Dave's Experiences Tuning His 1973 Super Beetle


Note: The emphasis of this section is on Dave's experience with tuning his '73 SB. A mystery plagued Dave's Bug for a long time; in Dave's words - "Why does this distributor/carburetor setup want 7.5 degrees BTDC when it should want 5 degrees ATDC? Thereís no question," Dave says -- "I almost canít keep it running if itís advanced any less, and even then it runs very poorly." Also: "Why does it want to idle so high? If I turn the bypass screw in at all (idle lower than 1200 rpm) the engine dies." With Rob's very able assistance, Dave worked on this problem over a period of several very frustrating years.


Bottom Line -

Tuning of Dave's system proved to be impossible because of a severe air leak around the carburetor throttle shaft. This leak, which remained undetected for a long time, caused the fuel/air mixture entering the intake manifold to be too lean, which meant that the idle had to be set excessively high just to keep the engine running. This higher-than-normal idle made it impossible to time the engine properly. See our article on Air Inleakage for more details. A new SVDA distributor and a new 34 PICT/3 carburetor finally! solved Dave's problems, and now his beautiful little black Bug now purrs like a kitten and fweems down the highway without any hesitation.

The following notes are a condensation of conversations between Dave and Rob as they attempted to resolve this problem.

Elsewhere on this Web page we have included Complete Tune-up Procedures. It is with Step #5, "Adjust Idle," and Step #6, "Timing," that Dave had trouble. In retrospect the solution to this problem is very obvious, but it took Dave the better park of three years to discover that he had air leaking into his system.

There was a considerable amount of interchange between Rob and Dave (and others) about this problem; we will just touch on the highlights to give you a feel for this learning experience, hopefully to keep others from having to repeat it!


Documentation of Dave's Frustrations

Idle -

From day one Dave found it virtually impossible to adjust the idle on his '73 SB (34 PICT/3 carburetor, dual vacuum distributor) to the specified 800-900 rpm. He found it necessary to set the idle at 1200 rpm or more just to keep the engine running at idle. Rob pointed out that with the faster-than-normal idle, the retard vacuum would not be working and the airflow/fuel flow through the idle circuits would be incorrect, making it impossible to properly time the engine.


Timing -

Dave went through three different distributors during this period, as follows -

  1. The car was originally equipped with an 009 centrifugal advance distributor; the correct timing for this distributor is roughly 7.5 degrees Before Top Dead Center (BTDC) (actually 28-32 degrees at 3000+rpm), with any vacuum ports on the carburetor plugged. This means the TDC mark on the pulley will be about 11mm to the LEFT of the crankcase split.
  2. Later Dave installed a vacuum-advance distributor (two vacuum hoses), for which the correct timing is 5 degrees After Top Dead Center (ATDC) with the vacuum hoses attached.
  3. Finally, after much research and on the advice of experts like John Connolly (Aircooled.Net), Dave installed a Single-Vacuum Dual-Advance (SVDA) distributor. As with the 009 distributor, the correct timing for the SVDA distributor is 7.5 degrees BTDC, with the vacuum hose plugged to prevent air from being sucked into the carburetor during the setting of the timing.

Rob has repeatedly advised that when using the 009, it is the timing ADVANCE that is the most important; this should be 28-32 degrees advanced at 3000+ rpm (the TDC mark 43.5-49.5 mm to the left of the split).

The timing at idle is not nearly as important as the timing at maximum advance. If it will take 32 maximum advance without pinging, and the idle comes to 11 or 12 BTDC, it doesn't matter much to the idle, but it will help with the 009 hesitation problem. In fact in your cold weather (northern USA), it might even take 33 or 34 maximum advance without pinging at full throttle, but if you do that, don't forget to reduce it a touch when the weather warms up again (detonation is a function of heat remember, so it's more likely in hot weather with an advanced ignition).

The 7.5 degree BTDC standard recommended by VW for single vacuum distributors simply matches the vacuum distributor's maximum advance characteristics. Some of the early 1200s used 10BTDC anyway, to suit their particular distributor design.

You could then experiment a bit with the maximum advance. Try 28 then 32 (but don't go outside this range -- 28-32 degrees maximum advance is about the best range for Beetles). See if this makes any difference in the car's performance (e.g., hestitation).

And see how this pans out at the idle speeds. For example, if the 28 degree maximum advance gives an idle setting of anywhere between 5 degrees ATDC and TDC (hopefully closer to 5 degrees ATDC), and the car performs well at this setting, I think that's the one to use. If you find the car performing better at 30 or 32 degrees maximum advance, and the idle is still just 5 degrees ATDC or TDC, then use that. But if the idle setting is coming out BTDC with the 30 or 32 maximum advance -- then I'd use a slightly lower maximum setting, to make sure that the idle is not any more than TDC -- for emission control. What this is doing is playing with the advance/rev curve of the distributor while making sure it remains slightly retarded at idle for emissions control.

Note added by Rob - The 009 distributor will normally provide between 5 and 10 degrees BTDC with the maximum advance set at 28-32 degrees. We were working through a problem with Dave's car that we didn't quite understand at that point, and SOME VW models in the US were originally timed at 5 degrees ADTC or TDC where the rest of the world used 7.5 degrees BTDC or 10 degrees BTDC for almost all models. The retarded ignition on some US models was one attempt by VW to meet stiffer US emissions standards, but the 009 distributor doesn't lend itself to retarded ignition -- the maximum advance would then be much too low.)


The foregoing is condensed to -

  1. Make sure the throttle lever is returning firmly to the cam and that the throttle screw is adjusted properly.
  2. Check to make sure the timing is at 30 degrees BTDC at 3000+ rpm.
  3. Try to set the idle speed at 850 rpm.
  4. Look at the maximum advance at 3000+ rpm again.
  5. If it's now MORE than 30 degrees, retard vacuum had not been overcoming the main vac at the 850 rpm idle speed -- in other words the retard vacuum is not working as it should.

  6. If no change in the 30 degree maximum advance, the idle speed of 850-900 rpm is not critical to the retard line doing its job, so set it where it feels right.
  7. Experiment with the maximum advance:
    • First try 28 degrees, then 32 degrees advance. See how this affects timing at idle.
    • Use the maximum setting that gives 5 degrees ATDC - TDC at idle. If the car performs well at this setting, use it.
    • If the idle setting is coming out BTDC with the 30 or 32 maximum advance, use a slightly lower maximum setting (idle setting no more than TDC) for emission control.

The 009 distributor should be timed at 28-30 degrees advance at 3000+ rpm. THEN you check the static timing, which might be 7.5 degrees BTDC, but can end up anywhere from 5-10 degrees BTDC, because the 009s vary in the total amount of advance they make, and the FULL advance is much more important than the idle advance. Once that particular 009 has been set at 3000 rpm and the idle timing established, the idle timing can be set statically for that particular 009 distributor (be it 5, 7, or 8 degrees BTDC etc.). 7.5 degrees BTDC should only be used as a rough starting point when setting up a 009, then you set it at 28-32 degrees advance at 3000+ rpm, and after that you can check the idle advance if you like, and then use THAT advance for THAT 009 distributor on THAT engine to set it static in future, if you prefer to do it that way.

And the MAXIMUM advance depends mostly on compression ratio and the available fuel. If compression is high or fuel quality (high oxygenates) is low, use 28 degrees maximum advance. If compression is low or fuel quality is high enough, use 30, then 32 maximum advance.

Short story: Try setting the maximum advance at 32 degrees advance. If it detonates, back it off to 30, then 28 maximum advance and/or try another fuel.

With the single vacuum distributors, setting the (static) idle meant the vacuum decided how much advance was introduced, and when. These can top out at around 40 degrees maximum advance under part load, but 40 degrees is way too much for full throttle high speed, where 30 degrees maximum advance is about right. So the 009 distributor (which has varying amounts of internal advance) has to be carefully limited to 30 degrees maximum advance so it will won't be over advanced in "worst case" conditions. This means it's not ideal in part load conditions of course, so it's not quite as economical as the vacuum units either. And once the 009 maximum advance timing has been established and the total amount of internal advance noted (checking the static timing) this static timing can then be used to set that particular 009.

So -- For the newbie's benefit, set the advance timing with a stroboscopic timing light, 30 degrees BTDC at 3000+ rpm. Then return to idle (850-900 rpm) and note the point on the pulley that crosses the split in the crankcase when #1 fires. THAT is the idle timing point for that particular 009 distributor (be it 7.5 degrees BTDC or whatever) and from then on it can be timed statically to THAT point.


Some of the conversations between Dave and Rob reflected Dave's frustration -

Dave wrote - I absolutely could not time the car to spec! When I tried it simply would not run. I ended up setting it at about 5 degrees BTDC (10 degrees advanced from spec!), and it ran wonderfully, with no hesitation and lots of pep. I fiddled and fussed with the timing, and finally gave up and just timed it where it ran the best (felt like I was emulating our meatball mechanic friend!). I am more than a little bit confused. It seems to want to run 7.5 degrees advanced (BTDC) at idle.

Rob responded - It will all come down to ONE faulty part you know -- PROBABLY the carburetor, from all the other things you've tried. Sometimes, Beetles being able to run when everything is wrong is a pain -- harder to find the problem than if it didn't work at all.

Dave wrote - Yesterday afternoon I went out and painted the timing marks more clearly on the pulley -- one at TDC (distinct "V") one to the left at 5 degrees ATDC, one to the right at 7.5 degrees BTDC, and another to the right at 30 degrees BTDC. According to the book, I should be timing the car at 5 degrees ATDC at idle (850-900rpm) and 30 degrees BTDC at 3000+ rpm. The engine "wants" to run at 7.5BTDC at "idle", which it also seems to "want" to be 1100-1200 rpm.

There is SOME advance mechanism in the (dual vac) distributor, because it advances very nicely to 30 degrees at 3500 rpm. I've verified that with the timing light many times.

Rob responded - These distributors (SVDSs) are supposed to provide the best of both worlds for the VW if you can get it working right.


Vacuum -

Dave asked - There's a way to determine whether the retard vacuum line is inoperative, isn't there?

Rob responded - Yes. Take the cap off the distributor and suck on each of the lines in turn. You should see the points plate move in opposite directions. The retard line will only retard a little of course, compared to the main vacuum's advance.

During timing the vacuum hose connected should only if you are timing it statically (engine off so the vacuum ISN'T operating. Otherwise this (and my single vacuum distributor) are timed at idle with the engine running with the vacuum hose DISCONNECTED and plugged temporarily at the carburetor so that air isn't sucked into the system and making the fuel/air mixture too lean.

It doesn't matter how the carburetor is plugged (on the carburetor on the other end of the vac line) so long as it's the vacuum port on the carburetor which is plugged, not the distributor.

The aim is two fold -

  • To stop the advance plate from moving so you can set the 7.5 degrees BTDC idle unaffected by any vacuum. Plugging the distributor end of the vacuum line is irrelevant at this point - it's inoperative anyway.
  • Prevent the carburetor from sucking air through the vacuum line so it idles evenly whilst making the idle adjustment. So you pull the vacuum line off and plug the carburetor end. The carburetor creates the vacuum, the distributor uses it.

Dave wrote - If I ever need to replace my distributor, I'll probably be going for one of these SVDA types too. I suspect it will work just a little better than the vacuum-only unit I have at present. The only concern I have with them for my car is that the rate of centrifugal advance (the 009 part of it) may be slightly different to the vacuum only rate of advance at each point in the rev range, which may change the engine acceleration characteristics a little, since it's a single port 1600, with different head/gas dynamics to the 1600 dual port. And the 30PICT/2 carburetor produces a different flow rate through the venturi than the larger throat 34 too. It will be interesting to find out if any difference is noticable.

Rob responded - There definitely IS a difference in distributors. I'm using a distributor with a small vacuum canister designed for the larger 34PICT/3 carburettor, but I'm using a smaller 30PICT/2 carburettor, and in fact I get a fractional hesitation off the line (like a mini 009 flat spot), which I've almost eliminated by setting the static timing at 9 degrees BTDC instead of the normal 7.5 degrees BTDC (as much as the engine can take without pinging). If vacuum canisters were not so expensive, I'd buy a larger vacuum canister which works better with the smaller carburettor to get the acceleration advance in a little sooner.


Vacuum Retard -

Rob wrote - Plug the vacuum lines at the carburetor -- the distributor end doesn't matter. Adjust the timing to 30 degrees at 3000 rpm. Reconnect the vacuum lines. With the engine idling, check the timing -- it should be 5 degrees ATDC or thereabouts if the retard line is working OK. If it's 0-10 degrees BTDC, retard line is not working.

And this is the RETARD part of the test. Hopefully after this you'll fully confirm which parts of the distributor are working and which aren't, whilst the car is running (rather than just static testing). Once it's confirmed that the retard line is working, slowly open the throttle and watch the advance with the timing light. It should change rapidly from 5 degrees ATDC to about 7.5BTDC as the retard line stops working.

If the advance line is not working the change might be gradual (revs vs retard only). If the advance is working, I would expect it to be fairly quick (revs + advance vs retard). In either case, the timing should certainly go positive since we know your centrifugal part IS working.

If the advance or retard are not working, you can either look for causes or start looking for a new distributor!

Just one final test to see if the actual LINES are plugged (rather than the distributor being faulty) -- pull each line off at both ends and see if you can blow throw them IN BOTH DIRECTIONS. This will check for any internal fraying or loose flaps of rubber inside the lines.

Then try sucking on one end of each with a finger over the other end to make sure they don't collapse under suction. Now THAT would be a "gotcha" wouldn't it! I don't expect any trouble here -- the Mike + Dave suction test indicates both lines are open (otherwise the retard suction would not have "stalled" against the advance suction). I'm feeling more and more inclined to think there is something amiss in the distributor itself like a lost screw under the points plate or something.

Reset the idle to 850-900rpm and the timing to 5 degrees ATDC at idle if necessary.


Accelerator Cable -

Dave wrote - I fiddled with the accelerator cable one more time. That 1mm of clearance between the throttle lever and the carburetor body is very important. The problem is, if I adjust it that way it won't return all the way the the stepped cam (lowest point). I thought I had solved this problem when I found the throttle valve binding against the inside of the throat in the carburetor, but it's doing it again. I guess I'll have to pull the carburetor and see. Any way to tell visually whether there may be air in leakage around the throttle shaft (upon inspection with the carburetor off the car)?

Adjustment of the accelerator cable is much more critical than I realized. If it's too tight, the throttle idle screw won't return all the way to the stop, and it is impossible to adjust the idle, and thus the timing, correctly. Once I got the accelerator cable adjusted properly, I found that the idle settings (which I had NOT changed during the engine removal process) were too low.

THEN I tore into the idle and timing. First thing I discovered was that the one reason the car was running so crappy was that the accelerator cable connection under the pedal was all screwed up. That connection is a very poor design, in my opinion. I bent the metal piece on the end of the cable around so that there's no way it can come loose from the pedal or get twisted around scroogy. Then around to the back and readjusted the accelerator cable on the throttle lever for the umpteenth time.

Note: See our Accelerator Cable Discussion for the correct attachment of the cable to the pedal.


Air Inleakage -

Dave wrote - Here's my plan -- check for air leaks. (See our treatise on air inleakage.) I'll spray around with starter fluid (ether) again, though I've never found leaks this way before. Then Bob Hoover says to clean the area around the throttle shaft with a "cancer causing solvent" :-) and then smear on a THIN film of RTV compound and allow it to cure. This sounds strange to me. He's talking about right around the throttle shaft where it goes through the carburetor on both sides, right? And the RTV will move with the shaft without tearing away, maintaining an air tight seal? Then if a test drive improves the hesitation, we've found the source of our problem air in leakage around the throttle shaft. Then it's time for new bushings (a job that's beyond my capability, I'm afraid) or a new carburetor (which I need a good excuse for anyway! : )

This time we DID find air inleakage around the carburetor shaft, and we DID replace the carburetor with a new Bocar model purchased from Desert AutoHaus. This solved all of our problems -- we've had no trouble setting the idle or timing the engine ever since. That others may learn from our experience!


Notes -

Some pertinent NOTES that came up during these discussions -

  • The points should always be set BEFORE setting the ignition timing, as just a 0.004" alteration to the points gap makes a 3-degree change in the timing.
  • It is preferable to set the timing on a cold engine using the static timing method. Using a strobe timing light on a cold engine will result in alterations over the whole setting range. The engine CAN be set when a little warm, so long as the oil temp is below 50C/112F.
  • I'm not sure why this is -- the book doesn't say. Maybe the gear lash increases with engine temp and affects the timing or something. I presume the bit about using the static method in preference to a strobe light would be the effect of the vacuum.

  • One thing I've discovered for sure--you must keep those electrical connections (distributor to coil, coil to cut-off switch and choke) clean and tight! We've had several problems in that regard -- all easily fixed once they were discovered, of course, but sometimes you look right past such things. I've replaced every one of those wires now.


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