1quickirocz

Ok im helping a friend with his 400 swapped S10. Im an EFI guy and dont have alot of carb experience. We are having an issue with the vac advance on his Holley 750. It would make since to me that on the timed vac port on the metering block should NOT have vacuum at idle?? Am I thinking correctly on that? I have not put a guage on it but it feels like its pulling as hard as the full vac port at idle. We have the timing set at 14* with the vac advance unhooked and plugged in at an idle it jumps to 35* which im thinking is full advanced. What could possibly be causing my timed vac and my full vac to be the same? Could it be the metering block gasket or is there something else im missing? Any help would be greatly appriciated. Thanks in advance!!

skinny z

The ported vacuum will not have vacuum at idle. Holley carbs have several ports (depending on the model). Only one is ported vacuum on my 4150 style carb.

1quickirocz

Thats exactly what i was thinking. There must be some issue with that gasket or something. I have vac on the ported and full vac ports at idle.

skinny z

I reread your post and am checking out the Holley website for some info.
Do you have a model number? Have you checked for another timed/ported vacuum source?

1quickirocz

Thanks....I did a search on here but did not check the one most important site to get Holley info haha.....thanks checking that...

skinny z

Yes, some models provide a timed port on the metering block. Mine is in the carb base. I would think that if the metering block port is delivering full manifold vacuum, then there's an internal leak. Perhaps the carb isn't as fresh as it could be?

1quickirocz

My friend bought the carb new about 8 years ago and in that time it has been on 3 other peoples cars. As far as i know it has never been into and it has very little run time on it. Im sure some of the gaskets and seals are not in good shape. Truck runs great with it minus the ported vac port having full vac at idle. I think its a 4150 but i could be wrong. I know its that same style. Its a DP and mechanical secondaries. The picture i saw showed the timed vac port being in the metering block but that may have been for a different model. I did check both of the front vac ports and both seem to have full vac at idle. The pic that i saw showed the port at the base of the carb being full vac. I have another 750 Holley that has a float issue and i took that one appart today and it looks like the vac port in the base of that one is plumbed to a hole on the bottom of teh throttle blade which i assume should be full vac. The port on the metering block was plumbed to a hole above the throttle blade which i assumed to be the ported vac source. Is that correct?

skinny z

Above the throttle blades would be timed/ported vacuum. Below the blades is full vacuum.
Get the part number from the choke horn. The small amount or research I've done says that not all models have a ported vacuum supply. I can't confirm that however in a pure racing application, the vacuum advance is often excluded from the setup.
A part number will get you to a decent diagram. The rest is easy. After 8 years and liitle use, I suspect it may need a refreshing anyway.