes the problem.

Thanks for all your advise and postings on this topic, I have spent a lot of time on here just reading posts and got a pretty good idea of what I am doing now. Ill let you know how it goes when I get another booster.
 

·
Registered
Joined
·
2,948 Posts
You have very good vacuum it seems so you should get get results from a new booster.

I would also, when the new booster goes it, replace the check valve, filter, and any of the engine compartment vac lines.

I've seen engine vac lines that looked great on the outside but were cracked and leaking on the inside, where they slid over nipples.

Good luck, keep us posted.
 
1 - 8 of 8 Posts
Top