I have a 460 cu in recently rebuilt engine in my vette and was checking my engine with a vacuum gauge today and noticed that there were different readings from different parts of the manifold/carb, i do have a large duration cam which is matched to my edelbrock performer rpm intake (cant think of the duration, but lift is .540 i think).
From the small port on the 750 performer series carb (not the one for the dissy vac adv) At idle, i was getting a steady 9 hg, from the large port to the pcv i was getting an very irratic 4-9 hg, and the same to the manifold port to the vacuum system (lights etc)
Question is why would the readings be different from the carb to the manifold, engine runs very well and i have no issues. Just wondered why there was this difference.
I did adjust the static timing from 12 btdc to 22 btdc and the vacuum did increase as did the idle speed (obviously) but only a small increase in vacuum was gained when the idle was dropped back to 850 rpm.
If i increased the engine speed slowly vacuum would rise to 15hg by about 1500 rpm (Guesstimate) and did rise further the more i increased the rpm.

P.S. I did disconect the lights (vacuum system pipe) and block it at the manifold, when i did the test as i know there are faulty headlight relays which leak.
Is the irratic needle just down to an intake leak? inwhich case why was the needle steady when on the carb port. Was this due to the fact that the leak is after the carb ie in the intake!!!

 

True vacuum readings are found at the manifold port. When you take readings off the carburetor, some ports are full time vacuum and others are ported with little to no vacuum at idle.

The port on the side for the vacuum advance is full time.

cc

 

I was wondering why the readings were so irratic, constant flicker from 4hg - 9hg is that due to the big cam?

 

no, the erratic reading is the small port will not be as sensitive to the gauge, as the large port,the vacuum bleeds off more slowly on the small port as far as the gauge is concerned. as far as the difference from 4-9 inches, it is either the cam duration or valve action is too late, there is a sweet spot where the needle should be somewhat steady and the more cam you run the higher the rpm for a steady needle.....the latter(bigger port) is where you should take the reading .....

 

The erratic reading was from the large port, and the steady reading was from the small port.

P.s. think i misread your answer!! are you saying it should be more erratic on the larger port?

 

yes!!....good luck.....it would be more erratic on the large port...my answers are easy to mis read..hell at my age i can't read em' either..lol....

 

Would it be possible to advance the cam a couple of deg to solve the problem, if the valve action is too late, and what benefit would i gain by doing this? any more hp or will it just smooth out the the lumpy idle.

 

 

Doubt it, I've got a big cam. Stays at a steady 7 inches on my boost guage.

 

so is it down to the valve action being too late and how can i solve this and will there be any hp gains or throttle response as i haven't got any problems with it at the moment?

Peter

 

Sounds like there might have been a problem with how you were measuring. If you hook it up to a manifold vacuum port and the vacuum is steady, don't worry about it. If you're measuring a line with other vacuum devices attached, they could be throwing the reading off.

Set the idle mixture screws to pull the highest vacuum at idle. Read a Lars article on the archives for how to set the timing, I'm used to doing it with Holley carbs.

 

I tested from the manifold pipe to the pcv valve, i disconnected the valve and just tested the vacuum there, and it was erratic, when i tested it at the small manifold port on the carb it was steady at 9hg

 

If any readings are steady, that means your motor is steady. The other readings are bleeding off somewhere else. You shouldn't ever tap into a vacuum t somewhere, you should always be testing directly from a port on the carb.

Drive your car, worry about something else.

 

Thanks for your input guys and happy vetteing!!!