Don Terrill’s Speed-Talk

360ci LA Mopar (iron headed street engine) 11.3:1cr. Has developed a rapidly fluctuating vacuum gauge needle between 19 and 21", at idle speeds upto around 1000 rpm. Above 1k rpm it smoothes out nice and steady.
When the transmission is put in Drive the fluctuations increase upto a 4" sweep. Engine runs rough too and usually needs some throttle to keep up the rpm. The vacuum needle fluctuates right from the start on a cold engine and doesn't improve when hot.
Engine vacuum is normally 'high' around 17-21" during regular driving.

I understand this kind of needle behaviour could usually be pointing to a bad valve guide(s).

My question, can this rapid needle movement also occur when a lash is too tight on a valve and holding it open ever so slightly?


Some more info:
I've had this engine broken-in on an engine stand were the needle was a bit more steady.
Engine currently runs on a MegaSquirt digital ignition in the car where only ignition is controlled for now.

Did a cranking compression test with an old MotoMeter push-on tester, which didn't show much other then the readings went off its 12 bar (174psi) scale. Cam is CompCams XE256H.
The heads have had new bronze valve guides installed and all valves received positive valve seals, so I would think valve-guide related vacuum issues would be closed off by the seals. But I haven't been able to rule that out yet. Valve seats have been leak-tested with brakecleaner while watching for leakage.

Now for the curve ball in the mix;
The new hydraulic lifters have been converted to solids with a spacer inplace of the spring and check valve, because a few of them started collapsing on one side of the engine. I suspect too much/deep inner lifter bore camfer, and/or lifters with a too high oil band.
Valves were lashed (a number of times) at 0.003 to 0.006", and are currently at 0.004" cold (have not checked them hot yet). Iron block and heads. 1.6 rockers.

So my concerns about the lifters is I think they are still able to build some pressure inside and perhaps manage to close up the tight lash at idle and cause vacuum issues.
This seems not something I can easily test with a compression tester, unless someone else will prime the engine's oil system at the same time perhaps.

Leaky valve or lash too tight?......same thing.

How about a compression test?

Did a compression test, readings went off the scale on all cylinders (+12bar/+174psi). But I thought something would've shown up already given how bad the idle quality is.
Have ordered a better screw-in tester already.

I don't think I've ever had a leaky or too tight lashed valve in an engine so I wouldn't know how exactly a vacuum gauge would react to that.

You check your timing?

Timing is digital by means of a MegaSquirt ECU.
Engine idles with 30° ignition advance in Neutral. 25° in Drive. These numbers give best and most steady idling.

Yes, a tight valve will make the vacuum gauge needle twitch, same as slightly bent valve that only leaks a little bit.

Ran into this years ago, the vacuum gauge is to sensitive and it's
picking the intake pulsing, place a restrictor in the vacuum line
at or near the gauge with about a .032 thou hole thru it, it
should smooth right out.

If this is a multi-port injection you need to isolate to cylinder could have a dribbling injector at low RPMs

You can google vacuum gauge chart or diagnose and click images. This is one example.

bigfoot584 wrote: ↑Sun May 13, 2018 7:28 pm Ran into this years ago, the vacuum gauge is to sensitive and it's
picking the intake pulsing, place a restrictor in the vacuum line
at or near the gauge with about a .032 thou hole thru it, it
should smooth right out.

Somewhere along this I would comment. But would like to add: how is vacuum beeing measured? Taking from one runner only?

I have seen the same pulsing, taking vac from one runner (IR) only, and hearing the distributor vacum advance ticking. Diminishes as RPM goes up. Have tried restrictor, but does not clean up. Need take offs from all runners.

No Clue if this is yur situation anyway....

Vacuum is taken from the carb's base on a dual plane AirGap intake.
Putting restrictors in the line will only mask the problem I would think. The engine's idle suffers greatly too and this won't be masked by this.

The intake ports are drilled and tapped near the heads for (lpg) injectors to be installed later in time, so vacuum samples have been taken from each port, but no real distinction could be made in the readings.
If at all I would think the issue would be at one 'plane' of the intake manifold, so either cylinders no. 1, 4, 6 and 7.
Tried pulling sparkplug cables for each cylinder at a time while idling, but perhaps cyl. 6 would be my first guess where I would look first.

I've looked up a number of these vacuum charts but not many of them give useful additional info about what could be happening.
Will have to open up the engine and recheck the lifters because I tend to think that's where the problem is.

If the lifters were collapsing internally, you would have valve train noise. As long as the metal spacers you used in the conversion to solid lifters are hardened [ or s/steel ], there should be no problem. I have done the same thing many times, zero problems.
Check out your ign system. What type is it? Is a plug fouling at idle speed? Should be using a 5 heat range in NGKs. 6 or 7 is too cold for normal street/high perf use.

I tune multi carburetor bike engines with a manometer....Individual runner and carb for each cylinder...Yes, the readings bob up and down ,less so with a restriction that is properly placed in the hose at the source of vacuum ,not the gauge end..
But on the auto engine vacuum is usually checked common to all cylinders.....I think the OP suspects a leaky valve, yes?

Just as a quick check and problem direction indicator. Back off (open) all the valve lash settings about .020" on all the valves and run the engine at idle. I
It will be obviously have a clatter but if that smooths the vacuum readout then you just have a tight valve in a cylinder and you can reset the lashes and stop the problem.

Gday mate

Thirty degrees of timing at idle on a small cam seems a lot. Notwithstanding that LPG* respond well to a lot of initial, it seems an excessive requirement for best idle on a cam with 212 degrees at 50thou. (I use 24 degrees initial at 1200rpm on a Gas Research LPG setup, but that's with a 240@50 solid flat tappet)

I'd be looking closely for something ignition related, assuming that the conversion to solid lifter hasn't done something unusual to the effective cam specs.
Regards
Paul

* I'm assuming again that the engine is currently on carbed LPG- I note the reference for injected LPG later.