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TR6, DCOE, Cannon manifold and balance tube

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My 1974.5 US spec TR6 has a modified engine with 9.6:1 compression ratio and Goodparts GP2 cam at the core. As part of that rebuild several years ago I converted to the DCOE carbs which came with a Cannon manifold and linkage.

This configuration starts and runs well and the car dynos at 130 RWHP.

A chronic problem has been the inability to get the engine to reliably idle at anything below 1200 rpm. The carbs were new and I have checked and double checked the float heights, butterfly operation etc. I have an air/fuel meter with an o2 sensor in the exhaust to monitor mixture and I believe I have an acceptable configuration of jets, emulsion tubes, pump bypass etc etc.

I approached the Yahoo weber group and they hate the Cannon manifold so much that they initially said they would not even discuss my issues with the DCOE's until I either replace the Cannon manifold or block off the balance tube. The assertion is that the DCOE's want to see a strong vacuum signal to operate correctly and the balance tube is interfering with that.

I know there are others with the same Cannon manifold and DCOE's so I'm wondering what you guys think of this configuration and if blocking off the balance tube is worth a try ?




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Don't really know enough about the carbs myself to comment usefully, but my observations:

- Balance tubes on side draught manifolds seem to be relatively unusual.  That said, the Cannon manifold is offered by well respected suppliers including Webcon themsleves so I'd expect it to work. 

- There are some advantages to the balance tube - enough vacuum to work your brake servo properly being one significant one

- There are people out there who have them working and have had for a long time (Tom Fremont on TRR has two cars equipped with them), so clearly it's possible.

- Thinking about why they might be a problem..... on  a car with a wild cam you might get issues with reversion and "cross-talk" between cylinders upsetting the carbs.  I wouldn't consider your cam wild enough for this.  I'll assume the cam timing is correct.....

- Carb balance is always important, critical even, for idle and low speed running.  I've seen it written that the balance tubes make it less critical but I disagree.  How confident are you of the balance and what are you using to measure it?

- When you say you have trouble getting a reliable idle below 1200 rpm, do you mean you struggle to throttle it down that far (air leaks?) or that you can turn it down that far no problem but it just doesn't like to run there?  If the latter, what happens to the AFR when it's busy dying?

- What idle ignition timing do you have (actual measured)?


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Stan, On the basis of what I've fathomed on SUs  the balance pipe will allow ' suction to escape' to the other carbs, diminuishing the pulses of depression at the jet the iv is fed from. So suspecting the BP is not unreasonable. At present the pulses of depression from the iv  may be too weak to pull enough fuel form the jet when you try to slow the rpm below 1200. Blocking the bp will keep all the pulse from the one iv for its one jet, and you should then be able to slow the rpm. 



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The problem is with the throttle adjustment. If I unwind the adjusters so they are well clear of the carb body, the idle is still too high. I can control the idle by screwing in the idle mixture screws (normally 1 turn out from fully seated) but then it will stumble when coming off idle because at that lower setting it is not getting enough fuel.

I balance the carbs with a Synchrometer like this one from the Pierce web site. I also have the elbow to get to the front carb that is cramped by the wheel arch.



Chokes are 30mm


These are the Spanish DCOE's and they have the following config:

Chokes 30mm

Aux 4.5

Emulsion F2

Main 130

Air 160

Idle 55F9

Accel pump 50

Pump bypass closed

Float height 12mm/25mm

It pulls like a train, no stumbles or flat spots. Full height but stiffer springs, all poly bushes front and rear, Alu steering mounts, 13 inch Motolita steering wheel. A lot of fun.

Just cant idle below 1200 rpm.


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So too much air is getting in and you can only reduce idle speed by spoiling the mix.

First thing to check is that it is not the rest of the linkage holding the throttles (or just one or two of them - you have a balance tube!) slightly open.  It really doesn't take alot of air to raise the idle.

Some DCOEs at least have air bypass screws which bleeds air around the throttle plate.  These should be fully closed for initial setup and most are always left that way.  Maybe yours (or some of them) are open?


If you are confident it's not the linkage and the byspass screws are either absent or closed, either the throttle plates are not sealing properly (seems a little unlikely) or air is getting in elsewhere.  Might be worth trying your flow meter on each throat having backed off all the throttle stops as this will tell you if the air is going in through the front of the carbs or not.  If not then you need to think about airleaks from manifold-head gasket, carb to manifold gaskets / Misabs or other possible sources. Don't forget the brake servo plumbing as a possible source - or even a split in the servo diaphragm.


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Does sound like air leak, but also enough fuel, what are the AFR's like with throttle stops fully unwound, i.e. if very lean probable air leak on inlet past carbs, if about right tends to indicate throttle plates not closing enough.

NB.  Re effect of balance tube, at idle and up to around 1500rpm, my Vitesse will happily run on one carb, i.e. one with throttle plate fully shut and no fuel supply. In this instance the AFR's look fine on both front and rear cylinders (I run twin sensors so can see this) i.e. a balance tube can supply enough, and with 3 carbs, it could be anyone of them that could be holding the idle high. I think I made a comment on another thread recently that next time I'm setting up the carbs I'm going to try blocking the balance tube, just to ensure that cross talk via the balance tube isn't affecting idle mixture settings.

re idle RPM, the guy that runs the RR I use, who has a lot of experience with classic racing engines, always advises that he hates to see flat tappet, push rod engines idling at less than 1K.


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Re the linkage, I replaced the original cheap linkage components with better quality stainless and alu parts and I'm confident that the linkage is not holding the throttles open.

The air bypass screws are fully closed.

The Spanish built carbs do not seem to be the same quality as the Italian built carbs and I there is a good change that one carb is behaving badly with the balance tube allowing that to affect the other two.

I did try disconnecting the servo and the ventilation connections to eliminate them as the cause. After pulling the hoses and blocking the banjo ports it made zero difference.

Carbs are connected to the manifold with misab plates.

Distributor is a 123 unit, the one with the dip switches not the later model with the programmable feature.

I would be fine with a 1000 rpm idle.

The TR6 is in the trailer, snowed in, and its battery is in the garage on maintenance so I wont be able to do anything hands-on for a few weeks.



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Sounds like you have covered most bases already.

When you can get to the car again it might be worth seeing if you can nail down which carb throat(s) are letting the air through.  Occasionally you'll get one with a twisted spindle and it really doesn't need to be open much.  A leak further back remains a possibility, but a fairly low probability.  I have read that the Spanish DCOEs are not as well made as the Italian ones, but have had no dealings with either personally. Dave, the original creator of this forum is a Dellorto man and not a fan of Webers.  I own a pair of Solex 40s from an old Alfa but I've never found a use for them.

The PI throttle bodies suffer from this leakage quite often, poor fit of throttle plates (waiting for them to soot up and seal themselves is a known cure) and, occasionally twisted spindles.  The OE linkage is rubbish too.


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1 hour ago, Nick Jones said:

When you can get to the car again it might be worth seeing if you can nail down which carb throat(s) are letting the air through. 

May be try getting a bunch of the rubber bungs they use in wine making jars, and play blocking off combinations, if its one or two leaking, you should see a dramatic drop, otherwise just a small one. 

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The GP2 is not really a wild cam.
 I'm amazed, that you need such a rich idle jet with with a GP2

I've just had a look into my records:

In my 2,7l, I have a WBC518v2 cam ,which is somewhere between the GP2 and the GP3. and with the 55F9 my Spanish Webers were very rich all over.

I ran a 50F8 idle jet after finding the 45 with small air bleed  was just slightly too weak.

But the 50F8 I ended up, became still rich at the end of the transition before  the main circuit took over.

I was so fed up with wide steps between Weber idle jets and the wrongly positioned transition holes in cores of the Spanish Webers, that I eventually converted to 6-throttle EFI...

I could imagine, that your need for such rich idle jets is a compensation for an air leak, thus generating high idle rpms.

I assume your throttle plates can close completely...


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  • 10 months later...
  • 4 weeks later...
On 12/7/2018 at 1:39 AM, graeme4a said:

Hi Stan,

Did you ever bottom this problem with the idle speed?



No Graeme. The TR6 is still running with the balance tube at least up until the point that I tucked it away for hibernation and is still idling a bit higher than I would like. There will be nothing much happening with TR's here until late April and I will re-visit this next summer.


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