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Idle vacuum in neutral dropped from 20 to to 16+ on carbed 87 Dak

Hi, back again with a new issue. For those keeping track, my brakes are operating flawlessly after the initial problem with a new master cylinder. However, without my noticing, over a period of maybe two months or so, the idle manifold vacuum in neutral has dropped from 20 to 16+. To diagnose I snugged up the four carb mounting screws and disconnected and plugged all vacuum hoses from the carb except for the vacuum gauge and the one going to the vacuum actuated diaphragm that regulates the spark advance on the engine control computer. With that disconnected and plugged the idle vacuum goes way down. I did this by disconnecting the larger hose going from the carb to the plastic manifold that distributes the vacuum to varies components that use it, and teeing off it to the vacuum gauge and the spark advance diaphragm. There was no change in the idle vacuum reading, still at 16+.

I also noticed that the engine seems to have less power (subjective), and freeway gas mileage on a 400 hundred mile trip has gone from 21+ to 17+. I suppose I should check to make sure all the spark plugs are tight. Other than that, does anyone have any ideas what could be causing this significant drop in manifold vacuum and mileage? I have a vacuum gauge hooked up full time in the cab, but failed to notice exactly when, or over what period of time the drop took place, but it had to be within the last month or two. I certainly haven’t removed and replaced any plugs in that time.


You may have an intake manifold gasket leak.

The vacuum drop might be caused by a stretched timing chain.

If the chain is stretched, it causes the valve timing to be off.

Here’s how to check for a stretched timing chain.


A restricted exhaust is another posssibility.

Thanks, I think I remember reading that one way to check for an intake manifold gasket leak is to use a propane torch (unlit of course) and run it around the edge of the intake maniold to see if the RPMs increase.

Thanks, that looks simple enough. I will definitely check and report back.

Quite a while back I manufactured a device to check exhaust back pressure. I will have to dig it out and see if it reads within specs.

Thanks to all,