Need help with vac guage readings

[David]

I was trying to adjust my air/fuel mixture today but couldn't get the vac guage to go above 11 in.HG It said I could have a vacuum leak or bad vavle timing. I also noticed that were the intake manifold and head mate the gasket was leaking oil. Could there be a vacuum leak in that area?

 

[68ChargerAG]

A trick I have seen mentioned several times on here is to take starter fluid and carefully spray it around the edge of the intake manifold and if the engine RPM increases when you spray you have found a leak.

Also, ensure everything else is plugged off to the intake such as PCV, vac to brakes, ... etc.

 

[David]

This may be a dumb question, but by plugged off do you mean disconnect the PCV and all of that?

 

[69 Runner]

could mean you've got a bigazz cam too. that gauge doesn't take that in to consideration

 

[David]

No the motor is completely stock except for the dual plane intake, 4 barrel carb, and headers I have put on

 

[69 Runner]

then you have a vacuum leak

 

[68ChargerAG]

This may be a dumb question, but by plugged off do you mean disconnect the PCV and all of that?

Yes, disconnect them and plug off where they connect to the carb or intake manifold any way you can. If the PCV valve is sticking open it will cause a large vac leak, and you could also have a leak in the brake boost diaphram, so disconnecting both of these (if you have them) and plugging them off will rule them out so you can focus on only the intake manifold. I would even disconnect and plug the line going to the distributor to rule out a leak there too.

Or, like 69runner said, if you have a huge cam in there you will see lower vacuum... but it would have to be pretty big to drop that much.

You should also acount for your altitude. If you are not at sea level you will see slightly lower manifold vacuum then recommended, but not all the way down to 11. I think it will be around 1 inch per 2000ft in altitude or something, but you might want to double check that with a google search. :grin:

 

[Meep-Meep]

What is your timing set at? This will affect vacuum readings as well.

I don't use a vac gauge to set mixture but rather the lean drop method. Make sure your timing and dwell (if running points) are correct and general good health of the ignition system. Carb float settings are spot on, no vac leaks as mentioned and engine at operating temp. Set both screws to 2 turns from lightly seated then go in the lean direction until you get a lean misfire, then back out to the point where it runs smooth. Go slow and pause at 1/4-1/2 turn increments to give time for the engine to react to the change until you find the line just before it smooth again. Go back in until you get a 20-50 RPM drop and stop. In some cases I run right at the point of where it runs smooth. If you have little to no change when you diddle with the screws then you have other issues and need to do some investigating.

Since we are in the age of crappy high vapor pressure fuel, the intake manifold temp will affect the fuel charge to each cylinder and that might mess with fuel distribution. Some cylinders might run very lean (causing a misfire on that cylinder) and others rich to normal. The effect will be a rough of erratic idle. Blocking or restricting the heat riser will help but cold weather drivability may suffer.

 

[65-440]

could mean you've got a bigazz cam too. that gauge doesn't take that in to consideration

hey .... sounds like my situation ... what vac. reading ? !!!!!

 

[David]

Thanks for the help, I'll check it out and get back to you guys

 

[scotts74birds]

I hope this helps.
http://www.visn2.com/UsingVacumeGauge.html

 

[David]

Thanks for the site it did help. I tried with the vac gauge and lean drop method. I wasn't noticing much change, so i'm gonna check into a couple of things and see whats goin on.