Intake vacuum

[Dean3326]

Does anyone know what the intake vacuum on a Lycoming Straight eight should be? Lycoming 8BIS42915 in a 1927 Gardner Imperial Sedan model 8B.

I with the help of many can not get this vehicle to run smoothly it is running extremely rich. I would welcome any conversation about this problem.

my number 484-553-2042

The Carb was rebuilt  vacuum fuel pump rebuilt  compression 70 # consistently     Vacuum is on 10#

[Curti]

I just tested the idle vacuum on a 288 CID Lycoming it is 18 inches. compression 112 - 117 .

Just because the carb was rebuilt doesn't mean it is jetted properly. Maybe someone has reamed out the main jet in a misguided attempt to gain more power.

[Rusty_OToole]

All engines have about the same vacuum, with the possible exception of diesels and some extremely hopped up racing engines. There are charts you can find on the net, print one off and go by that.

 

How rich is rich? A lot of old engines run rich at idle especially with the choke on. If you don't drive them but just let them idle or putter around, they never get up to full temp. It takes  10 - 20  miles to fully warm up some of those old engines, there is a lot of iron there.

[Spinneyhill]

Is the carb float level set correctly? Is the float needle valve closing properly? Is there a speck of dirt on the needle valve? Hopefully your rebuilder did a good job. Mind you, there is more than one description of "rebuilt" too.

[carbking]

A couple of possibilities for overly rich running:

 

(1) If the car has a vacuum tank, it is not uncommon for the vacuum control valve to fail, thus pulling fuel directly into the manifold, bypassing the carburetor completely.

 

(2) A weak spark will not burn all of the fuel, and the engine will seem very rich.

 

And of course the Schebler is an "automatic" carburetor that was designed for gasoline with a Reid Vapor Pressure of approximately 6.0, and btu content of 117,600/gallon; NEITHER of which are characteristics of modern "fuel".

 

Jon.

[Guest Rastus]

Manifold vacuum & running too rich are unrelated issues.

A gas engine in good condition @ sea level should read between 18-21" on a vacuum gauge.  

Expect reading reductions of 1" per 1000 feet above sea level.

 

http://www.secondchancegarage.com/public/186.cfm

[Rusty_OToole]

A couple of possibilities for overly rich running:

 

(1) If the car has a vacuum tank, it is not uncommon for the vacuum control valve to fail, thus pulling fuel directly into the manifold, bypassing the carburetor completely.

 

(2) A weak spark will not burn all of the fuel, and the engine will seem very rich.

 

And of course the Schebler is an "automatic" carburetor that was designed for gasoline with a Reid Vapor Pressure of approximately 6.0, and btu content of 117,600/gallon; NEITHER of which are characteristics of modern "fuel".

 

Jon.

Interesting point about the fuel. Is it possible adding kerosene makes modern fuel more like the 30s stuff?

Edited August 28, 2015 by Rusty_OToole (see edit history)

[Guest Rastus]

Certainly does.

Kerosene, heating oil or diesel are all similar & will reduce possibility of vapour lock issues in very hot weather & addresses the negatives of ethanol. 1 litre per 60-70 litres