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SU throttle disc binding

spock345

Well-known member
Joined
Nov 3, 2016
Location
Livermore, CA
I've got my HS6 carbs back on the car after fixing the worn linkage parts and I've run into a new problem. Whenever I touch the throttle once the car is warmed up the throttles seem to stick a little bit. If then press just a bit on the lever where the return spring is the idle drops back to normal. So when I set the idle to 900 with the valves fully "closed" and then hit the gas the idle will stick around 1000-1100. Sometimes it comes back down if I sit there and wait a bit.

Bushings are just about new with no wear I can see, throttle shafts are new, springs are almost new (the sort concentric with the throttle shaft from a later set of carbs).

I've taken the carbs off the car already to see if something was off center, it all seems fine and the shaft rotates freely. The linkage parts are all setup right with even a little bit of grease on the spots that were worn on the old stuff.

Maybe a bur somewhere that something is catching on?

At least this car keeps me busy.
 
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I back off the idle screw, with the shaft and plate in with screws loose, hold plate closed, tighten screws. I then check the plate to the bore by holding it up to bright light to see any gaps around the plate, open and close recheck.
 
I back off the idle screw, with the shaft and plate in with screws loose, hold plate closed, tighten screws. I then check the plate to the bore by holding it up to bright light to see any gaps around the plate, open and close recheck.

/\ /\ /\

Although hanging open, then closing with a little persuasion usually happens when there's some slack in the shaft, it will hang on the side of the carb, then close more fully once wiggled. Shouldn't be the case with some newly refurbished carbs.

But if it's nothing on the external linkages hanging it up, then it's going to be an issue with the throttle plate and the carb bore.
 
/\ /\ /\

Although hanging open, then closing with a little persuasion usually happens when there's some slack in the shaft, it will hang on the side of the carb, then close more fully once wiggled. Shouldn't be the case with some newly refurbished carbs.

But if it's nothing on the external linkages hanging it up, then it's going to be an issue with the throttle plate and the carb bore.

I am guessing that is the issue. Carbs are coming off again. I am guessing a burr of some sort on either the plate or the bore. Maybe it just needs a really good cleaning.
 
If the jets are centered proper the piston should drop with a clunk, when you lift them and release. I have encountered off center jets and miss aligned throttle plates.
 
If I'm not mistaken, some HS6 cars (pre 1968) had an extra throttle return spring from the factory that went from a bracket on the throttle shaft to a bracket located on one of the intake/exhaust manifold studs.

I just can't remember the details at this time.
 
We've found the Volvo and aftermarket return springs for the pre-1968 HS6 carbs are now a little longer and don't always pull on the lever with sufficient force to return the throttle flap properly so we've had to shorten the springs a little.
 
I found a spare brand new throttle disc in my bin of assorted parts. I tested how well it sealed against the inside of the bore after I cleaned it out a bit by sticking a unisyn on one side and blow compressed air at the other. It did better than the old disc by a decent margin when the idle screw was backed off all the way and when I would set the idle screw and wiggle the throttle shaft.

Seems to be an improvement so I'll try that first.
 
We've found the Volvo and aftermarket return springs for the pre-1968 HS6 carbs are now a little longer and don't always pull on the lever with sufficient force to return the throttle flap properly so we've had to shorten the springs a little.

I've got these carbs set up with the later type of springs.
 
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