I have a 350 SBC Just rebuilt a year ago. I installed new dart iorn eagle heads with 202/160 valves 67cc chambers. A comp thumper cam with 1.6 rockers, 510 Intake lift and 490 Ex lift. 227 intake duration and 241 ex Elderbrock RPM air gap intake and a MSD street fire dist and wires. I added a holley 750 vac secondary carb ( 80508s ). The car off idle runs very strong with lots of power. It will idle in park at 1000 RPM but when you put it in drive it stumbles and cuts off. I have a 2000 stall converter also. There are no vacum leaks, it holds a steady 14-15 at idle in park. The plugs are sooty black but the tips are firing good, Any help will be appericated......Thanks Steve81Z

What is your initial timing set at?.....Remember, those sooty black spark plugs could be causing a lack of a good ground resulting in a weak spark @ idle!....Clean all the carbon and oil off the threads of the plugs and the cylinder heads.

Sooty black... u have acam designed for higher rpm , so at low rpms it doesnr burn fuel well....up the heat range of the plugs, and if the cnr plugs are real bad up them 2 steps if one is not enough.
Is the vac advance all in about 1 " below your idle vaccuum, and is it connected to ported or manifold vaccuum.

check your modulator valve on the tranny for vacum leaks
the diaphram will go bad and also check the rubber vacum hose for leaks

I disconected the modulator line and pluged it and it made no diff. Im using ACR44ts wich was recomended for the dart heads. It ran fine with the stock heads, I guess with the compression increase and the larger valves on the dart heads it made it totally different. would it be wise to open the secondary plates a little?

I was also wondering if the fuel pump would make a difference. Im only using a stock pump

Sooty black is running rich. What is the initial timing? I suggest around 18*-19* initial, especially with that cam. When you adjust the mixture screws, do they have any effect on the idle? If not, then you are not idling on the idle circuit and are actually into the main circuit. That is not uncommon with a larger cam, but I would still think that you should be able to get it onto the idle circuit. Try getting to idle under 1000rpm. Say around 850-900 in park. That way you won't be pulling fuel from the main due to the increased air velocity from the butterflies being open too far.

I do not think the pump has anything to do with this. I could see if you were having a problem at high RPM, but your issue is with idle and the engine doesn't require that much fuel to idle. Once you get the idle fixed, you may find that the pump won't be able to keep up at WOT, but even then you will want something that supplies volume, not so much pressure. Carbs respond to volume more than pressure. What is the compression ratio on the engine? Are you running the right octane? I don't think you need to open the secondaries just yet.

Yes the mixture screws do have a effect on the idle and the compression ratio is 9.4 to 1 and im running 93 octane. Would temp have this effect, it was ok till it got colder...... Thank you all so much

How long after you put it in drive does it begin to stumble and shut off? Does it take it about 25-30 seconds? If so, then it's loading up with fuel and that is usually and pressure side issue(needle and seat leaking or float issue). Is the carb brand new? If it shuts off right away, I would start looking for a vacuum leak somewhere. Intake or maybe valves adjusted incorrectly.

Also, IMO, running 93 octane is just throwing your money away. A 9.4:1 engine will be more than happy with 89. I run 9:1 and run 89 and it's perfectly happy. In fact, I tried 93 and there was absolutely zero change in performance whatsoever.

After reading symptoms given,
1. What is vacuum in Drive?
If it is 14-15 in. in Park, and place a oad onto it, vacuum will drop down to 12 in., enough for a stock(?) carb having the power valve open up and dump more fuel. Generally, power valve number is half of idle vacuum.
2. Sooty plugs, more up a step in heat range. Apparently, the new heads cool better than stock.
3. Bump up initial timing, again, use vacuum gauge and tune for max, then back up timing to allow the engine to start normally when hot, no kick back against the starter motor - definitely will break the starter nose.

The carb is new and it does take about 10 or 15 seconds to die out. I changed the power valve to a 2.5

What type of plug would be best?

You're running 44's, I'd go 45's or even to 46's. Can't hurt.

Another troubleshooting aid, on the pass side base plate of carb, there is a setscrew to hold the secondaries open. Slide a 0.015"- 0.020" feeler gauge between the setscrew and linkage, basically opening the secondaries, to see/hear it run better?

If it runs better, then you might have to open up the secondaries just a hair to allow for the additional air needed. Then readjust curb idle and mixture using vacuum gauge.

Ill give it a try thanks so much......Steve

I opened the secondaries like you said and it runs great and will idle in drive now but the idle mixture screws make no difference when turned in or out. Is there a happy medium to get the proper air flow......... Steve

Close the primaries to allow the idle circuit to work.

As you have seen, the setscrew is from the bottom. You can screw it out and screw it in from the top for better access. I don't think you want to remove the carb to make a minor adjustment.
Experimentation is the key, trial & error.

Adjust everything for best idle characteristics. Any camshaft not stock isn't made to idle smoothly, unless there is a wide LSA, lobe separation angle between intake and exhaust.

Thank you very much, you have been very helpful. Should i still go with a hotter plug?

You never did tell us what your initial timing is set at. With that cam, you are going to need an initial of around 18*-19*. Then limit your total to whatever gives you the best WOT performance. Probably around 34*-36*. Check your timing and set it to the specs given. Anything other than stock cams need more initial timing. After bumping up the initial, you are going to have to reset your idle and mixture screws. Like Everett said, using a vacuum gauge, adjust for highest idle and vacuum reading. I don't know about 46's. If for some reason the engine runs lean, the 46 could potentially do some damage. I would go with the 45's.

Thank you, Im going to set the timing to the specs you gave me this weekend. Ill use the 45's also

I had a similar prob and fixed it with a 1" spacer and resetting the float levels. no smoke and Idles better although it has to be higher due to the cam..If I go below 1k rpm it will die

Which cam are you using?

Steve, I have asked at least 3 times now. What is your timing? Are you avoiding because you don't know? Nothing wrong with that if you don't, but we need all the info we can get to help you. Timing is the place to start.

Sorry, But a buddie was suspose to bring a timing light over but as usual he couldnt make it. The timing i do by ear for the highest RPM and no drag on the starter or pinging, so i really cant say what its set to. I opended the secondaries ang closed the primaries and for some reason i got a real good balance, all the plugs are burning a tan to light brown color and it will idle in drive at 650 and in park about 900-950. The plugs i changed to the R45TS and adjusted the float levels which were to high. So i guess all is good for now, and i want to thank everyone for all the exelent advice giving......Steve

Cool !!

I really hate to ask this but now its running right ,another issue is low vaccum and the power brakes, I isnsalled the vac. can and it helps but not enough. What else can i do to improve the stopping power?....... Thanks again

Original Z/28's created 9"-10" of vacuum and were equipped with power brakes and worked fine. Anything amount less and a reservior should be added or hydro-assist added - works with power steering oil pressure. Diaphram have a leak?

I'd add a vacuum gauge to inside the car and view it while traveling. Idle and stoplights should develop low vacuum. Light cruise, 15"-17". Decell, close to 18"-20".

You also might check to make sure the rod from the pedal is in the correct hole for a power equipped car, first hole from the pivot.

Im not really sure if there is a leak in the diaphram, Is there a way to check before replacing it ?

Either a hand vacuum gun or a longer hose so you can suck on it yourself, or your assistant.

Thank you for all your assistance..........

Firther to Everett comments
I have 8/10" in drive..it doesnt effect my normal driving, when one come up to a stop sign, the engine brake is working and hi vac.
When it does effect, is If im stopped, let the car move forward slighty, taking my foot off the brake, then do it again, then it looses the vaccuum...NOT the braking abilty...it just doesnt have full power assit...the pedel is higher and harder...no biggy.

I also have had an in car vac gauge for 20 yrs.

And further to Everetts , adjusting sec butterflys...close to near fully closed, adjust idle and mixture screws....check idle is withing fact specs 1 1/2 /2 turns down on prinarys, if not open the sec stop a smiggen, untill it is, and the mixture screws should now also be in factory specs.

yea thats what it does,and if you sit still whit your foot on the brake you can feel the pedal moving up and then it gets hard to push. With the vac. can it gives you another push also.

OK now think it thru...
are u moving when this happens? NO
And if u are moving what speed are u dong...less than walking? yes doesnt matter
How often u u actually have to pump the brakes? and when? no where where it matters
And when u slow down, do u take your foot off the brake just before stopping then put it back on? NO
Do the brakes stop working? NO u have the same brakes as if u where being towed without the engine running

Good point....Thanks

I know this seems a biy of harp on BUT
after 20 yrs of driving like this, there is only 1 place where one COULD BE caught out
So WARNING and BE AWARE:

At an intersection, the car infront starts to creep, u take you foot off the brake and creep forward also, then they go to take off, you just start to follow out into the gap, then they suddenly change there mind while u are still looking at the on coming traffic, then suddenly have to hit the brakes...and they need a far heavier foot as your engine vac hasnt yet come up.....

I drive daily, in 25 yrs this has happened maybe 4 or 5 times with no vac...I have never hit or realy got close..because I by habit keep a reasonable gap regardless
BUT someone who is not of this habit ( tail gater) is very likely to tap the car in front and derver the greif for being of a tail gater mentality.

I try to keep a good distance between me and everyone else out there. Someone pulled out in front of me at the gas pump one day and the brake was like pushing a rock to the floor from low vac at ideling through the parking lot, Thats why i was wondering if there is a way to improve the vac. to the booster ( vac. pump)

Yeah..before dropping into gear, give the engine a small rev, just to bring vac up for a second, keeping the foot on the brake if u wish...Then put into gear to move off
Even thu the engine Vac drops, the booster will hold the vac for quite a while, for a couple pedel pumps
If it doesnt, u have a vac leak in the booster...not good.

Run a temporary vacuum gauge inside. You will see at closed throttle, decell, max vacuum, brakes should work good.

As less vacuum is available, more pedal pressure is needed, i.e., sitting at a stop light idling with brake applied. Engine off, vacuum should be available for one stroke.

I did have one vehicle with power brakes, '87 S-15 Jimmy. Brakes woulod squeal the tires, put you through the windshield. Then, overnight, there would be one time, any time, you hit the pedal and it was like pedal hitting a brick wall - no stopping. I replaced the booster and master cylinder separate times and to no avail. I traded the vehicle in - removed my liability.

Hopefully, you don't have this problem. Run a vacuum gauge inside and read the vacuum at different times.

Do what I did and ditch the vacuum brakes alltogether and go with hydroboost. A hydroboost system is far superior to ANY vacuum braking system. The stopping ability far out performs ANYTHING a vacuum system could ever dream of. I will NEVER drive another car without a hydroboost system again.

And where would i find the hydroboost system ?

How long should the booster hold vaccum after you shut the motor down?

Give Mark a call at Savitske. He is who I got my system from and he is a great guy to talk with, but be careful. ONLY call him if you want to spend some time talking about what you want. HE IS NOT THE KIND OF GUY TO SELL AND RUN!!! He loves to shoot the s&^t. I spent an hour at least with him when I purchased my system. Great guy and knows his s&^t. Here is his number: 610-346-8154

If you do consider a hydroboost system, I highly suggest getting a P.S. pump that is designed to run a hydroboost system. They operate at a higher output pressure and also have a second return port on the resevior. That is not 100% necessary, but the less restrictions to flow you have the better the system will operate.

Thanks Ill give him a call. I know there expensive.$$$$

Is there a way to convert it over to manual brakes?, or would that not be a smart thing to do

Is there a way to convert it over to manual brakes?Kindda the wrong direction, make a U-turn.

Kinda figured that, but i needed to hear it from you. Thanks

I'm toying with the idea of ditching my hydroboost and switching to manual brakes...

You dont like the hydroboost system?

too much boost for my tastes...

There is no better braking system available today than the hydroboost. HANDS DOWN!!! I WILL NEVER DRIVE ANOTHER CAR WITHOUT HYDROBOOST.

I WILL NEVER DRIVE ANOTHER CAR WITHOUT HYDROBOOST.



Yeah you will, when you drive through a water puddle and your belt slips:)(if you are talking about hydraboost like diesels have that are belt driven).

just kidding with ya, Charlie:)

Kinda funky the first time you hit the brakes hard and you feel it in the steering wheel.

Where is the best place for the vacuum to the brake booster, From thr back of the carb or from the manifold?

If the manifold is drilled and tapped underneath the carb at the rear of the plenum, this is the OE location.

Don't "t" the PVC hose because of the valve is a calibrated leak and not enough volume of vacuum to actuate the booster. Some carbs, Edel's, are tapped for the vacuum port for brakes.

Yeah you will, when you drive through a water puddle and your belt slips:)(if you are talking about hydraboost like diesels have that are belt driven).

just kidding with ya, Charlie:)

Kinda funky the first time you hit the brakes hard and you feel it in the steering wheel.

I drive my car in ALL weather and I have never had the brakes or the steering go out. I am not sure about diesels, but I have never had a problem.

I just wanted to thank everyone for all the help