Message text written by Hugh Fader
>Well, after I replaced the diaphragms, I was able to get the carbs to
balance. I'm guessing the torn diaphragm on the rear was reducing the flow
to it. How, I'm not quite sure. Possibly because air is moving around the
I replaced the bypass valve on the rear carb, but may have screwed it up.
Idle seems to be hunting around a bit. If it persists, I'll probably do as
suggested by Dick Taylor and just block it off with a gasket.
1. A broken diaphragm reduces flow
2. The Gunson Carbalancer works fine
3. I satisfied my curiosity about what's inside the carbs.
Hugh, the piston is lifted by the partial vacuum created by the venturi.
If you look at the bottom of the piston there are two small holes that are
positioned at the downstream side of the venturi and the pressure on the
top of the piston is at this partial vacuum. The job of the diaghram is to
prevent that vacuum pleading back to the lower (high pressure) side of the
piston. If that diaghram is compromised there will be no (or reduced)
vacuum to raise the piston and it will not raise properly. (by
coincidence, I did the same thing to my TR8 last week and suffered from
significantly reduced power)
A wandering idle may be caused by a vacuum leak, vacuum advance/retard or
mixture. A vacuum leak can be located lightly spraying carb cleaner on the
gaskets and other joints on the intake manifold. Remove and plug any hoses
that connect to the mainfold. Disconnect the vacuum advance/retard lines
to the distributor to evaluate this possibility.
To check if the mixture is too lean lightly spray some carb cleaner (or any
aerosol product that uses propane as a propellent) intot he throat of the
carb. If the mixture is lean the idle speed will jump up. If the mixture
is fine the speed may rise slightly or not at all.
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