Trying to install ported vacuum on 780 Holley

[maguyvr]

I purchased a Holley ported vacuum body 134-155 for my stock list-4555 780 cfm carb and installed it but it has no vacuum at the port. Has anyone tried this? Do I need to drill the body of the carb to get vacuum to the plate? Thanks in advance.

 

[COPO]

Why would you want ported vacuum on a 70 Z28? It’s not a smog gutless engine. Manifold vacuum is the way to go on a performance engine. I think you should read up on Ported vs Manifold vacuum 101 before destroying a 4555 Holley.

Just buy a used 750 Holley that has a ported vacuum port.

 

[maguyvr]

I’m not trying to destroy anything that’s why I’m asking questions. The engine has a hesitation/stumble off idle I’ve been fighting for a long time. Accelerator pump, jetting, timing, etc has all been played with. It’s a stock LT1 with manifold vacuum to the distributor only in 3rd and 4th controlled by a solenoid. I was told by a carb specialist to try running ported vacuum to it.

 

[Mitch_Z]

I believe the ported vs manifold vacuum applies to what your distributor vacuum advance is connected too. It comes ported thru the TCS solenoid from the factory. To run full manifold vacuum ditch the TCS and tee into the hose that goes to the choke vacuum.

 

[maguyvr]

I believe the ported vs manifold vacuum applies to what your distributor vacuum advance is connected too. It comes ported thru the TCS solenoid from the factory. To run full manifold vacuum ditch the TCS and tee into the hose that goes to the choke vacuum.

I could do that I’m just wondering what that’s going to do with my idle speed.

 

[COPO]

I had a hesitation while cruising and just taking it apart to clean and installing a renew kit didn’t fix it. I took it apart a 2nd time and checked for warpage. Yup, I spent 4 hrs filing with oil and a machinist file to true all the warped parts. The last time I filed on this carb was in 1985.

I then cleaned as much as I could with brake clean and compressed air before putting the parts in an ultrasonic cleaner. Then blew it dry and reassembled. The carb now performs like a new carb.

Make sure you have .015 clearance on the accelerator pump when the throttle is wide open. With throttle held wide open with a coat hanger or something if your alone then using a flat head screwdriver, push down on the lever as far as it’ll go and measure the drag between the lever and the bottom of the adjustment bolt.

Also check the adjustment on the secondary flaps.

Secondary throttle stop screw adjustment Must have carb off car. Set as follows: Back off adjustment screw until the throttle plates are fully closed (secondaries). Turn adjustment screw until it just touches the throttle lever and turn 1/2 turn more to position the valve.

I would also set the idle mixture screws to achieve the highest vacuum reading then set idle at 800 rpm.
Timing should be 36* total with vacuum plugged. Or 12-14* initial.

 

[maguyvr]

I had a hesitation while cruising and just taking it apart to clean and installing a renew kit didn’t fix it. I took it apart a 2nd time and checked for warpage. Yup, I spent 4 hrs filing with oil and a machinist file to true all the warped parts. The last time I filed on this carb was in 1985.

I then cleaned as much as I could with brake clean and compressed air before putting the parts in an ultrasonic cleaner. Then blew it dry and reassembled. The carb now performs like a new carb.

Make sure you have .015 clearance on the accelerator pump when the throttle is wide open. With throttle held wide open with a coat hanger or something if your alone then using a flat head screwdriver, push down on the lever as far as it’ll go and measure the drag between the lever and the bottom of the adjustment bolt.


View attachment 201454




Also check the adjustment on the secondary flaps.

Secondary throttle stop screw adjustment Must have carb off car. Set as follows: Back off adjustment screw until the throttle plates are fully closed (secondaries). Turn adjustment screw until it just touches the throttle lever and turn 1/2 turn more to position the valve.

View attachment 201455


I would also set the idle mixture screws to achieve the highest vacuum reading then set idle at 800 rpm.
Timing should be 36* total with vacuum plugged. Or 12-14* initial.

Thanks for the info. Most of that has been performed already. When I bought the car it had a 750 Holley on it. That (besides headers) was about the only thing not original. It had the hesitation then and I couldn’t get rid of it. Holley (for a while) was remaking the List 4555 carb so I bought one. Still same hesitation. I did just find the primary jets smaller than original size and changed them and ordered another accelerator pump as now it’s not squirting correctly. I also fixed an issue with the distributor weights bushing missing. I’ll give it another go once I replace the accelerator pump but the pump used to work right and still hesitated.

 

[FS87LT]

I highly suspect the CARB is calibrated for ported vac (i.e., no vac advance at idle as the vac at the port is less than what it takes to activate the vac advance mechanism). This relates to where the holes for the idle mixture are located in the carb's base plate.

IF there is too much or too little transition port exposed at hot base idle speed, there can be hesitations/flat spots with too much or too little.

IF you put the vac adv at full manifold vac, you'll need to turn down the base idle screw to get the normal-spec base idle speed. Which can decrease the transition port exposure as a result. You'll still need to set the initial timing with the vac advance unhooked, big time.

Once the engine rpm is higher than hot base idle, the ported advance signal should approximate that of the full manifold vac signal, so the main difference is at hot base idle, as to vac advance. Does it do that?

Using manifold vac to run the vac advance means that you are getting full vac advance + initial advance at idle. Some use this to get a better off-idle throttle response with wild cams that might be too radical for street use, by observation.

To me, IF the factory could get the car to work well with ported vac, which was the standard way to do things back then, then it ought to be good now. The issue might be that with E10 fuels, some prior good carb calibrations (jets and such) might need to be a tad richer to work right? How is your idle mixture adjusted? As the factory recommended?

Just my experiences and observations,
FS87LT

 

[maguyvr]

I highly suspect the CARB is calibrated for ported vac (i.e., no vac advance at idle as the vac at the port is less than what it takes to activate the vac advance mechanism). This relates to where the holes for the idle mixture are located in the carb's base plate.

IF there is too much or too little transition port exposed at hot base idle speed, there can be hesitations/flat spots with too much or too little.

IF you put the vac adv at full manifold vac, you'll need to turn down the base idle screw to get the normal-spec base idle speed. Which can decrease the transition port exposure as a result. You'll still need to set the initial timing with the vac advance unhooked, big time.

Once the engine rpm is higher than hot base idle, the ported advance signal should approximate that of the full manifold vac signal, so the main difference is at hot base idle, as to vac advance. Does it do that?

Using manifold vac to run the vac advance means that you are getting full vac advance + initial advance at idle. Some use this to get a better off-idle throttle response with wild cams that might be too radical for street use, by observation.

To me, IF the factory could get the car to work well with ported vac, which was the standard way to do things back then, then it ought to be good now. The issue might be that with E10 fuels, some prior good carb calibrations (jets and such) might need to be a tad richer to work right? How is your idle mixture adjusted? As the factory recommended?

Just my experiences and observations,
FS87LT

 

[maguyvr]

Thanks for your response. Interesting observations. This carb did not come with ported vacuum. It gave the distributor manifold vacuum only in 3rd and 4th gears. I was attempting to add the Holley 134-155 metering block that has the metered port on it but it doesn’t work. I’m thinking the body was not drilled for it.