Power Brake Assist is weak

[kcmash]

Since putting the new engine in with the cam specs above I have had poor vacuum assist on my brakes.

Should I hunt for a vacuum leak, or am I just SOL with this cam? What happens if the booster goes bad? Do you get poor assist?

Thanks, kcmash

 

[71ProjectJunk]

Usually 220 degrees at .050 is about max for power brakes. You are a little over that, but you should be able to make it work. Have you checked how much idle vacuum you have? That is probably the first thing to do to see where you are at. Second thing is to recurve your distributor to get a lot more initial advance than stock. I would shoot for 20 degrees of distributor advance, and set my initial timing to 14-18 degrees, you can shoot for less advance at the distributor and more initial timing, but eventually it will be hard to for the starter to crank the engine with too much initial timing. The more initial timing you have the more vacuum you will pull. Also, set you vacuum advance to direct manifold vacuum instead of ported, that will also increase your timing at idle and cruise and give you a bit more vacuum. You need to try and get your vacuum to about 15" for the power brakes to work acceptably. After you recurve your distributor reset your idle mixture screws with a vacuum gauge to max vacuum. You can get a vacuum canister to help out the situation if everything else fails.

 

[Ron Tanzi]

Did you confirm what your intake vacuum reading is at idle with a gauge? That will tell you what is going on with your brake booster.

Ron

 

[Stanglover]

A buddy of mine has a 73 Torino that has a transplanted 429. With the cam that was installed, he had to add a vacuum booster pump (if that's what you call it). After that, no more issues with brakes. Might be worth looking at buying.

 

[Hemikiller]

My Howards roller cam is a fair bit larger than that, but has a 112° lobe separation. The 108 LSA is going to hurt your idle vacuum a bit. I'm getting 11" at idle and ~15" at cruise. I feel the assist I have is not that much different than it was with the stock 302 that was in the car, which produced 18" at idle.

First step is to check your idle vacuum, make sure you're getting the maximum possible. My initial timing is set at 16°BTDC, with 18° mechanical. I'm currently not running vacuum advance.

With the engine off, listen for air leaking into the booster. Next, make sure the booster check valve is good. Run the engine for a few seconds, blip the throttle a bit to build some vacuum, then shut it off and immediately pull the check valve out of the booster. You should hear the whooshing sound of the booster sucking air in. If not, replace the check valve.

Check the grommet and hose for cracks. Make sure you're using a vacuum rated hose, not fuel line.

Another check on the booster is to depress the pedal a couple times with the engine off to deplete any vacuum. Hold the pedal down, and start the engine. You should feel it drop a bit. If not, the booster has failed internally.

 

[kcmash]

I have not checked my vacuum lately. When breaking the engine in and trying to tune it, I had a lot of jump in the needle, so much that it was impossible to get a reading. Also the Ram Air flappers would clatter as they open and close.

I have re-torqued the intake a couple times, set the timing and tuned it so it runs well and idles well with no hesitation. The only thing wrong with engine operation is 1) power fluctuation while driving( Slight variations in power when cruising along. The tach stays solid) 2) I am unsure if the vacuum secondaries are opening up when I get on it. This occurs at all speeds when cruising, 25, 45, 55, 75, but when I accelerate, it accelerates smooth. The Brake Booster line is new, the booster is original, but it seemed to be more solid with last year's engine.

I'll try some suggestions. Right now the vacuum at idle is so jumpy I cannot use the vacuum gage to dial in the carb.

kcmash

 

[Stanglover]

Put a paper clip on the rod of the secondaries. IF the vacuum is working, the paper clip will have moved. Off hand, I don't remember if it's at the top or bottom, but I think this is in the Holley carb adjustment sheets I have. I can check if you like.

 

[vintageman]

I think the paper clip goes on the rod under the "cap" for the vacuum secondaries. Place the clip as high as you can on the rod and go for a ride. When you get back, check to see if the clip is lower, if it is, the secondaries have opened.
As far as the vacuum gauge reading jumping around, I have always found that to designate a vacuum leak somewhere. Sometimes it is even the hose to the gauge itself. Good luck, I am tuning mine up with a vacuum gauge as well and it can be challenging. My high idle cam keeps sticking and I will be removing it tomorrow night to do some light cleanup on the flash still on the plastic edges.

Tom

 

[Don C]

To me, a vacuum gauge needle that bounces that much indicates a valve(s) not closing or a very radical camshaft. The other possibilities include cam timing off and a bad vacuum leak. I would start with the vacuum leak.

 

[71ProjectJunk]

If the vacuum gauge is jumping all over the place I would check the valve adjustment. Usually when the needle jumps rapidly from a low point in the gauge to a high point it means that one or more valves are not sealing. The rocker arms could be adjusted too tight and not be letting one or more valve seals. You could do a quick compression check, you may find one or more cylinders that have little to no compression because the valves are not sealing.

 

[Hemikiller]

I have not checked my vacuum lately. When breaking the engine in and trying to tune it, I had a lot of jump in the needle, so much that it was impossible to get a reading. Also the Ram Air flappers would clatter as they open and close.

I have re-torqued the intake a couple times, set the timing and tuned it so it runs well and idles well with no hesitation. The only thing wrong with engine operation is 1) power fluctuation while driving( Slight variations in power when cruising along. The tach stays solid) 2) I am unsure if the vacuum secondaries are opening up when I get on it. This occurs at all speeds when cruising, 25, 45, 55, 75, but when I accelerate, it accelerates smooth. The Brake Booster line is new, the booster is original, but it seemed to be more solid with last year's engine.

I'll try some suggestions. Right now the vacuum at idle is so jumpy I cannot use the vacuum gage to dial in the carb.

kcmash

The jumping needle needs to be your first problem to solve. Like others suggested, I would run through the valves again to verify that they are properly adjusted. My needle wanders a tiny bit, maybe 1" at most. I plan on readjusting mine now that I have some miles on the engine. The hydraulic roller lifters I received from Cam Research were Howards, and they recommend a 1/2 turn from zero lash on a 7/16"-20 rocker stud.

The power "variation" can be a bunch of things. However, the first task should be addressing your vacuum problem.

 

[kcmash]

Ok, I checked the vacuum again today. The gage jumps rapidly between 0 and 15 when idling at 900. When I bring the idle up to 1300, the vacuum normalizes around 10.

I tried spraying starting fluid around the intake and found no change in idle. This has me thinking I need to go back to a compression test, then reset the valve lash on everything. I must be losing vacuum at the valves, right. And this would help with power fluctuation too.

Kcmash

 

[kcmash]

Well, my curse continues. Checked compression on all cylinders today. Everything was at 175 except #7 which reads zero. I have all the plugs out and there is no resistance at all when I have the compression tester on #7.

Maybe 1000 miles on the new engine. Farrea single piece valves. I reset the lash, all valves are moving.

Definitely my vacuum fluctuation.

Kcmash

 

[Hemikiller]

Well, my curse continues. Checked compression on all cylinders today. Everything was at 175 except #7 which reads zero. I have all the plugs out and there is no resistance at all when I have the compression tester on #7.

Maybe 1000 miles on the new engine. Farrea single piece valves. I reset the lash, all valves are moving.

Definitely my vacuum fluctuation.

Kcmash

Yeah, we just can't catch a break, can we?

Back off the rockers on #7 and see if you have compression turning it over by hand.

 

[kcmash]

So I backed off the rockers on #7 and bam I have 150 compression on #7. I am wondering what to do next.

Kcmash

 

[Hemikiller]

The rocker must be holding the valve open if you're getting compression. I'd re-adjust the preload on #7 and test again. Instead of spinning the pushrod to feel for zero lash, move it up and down. You'll feel the slack being reduced as you turn the polylock. I'd try the compression test at zero lash, and again at whatever amount of turns you're using on the others. If you lose compression at zero + X turns, then you may have a bad lifter and I'd contact Cam Research.

 

[kcmash]

Hemikiller, your input is greatly appreciated!

My pause at this point is why I am only getting 150 on this cylinder with the rockers backed off versus 175 on all other cylinders. I will try your suggestion tonight.

I must admit I am embarrassed by this deal. I feel like I am making a bunch of stupid mistakes.
Kcmash

 

[kcmash]

So I tried your suggestion tonight.

Vertical pull on the pushrods to set ploylocks. With no extra turns, cylinder 7 stayed at 150.

Cranked the poly locks a 1/4 turn, still at 150.

Cranked another 1/8 turn, still at 150.

So I am 3/8 of a turn past 0 lash on #7, all others are at 1/2 turn past Zero lash. Should I see if I can keep compression at 150 at 1/2 turn on #7, or button her up and see how she runs?

Kcmash

 

[Stanglover]

I must admit I am embarrassed by this deal. I feel like I am making a bunch of stupid mistakes.
Kcmash

No you're not making stupid mistakes, you're learning.
That is why my signature is "I learn something new every day".

 

[Hemikiller]

I'd try 1/2 turn. If it holds compression, button it up and try a test run.

What are you running for oil?