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Timing Question


HappyZ

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  • HappyZ changed the title to Timing Question

Well I'm no expert on the subject, but my answer is "In theory, it shouldn't matter whether the vacuum advance line is connected or not."

Why? Because the source for the vacuum advance is a ported vacuum source that should provide no vacuum at idle. So, if your car is tuned and idling properly and your throttle butterfly is where it belongs when your engine is idling, then you should have no advance at idle. And because of that, it shouldn't matter whether that hose is connected or not.

However, if your car is out of tune in some way and you need to have the throttle open further than normal just to get the engine to idle, then all bets are off.

So, here's a way to check... Put the timing light on it at idle with the vacuum line connected. Then disconnect the line and see what happens. There should be no change in engine RPM and no change in timing advance.

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If you're tuning with points, do you have a good dwell meter? I once tried 3 dwell meters on one car and got 3 different readings. (To make it even more fun, the distributor was missing one of the adjustment screws, so I had to wing it while adjusting it.) I since got a 4th dwell meter so I could confuse myself more. The old school dwell meters are NLA. I might eventually break down and buy a Fluke 88V. I think it will measure dwell, and I only have about 8 or 9 multimeters, so there is a need to buy another meter.

Once you switch over to an electronic ignition, you can probably retire the dwell meter. Sometimes I still work on a luddite's car, though, so I'm keeping them around.

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You could run the "pull the hose off and look for differences" now. That way, you would know if it really mattered before you threw another variable into the mix.

In other words... When your car won't start after the points conversion (because of some detail somewhere), you may know if it could potentially have anything to do with the vacuum advance system. Or not.  LOL

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@Captain Obvious I set timing 10 at 800 rpm with vacuum connected.  I then disconnected and plugged the vacuum hose.  There was no change in timing.  I then reconnected the vacuum and rev’d the engine…the timing advanced.  It appears it’s working as advertised.

I did notice that whenever I rev’d the engine, it would die when I released the throttle.  It was sort of a smooth die where the rpms drop smoothly and then it just stops…No sputtering or attempts to keep running.  It idles good when I restart.  Perhaps my floats aren’t set correctly?

Edited by HappyZ
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Cool. Good to confirm that your vacuum advance isn't doing anything at idle and that it does work when you rev the engine. Sounds perfect.

As for the stalling on throttle release... Long standing automotive issue. First thing I would do is check the mixtures. I've seen that dying at idle and a hunting idle when the carbs are running rich.

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  • 3 months later...
On 2/16/2022 at 10:34 PM, Captain Obvious said:

Well I'm no expert on the subject, but my answer is "In theory, it shouldn't matter whether the vacuum advance line is connected or not."

Why? Because the source for the vacuum advance is a ported vacuum source that should provide no vacuum at idle. So, if your car is tuned and idling properly and your throttle butterfly is where it belongs when your engine is idling, then you should have no advance at idle. And because of that, it shouldn't matter whether that hose is connected or not.

However, if your car is out of tune in some way and you need to have the throttle open further than normal just to get the engine to idle, then all bets are off.

So, here's a way to check... Put the timing light on it at idle with the vacuum line connected. Then disconnect the line and see what happens. There should be no change in engine RPM and no change in timing advance.

He’s lying to you….he is an expert ! 

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The way I understand this after rebuilding my distributor.   You have 17 total degrees of mechanical advance with the 8.5 counter weights in the distributor (btw some have 9)  The engine rotates 2x for one distributor rotation.   This equals 8.5 x2 = 17 deg mechanical advance, and according to the FSM you should get full mechanical advance at 2500 engine rpm (1250 distributor rpm). Distributor vacuum advance should have about 10 deg of advance at 2500 engine rpm.   So at 2500 rpm you should have a total of 37 degs of advance and you can check that with a timing light by setting 2500 rpm and you should see the light flashing at 37.   If it’s way off of that at 2500 you could have issues in your distributor (breaker plate bearings missing or rusted) or a blown vacuum dash pot.  BTW….I’m definitely not an expert.  

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I had forgotten about him but you have successfully dusted off some old brain cells. Isuzu kind of fell of the US map. That had some semi-cool vehicles back in the day. They should have never called that small SUV an Amigo.

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  • 2 weeks later...

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