Subject: 1600 carbs
Leigh wrote:
> I took a good look tonight at the carbs for the Rat.
>
> There is some side-to-side play on the butterflys (the front more than
> the rear). I sprayed a bit of WD-40 on the outside of the bushing and
> after some jiggling, there was some seepage into the inside of the
> carbs. I assuming that they will leak air and need to be rebushed (or
> get the Z-therapy treatment).
>
> Should there be any side-to-side play, and is this a good bench test of
> leakage?
Hi Leigh,
All throttle shafts will leak a bit of air when cold, though the amount
decreases when the carbs warm up and the metal of the carb body expands
causing a decrease in the shaft to bushing clearance.
Unfortunately, most bushing clearance inspections are made when the
carbs are cold.
Few of us want to hold onto a fully heat soaked carb to perform such
tests, though that is the proper time to check for bushing clearances.
Simply put: Cold clearance checks are not accurate.
The fact remains that the only time shaft leakage could possibly be a
detrimental factor to carb or engine function is at idle.
The moment the throttle is opened, a huge vacuum leak is induced via the
open throat of the carb (and bushing leaks become a non-issue).
The throttle shafts are just one source of false air which combines with
all other sources of air regardless of how it gets into the intake manifold.
Other vacuum leaks which are not caused by throttle bushings:
1. Most R and U series Roadster engines on the road today, have eight
additional intake vacuum leaks caused by worn intake valve guides, yet few
of us will tear down the engine to remedy this vastly greater false air
entrance point (into the intake manifold area) as a remedy for idle
fluctuations.
2. Cars equipped with a positive crankcase vent system (a large vacuum
leak as compared to a throttle shaft leak) are not adversely effected by
this false air source.
3. All late model cars constantly recirculate air from inside the entire
engine as a whole (closed circuit venting and PCV systems), and these
engines also idle correctly.
Fuel mixture adjustments can compensate for any additional false air
which enters the intake side of the engine, regardless of the source.
We simply add enough fuel to the mix to make the air-fuel mixture
correct, no matter where the air comes from.
IMO, normal throttle shaft leakage is not as serious a problem as some
carb remanufacturers would have us believe.
Most idle fluctuations which are actually created by worn throttle
bushing are NOT the result of false air, but instead they are the result of
the butterfly itself failing to come to rest properly (failing to seal the
venturi throat consistently).
If you can lift up and down on the throttle shafts (without rotating the
throttles) and effect or create an idle speed change, then the bushings may
need to be serviced.
A simple test to see if the air leaking by the throttle bushings is
indeed causing an idle fluctuation problem is to coat the ends of the shafts
with any heavy grease.
This will temporarily seal any leakage which may be present.
If you do not see a significant change in idle speed, the amount of air
leaking by the shafts is not a problem.
The old saying goes: If it isn't broke, don't fix it. :)
Kim Blough
Idaho Z Car
2913 Garrity Blvd.
Nampa, Idaho 83687-2629
1-208-466-0004
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