Source: Lotus West
Author: John Kouba
Title: DIAGNOSIS OF INCONSISTENT IDLE PROBLEMS
A vacuum leak is not a particularly common problem, but when it occurs it is a most annoying and difficult problem to discover. I am, here, referring to a loss of intake manifold vacuum seal. In other words, there is a leak somewhere between the carb(s) and the head, and air is being sucked in. The problem may be identified by any or all of the following:
- a hissing noise around the carbs (not air intake and not a snake) - a wandering idle speed
- a high idle speed which cannot be reduced by the idle screw
- loss of performance
Detecting the problem involves using an ether based starting fluid (aerosol), available in most auto parts stores. Spray the mating surface joint between the carbs and intake manifold, and watch for an increase in engine rpm concurrent with your spray...Check also around the butterfly shafts. An increase in rpm indicates that a vacuum leak exists in the area sprayed. Gaskets or O-rings should be renewed.
If your car exhibits a high idle and no vacuum leak exists, you should suspect a linkage problem. Renew your throttle cable. The original part seems to break at the most inopportune time. A new one is very inexpensive at your local bicycle shop and of far better quality. While installing the cable watch to see that the pedal pulls the cable down the center of the hole, rather than chafing it along one side. Check also at the carbs, then inspect the carb linkage. A little WD-40 to lube the joints and give surfaces a shiny finish. You want the shiny finish to look for cracks in linkage bits. Stromberg interconnecting links are especially susceptible. TR-6 links are identical.