Power assist gone when stopping...blew through a stop sign yesterday!

[utahcarguy]

So I am shaking out all the problems with my new '70 RR. It has GREAT disk brakes on the front when the RPM's are high. Stops better than my new truck! However, when the RPM's are at 1,000 or less (idle), I have NO power breaks and standing on the pedal helps, but does not safely stop the car. I'm affraid of running into someone or something at a stop sign if I don;t figure this out!

The power assist works on vacuum, right? That's why I don;t have power with low RPM's? How would I fix something like this?

 

[quikbird]

separate vacuum pump. this is a fairly common problem

 

[Big John]

Check the hoses etc. first, but most likely you have a bad booster. There are shops that rebuild them.

Is there some reason why you don't have good manifold vacuum? Hot cam or the like? That's the only reason why you would need a vacuum pump.

 

[utahcarguy]

I should also mention that the car came from sea level to 7,000 feet and I am running a bit rich and having idle issues. Is it possible that if I re-jet, this issue could go away? I have not checked vacuum (where is the best place to check this?) but I planned on following a step-by-step guide for tuning the 6 pack this weekend.

 

[Big John]

I've never had to deal with high altitude, but I'll share what I know.

As I understand it, the rule of thumb is manifold vacuum will drop about 1 inHg (inches of mercury) for every 1000' of altitude. In other words, if you have 22 inHg manifold vacuum at sea level, you are going to 15 inHg at 7000'.

It may be possible that you don't have enough vacuum for the brakes because of the altitude, but I've driven some really bad running cars that still had power assist.

I don't know how bad your car is running so I can't even guess what changing the jetting will do. BTW, you will only be able to re-jet the center carb as the end carbs have metering plates. I used to run a #65 jet in the center carb my 6 barrel setup.

Try this... Let your car idle for a minute and see if you have power assist for the brakes. Now, let it idle again and without touching the brake pedal, turn the key off. Now try the brake pedal. Does it go hard right away or do you still have assist for at least one press of the pedal.

If the pedal gets hard right away, it's a bad booster.

Keep in mind, if you have a bad brake booster, you will also have a big vacuum leak!

One other thing... and I'm a stickler for calling things by the correct name. There is no Plymouth "6 pack". That's a Dodge term. Correct term is "6 barrel" or 440 6 bbl.

 

[droptop]

This has all the classic symptoms of to much overlap in the camshaft, especially after you stating "having idle issues". Do you know the spec on the cam? I would not think that the brake booster is bad due to working fine at higher rpm.

 

[69hemibeep]

I once read an article by an A12 , 6pak, 6bbl guru that said it was real easy to over cam a 6bbl car. This might also explain your eyes bleeding in the fumes and low vacuum

 

[Big John]

This has all the classic symptoms of to much overlap in the camshaft, especially after you stating "having idle issues". Do you know the spec on the cam? I would not think that the brake booster is bad due to working fine at higher rpm.

I think you are right. After hearing that he has a bad idle and thinking a little more, it's very possible this is what's going on. I think his altitude is a factor too.

 

[Ranger]

I should also mention that the car came from sea level to 7,000 feet and I am running a bit rich and having idle issues. Is it possible that if I re-jet, this issue could go away?

I drove a '69 Torino cross country and never had a brake or idle problem crossing the mountains.

This has all the classic symptoms of to much overlap in the camshaft, especially after you stating "having idle issues". Do you know the spec on the cam? I would not think that the brake booster is bad due to working fine at higher rpm.

I agree with the cam concept. Especially based on the vacuum I had before I pulled the radical cam that was in the car when I bout it and went to a milder one.

 

[utahcarguy]

So here's the thing, I test drove the car in New Jersey where I bought it. No brake boost issues at all. The owner has been straight with me on all issues with the car, even mentioning to me that it pops out of 2nd gear sometimes (which it does). He claims to have not had any brake boost issues while owning it for 3+ years. I'm wondering if I re-jet and look at initial timing, if that could help.

Also, he mentioned that he has upgraded the metering plates to something that allows for adjustment at a 45* angle. I've yet to dive into that yet, but he made it sound like I can tune all 3 carbs now.

 

[utahcarguy]

More helpful info (maybe)....The car seems to want to die at idle of 900. At the lights I would hold the gas to 1000-1100 RPM so I didn't run the risk on her dying at the light.

 

[utahcarguy]

Previous owner just told me it has a "mild cam Mopar 509 lift" with a new brake booster from Ramman installed with a new master cylinder, just a couple years old. He seems to think it is all in the carb setup. Seeing that elevation is the only thing that has changed, maybe he is right?

 

[Big John]

Yea, there are plates that you can screw jets into. I've never used them, but they have to be easier. Still have to take at least two carbs off.

 

[69hemibeep]

Mopar 509 is not all that small especially in a 383. The 108 centerline version have very little vacuum at idle but then every engine is different. Id plug a vacuum gauge into it and see what you have.

 

[utahcarguy]

Mopar 509 is not all that small especially in a 383. The 108 centerline version have very little vacuum at idle but then every engine is different. Id plug a vacuum gauge into it and see what you have.

It is a 440 motor. I will get a vacuum gauge on it and report back. What should vacuum be at 900 rpm with a 440 6 barrel?

Thinking of plumbing a T fitting on booster hose. Best place to get vacuum reading?

 

[Ranger]

I'm wondering if I re-jet and look at initial timing, if that could help.

No. Jetting will only affect fuel/air ratio.

More helpful info (maybe)....The car seems to want to die at idle of 900. At the lights I would hold the gas to 1000-1100 RPM so I didn't run the risk on her dying at the light.

That sounds like a pretty wild cam that won't idle at 900. Even my previous one would and I believe it had a 108 centerline.

Thinking of plumbing a T fitting on booster hose. Best place to get vacuum reading?

OF the left (drivers) side vacuum port at the base of the carb or where the booster is plugged in.

 

[droptop]

Change the cam. I recommend the stock 6 bbl cam (P4529270). Good vacuum at idle for power brakes, acceptable idle, and good low end torque and mid range. These are all necessities for street driving IMHO.

 

[69hemibeep]

Change the cam. I recommend the stock 6 bbl cam (P4529270). Good vacuum at idle for power brakes, acceptable idle, and good low end torque and mid range. These are all necessities for street driving IMHO.

Call the previous owner and see if your cam is the 108 or 114 centerline cam big difference. I would agree with the cam change on a 108, with a 114 some sort of vacuum pump to assist if you keep the cam but still check all your brake components first and put a gauge on the non ported carb fitting it will be the bottom one on the center carb.

 

[mac]

put a vacuum gauge on it for sure and what are your timing settings?

 

[utahcarguy]

OK, reporting back....I was getting about 3" of vacuum at idle. After messing with the idle and air/fuel screws, I am now getting about 5.5" of vacuum at 950 RPM. What else should I adjust to get the vacuum higher?

I checked the fuel floats withe the clear sight glass (which subsequently broke after about 60 minutes but I was able to stop the plastic form falling inside (I THINK!)) and those are set correctly. I'm not sure what the timing is but the car starts easily and has no detonation or backfire issues. I'd like to jet down a bit so I'm not running so rich, but I haven't gotten there yet.