Vacuum gauge diagnosis showed:

[gr_diver]

Hello everyone!

Today, I hooked a vacuum gauge to my manifold and noticed this:

At idle it gave me a reading af about 19" Hg, which is supposed to be good. Though, the needle at idle is very fastly and continously fluctuating about 1"Hg, with the middle point being the 19" reading. If I steadily rev the engine (anywhere above 1500 rpm), this fluctuation get's very small, about 0.5" but remains...
What would this suggest?

In addition, as I would open the valve-cover today to change the gasket, I tested the adjustment of my valves today (solid rockers). While turning the engine by means of the fan (very slowly, to get the valves closed), I could hear the pressure of the cylinders discharge in the crankcase, is this normal?

[David McCollam]

The guage reading and the slight fluctuation in the reading are normal. This fluctuation is caused by the timing overlap in the valvetrain. The camshaft ends the timing cycle and the piston is on the downstroke when the valve is opening. This slightly disrupts the vacuum flow and causes the needle to move. As you increase engine speed the overlap shortens and the varience is smaller.

The compression that you hear in the clyinders is normal. If you didn't have any compression during the manual turning, it would indicate that the cylindes had little or no compression and the engine wouldn't run. If you need to reduce the compression to turn the engine by had, remove the spark plug while adjusting the valves. The compression will be less during your adjustment. Once all the valves are adjusted, reinstall the plugs and start the engine.

Good luck.

[gr_diver]

Thanks for replying David!

I had noticed this fluctuation about a month ago, and some vacuum-gaguge-diagnosis charts on the web described such but bigger fluctuations, in combination with low vacuum, to be a valve problem. Today I set my valves (in fact they were ok, just a couple of them needed hair-like adjustments (about 10 degrees rotation), and I didn't know if I should expect the fluctuation to dissapear completely.

I didn't have much of a hard time to turn the engine with the plugs installed, in fact I know of this "trick" and I have used in the past when adjusting static timing. Last October that I tested compression it was well inside the specifications, and didn't have above 10% difference between cylinders, so I am happy to know that my piston rings and bores are in good condition! I just noticed that today (first time self-adjusting valves), and could not tell if this would happen in the past and made me curious if it was normal.

Thanks again!

[David McCollam]

Not a problem for the reply. The 1975 fuel injected US Opels have this problem and it puzzled me at first. I looked at several charts to attempt to obtain information on the adjustment. The US engines have self-adjusting lifters (hydraulic) and not the solid. The fluctuation on the gauge is remedied for the US cars in part by an adjustment, but it is mostly due to camshaft overlap.

The guy in Athens that posted to Classicopels is a good fellow. Find the thread on this site and contact him. I have received parts here in Texas that he has shipped from Athens.

Cheers,

Dave