elementary manifold vacuum question

Clarence L.Snyder clare.snyder.on.ca at ibm.net
Mon Nov 9 01:23:34 GMT 1998


Stowe, Ted-SEA wrote:
> 
> I have a question for the collective. I think I have forgotten vacuum 101. I
> am trying to make sure everything is hunky dory before I strap my gm
> throttle body onto my car.I have a 77 mgb 1800 cc.
> 
> this has siamesed intake and exhaust ports unfortunately.
> 
> when it is cold and I hook up a vacuum gage to the intake manifold, at about
> 600 rpm the needle fluctuates between 15-19 inches. meaning that the needle
> itself is nearly invisible I can see the trend. if I pinch the hose to the
> gauge, I can dampen down the 'pulses' and get it to stay steady. at higher
> rpms this effect goes away completely.
> 
> as it warms up ,at 1000 rpm, it will fluctuate between 16-18 and eventually
> just settle down to a buzz at about 17+.
> 
> the engine runs fine and idles fine by all appearances, no shaking or
> anything.
> 
> I have no idea if the cam is stock or not.
> 
> I remember that one is supposed to actually restrict the vacuum line to
> resolve this issue, I am just wondering if this sounds like word valve
> guides or seals.
> 
> I don't know if the gm 1800 cc I have this from had this type of issue and I
> am wondering how this would affect the sensors on the throttle body/ECU. I
> don't seem to remember this as an issue on other engines.
> 
> thanks for all the great support btw.
> 
> Ted Stowe
 This is normal for that engine, from what I remember. If you are using
a MAP, you will have to put in a restrictor to damp the signal - for MAF
there should not be an issue.



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