I have recently rebuild my HD6 carbs on Mk IX. new throttle bushings, & kits. and that is all running fine.
But now I am looking at Thermo carb, as starting is variable.

My first question is around the Thermo Needle adjustment.. it is quite loose, that even when I see it starting the nut seems to move. is it meant to be stiff?

Second, tuning the Thermo carb, I earthed the connection, hearing it click etc.. and gave bit of rev, but the thermo is not kicking in?
I tried tuning while cold.. but the idle barely changes.. is this another indication that the needle needs replacement?

Thanks in advance

 

Hi Dean. In the literature, that's referred to as the Starting Carb. To answer your direct question, yes, that nut turns freely, BUT there's supposed to be a metal tab screwed against it to keep the nut from moving once you hit the right adjustment spot. If you're missing that, you need to get or make one. I can't remember what it's called, but it's a common way Jag used to keep adjustment nuts from moving. See the attached photo of mine.

I also attached a blow-up of the Starting Carb, as well as adjustment instructions. I'd be surprised if the behavior you're seeing is due to the needle needing replacement; maybe just cleaning? My understanding, by the way, is that most people disconnect the thermal switch in favor of an under-dash switch so you can control when the Starting Carb is engaged. On my car, I only need it on cold starts; in the summer just to get it going, and in the winter for maybe 30 seconds. The revs are high enough, even when adjusted all the way down, that I don't like it on for long. It also produces a fair amount of soot in the exhaust.

/Doug

 

Thanks Doug, I had my metal tab screen, set upside down.. and is slightly broken.. so I need a new one.

I earthed the solenoid.. and pumped accelerator.. and that triggered the choke. which allowed me to tune starting carb.
The tacho cable was not connected.. but I couldn't notice a huge change in RPMs

 

Hi Dean. Any progress on this? As I re-read your last note, I was wondering if your stop nut under the needle is just screwed in too far, I wonder if you'd notice RPM difference if you backed out the stop nut a bit (and thus enriching the mixture).

On my car, I have the opposite situation. My stop nut is screwed down all the way and the RPM still rises a bit more than I'd prefer. Perhaps *my* needle valve is worn down, letting too much fuel slip by, and thus requires replacement.

/Doug

 

Just an FYI. Pumping the accelerator does nothing on SU carbs. SU carbs do not have accelerator pumps, and do not have stepped cams that are connected to the accelerator pedal that set the choke. This is a habit from earlier American cars that required pumping the gas pedal to feed fuel into the throat of the carb, and for setting the choke when cold. The accelerator pedal does not need to be depressed when starting the car with the thermostatic starting carb. Turning on the ignition or the dash activated switch for the carb is all that is required.

 

Hi Doug, part of the problem after earthed was to trigger the choke by a deep throttle press.
While I got some variation of RPMs, it wasn't huge.. added to fact the RPM counter was removed.. and I didn't have any other gauge. But I think I have set ok now

That is interesting about not pumping accelerator, my old English ford zodiac had a Zenith, that would squirt gas when pumped.. so should foot simply not be on accelerator at all? I will try tomorrow

 

Rotomike,

I agree pumping generally doesn't do anything. And yes, before the car is started, energizing the electrically-controlled starting carb will activate it just fine. But at least on my car, once the car is running and the starting carb is 'off', if I flip the switch 'on', it doesn't actually activate until I depress the accelerator a bit. Guessing there's a vacuum that is preventing the starting carb from activating until you break it with some accelerator movement.

/Doug

 

Well I tested this on jag and perfecto
the car sits in a shed that gets cold especially in winter. And it’s winter now in NZ

my father in law has always had trouble starting. Said he used to pump pedal also

but now the carbs are rebuild incl the starting carb. It starts first time

thank you