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Tue Mar 18, 2025 1:48 pm

Okay.
I will be checking this! Tnx

Tue Mar 18, 2025 2:25 pm

seeing we are gone into the woods with timing ,this may be of use also the link with in this link is interesting
https://www.itinerant-air-cooled.com/viewtopic.php?t=2198#p175473

Tue Mar 18, 2025 4:54 pm

Davy_B. wrote: Ok so if I remove retard when i'm at 2000rpm it should not change on the Strobe.

But when i remove vac advance I should go down at 2000rpm.

YES 100%. Retard must not (it functionally can't really but anything is hypothetically possible) be affecting the timing at any other throttle position than throttles closed (idle).

The retarded timing is only designed into the system to allow the engine to idle cleanly. With a carb setup with two throttles feeding 2 cylinders each, the engine will idle mostly only on the left carb unless the timing is super retarded (and unless there is a circuit between the left and right carb for idle charge). Without the retarded timing the engine will run fine at all throttle openings but will not idle well. The central idle circuit requires the retard to work.

Wed Jun 04, 2025 11:12 am

"Okay, time to blow off the dust... After a few short rides, it became clear that there was already too much play on the left throttle valve again. And this after a rebuild, unfortunately... I sent everything back to the company—thought it would be a quick fix! Unfortunately, they came back and a part around the throttle shaft had broken off. So I sent it back again, and now they seem okay—I thought! But after a closer look, I noticed there was a gap between the two halves, both left and right. This became clear when I used a thinner gasket. I’ve now leveled them myself using a glass plate and 320 wet and dry sandpaper. Let’s hope we’re almost there now... I’ll put them back on soon, check the levels, and readjust everything. Bit of a bummer, but I guess that’s just how it goes sometimes..."

Fri Jun 06, 2025 5:42 am

I have been running my PDSIT carbs without the retard end of the vac can hooked up. It doesn’t seem to affect the idle and I’m certain that it’s idling on the central idling system. Of course, the timing at idle is more advanced compared to the stock setting. I have also run it with a good quality German 009. It also idled well with this distributor.

Fri Jun 06, 2025 6:55 am

Davy_B. wrote: "Okay, time to blow off the dust... After a few short rides, it became clear that there was already too much play on the left throttle valve again. And this after a rebuild, unfortunately... I sent everything back to the company—thought it would be a quick fix! Unfortunately, they came back and a part around the throttle shaft had broken off. So I sent it back again, and now they seem okay—I thought! But after a closer look, I noticed there was a gap between the two halves, both left and right. This became clear when I used a thinner gasket. I’ve now leveled them myself using a glass plate and 320 wet and dry sandpaper. Let’s hope we’re almost there now... I’ll put them back on soon, check the levels, and readjust everything. Bit of a bummer, but I guess that’s just how it goes sometimes..."
Please don´t take me wrong.
I hope you can find the perfection, you are trying to find.

First, I´m wondering what kind of engine and carbs you have installed, because the GD is 2,0 l with Bosch L-jet. The distributer 021 905 205 E is from 1700 ccm CA dual carb engine.
Your carbs and linkage look more 1,8 l or 2,0 l which used the 021 905 205 P Distributor with SVDA. So, imo the set up is a bit unclear and not original.

When I´m reading old car magazines of the early and mid 70ties, The testers had found out, many of these cares did not run perfect under all circumstances of temperature and load. Not even the ones with early systems of electronic fuel injection.

A dual carb system only works perfect, when the air flow on both butterflies and throttle linkage works perfectly synchron and also the beginning and volume of the acceleration system. The bus has the central idling system in left carb and also 1 screw in each carb, so in total 3 + 1 adjusting screws influencing idle and partial load.
The value of CO emission should be in the range of 2% up to 4 %.
The next thing is the mechanical resistance of the ground plate inside the distributor for vacuum advance. Sometimes a drop of oil to the splitted shaft of distributer can make a difference. Even if all single parts are in factory tolerance, the sum of tolerances in the whole system make the system a perfect GO or NOT. (Murphys Law!)

Imo, we, you should not expect too much of a more than 50 year old rather difficile and already emission controlled system. New modern cars are a different world!

Do you know a really perfect runnig original dual carb bus, the same model and carb-dizzy-set up like yours for reference?

(when I drive my 49 Standard with it´s early post war 26 VFI Huf carb, with hand choke, but without acceleration pump, I´m positivly impressed, how good and smooth that old system still works, including 6 Volts and the original, but rather low voltage and weak "star" coil) (Fuel is allways modern E 10 without any extras)

Fri Jun 06, 2025 12:34 pm

He just needs to run smooth and safe — not chasing perfection here, especially knowing not everything’s original.

I leveled the carbs on a glass plate and cleaned up all the imperfections. Float height is set at 13mm for now — we’ll see how that holds up. Everything’s back together and the carbs are synced at both idle and high RPM. Planning to go over the rest tomorrow and double-check

everything.

Fri Jun 06, 2025 2:10 pm

Looking good, hopefully that was the issue all along.

Sat Jun 07, 2025 2:17 am

Alright, just did a bit of tweaking. According to Colin’s method. So now I’m back to 3.5 turns out on the mixture screws... I still notice a slight difference, feels like to many turns to me. So here’s what’s happening next: if I disconnect the right cutoff, the engine keeps running 550 rpm— but if I do the same with the left cutoff, the whole thing stalls?? It didn’t do that before.

Sat Jun 07, 2025 9:09 am

Now you have to find out if the RH carb is too rich, or too lean, try a turn in on the screw and repeat the disconnect the LH test, worse?, better?
If worse turn the screw back and go a turn out, one direction should show signs of trying to idle.

Sat Jun 07, 2025 1:49 pm

the first step in tuning any carb, or dual carbs is to make sure there is no warpage or blockages - and you have done that.

the second step is to look carefully at the bottom butterfly and make sure it closes evenly all the way, every time. If it is off center or crooked, that must be remedied by adjusting it. If the shaft is worn, that must be rectified too or air leaking thru the shaft will cause issues tuning it.

The next step is to make sure the bottom butterfly closes in such a way that ONLY the idle mixture port is exposed. If the butterfly is too open, off center, not closing properly so that ONLY the idle mixture port is exposed, then the engine will be running off the idle and other circuits. That will mess with everything. If transition ports are exposed then the engine will be partially running off them, and they will not be available for extra fuel when the butterfly is opened.

once the items above are done, the carb can be adjusted to factory tuning. We used to use a thin piece of carbon paper in a strip to adjust the idle stop. The butterfly should not dig into the side of the carb either when it is closed. Then we could set the idle stop to whatever the factory setting is. If the carb has an idle bypass that controls air going around the butterfly, then the carbon paper clearance was where the stop stayed, as long as the transition holes were not exposed. I have on rare occasions had to tweak (bend) a butterfly just a tiny infinitesimal amount to make sure the transition ports are covered or just barely above the butterfly. When the butterfly is opened, those ports offer additional fuel to enrich the mixture in combination of the accelerator pump.