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Dual weber 34 ICTs and manifold balance pipe idle issue
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uladk84
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PostPosted: Sun Sep 11, 2022 2:30 pm    Post subject: Dual weber 34 ICTs and manifold balance pipe idle issue Reply with quote

Hello all,

I synced up my weber 34 ICTs today and ran into an odd issue I hope you can enlighten me on.

I have a 1/4" balance pipe connected between the CB Performance intake manifolds and had it pinched off while I synced the carbs (I read somewhere that this must be done, is this correct?). Both carbs synced up nicely, and the two screw adjustments were similar on each side. I then unpinched the balance pipe, and the rpms dropped from 900 to around 650. I had to open up the idle stop screws more and play around with the mixture screws to get back closer to 900. This doesn't seem right though as it's not recommended to have the idle stop screws more than 2 turns in. Also with this, letting off the throttle didn't drop the rpms quickly like before.

Any ideas on why the rpms would drop after unpinching the balance pipe? If I have to open the throttle more at idle to compensate, then where is that missing air going?

Vacuum or pressure related? Can the air/fuel mixture be sucked into the balance pipe to the opposite intake manifold? That could mean a leaky valve? Could the dizzy vacuum advance be affected by having a balance pipe?

Valve clearance has been set, and timing done. Engine is a new Type 1 1776 with 12k miles done.

Any insight is appreciated.

Thanks
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bsairhead
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PostPosted: Sun Sep 11, 2022 2:39 pm    Post subject: Re: Dual weber 34 ICTs and manifold balance pipe idle issue Reply with quote

No answer for you. Just that I ran ICT's for years without a balance tube with little to no issues.
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SGKent Premium Member
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PostPosted: Sun Sep 11, 2022 2:47 pm    Post subject: Re: Dual weber 34 ICTs and manifold balance pipe idle issue Reply with quote

could you have introduced a small vacuum leak when you pinched off the lines?
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volkybus
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PostPosted: Sun Sep 11, 2022 9:02 pm    Post subject: Re: Dual weber 34 ICTs and manifold balance pipe idle issue Reply with quote

I run Twin 34 ICT's on my 76 Westy w no balance tubes for years.I have had no no problems but did lose about 5% mileage over FI I got 20.5 mpg on a recent trip to Maine.& around town I got 18.6 with a kayak on the roof it does often idle high but it linkage problems but my carbs are over 30 years old.
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Multi69s
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PostPosted: Mon Sep 12, 2022 12:26 am    Post subject: Re: Dual weber 34 ICTs and manifold balance pipe idle issue Reply with quote

As SGKent suggested, it sound like their is a vacuum leak in your balance system. Each engine is different. It is not written in stone that you need a balance tube for ICTs. Some engines do, others don't. If yours runs fine with out it, don't use one.
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timvw7476
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PostPosted: Mon Sep 12, 2022 12:59 am    Post subject: Re: Dual weber 34 ICTs and manifold balance pipe idle issue Reply with quote

never tried 34 ICTs on a non-Type IV bus engine. The old set up manual stated setting the throttle blades with a .004" feeler gauge as your gap, putting both blades in the position of the idle orifice/drilling.
Then, bolt them on, being careful when attaching throttle linkage to not tweak that .004" setting. Start it, check timing, adjust the idle speed with the mixture screws, at each carb base. Mine are now 26 years old, & hooked up to a power brake booster on a '76 Transporter, I suppose that almost serves as a 'balance tube' of sorts. There's got to be someone here running similar to you with the Type I bus set up....... ??? The idle mixture effects response at more than just idle speed on the ICT. Should be called the 'up to 5/8ths throttle mix screw'.
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Zed999
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PostPosted: Fri Sep 23, 2022 6:06 am    Post subject: Re: Dual weber 34 ICTs and manifold balance pipe idle issue Reply with quote

When I added a balance pipe to ICT's the idle speed increased a little, no doubt because it was idling better. I wouldn't add one if it was idling smoothly without one when you set it up.
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uladk84
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PostPosted: Sun Sep 25, 2022 11:08 am    Post subject: Re: Dual weber 34 ICTs and manifold balance pipe idle issue Reply with quote

Thank you all.

I plan on temporarily removing the balance pipe to see the effect on idle and fuel economy. It has been on since I built the engine so it will be interesting to see what happens.
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uladk84
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PostPosted: Tue Dec 20, 2022 2:23 pm    Post subject: Re: Dual weber 34 ICTs and manifold balance pipe idle issue Reply with quote

An update on this:

I removed the balance pipe and was still able to get a nice, smooth idle. I'm still tracking fuel economy but there is an improvement when driving around town (17 - 19). Motorway MPG hasn't changed much, but I have got near my best of 25 MPG. Looking forward to seeing how she does in summer though.

Still unable to confirm the cause of the issue however.
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timvw7476
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PostPosted: Wed Dec 21, 2022 12:09 am    Post subject: Re: Dual weber 34 ICTs and manifold balance pipe idle issue Reply with quote

uladk84 wrote:
An update on this:

I removed the balance pipe and was still able to get a nice, smooth idle. I'm still tracking fuel economy but there is an improvement when driving around town (17 - 19). Motorway MPG hasn't changed much, but I have got near my best of 25 MPG. Looking forward to seeing how she does in summer though.

Still unable to confirm the cause of the issue however.


Nice. I'm not one for advocating mpg as a hallmark for proper tune, but just less than 24mpg is the best I could generate with 34 ICTs & a stock 7.3CR 2.0L, with 091 (stock tallish) gearing & an occasional sparkplug color check.
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orwell84
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PostPosted: Wed Dec 21, 2022 6:35 am    Post subject: Re: Dual weber 34 ICTs and manifold balance pipe idle issue Reply with quote

timvw7476 wrote:
uladk84 wrote:
An update on this:

I removed the balance pipe and was still able to get a nice, smooth idle. I'm still tracking fuel economy but there is an improvement when driving around town (17 - 19). Motorway MPG hasn't changed much, but I have got near my best of 25 MPG. Looking forward to seeing how she does in summer though.

Still unable to confirm the cause of the issue however.


Nice. I'm not one for advocating mpg as a hallmark for proper tune, but just less than 24mpg is the best I could generate with 34 ICTs & a stock 7.3CR 2.0L, with 091 (stock tallish) gearing & an occasional sparkplug color check.


That’s quite respectable gas mileage. My ‘16 Subie Forester only gets 27 mpg highway. Stock Solexes average about 18-20 mpg. Some people hate the ICT’s. I did quite well with mine out of the box until they began to wear. Setting up any carb takes work to dial in.
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Jalabert
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PostPosted: Wed Dec 21, 2022 11:55 pm    Post subject: Re: Dual weber 34 ICTs and manifold balance pipe idle issue Reply with quote

There's a really comprehensive guide here - https://www.volkszone.com/threads/setting-up-34icts.664575/ - to setting up ICTs. Originally posted on the justkampers forum a zillion years ago when they still ran one. (The return springs bit may make your life a load easier, anyone with a high idle...)

I used it to set up mine - I think 2003? Never really had to touch them since, occasionally a twist in the linkage to balance them up, but that's it.

My inlet manifolds are teed for the booster take off, I take the vacuum for the dizzy off just the one - running a stock late mexi beetle svda.

Mileage wise, what you're getting is about what I get on a run...
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69BahamaYellow
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PostPosted: Mon Dec 26, 2022 6:07 pm    Post subject: Re: Dual weber 34 ICTs and manifold balance pipe idle issue Reply with quote

To the OP. A 1/4” balance pipe seems small; most I have seen are 1/2”. That is also the size balance pipe I used for a 2.0 bus motor with 34 ICT’s and it does make a smoother idle. Tee your balance pipe into your brake booster; the vacuum from both carbs will make your brakes work better. Check your brake servo for vacuum leaks, as this is a common place for them. If you’ve got a leak, RPM’s will drop.
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Multi69s
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PostPosted: Fri Dec 30, 2022 11:20 am    Post subject: Re: Dual weber 34 ICTs and manifold balance pipe idle issue Reply with quote

69BahamaYellow wrote:
To the OP. A 1/4” balance pipe seems small;


Good catch Bahama: I didn't catch the size at first. You are right, a 1/4" tube IS too small to make an effective balance tube, there is just too much restriction. As an example: when I just want straight vacuum for my auto trans, I run an 1/8" pipe. I am actually running dual 40 DCNFs now, and I have each manifold runner tapped (4 total) and connected to 1/8" pipe, and I get no balancing action, just vacuum.
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