Weak Vacuum problem

[sleepy]

I have a 1981 Datsun 210. The vacuum from the port at the base of the carburetor is very very weak. It supplies vacuum to the EGR which is why it is not working and will not pass smog. I sprayed carburetor cleaner around the base of the carburetor and it indicated vacuum leak. In the past I've had the carburetor off and soaked in solvent. I'm sure we would have installed a new gasket and that the mounting bolts were tight as I could get them. My family has owned the car since new so I have no reason to think the surfaces are bad. I plan to at least recheck the torque at the base of the carburetor. Any input you can offer on stopping the leak at the base would be appreciated. Also I've read to check for intake manifold leaks. Any other input for increasing the vacuum from the port at the base of the carburetor would also be very much appreciated

[datzenmike]

That is a ported vacuum signal and there is none at idle. The port is located just above the throttle plate so at idle it's exposed to ambient air. Then you step on the gas the throttle plate rises to and above the port and it is exposed to intake manifold vacuum. At full throttle the intake vacuum is at it's lowest and the signal is extremely weak or non existent. There is no EGR at idle because basically the engine wouldn't idle and no EGR at full throttle because you want maximum power.

 

I suppose you could maximize intake vacuum by checking the valve lash. A tight intake or a bad valve might not seal the valve and leak vacuum. Use a vacuum gauge and adjust the idle mixture and speed to maximize the vacuum reading. I would expect a vacuum of 16-18 at idle. 

 

If you suspect a vacuum leak around the carburetor base replace the gasket. A leak here lowers the intake vacuum.

 

Does it idle alright? The EGR can also be a vacuum leak if the valve does not fully close at idle. Remove and wire brush the exhaust deposits off.

[sleepy]

On 1/5/2023 at 1:51 PM, datzenmike said:

That is a ported vacuum signal and there is none at idle. The port is located just above the throttle plate so at idle it's exposed to ambient air. Then you step on the gas the throttle plate rises to and above the port and it is exposed to intake manifold vacuum. At full throttle the intake vacuum is at it's lowest and the signal is extremely weak or non existent. There is no EGR at idle because basically the engine wouldn't idle and no EGR at full throttle because you want maximum power.

 

I suppose you could maximize intake vacuum by checking the valve lash. A tight intake or a bad valve might not seal the valve and leak vacuum. Use a vacuum gauge and adjust the idle mixture and speed to maximize the vacuum reading. I would expect a vacuum of 16-18 at idle. 

 

If you suspect a vacuum leak around the carburetor base replace the gasket. A leak here lowers the intake vacuum.

 

Does it idle alright? The EGR can also be a vacuum leak if the valve does not fully close at idle. Remove and wire brush the exhaust deposits off.

Many thanks for the knowledgeable details and insights you gave me. There is a leak at the base of the carburetor so I'm going to try fixing that and go from there once again sure appreciate your help

[datzenmike]

The ported vacuum is not going to be that strong anyway. How are you determining that it's weak?

[sleepy]

Hi Datzenmike,

I plan to retire my 1981 Datsun 210. I also have a same exact spare engine with less than 40k, a spare passenger door, a spare hood, and a number of spare parts -alternator etc. 

It was really good of you to help me out with the knowledgeable input you provided. So if there's any of this you'ld be interested in, let me know including the entire vehicle, some if not all I'ld be possibly willing to give to you. I don't know how you respond to a message sent just to you but you're welcome to send me an e-mail or put another response in my Weak Vacuum problem question.