13 replies to this topic

[finch661]

hi there, i am fitting a set of twin hs carbs to my project car, and just wondered how to initially set up the mixture/idle speed enough to get the car running so that i can then tune it?

cheers

[finch661]

bump

[dklawson]

You really need to read through the complete procedure in a Haynes or SU manual to catch everything.

However, a quick overview would go something like this.
Fit both carbs to the intake with the linkages installed but clamping fasteners on the actuating arms "loose".
After tightening both carbs to the manifold, make sure the air filter housing fits and that the linkage rods are free to move without binding.
Make sure the choke mechanism is loose and not connected (both the cable and the actuating arms).
Bring the jets up flush with the bridge in each carb.
Center the jets on each carb. Make sure the piston can fall freely to the bridge with a clunk with the damper removed from the top of the carb.
When the jets are centered, lower the jets two complete turns of the adjusting nuts (12 flats).
Back off the idle adjustment screws completely, then screw them in one full turn after they contact their stop.
Hook up the choke (temporarily) and tighten the choke actuating arms. Start the engine and bring it to operating temperature.
Once at temperature disconnect the choke again (both cable and arms) and go through balancing the airflow.
Lock the actuating arms as per your manual, then set the mixture as described in your manual.
With the mixture set, re-set the idle speed.
Now follow your manual for the fast idle (choke) adjustments.

[finch661]

cheers mate, just what i am after. just want it running so i can tune it

[finch661]

hey had a go tonight as suggested above.

i can get the car started and warmed up. but getting rest done is really annoying me. i tried to use the carb balancer, but one carbis just not having any of it. when i lift the pistons up, one causes the engine to increase in rpm, the other doesnt do anything, as if its not actually working (although if the throttle screw increases engine rpm when adjusted)

its really pee-ing me off

[dklawson]

With dual carbs it is really important to make sure the jets are free to move down and (more importantly) UP when you actuate the choke cable. It's very easy for the choke to interfere with adjusting the mixture and setting the idle speed. That's why you need to have the choke disconnected, loose, and the fast idle screw backed out when making mixture and idle speed adjustments.

I suggest going back and trying the mixture and idle speed adjustments with the choke completely disconnected. However, with HS carbs you DO have to have the jet lowering linkage attached between the choke and jet (to make sure the jets are up). This will make some sense when you look carefully at how the jet is lowered when the choke is actuated.

Remember that balancing the air flow is done with the throttle linkages 100% loose. Both arms on the throttle linkage rod must be totally loose. What you want to do is adjust the idle speed screws on each carb until the carbs are flowing the same amount of air. You cannot do this with the throttle linkage connected. Once they are flowing the same you bring the arms on the throttle linkage rod up until they are JUST touching the inside of the forked arms on the throttle spindle. Then you tighten both arms. There should be about 0.020/0.030" clearance between the stop finger on the throttle linkage rod and the choke linkage rod. Again, this will make sense when you look at the parts and/or read through a carb manual. With the carbs adjusted to flow the same air you are setting the linkage so they "pick up" at the same time AND setting the throttle stop finger so it is not touching anything at idle. Once the linkages arms are set properly you reset the idle speed to where you want it using the idle screws.

At that point you can adjust the mixture. If in doubt about the jet positions, remove the damper from each carb (do not mix them up). Put a short wire rod down into the dashpot hole and run the jet "UP". Hold the wire in one had while you raise the jet with the other. Stop raising the jet when you can just feel the jet start to push the dashpot (wire) "UP". Do this for BOTH carbs, then run the adjusting nuts down 2 full turns. Put the dampers back in, start the engine and start making your mixture adjustments.

As I mentioned in my last post, to do these adjustments the jets must be centered and everything must move freely. If anything is binding, resolve that problem before trying to make adjustments.

[finch661]

cheers for the indepth reply, really appreciate it . little bit confused about the wire technique, but will give it a go tomorrow night.

starting to like the idea of fuel injection lol

[dklawson]

Sorry about the wire thing. I was afraid it was a bit unclear.

The purpose of the wire is simply to give you a "handle" that you can hold onto while you are raising the jet. With the wire inserted to the bottom of the dashpot (the bottom of the oil) you will be able to feel when the jet touches the bottom of the piston. You could also measure this with calipers or even do this visually if you have good eyes.

SIDEBAR: Not to confuse you, but for carb balancing they USED to sell a kit where you were supplied 2 plastic sleeves and 2 pieces of wire that looked like bent paper clips. You would remove the dampers from the top of the carbs and insert the plastic tubes in the top of the pistons. The bent paperclips were then inserted into the sleeves and their "tips" bent until they were pointing towards each other. This gave you a visual reference to tell when the piston was starting to lift as the jet was raised. Then... when you were balancing the airflow, you would adjust each carb's idle speed (independently with the linkage disconnected) until those bent paperclips were again pointing at each other. Basically this method measured the height of the two pistons as a method to confirm the air flow through each carb was equal. (Sorry, I know that's probably confusing but it is an interesting alternative to the air flow meters and the piece of tubing stuck in your ear).
For a picture of the tuning wires/rods, see this link: http://www.british-c...22414817859.htm

[finch661]

cheers, much better idea now! thanks

[finch661]

right i had another go, and found that one of the pistons wasnt moving freely, so replaced it. managed to get the carbs balanced, but setting the mixture was a problem. i have got one perfect (interestingly enough it was the one that wasnt working) and cant get the other one right. I used colortune and got a big yellow flame, and even with the nut right up! dont know whats going on!

[cooperrodeo]

Don't rule out the possibility that you might have a very worn jet and needle in one carb, in which case you would have to wind the jet up much higher than normal. I had this with one of my pair of HS2s and ended up buying new parts. I was able to prove I had a worn needle by swapping them over and noting the change in jet height after adjusting for correct mixture at idle.

[finch661]

sigh i was afraid of that

[dklawson]

Worn jets and needles are indeed a possibility if you are talking about the spring-loaded swinging needles used in later HS and HIF carbs. If you have early HS carbs the needles are "solid" and do not move sideways. When the jets are properly centered on an early set of HS carbs, the needles never touch the jets during operation.

[finch661]

its the later spring needle. will have a look at the jet tonight