The HIF uses some sort of cold start or idle bypass doesn't it? I would
strongly suspect your problem lies
in that circuit.
>>> "Terry L. Thompson" <tlt@digex.net> 08/22 12:29 PM >>>
I've been using a set of HIF4's from an MGB on my spitfire, and recently had
a problem with the idle speed jumping up to around 1500 rpms and not wanting
to come down.
I knew that I had leaky throttle shafts on both carbs and one of the
throttle disks didn't look like it was sealing well (on the front carb).
Also the throttle discs (butterflies?) had the spring valves on them.
The pistons weren't sticking and the throttle springs appeared to have no
trouble returning the butterflies to the closed position, so I assumed it
was the leaky carbs (using carb/choke cleaner on the outside throttle
shafts, the engine would shutter and the idle would fade).
So In an effort to fix that 1500 rpm problem. I decided to put new throttle
shaft bushings in the carbs and replace the valved throttle discs with solid
no-valve throttle discs.
Once that was done the car idles at 1800 rpms. And I'm beating my head
against a wall trying to figure out what the hell it is that I'm missing
here. The throttle shaft bushings appear to have cleared up the problem
with the air coming in from the throttle pivots (Spraying carb cleaner into
the pivot shafts has no affect on the idle any longer). Good right?
But the symptoms I have now are as follows:
The rear carb can't seem to be leaned-out enough. With the idle speed screw
completely out of the carb (The butterfly COMPLETELY closed on the rear
carb), my UNISYN carb tool measure the carb sucking air faster than the
front carb with the fast idle screw turned in 3 complete rotations.
I've pulled off both carbs three times now and the rear carb butterfly is
seated in the throttle chamber as well as it will ever be seated. I can't
even see light coming in around the edges when I hold it up to a light
source.
Pushing up on the lift-pin on the rear carb makes the engine start to die
(indicating a lean mixture), but no matter how rich I make the mixture it
continues to act lean. It's as if there is some other way that the 3&4
cylinders are getting air and fuel.
Any suggestions as to what is causing this? My head is getting sore from
beating it on the wall.
-Terry
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