Triumph Repair: rough idle on spitfire 1500, vacuum leak, spitfire 1500

Question
I have tuned up my 1500, set values, checked plug gaps, checked timing and at idle speed, the tach shows that the idle speed varies from approximately 850 to approximately 925 or 950. some other people say there is a vaccum leak some place. are they right or is there something else wrong.

Answer
Hi Lester,
You missed an important one. You need to check compression first. Then after you check compression you can look for a vacuum leak. Compression should be about 125 PSI to 160 PSI with little difference between cylinders.

AFTER you have confirmed good compression you can test for a vacuum leak by slowly restricting the air going into the intake. This must be done very slowly and smoothly. Have someone watch the tach and you listen to the engine.

As you restrict the air flow, the engine should steadily slow until it quits. If at any time during the restriction process the engine smoothes out or picks up in RPM, you have a vacuum leak somewhere.

Even if you find that you do have a vacuum leak this does not mean that there is not another problem helping to cause a RPM fluctuation. A worn distributor drive or advance system can add to the RPM movement. Even a lean carburetor can do that. EGR is another cause of a RPM change.

Howard