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TR4/4A Baffling Carb Issue

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RJS

RJS

Jedi Warrior
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Hi

I need some help. I spent all day yesterday trying to get my rebuilt carbs dialed-in (SU HS6s). What should have been a 2 hour job became a very frustrating all-day affair.
My issue is a hanging throttle. I got the carbs balanced, mixture set and idle at 750 RPM. But, if I slowly take the RPMs up and let off the gas (i.e. crusing and coming to a stop sign) it will hang at 1,500 to 2,000 RPM. If you snap the throttle, it will drop back to a nice 750 RPM. The car runs great otherwise but, this one issue really spoils the driving experience!

Sitting in the driveway, I can clearly see that the throttle lever on the front carb isn't returing fully closed. With the engine running, I can push the throttle lever back closed with my finger. It only moves about 1mm to close - and it feels like it "releases" almost like something is binding. Idle returns to 750 RPM.

Here's the baffling part - I inspected and checked everything mechanical and I lubed the shafts. With engine off, the throttle lever and throttle disc work perfectly and close fully. It is only when the engine is running that the problem manifests itself. To confirm this, while experiencing the problem (idle at 1,500 RPM), I killed the motor and could watch the throttle lever return that 1mm fully closed as the motor died. Then when I immediately restart the motor, it goes straight to a 750 RPM idle.

At this point, I can only think it is something vacuum related...but what?? I have a new cylinder head and manifold gaskets. Carbs just professionally rebuilt (carb body rebushed and new throttle shafts among other things). Engine has 180psi dry compression across all four cylinders and pulls 20"-22" Hg of vacuum at the intake manifold.

Any ideas?

Bob
 
With HS6, there should be two throttle return springs (Moss P/N 371-930) -one to each carb lever. Are both springs installed? Is one weak?
 
TexasKnucklehead said:
With HS6, there should be two throttle return springs (Moss P/N 371-930) -one to each carb lever. Are both springs installed? Is one weak?
Click to expand...
That is my first guess as well.
Second guess is that on these HS6 carbs the location of the throttle connecting rod ( between the carbs) and the attached levers ( one on each end) is adjustable. Could the lever to the front carb be adjusted so that it is causing this problem? Or maybe adjusted to stop the problem? Two good springs are a must on these carbs.
Charley
 
Probably not relevant, but I had a somewhat similar problem with my H6 carbs, caused by not getting the stop lever in just the right place on the shaft. There was just a little play left over between the carb body and lever, which was letting the shaft move sideways in the bore when the throttle was open slightly and then the throttle plate would drag on the side as it closed. Somehow the suction from the intake manifold was creating enough friction that it wouldn't slide closed reliably. The sharp edge of the plate had actually cut a small notch in the carb bore.

I wound up cutting a small shim to fit on the throttle shaft, so the plate could be centered with the lever firmly against the carb body.
 
throttle connecting rod ( between the carbs), only the pin need to be engaged and situated at rest in the middle of the fork, the bracket must not touch the carbs, minimum 1 millimeter on each side of the rod. IMO
 
Thanks everyone,

It's definitely not the springs. Been there , done that. Stronger springs only turn the throttle into an on-off switch.

Randall and Tr-Beg I'll try you ideas and report back. Probably not til the weekend.

Bob
 
Bob,
The two springs are attached to the lever arms of the butterflies. My guess is that the butterfly is not closing. On my carbs the springs make that closing of the butterflies VERY POSITIVE? A misadjustment of the levers on the ends of the connecting rod could also influence the butterfly movement. Were you able to center the needles during the rebuild?
Charley
 
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