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blacktruck
Samba Member
Joined: September 23, 2005
Posts: 377
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 Posted: Thu Nov 05, 2015 6:16 pm Post subject: |
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anyone make one of these work on dual carb setups?
seems like it might be gooder than without |
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airschooled
Air-Schooled
Joined: April 04, 2012
Posts: 12722
Location: on a bike ride somewhere
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| Posted: Thu Nov 05, 2015 7:46 pm Post subject: |
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Volkswagen used a dashpot on the factory dual carbs. That might be easier to incorporate.
And yes! I would highly recommend a dashpot on a setup of duals. If not for our planet, then for your cylinder walls.
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Last edited by airschooled on Thu Nov 05, 2015 10:39 pm; edited 1 time in total |
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Luft kühl
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Joined: January 21, 2009
Posts: 1178
Location: Allentown, PA
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| Posted: Thu Nov 05, 2015 9:48 pm Post subject: |
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I was not aware that any air cooled VWs ever came from the factory with dual carbs.
Which years/models had them ? |
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airschooled
Air-Schooled
Joined: April 04, 2012
Posts: 12722
Location: on a bike ride somewhere
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| Posted: Thu Nov 05, 2015 10:35 pm Post subject: |
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| Luft kühl wrote: |
I was not aware that any air cooled VWs ever came from the factory with dual carbs.
Which years/models had them ? |
The 72-74 North American buses had them on the T4 engine, (though some 74 buses came with fuel injection.)
Early Type 3 cars also had dual carburetors, before they became the first ever mass produced commercially available car with gasoline fuel injection some time around 1969. The dual carb setup was borderline mandatory to make the 18" height requirement of the compact engine bay.
Here is a Type 4 bus carb with the dashpot for slowing the throttle arm. It's a crude spring-based mechanical only throttle positioner.
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Juanito84
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Joined: March 17, 2012
Posts: 2436
Location: Colorado Mountains
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| Posted: Fri Nov 06, 2015 1:09 pm Post subject: Re: 1968-69 throttle positioner disassembled and repaired |
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This is an awesome thread!
| Amskeptic wrote: |
| In serious mountains, I disconnect it, because it takes away badly needed engine braking with these four over-worked brake drums. |
Disconnecting the vacuum advance should also help induce better engine braking if you're using a throttle positioner. But it wouldn't help to disconnect both. Either one or the other.
I'm going to hook up a switch to my idle shut off solenoid. I figure that no fuel would not only help with engine braking but also cause zero NOx, HC and CO emissions.
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blacktruck
Samba Member
Joined: September 23, 2005
Posts: 377
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| Posted: Fri Nov 06, 2015 6:25 pm Post subject: |
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seems like a 1600 with dual weber ict carbs an SVDA and one of these dashpots might be very "stocklike"
didn't the euro market get a dual carb1600?
these get a vacuum from right below the carb like the brake booster? |
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airschooled
Air-Schooled
Joined: April 04, 2012
Posts: 12722
Location: on a bike ride somewhere
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| Posted: Sat Nov 07, 2015 8:38 am Post subject: |
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| blacktruck wrote: |
didn't the euro market get a dual carb1600? |
I'm not sure. The Euro market DID get the 1600 engine in the vanagon as an option, and the T4 engine at that time also had dual carbs..... I don't know, though.
| Quote: |
these get a vacuum from right below the carb like the brake booster? |
Yes, the one-piece positioners get full manifold vacuum from anywhere on the intake, and the two-piece positioners get that AND an angled port on the back of the carb.
http://www.thesamba.com/vw/forum/viewtopic.php?t=641337
^ for too much info.
Robbie
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Juanito84
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Joined: March 17, 2012
Posts: 2436
Location: Colorado Mountains
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| Posted: Sat Nov 07, 2015 12:33 pm Post subject: |
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That's a pretty good article about it. But it's a little off I think.
When you let off of the throttle suddenly it goes rich momentarily. This is because air, which is less dense, slows down much quicker than liquid fuel. It's the opposite of why you need an accelerator pump. Without an accelerator pump or a throttle positioner (or dashpot) the engine would go lean momentarily every time you floor it and rich momentarily every time you let off the accelerator pedal.
However, at closed throttle I understand that the strong vacuum during engine braking causes the AFR to go lean if the throttle is completely closed. This is because the idle and off-idle ports can only spit out so much fuel, yet the high vacuum pulls in more air. The throttle positioner holds open the throttle and therefore uncovers some off-idle ports allowing in more fuel. I could be wrong but that's how I understand it.
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airschooled
Air-Schooled
Joined: April 04, 2012
Posts: 12722
Location: on a bike ride somewhere
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| Posted: Sat Nov 14, 2015 12:55 am Post subject: |
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| Juanito84 wrote: |
That's a pretty good article about it. But it's a little off I think.
When you let off of the throttle suddenly it goes rich momentarily. This is because air, which is less dense, slows down much quicker than liquid fuel. It's the opposite of why you need an accelerator pump. Without an accelerator pump or a throttle positioner (or dashpot) the engine would go lean momentarily every time you floor it and rich momentarily every time you let off the accelerator pedal.
However, at closed throttle I understand that the strong vacuum during engine braking causes the AFR to go lean if the throttle is completely closed. This is because the idle and off-idle ports can only spit out so much fuel, yet the high vacuum pulls in more air. The throttle positioner holds open the throttle and therefore uncovers some off-idle ports allowing in more fuel. I could be wrong but that's how I understand it. |
Interesting thoughts, and yes, your observations are exactly in line with what happens to our engines. I left out the second part, and I think I'll add it since you brought it up.
Here's the catch; Volkswagen was stuck in between a rock and a hard place due to emissions standards. They could not allow the hydrocarbons of a rich blob of gas every time throttle snapped shut. They could also not allow the lean effects of the idle circuit being insufficient to make complete combustion events at high RPM overrun. Too much NOx is produced in an overly-lean condition, but if the fuel mixture goes too lean, a misfire occurs then you're back to hydrocarbon smog city. So the positioner is a neutral mediator between too much fuel and too much air in all throttle decel conditions.
The engine is doing approximately zero work in either case here, so fuel economy is not a concern. Anyone who sees a VW bus get 22mpg and a Chevy Suburban get 12mpg can read the emissions stickers to see that fuel consumed is NOT equivalent to tailpipe emissions produced. So you might lose a drop or two of fuel per drive, but I'll take that instead of washing my cylinders with solvents!
Marvelous stuff, this German engineering… 
Robbie
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