Idling circuit

[Tom1]

I'm hoping that someone can shed some light on a carburettor problem that I'm baffled by.
I have a 3.8 fitted with a straight port head with a little porting and slightly larger inlet valves. Piper fast road cams and a classicsfabs exhaust system with 1 7/8" headers.
I have a pair of professionally rebuilt HD8s fitted on to a xj6 inlet manifold and 9:1 Pistons.
My problem is that I can't get the engine to idle on the idling circuit. I can only get the engine to idle if I connect up the linkage and set the butterfly valves very slightly open.
I've blocked off the port for the brake servo to eliminate a servo leak. I've blown through the idle circuit to eliminate a blockage, checked float levels set idle screws out 3 1/2 turns, set jets down .065".
I have got the engine up to temp with the butterflies slightly open and then tried to introduce the idle circuit but the engine stalls as soon as I shut the butterflies.
The engine runs fine by holding the butterfly valves open on the throttle stop linkage and setting the mixture using a colourtune and checking the spark plug Colour.
The engine pulls like a train right through the rev range and the car goes like the clappers but something is fundamentally wrong if it won't idle on the idling circuit. Any ideas that may help me resolve this problem would be much appreciated.
Tom1

[PPV]

Are you sure you do not have an air leak somewhere? Having said that not sure about HD8's as my car is standard but the AH site has a good guide on how to set them up as per the link :http://www.healey6.com/Technical/HD8.pdf
Do the pistons lift and clearly drop with a clunk? Just worth checking as they may be a bit tight at the lower ends of travel.
Not exactly sure what you are saying when you refer to 'then tried to introduce the idle circuit but the engine stalls as soon as I shut the butterflies'. Are you sure that the diaghragms are Ok as well as sometimes these move during re assembly.
Regards,

[piman]

Hello Tom,

where did you get the 0.065" setting from, that's not a conventional S.U. setting, 2 .5 turns down is their convention?
Despite being professionally rebuit, I would double check the needles are correctly fitted in the piston, (shoulder flush with the face of the piston.)

Alec

[Tom1]

Thanks gents for taking the time to reply. As PPV suggests I also strongly suspect an air leak. I'm confident that the mating surfaces between the manifold and cylinder head are good and flat with new good quality gaskets fitted. I have blanked the vacuum take offs. The only other thing I can think of is a leak through the auxiliary starting carb circuit or that the plunger is not shutting completely.
In response to Alec, using a digital vernier caliper 2 1/2 turns is 65 thou give or take a couple of thou.
The Pistons/needles rise and drop as they should with no evidence of rubbing and a audible clunk when they drop. The needles are dead flush.
By introducing the idling circuit I mean that once the the engine is up to temp and while the engine is idling with the butterflies held very slightly open on the throttle stop I try to close the butterflies by backing off the throttle stop so that the only path for the air/fuel mix is through the idling circuit but the engine stalls. Because I can't get the engine to run on the idling circuit I can't follow the normal set up procedures.
When I get a chance I'll check the auxiliary carb system for leaks and hope that maybe someone reading this knows exactly what the problem is and is happy to share their knowledge.
Tom.

[piman]

Hello Tom,

OK, it's just that when I have a difficult problem my first step is to double check all settings.
I take it you have opened up the idling screws to try and get it to idle? What did you use to check the mixture at idle?

One of my other cars is a petrol injection Triumph and I know how much difference different cams can make to the idle vacuum. (Which is what controls the fuelling so it is important) I am wondering if your modifications have reduced that and hence air flow at idle? I would suggest contacting Burlen Services to discuss this problem with them. It may need, for example, a lighter piston spring?

Alec

[PPV]

Hi Tom, when the car is warmed up the aux carb should shut off so you could disconnect it and blank the connection to the inlet manifold and see if it runs then. If so it could be aleak in that pipework somewhere.
Another leak source would be the brake servo pipework and likewise you could blank this off at the manifold and see if that makes a difference with the slow running.
Having said that I see from the Internet that when changes like yours have been made then the standard needles will require changing and as Alec suggests talk to a carb supplier and see what they say also.
Regards,

[Tom1]

<t>Thanks again for your responses. I haven't had a chance to experiment yet and it will be next weekend before I get back into the garage. I have another starter carb solenoid/ needle assembly in good condition that I can try as well as checking for leaks in that circuit. As mentioned before, I have blanked off all vacuum take offs on the manifold.
In response to Alex, using lighter springs is an interesting idea. From my understanding the cams I have produce a larger inlet valve opening for longer and same for the exhaust valve opening and duration thus more power. Theoretically I would be drawing far more air. It certainly appears that the carbs are sucking well by the balance gauge readings.
I should say that I've been driving this car despite the idling circuit issue and have had a chance to experiment with various needles and fuel. Shell v power being the best fuel and needles had quite subtle differences, UM were too lean from mid rev range onwards. UN Were better. UR seemed to be the best all rounders. UVO were fantastic from 3500 rpm onwards and perfect for track days, far too juicy for touring or poodling around.
With reference to my issue, all the needles mentioned are exactly the same dimension at idle, so that's not my problem.
I have set my mixture in the past by using a gunson colour tune kit, checking exhaust tone and monitoring spark plug colour. Lifting the pin method does not really work for me although I have tried it but just seems very vague. I find that using the colourtune at 1200 rpm gives the best results. There's no point in me trying it at idle.
I'm hoping my problem is a silly over site that I will resolve soon.
I'll keep you posted.
Tom.