Vacuum advance at max at idle

[GL03]

Symptom: transition from idle to part throttle hesitation under load and stalling under snap of throttle

Checked and verified 63 Service manual specs 215V8:Timing set 5 degrees, no vacuum advance, plugged line from aluminum base of 4gc rochester. Tach 500 rpms for automatic.
Vacuum line from carb base showing 17 in. ,with specs calling for min. @14 in.

Engine is fitted with StreetRace cam, just one above stock, mild increase therefore 17 in. vs. =>18 in. vacuum. Pertronix igniter fitted instead of points. Carb, has been kept up, base set,mixture, etc.

Concern: vacuum advance begins to pull(doesnt leak) at 10 in. and max pull at 18 in ,this is normal. Now, the problem is at idle my vacuum advance is applying max pull, because i have 17 in. Vacuum from my throttle valve the base plate the only fitting for this on 4GC. So, my idle rpms go to 600 from base of 500 with advance plugged in. I can see it in the timing advancing more than 10 degrees. (irrelevent) dont know amount i have Sears inductive with no adjustable reading.

SUMMARY: The vacuum advance comes in at part throttle and turns off at WOT, and it should not be at maximum advance at idle because it will prematurely activate the advance.


1963 Buick Chassis Service manual, page 10-33.


Timing & Vacuum Advance 101 -good article by GM engineer

 

[Bob]

When you set your timing with the ported advance plugged, it should not make any difference to the idle when you plug the vacuum advance back in. In fact, if everything is working properly, in theory, you don't even have to plug the port to set the timing. The fact that it does means your carburetor throttle valves are sitting too high in the venturi. You can also detect this problem by plugging the carburetor port with a vacuum gauge. It should read 0 at idle and show a signal only when lifted off idle.

The carburetor throttle valves sitting too high in the venturi is a common problem with a heavily cammed engines, but yours does not sound too much. With your carburetor port plugged (preferably with a vacuum gauge), give the engine a little more timing to bring the rpm up, then try lowering the idle with the idle screw until the vacuum gauge is reading 0. Do not be afraid of running a higher initial timing (10 degrees is common) to get the throttle valves to sit down. In extreme cases it can be necessary to drill bypass holes in the throttle plates to get them to sit properly.

 

[LARRY70GS]

I'm pretty sure in 1963, vacuum advance used manifold vacuum for advance at idle. That is normal. Ported vacuum advance was used in later engines for emissions reasons. Manifold vacuum advance results in a cooler running engine.

 

[Bob]

I'm pretty sure in 1963, vacuum advance used manifold vacuum for advance at idle. That is normal. Ported vacuum advance was used in later engines for emissions reasons. Manifold vacuum advance results in a cooler running engine.

I don't know, but the clip included with the posting says the opening is at the "atmospheric side of the throttle valve". To me, that sounds like above the throttle plate.

 

[LARRY70GS]

I don't know, but the clip included with the posting says the opening is at the "atmospheric side of the throttle valve". To me, that sounds like above the throttle plate.

Right, but he also says that he is getting full vacuum advance at idle. That means a manifold source as it should be in 1963.

Concern: vacuum advance begins to pull(doesn't leak) at 10 in. and max pull at 18 in ,this is normal. Now, the problem is at idle my vacuum advance is applying max pull, because i have 17 in. Vacuum from my throttle valve the base plate the only fitting for this on 4GC. So, my idle rpms go to 600 from base of 500 with advance plugged in. I can see it in the timing advancing more than 10 degrees. (irrelevent) dont know amount i have Sears inductive with no adjustable reading.

 

[GL03]

I threw everyone a curb ball. I mistakenly read the vacuum, it was actually less at ~14in. Hg. this image was taken before the post. Snap on gauge reads backwards.





Anyways, I pulled the carburetor and replaced gaskets and found some gum in the power valve from water tank fuel contamination lack of driving car. Blew all the passages, checked floats, base plate settings, the power valve and pump operation. By spraying some spray. the vacuum passage shows me it's path on the base.



Redid the timing procedure as suggested, went to 10 degrees instead of 5 for initial setting, but the vacuum was hanging around 14. Set at 7.5 degrees. Before I continue, I have to replace the carb base gasket because of a vacuum leak I created reusing old gasket.

Work in progress.

 

[GL03]

At completion of the base gasket replacement, the vacuum sits at 16 in. Hg. Reset timing, didn't drop vacuum at 10 degrees, so returned to the 5 and no difference so settled at 7.5 degrees. Runs better, but the hesitation and stall at hard acceleration is still present. The vacuum advance continues to come in more than I'd like to believe, so I've decided to look somewhere else.
The fuel pressure calls for 4-5.5 but test is showing 7 and intermittent 8. Another thing I noticed after turning off the engine, the pressure doesn't hold, goes to zero instantly. I can see two valves in the pump, so will replace the pump and retest.

 

[kensgs01]

Running manifold vacuum is fine and will deliver great off-idle response and instant response at cruising speed, 35-40 mph. This is because after you start the engine you will pull most if not all of your vacuum advance from the distributor cannister. Therefore, you will be running your initial timing, (5-10 degrees?) plus the vacuum advance timing which may put you as high as 20 degrees at idle. This is fine with the engine running, just set your idle to where you want it and don't worry about where the advance is. Your engine will actually run cooler because you are burning all the cylinder mixture before the exhaust valve opens. The secret here is to have your distributor curve set to max out at 30-34 degrees (you will need a dial-back timing light to check this) so you don't exceed what its's supposed to be at for your particular engine. If you exceed total advance numbers for your engine by not recurving the distributor to limit total advance, you can damage your engine with too much advance resulting in pre-ignition (Pinging.) So, by running manifold vacuum and recurving your distributor your engine will give you excellent response and actually run cooler. It's a win/win...

 

[GL03]

I've taken notes on timing and the advance and will work on it further. The car has the late produced disposable crimped type fuel pump 40001 part, it continues to be sold for the skylark as the correct part, but it outputs too high of pressure. I've replaced it with a NOS 4827 fuel pump, and the output fuel pressure was within specs.

I narrowed it down that the fuel pump is not squirting consistently. There appears to be no squirt at the top and bottom of the plunger path of travel. I was in the last hole for testing and went back to the second hole as the manual recommends. I adjusted several times and decreased the float adjustment from .140" because the float is not parallel to the gasket. Simply that I increased the amount of fuel entering the pump plunger chamber. Also, I removed some of the play in the linkage. I have a good squirt from the Rochester.

Symptom: Transition from idle to part throttle hesitation under load and stalling under snap of throttle
Cause: Incorrect float setting, excess play in pump linkage, ethanol byproduct contamination in carburetor
Correction: Adjust floats and linkage, and clear carburetor pump circuit using wire and air