hard start when cold

[VTHealey]

Its getting colder and I am having trouble with hard start. Have not had my Healey through a winter yet, so I have not seen this problem before. (66 mkIII). Idle is set at around 1000, and once warmed up to 170F, she idles nicely. Getting it going however takes a lot of starter turns and messing with the choke. I am pretty sure it is not flooded, opposite problem-- seems that the extra fuel from the choke that it is supposed to be supplying a richer mixture on cold start may in fact not be happening. The "choke" (fuel enricher might be a better description) mechanism seems to have a natural pause about half way through the stroke. What is the first part of the stroke and what is the second? I am guessing, that first part is additional fuel and second half is higher idle? On cold start should the choke armature be pulled all the way to the top? Not sure of how the mechanism works or where it should be. Want to start out by understanding the position of the armature and then figure out what is going on.
Thanks!

Fun to read these posts everyone. Great community out there!

 

[Keoke]

It seems your initial cold start choke setting is too low,should be around 2500 RPM.

 

[Legal Bill]

I'm not at home,but I'm pretty sure the manual explains how they work. If I recall correctly, the cable pulls up on a lever that lowers the jet at the bottom of the venturi which allows more fuel to get past the jet needle thereby enriching the mixture. At the same time, a lever opens the throttle slightly for a faster idle. You can't really see the jet being lowered unless you remove the air cleaner or the pot and look down at the bottom of the venturi.

From outside the car, you can see how far the cable moves the lever and then see if you can move it further by hand. The cable may be stuck and not opening the seat far enough.

Here is a nice article on a Jaguar web site that discusses the carbs in great detail. About two thirds of the way down is a brief description of the choke actuation.


https://www.jcna.com/library/tech/tech0006.html

 

[Bob_Spidell]

There are two stages to the 'choke' mechanism on SU carbs (it's not really a 'choke,' but is mixture augmentation):

1) the first part of the travel of the choke only opens the throttles; this is the 'fast idle' setting (actually not very fast--1,500RPM or so--when the engine is cold)
2) the second part acts on levers that lower the jets (HD carbs use diaphragms, others use tubes with packing). As mentioned, lowering the jet increases the annular gap between the needle and the jet (assuming needle remains stationary). This provides more exposed fuel for a given airflow and hence a richer mixture.

That's why you feel more resistance on the choke lever about halfway out; that's when the 'choke' levers start working against (additional) springs to lower the jets.

Conversely, when you accelerate the spring and oil in the vacuum dashpots prevent the vacuum piston from rising as airflow increases, so you get more vacuum over the same annular area between the needles and the jets. In effect, you are 'choking' the engine when you accelerate, but the richer mixture is required for acceleration and SU's don't have accelerator pumps.

 

[daily]

howdy, i agree with mr spidell, also i learned on my HD8's
that it's crucial that all link points [taper pins] are firm,true on thier shafts
with respect 59er

 

[RAC68]

I agree with Bob that the choke goes through 2 stages. My BJ8 originally had a single rather heavy spring working through a V wire attached to both choke arms. The resistance was quite heavy through the full range. Replacing the single heavy spring and V with 2 sets of 3 pen springs mounted between each choke arm and mount made the initial pull much lighter, however, the last ¼” of pull that lowers the seat remains quite hard. It is that last ¼”, and only that last ¼” that set up my Healey to start cold. Anything above and my car will not start.

Another possibility. Are you getting sufficient fuel to the carburetors? In early Spring, I was experiencing cold start difficulty although the car ran well when started. After a few of these instances, my pump stopped about 200 feet from the driveway. After correcting the pump problem, I had no further difficulty starting cold.

Hope this helps,
Ray

 

[VTHealey]

Thanks for the information, I think I had the purpose of the choke armature backwards: the first part of the travel is idle, then fuel enrichment.

I appreciate your answers and will give this some further testing to see if I can get the right settings. I may disconnect the cables and play around with them (in unison) to see what is going on and make sure they are balanced.

 

[Johnny]

I had a similar problem with my HS6 carbs. I had replaced the choke cables (bike shop cables) and still couldn't pull the choke all the way out. I eventually had to reduce the length of the outer cable from the choke handle to the inner side of the firewall about 2 inches, that seemed to help a lot as the cable before formed an "S" shape from the back of the choke handle to the inner firewall.