Center mount carb preheat question help

[Rob Combs]

Sorry folks just realized I went off topic and should have put this in the venturi cutting thread, not the preheat thread. The preheat part of this is still undefined, but after the fixes above the preheat tubes are starting to discolor from heat and the carb sweat is reduced, for whatever that is worth.
Rob

[Alstrup]

OK. When you have a freer flowing muffler, a stock cam, and a Z70 main air correction I would not be surprised if you need to go 145 main jet. The stock cam creates relatively low vacum in the inlet manifold, so when you enlarge the venturi, and may not have hit the transition (vacum) point spot on it will require more main air. The idle should not be affected of this. It shoul d be in the ,55 max 0,57. If it wants more something else is out of whack.
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https://www.thesamba.com/vw/forum/viewtopic.php?t=435993

[Rob Combs]

All right everyone, it's been a little while but here's where we are...

Opened up the Tri-Mil's heat riser tube-to-exhaust tube passages, driver side larger than passenger. Not sure if this will help support cross-flow or if the pulses will cancel each other out but we'll see.

Bored out the 34 PICT 3 venturi with a dremel to ~ 27.5mm. Was careful to maintain its shape. Polished the discharge tube, choke housing, and top of the air inlet. Went up to a 132.5 main at the beginning, which was sluggish but far better than the 130. So I re-read everyone's input and made a judgment that a 140 would be a better starting place (also got a 137.5 and a 135 from Berg just to have on hand). The 140 really brought it to life, but the low speed transition was still really bumpy and sluggish. Then I found out through all of this I lost the advance vacuum canister, leaking vacuum and no advance. Since a vac canister is more $$ and a longer wait than another distributor, I just got another distributor off the shelf (Kuhltek 034 - don't shame me that's what I could get on the fly and it's better than nothing. Plus the dead one gave me a core to lock out in preparation for a Black Box. That's another topic, as is the failed attempt at getting the DCNFs on there this weekend - I'll start another thread for that.)

The attempt at the DCNFs had the unintended benefit of forcing me to realign the intake center section, which turns out was also causing some of my problems. I could never get the heat riser tubes to align without force, and it kept blowing riser tube gaskets. Putting the manifold back on, I found the minor realignment to allow the heat riser tubes bolt up without force or issue. Had one set of riser gaskets left (for the blockoff plates, actually). No leaks and no excessive force required. I thought my manifold tubes were warped but it looks like the whole manifold was just in a bind.

Installed a tach under the dash, picked up some tires to make it safe to drive hard up in the hills, raised the front a little to stop the fender rubbing from the slightly larger tires, and went to work on the minor transition hole that was left over after replacing the distributor. Pulled the 61 drilled-to-size idle jet and installed a measured 55 idle. Ran better but still had hesitation, this time almost as if the engine was still cold (after it was fully warmed up). Drilled the 55 to a 57 (that's what I had drill bits to do). Even better now. Started to work on the pump lever adjustment. Pulled back to minimum then slowly and incrementally increased the pump squirt until the transition is nearly butter-smooth, about ¾ of the way to full stroke. It feels almost as if I'm covering up an overall lean low speed circuit with pump factor. Went abck to the 61 and pump adjustments for a few minutes, then settled on the 57 as best overall. That's all I can do today, but this thing is running great!

Stats:
Elevation: Sea level to 1500 ft, depending on whether I go up the hill or not. Can tell a minor difference between top and bottom of the hill. Running nearly perfect up top with butter-smooth transitions and throttle roll-in. I need that at sea level and let it run a bit rich up top.

Carb:
Vent cut to ~ 27.5
Idle Jet: 57 drilled-to-size
Main Jet: 140 from Berg so I trust it, plus the actual hole sizes between the 130 to 140 at each step all looked to be incrementing in plausible amounts. Not scientific as a measuring kit but I'm satisfied that they're close enough.

Distributor: Kuhltek 034
Idle timing: 14 BTDC no vacuum
All-in timing ~ 3000 RPM: 31BTDC no vacuum
Vac advance-added on new distributor: Not yet measured but irrelevant because little can be done about it. Failed unit was about 10 dagrees vac advance and I'm assuming this one is somewhere close to 10 as well. (Before I'll start messing around with the vacuum can or advance plate I'll get a Black Box - I would already have one if they ran on Mac OS, just hate to have to buy another laptop to get timing sorted out. One day I'll crack and just do it...)

Conclusion for today:
I think the sweet spot is a 58 or a 59. Again, not super scientific but can get close with plug readings. Perhaps a wideband is in the cards, but for now the butt-dyno, smells, sights, and sounds seem to be giving me progress. But now we're picking the fly excrement from the pepper. We're close on this.

Thanks everyone for your input and hopefully this helps someone else in their journey.

About the time I get this sorted it will be time for the Black Box and the Webers. I guess it never ends...

Rob

[Rob Combs]

Thanks for the input guys. I'll alter the filter if necessary, but trying to focus on the header mods first, before I install it. I'm also working on a velocity stack into an air cleaner, just for experimental purposes. I believe the climate here may be moderate enough to get away without heating the carb top, only the inlet manifold, but I'll soon find out.

Going to go ahead with the large hole on right preheat stack/smaller hole on left preheat stack as Alstrup mentioned earlier in this thread, rejet, and see how it goes. If it doesn't work out I'll go visit a muffler shop who can weld in a preheat tube from the left side down to the collector and refinish, then start the rejetting again. Time consuming but fairly cheap, and this is more of a hobby car than a daily-use vehicle.

I'll report back with my findings in case it would help someone else in the community.

After that we'll go forward with the enlarged venturi 34-3 and see how that goes, then at some point will go with the DCNFs I have on the shelf, maybe next year.

Thanks again to all,
Rob

[oprn]

Sweet! The best of both worlds, better carb without loosing the all weather drivability.
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Our cars get old, we get old but driving an old VW never gets old!

[mikedjames]

I use this - its basically an air box built to sit over a progressive, connected to a 34PICT-3 oil bath air cleaner, with the thermostatic vacuum flap in the air cleaner intake.

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Ancient vehicles and vessels

1974 VW T2 : Devon Eurovette camper with 1641 DP T1 engine, Progressive carb, full flow oil cooler, EDIS crank timed ignition.
Engine 1: 40k miles (rocker shaft clip fell off), Engine 2: 30k miles (rebuild, dropped valve). Engine 3: a JK Preservation Parts "new" engine, aluminium case: 26k miles: new top end.
Gearbox rebuild 2021 by Bears.

1979 Westerly GK24 24 foot racer/cruiser yacht Forethought of Gosport.
1973 wooden Pacer sailing dinghy

[Rob Combs]

[Fair-cooled]

I know the answers juniorberger. You are the one that wants to know, otherwise why are you wasting peoples time.
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‘74 Super beetle, originally from California until 5 years ago.

Seize the day...not the Engine...

[Fair-cooled]

[ccowx]

Vamram,

I have seen those before and I think I might have even considered buying those headers, or a set just like them. The main reason I did not was that I needed a better intake center section for my purposes and they would not have hook ups for the dual pre-heat pipes.

I would imagine that it would add some extra heat to the system and help in serious cold. California added it to improve fuel atomization and therefore combustion efficiency. It would also help in extreme cold I am sure, how much or how neccessary it is are probably functions of your overall system. Probably an insulating wrap would serve a similar purpose in practice.

Chris

[vamram]

Sorry if this is a dumb question that doesn't contribute much to this thread, but the topic makes this a good place to post it....I would like my stockish '74 Super to run well once I get it back n the road. Do any of you have experience w/the '74 CA dual pre-heat intake manifolds? Are these better than the single pre-heat manifolds? I was planning on using it along w/this header:

Thanks,

Víctor
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'74 Super 9/16 - present, in refurb process.
'73 Super - 6/18 - Present - Daily Driver!
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[ccowx]

OK, I have no interest in joining whatever p#ssing match seems to be started here, but in the interest of clarity I do need to put out a fact or two.

-First of all, what you describe, fair-cooled, is perhaps theoretically possible with the heat soak, but in practice of having driven ACVW's for roughly 20 years as daily drivers, it has never happened. If the heat in the engine bay was hot enough to cause the issue of low oxygen in the air, you would have other issues first, probably including a fire. I would estimate you would be hitting something close to 200 F degrees and the engine bays are just not that air tight to hold that. Most likely what would be the reason for the engine to stop would be the oil breaking down and the engine seizing.

-The one time I did experience a heat soak issue with my engine was when i was climbing a 2500' hill, behind a slow moving truck, in a fully loaded 4000 lb+ camper, in temps over 100F/35C. At that point, what actually happened was not an issue of lack of O2 but that the fuel pump itself heated up via heat obsorption from the block and fuel vaporized there, causing the pump to stop working. The engine itself ran fine, as intended by Porsche, but the pump stopped working, just as any liquid pump will stop at any temperature if there is a no liquid in the pump. The issue was not combustion but a mechanical failure that prevented the fuel pump from delivering fuel to the engine.

-Just for clarity, there is in every factory acvw engine I am aware of a 2" hose that picks up air at the cylinder head via an L shaped pipe and feeds it into the air cleaner and into the carb. It is thermostatically controlled either by connecting to the engine thermostat/vane assembly over the cylinder heads or with an independent thermostat. Either way, there is a factory designed system to deliver warm air to the carburetor for the purpose of preventing carb freezing.

-A note on the system of air pick up for the air cleaner. It is thermostatically controlled so that it does not function when the heads are warm enough or when the carb air temp is high enough. It is worth noting that the VW engineers constantly developed this system and eventually made it independant of the head temp. Effectively, the final 1971 and up system was an independant thermostat that would keep the air temp at a minimum of 50-60 F/15 C regardless of the outside temp.

-Also worth noting is that the carb pre-heat system is adequate to around -10C or 20 F or so. After that the pick up at the heads is not efficient enough. This might make for a slightly rougher engine or in extreme cases allow for carb icing. I suspect that in practice most situations that are cold enough to make that happen probably don't have much moisture in the air anyway, but it could be an issue. More likely is that the combination of the intake and carb pre-heat would simply not be able to keep the fuel/air mixture in suspension and you would have a rough running engine.

My personal opinions are this:

-The carb pre-heat is nice to have for carb icing and has the effect of taking some of the load off of the intake pre-heat. The warmer the air going in the less freezing issues in the carb/intake and the less intake heat needed to keep the fuel in suspension.
-The most important heat issue to address is the manifold pre-heat. This is because no matter what the outside temp and humidity the fuel air will chill due to the venturi effect and this will likely cause fuel distribution issues.
-There has been a lot of talk over the acvw world about heat being conducted through the heads and intake manifolds and that being sufficient to insure fuel vapourization. I can see this working to some extent in warmer conditions where most of these cars operate now, especially for dual carbs. I can not see that working at -40 on any set up and I absolutely can not see it happening on a factory center mount system. No way will heat move against the airflow for the 18"+ per side needed for it get anywhere near the carb or the bend in the manifold below it.

Not trying to fuel any fires(pun inintended!), but practical experience and theory both tell me that the pre-heat is needed both at the carb and the intake to insure smooth operation at much less than room temperatures. How smooth or otherwise is a function of temperature, humidity and some variation in each system. However, having operated acvw's for years in temps from 40C to -40C (110F to -40F) with and without either or both pre-heat systems, I assure you they are there for a reason.

In the OP's case, the lack of humidity and extreme temps means that it will be fine overall, but there will be some roughness at colder temps and warm up will be slower. Mileage may suffer a bit at times. Otherwise it will run fine I am sure.

Chris

PS: Fair-cooled, no one is hating on you, but on a forum like this it is fair ball to disagree with you. In this case I respectfully submit that my experience does not support your statement about heat in the engine bays and in the interest of helping the OP I feel that I need to say that. I did enjoy the historical note on the development and thanks for that.

[oprn]

Has no one on this thread ever seen the stock air cleaner with the hose that runs down and connects to the tin elbow fitting under the right cylinder bank? Have they all hit the garbage so long ago that nobody has seen one?

Maybe our Tupperware guy has a picture, I suspect that is what he is using. Stock VW parts guys, no need to reinvent the wheel.
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Our cars get old, we get old but driving an old VW never gets old!

[jberger]

[Fair-cooled]

Hi Hnoroian.
Your question of how an engine ( in this case the VW air-cooled Beetle ) can create an air intake tract that is operating at an optimium temperature is a very valid one imo. Since the Beetle was to be air-cooled, Dr Porsche had to devise a way to achieve a warm intake without coolant, which is the most common method for water cooled cars, motorcycles etc.
So, since he had no thermostatically controlled water to use, he picked the most obvious source of an air-cooled cars, the exhaust heat. He simply connected an aluminum tube from one muffler up through a hole in the engine tin, over to one side of the intake manifold, through on to the other side of the manifold ( but never are the two pipes open to each other, only touching each others outer tube, and thus heat was conducted from the heat riser tube
{ the narrow tube that connects between the two mufflers } to the intake manifold tube thereby raising the intake tube to a warm temerature ( thanks to the heat taken from the mufflers ) that assured that the fuel-air mixture would stay a mixture of fumes and NOT condense and then freeze at the 90 degree bend in the intake manifold, as the mixture splits to feed both the left hand side cylinders, and also the right hand side cylinders.
So that is how the Volkswagen air-cooled flat four cylinder engine keeps the fuel-air mixture from freezing up and stalling the engine. Quite clever I think.

Cheers Hnoroian.

Darren.
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‘74 Super beetle, originally from California until 5 years ago.

Seize the day...not the Engine...

[jberger]

Late to the party... but that Tupperware is brilliant.. I used a cookie tin on a center mount progressive Type IV years ago.. with a heat riser off the exhaust crossover. Was a game changer, until ambient temps got into the 90's.. then you needed to "pop-the-lid".. Ended up using gutted 1600 DP bug oil bath with automatic "lid-popping"! Still runs that way.. no manifold heat.. but plenty of pre-heat.

J

[modok]

With the heater.

"but I took the heater boxes off"

hmmmm

[Hnoroian]

Never mean for anyone to be in a tiff, especially when we talk about different experiences and situations. Everything that has been stated holds a valid truth for individuals. There are some great ideas being thrown around, and thank you for all of them.

Much of my particular problem I believe will be alleviated because of the generalized operational and personal temperature limits in my location.

So then comes to question how would one in this situation achieve warmer air to the carb?

Remembering this is a non-stock Baja (open engine compartment) tri mil exhaust, center mount IDF, etc. not ideal for perfect operation.
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Stupid people do stupid things.

[Fair-cooled]

You are a phunny little guy, but I've been wasting my time with you. I won't be replying to you again, but feel free to write ( type ) your hate out! I mean write your heart out. Good night.
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‘74 Super beetle, originally from California until 5 years ago.

Seize the day...not the Engine...

[oprn]