Dual 34 ICT’s work great up to 3k rpm, then seem to go “flat”

[Madscientist2]

I’ve tried my best to tune these carbs. Right now I have 60 idles, 150 mains, 180 airs, and F6 emulsion tubes. Acceleration and power is great up to 3k rpm, but above that seems like there is very little power in any gear. Airs were 170 before and change to 180 seemed slightly better. I’ve got 8:1 compression, a scat mild cam with 1.35:1 ratio rockers ( total lift of 0.410), ported and polished heads and intake manifolds. Is this just a case of I need larger air correction jets?

[modok]

When airspeed in the venturi approaches sonic, some carbs can start feeding fuel in ways not really intended, as, usually they are designed for airspeed is...oh, perhaps .75 mach

Kardons, or setups like kadrons especially on tall manifolds, can work really well but also may need a larger venturi than you expect.

So, like how kadrons need the acc pump tube raised on souped up engines...... maybe not that exactly but something LIKE that.

Also, like kadrons, won't handle high fuel pressure. Nothing above 3psi if I remember right

I don't recall what venturi size is typical in an ICT and if all ICTs are all the same or not, perhaps you could help me remember.

[rayjay]

OP, is this still the combo you are running? Thread from 2021.

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I just got done rebuilding my 1600. Have original (30 years old) Weber 34 ICT’s installed on short manifolds with a phenolic spacer. Have a balance tube between the manifolds. Ported and polished heads, 8:1 compression ratio, Scat C20 cam, Empi 1.35:1 ratio rockers. 009 dizzy with pertronix module and 45 k volt pertronix coil. Dual quiet pack exhaust. Current jetting is .055 idles, F6 emulsion tubes, .145 mains, and .165 airs. Running plug gap at .032. At 3k rpm, exhaust seems to not be a solid sound, but rather seems to have about 30% “weak” tones, like 70% of the time it runs strong and the rest is weak. Any suggestions?
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[Madscientist2]

Yes, same setup. Timing is set for 30 degrees max advance. I’ll have to check but I believe the 34 ICT Venturi is 28

[Madscientist2]

So, modok are you saying the carbs might be too small? On the accelerator pump lever, there are 2 holes that the link from the throttle shaft can be connected to. I have it on the furthest out hole. Not sure if that is right…[/i]

[Madscientist2]

Fuel pressure regulator is set at 2.5 lbs

[Zed999]

[modok]

[Alstrup]

I think there is an error on Eurocarbs website. ICT´s normally comes with 27 mm fixed venturis. The jetting looks about right, for a 1500-1600 cc engine.
Weber ICH carbs can come with 29 mm venturi´s.
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https://www.thesamba.com/vw/forum/viewtopic.php?t=435993

[modok]

I see
For me the #1 rule of carb blueprinting is..... -measure the holes-

and it's served me well

[Madscientist2]

Well, stupid me…. That thing known as WOT. Found out the throttle cable only pulls about 1/2 inch total travel. Moved it from the furthest hole away from the hexrod to the closest. Trial run tomorrow…

[Madscientist2]

So I spent quite a while adjusting the crossbar linkage and throttle cable tube angle at the pedal end. Originally if I floored it the butterfly was only opening to about 45 degrees ( only 1/2 open). The pedal range of motion was restricted by the throttle cable dragging on the side of the tube. I ended up setting the carb “arms” on the crossbar at the furthest out holes, and the throttle arm at the closest hole. Performance is night and day improved. Solid power right up to 4500 rpm (I don’t have a counterbalanced crank). Thanks for all the help!

[oprn]

Sweet! Years ago a Corvair powered trike came into the shop for some work. I asked the Boss how it was for power. He drove it into the shop from the owner's home including some highway distance.

"Surprisingly gutless!" he said.

Working on it I noticed that it had a set of typical "eye candy, performance" chrome air cleaners and they seemed close to the heads, low profile... and the carb linkage was hitting them at about 1/3 throttle. I took the air cleaners off and sent the Boss out for another run.

He came back with eyes the size of dinner plates and a huge poop eating grin!
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Our cars get old, we get old but driving an old VW never gets old!

[Madscientist2]

I now have a similar grin. Talk about missing the blatantly obvious… On hills that I used to floor it, and speed went down, now at fairly light throttle, speed increases. Now I know all the engine work I did has the positive effect I was hoping for.

[rayjay]

The 2 basic laws of throttle cables are
1- Verify WOT
2- At WOT don't be stretching the cable. The throttle pedal should be flat to the floor or against it's positive stop and the carb butterfly JUST at WOT. If there is any significant tension on the cable it will eventually fail.

[Rome]

[Madscientist2]

Yeah, I did the brick on the pedal trick to finally see where that pulled the linkage to, relative to WOT. Adjusted it so at full throttle I was just a touch away from WOT. Vibration and just a bit of dust/sand getting stuck in the throttle bearings in the carbs will very quickly egg shape the bearings if it is hard jammed at WOT.

[Madscientist2]

I also validated the 34 ICT Venturi size is 29. Looked at my original paperwork from Weber that came with the carbs. I am slapping myself daily for assuming I had the linkage set up right…