First some history. Car is a 1966 mustang that had a 289 2bbl engine in it when I got it. Engine would only produce 13" of vacuum resulting in little power brake assist.

Recently the engine was professionally rebuilt. It has a slightly larger cam measuring duration @ .050 of 204 intake and 214 exhaust. Rest of rebuild is pure stock. Engine checks out with compression of 140 to 150 PSI. Car now has a 4 bbl carb with original ford intake manifold. Carb is a used one that I put a kit in. Float level is correct and jets are stock. Venturi is a 1.08. Exhaust is new stock dual exhaust.

No matter how much I tweek the timing and idle speed I cannot get more then 15" of vacuum.

I have checked the balancer to see if the ring has slipped using a piston stop. It appears to be correct.

I have set the timing to 10 degress BTDC. Idle is set to 600 rpm is drive.

Ignition system is new except for the distributor. Bearing in it feels snug. Vacuum advance is disconnected and plugged.

Brake system is new including the booster. Disconnecting the booster has no effect on engine vacuum reading.

I checked the intake for leaks with a propane torch and found no leaks.

I have the transmission modulator disconnected to measure the vacuum.

What am I missing. I sure want to drive this car but with poor braking I really don't want to.

 

Is your vacuum gauge accurate? Have you tried driving it yet? Perhaps what is indicated by the gauge (wrong or right) is enough to give you the power braking you want.

 

I will check it with a second guage I have. I have driven it around the block and it requires more pedal effort than a car with manual brakes.

 

"No matter how much I tweek the timing and idle speed I cannot get more then 15" of vacuum."

and you probably never will...dusty roads buddy

 

So Gun Jam how did the factory power brakes ever work or is that booster different than after market ones?

 

I suspect your vac is at 15 because of the cam choice...Although I would think 15 should get your brakes to work..15 is not terribly low. I know little about brake boosters I decided that they were not for me long ago went manual setup. Again I would be surprised if 15" was considered insufficient for power brakes.

I pull about 13 to 15" at idle

 

high performance cams are gong to produce less vacuum although 15 sounds like plenty to me. hipo cam also like a lot more advance than stock. I'm running 20 deg of initial advance and can only generate 10 inches of vacuum at idle. Car runs great but I don't have power brakes.

 

13-15" of vacuum should give you some sort of assist with the booster. I suspect that the problem with the hard pedal is from the brakes themselves. One thing you can do is to hold your hand over the airhorn to restrict the air going in, or if you have a open filter HiPo air cleaner wrap it up to restrict air flow and you will be able to watch the vacuum come up. Then have a buddy press the brake pedal and see if it feels better. Of course this is a bit risky if the engine backfires and definatly no rings on your fingers But it will force the vacuum higher and it will let you know if you have a engine problem or a brake problem.

Again this is slightly risky and all on you if you choose to do it, but it will determine if its engine or brakes.

 

well i have manual brakes, so I can't add much info either than 15 is probably normal for such an engine ?! I haven't checked mine for ages, but I'd be surprised to see more than 13.

 

Take it for a drive. While coasting, drop it a gear lower and let the engine slow the car down. NOW hit the brakes. Better brakes? It's a vacuum problem. No difference? It's the booster. This procedure will produce the most vacuum for brake testing purposes regardless of cam choice and whatever vacuum level reached will remain in the booster until you hit the brakes as long as the check valve is working and there are no leaks in the booster.