Bosch 034 timing anomaly

[Tim Donahoe]

When I time my Bosch 034 distributor, it seems to make no difference if I plug the vacuum line at idle, or don’t plug it. Either way, at idle, say if I time it with the vacuum plugged, when I reinstall the vacuum line, I still end up with 7.5 degrees. Advance is between 30-32 degrees at maximum rpm’s.

Is it possible that the vacuum port (I’m using the correct one) is plugged, and I’m only getting centrifugal advance, and no vacuum advance at all?

Tim
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[gt1953]

Hmmm interesting. So with the engine idling and timing light connected suck on the vaccum line and see if the timing changes, yes or no. If yes then the vaccum pot is working so you may have the vaccum line plugged in the wrong port. No the vaccum pot coulb be devective. Oh what carb is in use?
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[busdaddy]

Idle timing should not change hose on or off, what happens when you leave the hose connected and rev the engine to the same speed you did for the hose off 30-32?, still 30-32?, or more? How about if you blip the throttle from idle while watching the mark with the light?, slowly advance with engine speed?, or a sudden big jump?
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[Tim Donahoe]

Gt, I have a Mexican Bosch SVDA 034 which I bought new a few years ago, and it has very few miles on it. The vacuum pot is fine.

I have this distributor coupled with a Solex 34 pict 3 carburetor. I bought this carb from a Samba member, then had Volkzbitz restore it. The carb is wonderful, and is a perfect match for my Mexican Bosch. However, as I said, the 7.5 mark seems to stay where it is, whether I disconnect and plug the vacuum line, or not.

Also, the vacuum nib I use is on the left side of the carb where you’re supposed to plug the vacuum hose.

I don’t have a degree wheel pulley, and when I check full advance, I’m only used a good guess as to exactly how far I’m advancing at 3000-3500 rpm’s. And it does advance.

Even so, I suspect the vacuum port on the carb is plugged for the reasons I stated previously.

Tim
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[67rustavenger]

I agree with BD.
The timing should not change at idle, vac tube connected or not.

When you open the throttle plate. The distributor should begin to advance. You can see it change with a timing light.
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[74 Thing]

Hook up a vacuum/pressure gauge to the line and blip the throttle just sitting in the back and you should see vacuum.
https://www.harborfreight.com/fuel-pump-and-vacuum-tester-62637.html

You can run a long line to the passenger seat and tee in the vacuum gauge while driving to see what kind of vacuum your engine pulls during driving conditions.

You can also get a Mityvac to pull some vacuum on the vacuum pot while sitting back by the engine with a timing light and see how the vacuum amount relates to the advanced timing.

https://www.harborfreight.com/brake-bleeder-and-vacuum-pump-kit-63391.html

[Tim Donahoe]

I just double-checked the vacuum hose while on then off at 875 rpm’s (idle), and the 7.5 degree notch stayed right at the crack in the crankcase.

However, I then hooked up a vacuum gage, and since I didn’t have my tach hooked up, I’d guess that I got the engine up to 2000 or so rpm’s. The vacuum gage on the HG scale went to 6, and climbed higher when increasing the rpm’s a little more. At idle, the Harbor Freight gauge showed nothing.

This tells me that I do, indeed, have some vacuum to the vacuum pot. However, I’m stumped as to why VW says to disconnect and plug the vacuum hose at idle when adjusting the timing. The timing at idle remains exactly the same, whether the hose was disconnected and plugged, or not.

Knowledgeable folks on this forum also say to disconnect and plug the vacuum hose when timing an SVDA and 34 pict 3 combination, so I’m left wondering what’s up.

P.s. I get 28 mpg’s on the highway, so something is going right. However, my plugs have always tended somewhat black, as opposed to tan or gray, when I pull them, despite what I believe is reasonably correct carb adjustment.

Edit: a search revealed that I should have a 11-17 reading at idle. I then attached the vacuum gauge to the manifold pipe that is used to provide vacuum to the air cleaner valve. This male fitting is located just under the carburetor on the left side of the manifold. At idle, I got a 17 reading, and the needle was a little jiggly.

Tim
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[busdaddy]

We disconnect the vacuum because no two distributors advance exactly the same, especially after ~45-50 years, and the recent Chinese versions add a whole new level of WTF to the mix. By disconnecting the vacuum you ensure the mechanical advance is where it should be, the vacuum part doesn't come on under heavy loads so setting to max mechanical (hoses off) guarantees you won't over advance but are still in the sweet spot for good power. In a perfect world you could set it to 7.5* BTDC @ idle like VW specified for brand new cars, but as rubber parts harden and springs age that advice goes out the window, even after a rebuild it's safer to set to maximum mechanical and connect the hose after regardless of where the idle ends up.

On the road is quite different from in the driveway, add some wide open throttle under a heavy load and the vacuum part pretty well doesn't do anything, it only works under light loads and city driving.
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[kreemoweet]

The carb throttle valve setting has a big effect on vacuum at idle. When correctly set, you
will have ZERO vacuum at the side vacuum port. Disconnecting the vacuum hose, and plugging,
eliminates this variable, so you properly set the timing no matter what's going on at the carb.
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[ashman40]

Hi Tim, found an old thread that may help your understanding…
https://www.thesamba.com/vw/forum/viewtopic.php?t=504060

From that thread….

This illustrates where the “ported vacuum” on the left side of the 34Pict carb gets its vacuum signal from. You can see it taps into the carb throat just above the closed throttle plate. So at warmed up idle (with the throttle plate properly adjusted) the port is blocked off and only sees the vacuum that exists above the throttle plate, which at idle is very small, or none.

As you would imagine, as soon as the throttle plate is opened, ported vacuum is exposed to the same vacuum as intake vacuum.

This high vacuum exists just as the throttle is opened as well as when the engine is holding a steady rpm while cruising. The vacuum in the intake goes high as the throttle is mostly closed, but unlike idle the ported vacuum port sees the strong intake vacuum.

One of the best diagrams from that thread shows the relationship between throttle position and the difference between ported vacuum and intake vacuum.

Here you can see that anytime the throttle plate is closed (yellow line at bottom of chart) the purple ported vacuum level also drops to zero. But as soon as the throttle plates opened even slightly, the purple line jumps up and follows the blue intake vacuum almost perfectly. Also, when the throttle is fully opened (WOT, yellow line at top) the intake and ported vacuum levels drop to near zero (bottom of the chart). This indicates the engine is under load.
What this means for the SVDA distributor, it sees a spike in ignition timing just as the throttle is opened and also when cruising. The off idle spike compensates for a lean idle air-fuel mixture. The cruising spike contributes to better fuel economy. But when the throttle is opened wide (under load) the vacuum advance timing will drop to near zero to avoid detonation due to preignition. This is why the SVDA/DVDA distributors are sometime called “load sensing”; unlike OO9 distributors which only respond to changes in rpm. This load sensing is exactly what you want when running a lean 34Pict carb.
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[Tim Donahoe]

Ashman, thank you. Now I understand a lot more on how this carburetor works.

I went to a friend’s house today to confirm his timing (he doesn’t trust his timing gun) with my timing gun, and on his stock 1600 DP engine with restored Bosch 034 and Solex 34 pict 3, he gets a different timing reading if we disconnect and plug the vacuum from the carburetor, as opposed to leaving the vacuum attached. According to what I just read, he surely has his throttle plate open a bit at idle, and that why we got different timing readings from attached to unattached and plugged scenarios. His WOT with the vacuum removed and plugged was 30-32 degrees. If we attached his vacuum line and tried to time it the same way, the 7.5 BTDC notch was in a different spot.

I suspect my 7.5 degree notch doesn’t move at idle is simply because my throttle plate is closed more than his; hence, when timing my engine with the vacuum line attached, no vacuum is at work until I open the throttle arm and increase rpm’s.

I know for certain that my advance is working, but I just don’t know how much because I don’t have a degree pulley. I have guessed around 30 degrees at WOT, and a quick jerk on the throttle will temporarily increase that by another 10 degrees. But I can’t really judge without any educated marks on the stock pulley.

Tim
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[ashman40]

Just a note... the proper throttle plate adjustment for the 34Pict carb is to rotate the fast idle cam so the throttle arm screw is resting on the lowest level of the cam. Unscrew the adjuster at the end of the throttle arm until it does not touch the cam. Screw it in until it is just touching, then screw it in another 1/4 turn. This extra 1/4 turn is not to open the throttle plate. It is just to make sure the closed throttle plate is not resting on the inside of the carb throat. The throttle plate should be fully closed, blocking off the ported vacuum port in the side of the carb throat. Here's a pic from glutamodo to show this:

Note how the dark colored line which is where the throttle plate rests is below the advance drilling. This blocks off the intake vacuum from reaching the hole when the throttle is closed.

For your friend's carb, try unscrewing the adjustment screw at the end of the throttle arm and see if it makes a difference in the vacuum available on the left side port at warmed up idle. If loosening the screw reduces the vacuum it means the screw was not properly adjusted to begin with. You will need to readjust the volume and bypass screws to reset the idle.

Note that if the throttle screw is resting on any of the fast idle cam steps the throttle plate will be open and the ported vacuum will be exposed to the intake vacuum. Only check after the choke is fully OFF.


I believe there are some models of 34Pict carb that have more than one hole near the closed throttle line. If part of these holes are below the closed throttle they may be exposes to some intake vacuum even with a completely closed throttle plate. In such cases, it makes even more sense why the instructions for adjusting the timing at idle has you disconnect and plug the vacuum hoses. Not all 34Pict carbs are the same.
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AshMan40
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'75 Beetle 1200LS (RHD Japan model) {junked due to frame rot}
'67 Beetle #2 {2019 project car - Wish me luck!}

[Tim Donahoe]

Great information, Ash.

Thanks,

I’ll pass along these teachings to my buddy, and keep some for myself.

Tim
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