TheSamba.com :: View topic - Dual 34 ICT’s work great up to 3k rpm, then seem to go “flat”

Sat Jun 14, 2025 1:34 pm

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?

Sat Jun 14, 2025 1:45 pm

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.

Sat Jun 14, 2025 3:21 pm

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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Sun Jun 15, 2025 5:07 am

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

Sun Jun 15, 2025 5:20 am

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]

Sun Jun 15, 2025 5:23 am

Fuel pressure regulator is set at 2.5 lbs

Sun Jun 15, 2025 5:27 am

Madscientist2 wrote: Fuel pressure regulator is set at 2.5 lbs At idle? I did the same thing but under constant load the carbs would run out of fuel and engine splutter until I took my foot off the gas. Obviously I turned it up a bit.

One time a pro set my fuel pressure he did it under full load on a rolling road, set it so the carbs were getting just enough under those conditions.

Sun Jun 15, 2025 12:04 pm

Madscientist2 wrote: 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]

I'm shooting in the dark here.
So I probably missed. :lol:
I'm assuming it's going too rich (which may be wrong)
But if they are then why? well, some reasons could be......
Maybe the main jets or acc pump jets are drilled bigger than marked or, maybe the high speed enrichment jet or venturi size is different than whatever is typical
Eurocarb says

29mm Venturi (fixed)
135 Main jet
F6 Emulsion tube
160 Air corrector jet
40 Pump exhaust valve
55 Idle jet
50 Pump jet
1.50 Needle valve

But it could also be anything.
Maybe your throttles are only opening half way, maybe your paper air filters got wet, maybe it's the ignition system, or a flat cam, maybe who the heck knows.....

Sun Jun 15, 2025 2:18 pm

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.

Sun Jun 15, 2025 2:21 pm

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

and it's served me well

Tue Jun 17, 2025 6:11 pm

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…

Fri Jun 20, 2025 12:15 pm

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!

Fri Jun 20, 2025 2:10 pm

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!

Fri Jun 20, 2025 6:07 pm

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.

Sat Jun 21, 2025 8:37 am

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.

Sat Jun 21, 2025 1:55 pm

rayjay wrote: 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.
A few years ago I also experienced much less power on my just-installed engine in my '77 Beetle compared to before its removal. I placed a brick on the gas pedal (engine off!) to keep the pedal floored, loosened the throttle cable barrel nut on the carb linkage, and pulled the cable end rearwards with a vice grip until I felt the resistance to pulling due to the brick's weight. The cable end pulled back at least an inch- there was some slack in the pedal and cable guide pipe... Then I simply rotated the carb linkage to its full throttle position, inserted the cable end into the barrel nut, and tightened the barrel nut. Full performance restored.

Sun Jun 22, 2025 7:16 am

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.

Sun Jun 22, 2025 7:21 am

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…