Too Much Vacuum At Idle - 30PICT1 and 205K

[67_Deluxe]

I wouldn't have thought timing my 67 Bug would be such an enormous headache.

I did a full tuneup on the car - rebuilt 205K distributor, points, & vacuum can, rebuilt 30PICT1 carb from Volkbitz, set the valve clearance, oil change, blew out idle jet with compressed air, the whole nine.

I had some trouble isolating the problem for why the car wouldn't idle properly, but I finally chased it down with a digital timing light and some sleuthing.

All the threads I've read on TS have advised that there shouldn't be much - if any - vacuum at idle. When I do the following on a warm, warm, warm, engine:

1. Statically time the distributor to 7.5° BTDC with a continuity tester;
2. Remove distributor-side vacuum tube and plug hole (while other end is still in carb);
3. Hit the pulley with timing light set to 7.5° BTDC;

Everything looks good. The pulley hits at TDC with the light, indicating that it is indeed at 7.5° BTDC. I use the (only one screw on the 30 PICT 1) fuel screw on the carb to set the idle to 875RPM on the timing light's digital tach (it bounces around from 850-890 RPM).

Here's the question: If there shouldn't be much, if any, vacuum coming from the carburetor at idle, then why does timing jump all the way to 15° BTDC as soon as the vacuum line is reinserted into the distributor's can? As you probably already guessed, the RPMs also increase to 1000. When under load (3000-3500 RPM), distributor still produces 32°-ish total advance, however, at idle something (the carb?) is adding 7.5 additional degrees of advance.

I think other posts have already answered my question that this much advance at idle shouldn't be occurring from the vac line, but if I'm wrong about that then please let me know. What I think I'm supposed to be asking here is: what could be causing this problem? Pertinent - and not so pertinent - information below.

Engine:
1500 SP Case (original to car), pistons bumped to 85.5 during rebuild in 2014 (so it's a 1600 now).

Current compression numbers
Cyl 1: 153psi
Cyl 2: 155psi
Cyl 3: 152psi
Cyl 4: 164psi

Distributor:
model -205K
points set to 48° on dwell meter

Carburetor
Western German 30 PICT 1 (105-1 on the flange), rebuilt by Volkbitz

Spark Plugs
Beru 175/14 NOS, gapped.

Any help would be greatly appreciated.

[heimlich]

Please take a picture of the distributor and vacuum can. If the wrong can is on there it can cause issues.

Can you put a vacuum gauge on the carb to see what pressure it is pulling at idle?
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[ashman40]

Even though your engine is warmed up, is the choke butterfly fully open (vertical)? If the choke butterfly is even partially closed it will create more vacuum in the venturi area which translates to more vacuum to the left side vacuum advance port.

The other possibility I can think of is the venturi vacuum port in the carb throat is plugged. This forces the vacuum to come from only the lower vacuum source which has a stronger vacuum. Check this by squirting carb cleaner into the left side vacuum port and see where is comes out from. You may need to take the carb off the manifold to see where cleaner is coming out from in the lower portion of the carb. If cleaner only comes out from a lower port (below the closed throttle plate) it explains the high vacuum at idle.

See the below table which maps the amount of vacuum required to get degrees of timing advance.

According to this table you need 28mmHg (1.1inHg) before the vacuum canister raises the timing by 7.5deg.
_________________
AshMan40
---------------------------
'67 Beetle #1 {project car that never made it to the road }
'75 Beetle 1200LS (RHD Japan model) {junked due to frame rot}
'67 Beetle #2 {2019 project car - Wish me luck!}

[67_Deluxe]

[ashman40]

Actually, reading the table a second time... it is a range of vacuum that could result in 7.5deg. It is between 28mm ~ 42mmHg.
_________________
AshMan40
---------------------------
'67 Beetle #1 {project car that never made it to the road }
'75 Beetle 1200LS (RHD Japan model) {junked due to frame rot}
'67 Beetle #2 {2019 project car - Wish me luck!}

[67_Deluxe]

Carb pulled, venturis clear all the way through, plate angle looking ok as well. Still searching for the culprit.

[glutamodo]

I was trying to post a detailed response to this hours ago, but either I hit a wrong button or something glitched and it got lost and I didn't feel like going over it again.

I have demonstrated how having the lower advance drilling in the wrong spot relative to the throttle butterfly will throw this all out of whack. I would not expect to see this on a stock carb, but I guess it's possible. My experience was the first generation of EMPI brand knockoffs.

https://www.thesamba.com/vw/forum/viewtopic.php?t=278191

A couple of comments. Originally some 66 30PICT-1 owner's manuals still specified the previous 550RPM idle speed. This was soon changed to 700-800RPM. Later to 750-900. If this is a single port engine, it should easily be handle lower speeds, you might just go with 750RPM and see how it works.
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Andy T.


IMAGE NOTE: It has been noted that Chrome based browsers may have issues in displaying my vast image library, which use non-secure links and are on an FTP server. Images should still be viewable if the link is clicked though.
I do not know how to fix this. All I can say is it all works fine for me with what I use, Firefox.

[ashman40]

[andk5591]

I would reach out to Tim...But look at Andy's thread on the carbs...might have plugged port. Tim's work is amazing, but maybe he missed something..
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[kreemoweet]

[67_Deluxe]

[mukluk]

[67_Deluxe]

[Scott65Beetle]

IIRC I set the timing with the vacuum hooked up to idle at roughly 8-900 rpm and if you pullrf the vacuum line it would idle way slow, This was on a 1300 with 30pict1 and vacuum only distributor. I would get approx 40-42 degrees at 300 rpm.

[mukluk]

[glutamodo]

Actually, 40-50mmHG at idle is way too high. Depending on the idle speed but still.

To me is sounds like it's not being properly balanced by the upper venturi fitting.

I linked above to a thread about how early EMPI 30PICT-1 carbs did not provide ENOUGH vacuum at the low end, and it was due to having the lower drilling just a millimeter or two too high. I was able to fix this, but it's all in that thread. However I will pull from that thread, this chart of what to expect for this sort of vacuum advance vs RPM. 40mmHg is a bit high.

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Andy T.


IMAGE NOTE: It has been noted that Chrome based browsers may have issues in displaying my vast image library, which use non-secure links and are on an FTP server. Images should still be viewable if the link is clicked though.
I do not know how to fix this. All I can say is it all works fine for me with what I use, Firefox.

[67_Deluxe]

I owe a debt of gratitude to everyone on this thread who has helped. You were correct and the "Idle Adjustment Screw" was indeed the culprit.

I think - for posterity (esp. folks checking out this thread in the future) - my takeaway is that step one should be "loosen or remove the barrel nut holding in the throttle cable." In hindsight; a really stupid mistake, but live and learn.

I set the idle speed to 775 RPM and that gives me 8 degrees of advance at idle, but preserves a full band of advance up to 31 degrees at 3500 rpm (I believe that's what it's supposed to be?).

The issue from what I can tell is that the throttle cable was putting too much "tug" on the idling arm, thus creating the vacuum in the carb's throat at idle.

Again, thanks very much to all. The old bug purrs like the day it rolled off the factory line.

[glutamodo]

Wow, throttle cable adjustment. I've talked a lot about various things here over the years, this has come up more than once, but I tend to overlook the possibility.

Me, I usually adjust the throttle cable by a different criteria, Full Throttle, which is laid out in the service manuals.

Now, after setting it up that way, I always make sure that the cable still has enough idle slack to make the carb rest properly that way.
_________________
Andy T.


IMAGE NOTE: It has been noted that Chrome based browsers may have issues in displaying my vast image library, which use non-secure links and are on an FTP server. Images should still be viewable if the link is clicked though.
I do not know how to fix this. All I can say is it all works fine for me with what I use, Firefox.

[ashman40]

[67_Deluxe]