Hey there vettesters. Finally got my motor mods all worked out and she's running like the beast she is. Please help with a few vacuum questions.
I have a 218-226* sub .500 lift cam. EPS intake. 750 Holly vac secondaries
Put the motor on a vacuum gage, and I'm pulling 14 on the manifold, 13 on the carb @ 900rpm idel..700rpm in gear. Pwr brakes hardly work, lights barely pop up. My cams not "that" radical......
#1. Do I need a vaccum booster? No intake leaks cause car runs/idels well
#2 Vac secondaries don't open... why?? Low vac the cause??
Any and all input welcome.... :cheers: :cheers:
Thanks
Eddie

 

You can tune your carb's idle mixture screws to get best vacuum. In Corvettefaq check out Lars' carb tuning guides. The vacumm you've got should be enough to run everything, but I would expect at least 18 in. vacuum with those cam specs. You might also be chasing a vacuum leak somewhere too. http://www.bokonon.net/corvettefaq/
To check for leak just pull the hose that supplies the accessories and take vacuum reading off intake again. If it improves you know to look "downstream" of the motor. Make sure your timing is set correctly then your idlespeed/mixture.

Good luck
Scott


[Modified by Scott78, 4:16 PM 7/28/2002]

 

A tip is to connect a "check valve" to the vaccum system as close to carb possible..Connect a gage after the valve,between the check valve and vac system..that will give you a clue if the system isnt airtight..You could also go for a test ride and engine brake from a little higher speed,that will pull much vacuum.. :cheers:

 

Thanks for you replies... any input on why secondaries aren't opening? Any trick out there to see for sure if they are or aren't?? Thanks Guys
Eddie

 

Are you sure they're not opening? To be sure you can put a paper clip on the secondary diaphram rod and push it up against the housing....go out for a "test" drive and check the paper clip to see if it's lower on the rod...if it is they are opening of coarse :cheers:

 

Eddie,when you were installing the holley did you use t pieces to get sufficient vac lines.....when i installed mine i used a spacer with separate vac lines tapped into it so that each line had its own connection...it may be that is where you are loosing the pressure...also there is a very useful vac trouble shooting guide at http://www.corvette-101.com/vacuum.htm......hope that helps......john :chevy

 

Your vacuum readings are about what I'd expect to see with the cam duration numbers you are running........

You should follow the vacuum trouble shooting guide as suggested, and you may improve your situation some by repairing the vacuum leaks you are likely to find.

I found both headlight control valves to be leaking and both headlight actuators needed the new grommet/witches hat seal, and my vacuum check valve was bad/needed replaced as well. I followed 'AWsvette' website instructions for repairing the, according to GM, 'non-repairable' headlight control valves, and got them working and holding vacuum without spending $60 to $80 apiece for new ones.........had to end up using some RTV silicone on inside lip of both ends of the rubber seal that goes on the shaft of the control valve, but after lots of trial and error, and cleaning and soaking to soften up the seal, then turning the seal lips inside out for a few hours, followed by the RTV treatment, and finally lubricating the seal with white lithium grease, the valves are now holding vacuum like it should.

AW's instructions are good, but my experience with this rebuild tells me to add that: 1) you can aid reassembly by use of vacuum gauge to hold the valve parts together while attempting to re-install the lip/ring, and 2) once the ring is installed, you can tighten to proper position easily with the use of a screw type hose clamp.

 

First, ensure that the booster check valve is holding a vacuum and the one-way check valve installed and working correctly in your primary system vacuum line for the headlight/hvac system. These often fail. Power brakes generally need about 14" hg to operate correctly. This ultimately depends on the size of the vacuum chamber, but that number is about the standard. I have found that number to be the minimum to operate the power booster at a marginally acceptable level. I wouldn't exactly call that a number that you wouldn't notice the difference. If your carb is properly adjusted, try bumping the base timing a degree or two or use manifold vacuum instead of ported vacuum. You'll generate a much higher vacuum signal this way and achieve a more stable idle to boot. You'll likely have to work the initial and centrifugal tuning to have the car perform the way you like without detonation, but using manifold vacuum instead of ported vacuum can have generous rewards.

Holleys use a fairly stiff secondary spring. For about six bucks, you can buy a spring kit that will allow you to tune the secondary opening. You should not be able to feel the secondary opening, but you'll certainly hear it. It should be gradual and smooth. Vacuum secondary opening depends on engine load (vacuum) and airflow to make the open and I've never been able to have them open soon enough as they are out of the box. The lighter the car, the more you need the spring kit. In my car, I had to use the lightest white spring to make them open.

 

jdunne: all my vac connections have their own port, (no t's or couplings)
the brakes are connected to the largest carb port, and I now suspect the check valve. Thank you for your suggestion :cheers: :cheers:

John: My lights were working perfect before the motor build. I might have jared something loose while crawling around in the engine compartment during the cam change and radiator install. OK.. don't laugh.. I did fit in there from underneith without the rad... I'll troubleshoot the lines before I check the acctuator. Thanks dude... :cheers: :cheers:

Gerry: Thanks for the spring tip for the Holly. I really need to be positive that they're not opening, so the paper clip trick is next to try. My trans kick down is not adjusted correctly, so the carb might not have the correct "kick" to get them to open. I will shift the trans manualy to get those R's up, with the paper clip on the linkage to be sure. i'll keep ya posted.... thanks a bunch... :cheers: :cheers:
Eddie