Center mount carb preheat question help

[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

[oprn]

Sweet! The best of both worlds, better carb without loosing the all weather drivability.
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We had the stone age, the bronze age, the industrial age and now we are in the age of mass deception and mind control for corporate profit. (The mass media age)

[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

[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

[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]

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