Weber 32/36 uphill idle issue

[alsimma95]

2 minutes ago, '76mintgrün'02 said:

 

Both ported and manifold vacuum go to zero at wide open throttle.  It's light throttle cruising where they tend to kick in.

 

Have you checked your all-in advance to see how much there is and at what rpm that happens?

So yeah initially what made me think about this whole blowby/vacuum thing was at light to mid throttle sometimes it feels a bit tired, or lacking in power, whereas when I open it up and press the pedal down fully the engine feels smoother and it accelerates better. I end up pretty heavy on the throttle driving around in normal traffic because that seems to be when it operates best. So the decreased performance from retarded timing due to reduced engine vacuum becomes a nonfactor as the mechanical advance is engaged. At least that's what my impression of what was going on is.

 

And no I've never fully charted out my timing. Before this thread I never really considered timing at all beyond setting it according to the manual.

 

 

 

 

 

 

 

 

['76mintgrün'02]

e21 distributors have been discussed a bit on this forum.  The early ones fit the 2002, but the later spin counterclockwise and have a backwards-skewed gear.  Which year are you dealing with?  Does yours spin clockwise, like the 2002? 

 

Dual function pods could add confusion to the equation.  Are you sure you're connected to the Advance side of the pod?  Some of your symptoms make it sound like you're on the retard side.

 

11 minutes ago, alsimma95 said:

I've never fully charted out my timing.

 

I was just wondering if you knew the total advance degrees/rpm, not suggesting you plot the whole curve.

 

 

[alsimma95]

57 minutes ago, '76mintgrün'02 said:

e21 distributors have been discussed a bit on this forum.  The early ones fit the 2002, but the later spin counterclockwise and have a backwards-skewed gear.  Which year are you dealing with?  Does yours spin clockwise, like the 2002? 

 

Dual function pods could add confusion to the equation.  Are you sure you're connected to the Advance side of the pod?  Some of your symptoms make it sound like you're on the retard side.

 

 

I was just wondering if you knew the total advance degrees/rpm, not suggesting you plot the whole curve.

 

 

In an exciting turn of events, you were completely right! It makes sense why my static timing without the advance was 10 degrees, and 2 degrees with it connected.. It was actually being retarded. The car feels much livelier under light load, revs more freely and doesn't feel like I have to 'work for it' when cruising around. WOT doesn't feel too different. I gave it a quick retune but I'll have to play around with it some more to get it feeling right for my driving style. My static timing is close enough that it's 'within spec,' maybe a bit advanced, so I'll keep an eye out for detonation.

 

Such a simple oversight I never thought to check. Yes my distributor spins clockwise, my car is a 78. I got the car 2 years ago and only got it 'running' last summer before I got busy with school and put it away. A previous issue I had was the p/o marking the distributor cap terminals with the plug firing order, but installing the distributor cap backwards, so the keyed cap wasn't in the proper orientation when the plug wires were in the correct order. This took me about a week before I realized that the plugs were in the incorrect order when the cap was on right....

 

And to be honest, I'm not quite sure what you meant by all-in advance I guess. I rarely bring the car over 3k rpm because it's a tired old engine, so I never really considered too far up in the rpm range. So I guess no, I don't know what the all-in advance is.

 

 

['76mintgrün'02]

Tired or not, it'll come alive after 3k rpm.  Rev it up!

 

All-in advance degrees is good information to have.  Check it out!

 

Off to work I go.

 

Tom

 

 

[Mark92131]

1 hour ago, alsimma95 said:

And to be honest, I'm not quite sure what you meant by all-in advance I guess.

 

It depends on what distributor number you are running, a combination of mechanical and vacuum advance at a set peak RPM.  Here's a chart for BMW motors that may have your distributor part number.

 

Mark92131

[John76]

3 hours ago, alsimma95 said:

From what I understand it should be plumbed to carb port not intake manifold?

There is a lot of debate on which vacuum advance source is best.

As @'76mintgrün'02 correctly noted above, there is no difference between ported and manifold vacuum when the engine is above idle speed.

If you have a vacuum gauge connected to the ported spigot on the Weber, you should see zero vacuum at idle. This is because the high manifold vacuum hole in the primary barrel is blocked by the throttle butterfly... which is why it is important to keep the idle speed screw at the correct setting. With zero vacuum to the distributor at idle, you are relying on your static advance only...and possibly a few degrees of mechanical advance depending on the idle speed. This is OK, but the engine will be much happier (cooler, smoother idle) with the vacuum advance adding 10 degrees to the static setting and slight mechanical advance. 

If the vacuum advance is plumbed to the carb, then static advance would need to be 10-12 degrees BTDC to get the cool, smooth (happy) idle... but you run the risk of too much total advance, not to mention harder starting. 

Static advance of 3 degrees is ideal for starting (no vacuum or mechanical at starter-motor speed), but as soon as the fire is started, the vacuum kicks in and idle is advanced (no load). As Tom said, once above idle speed, then both ported and manifold vacuum are the same. 

So why is there a ported vacuum source on carbs?  It is for emission controls to turn off the EGR system at idle (no vacuum). 

John

[TobyB]

Quote

There is a lot of debate on which vacuum advance source is best.

 

Possibly, but the right answer is always going to be ported, because...

 

best idle is at somewhere in the 10-15 degree range, as you note. 

However, best light throttle cruise (say, 25 in third or 35 in 4th) is in the mid- high 20's all the way up to 40.

So the extra 5-8 you get from the mechanical advance just isn't enough.

The stratified charge at light cruise is going to take a lot of time for the flame

to propagate, so while it still makes a nice 'pop', it has to be lit earlier to get the most work out of it.

This is even true up to 55+, at small throttle openings.

 

Remember, it's still a (roughly) 13:1 air/fuel mix, but there's not MUCH of it in there, since air is very restricted

by the throttle plate, and fuel's very limited by the transition circuit.  So it's not LEAN, it's stratified, thin.

Slow to spread flame, as there's a bigger jump from molecule to molecule...

 

Now, as you open the throttle to go 80, go up a hill, pass someone, etc, there's a lot MORE fuel and air in there,

it burns a lot faster, and even though the engine's spinning faster (less time per cycle) the flame front is also travelling

a LOT faster, and ideal timing drops back to just the mechanical curve.  Which works out, as the ported idle circuit

is now at almost atmospheric, so the vacuum advance drops out of the equation.  There's actually a restrictor

that meters how fast the diaphragm's allowed to move, to match engine inertia.

 

Any EFI that doesn't account for charge stratification somehow is leaving low- throttle power on the table...

 

t

likes throttle position sensors.

[jimk]

1 hour ago, TobyB said:

Possibly, but the right answer is always going to be ported, because...

 

Isn't it because additional advance not to exist at idle but at part an medium load where port vacuum occurs.

The operator's manual describes when the vacuum advance should start and stop, that isn't going to happen if connected to the manifold.  Seems like whoever started this idea of manifold connection with a mechanical distributor didn't read the manual.

The 123 distributor can be set up either way if the user understands manifold pressures and carb design.

Edited July 26, 2023 by jimk

[John76]

3 hours ago, TobyB said:

However, best light throttle cruise (say, 25 in third or 35 in 4th) is in the mid- high 20's all the way up to 40.

Yeah, but at light throttle, the manifold vs. ported vacuum is the same. 

WOT the vacuum drops off and you only have static plus centrifugal advance which should be about 40.

At light throttle cruise, you get static + centrifugal + full vacuum (light load) and can easily get to 50... same for both man/port.

The advantage of manifold advance is you get your full 10-15 degrees at idle with a better start-up at 3 (good for low compression engines).

 

[TobyB]

Quote

Yeah, but at light throttle, the manifold vs. ported vacuum is the same. 

 

Yup- and total advance can go well into the 40s, as you note.  But for starting and idle, 

the 10 degrees or so btdc of just the mechanical advance is about right.  If you connect the manifold

to the vacuum pod, then you'll get full vacuum as well at idle,

and THAT will be another 16- 24 degrees, depending on the pod.  So you'll be idling at 25- 30 btdc,

which doesn't seem to work too well.

 

I think where we're missing each other is that the mechanical advance alone at idle ought to be in the 10 degree btdc 

range, and increase up to 36 or so at 4500.  THEN the vacuum advance adds its advance on top of that,

BUT never at significant load.  

And my point is that at idle, if you're connected to manifold, you'll jump to 25+ degrees btdc

 

t

 

[John76]

3 hours ago, TobyB said:

I think where we're missing each other is that the mechanical advance alone at idle ought to be in the 10 degree btdc 

Toby, you are correct!

I get approximately 11 degrees MECHANICAL advance at idle, plus 3 degrees static = 14.

My car is a '76 Ca. spec with a dual -pod distributor (advance and retard). When the engine is at operating temp., the T2 coolant switch (under the intake manifold) is open, which switches the EGR relay to close the black electro valve (to the dizzy pod), turning off the vacuum advance (+10). So, idle advance stays at 14 degrees. No vac advance except on a cold start. 

This is the devil I have to deal with a '76 in Ca. which still requires biennial smog tests... however my car runs absolutely light-years better than when it was brand new.

John