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MG Midget and Sprite Technical - Vacuum Gauge Readings

After chasing down a somewhat variable idle ignition miss problem – I promptly attached my trusty vacuum gauge to the nipple port behind the SU dome. There is a small vacuum hose from there to the distributor vacuum port. Anyway, with only a paper receipt for work done in Los Angeles way back in the late 1990s for some hardened valve seats (and possibly some lapping and degreasing, etc) on the head – I have no idea of any other work done on the engine…

I currently read about 18 inches of vacuum (warm engine) but with that classic “fluttering needle” which ranges over +/- 3 units while idling. When I bring the idle up past 1200 to 1500 rpm the indicator needle is rock steady. When I blip the throttle I get what I consider to be a normal fluctuation of about 25 (high) to 5 (low) on the gauge.

The basic directions with the gauge indicate that this needle pattern (flutter at idle) is indicative of worn intake valve guides…And here is my question: Never having driven a new (or massively rebuilt engine in a midget) and acknowledging that some (or all) engines have some degree of wear – how much of an improvement would I gain by spending several hundreds of $$$ to take the head to a local shop where they want to crack test / degrease / disassemble / replace value guides (both intake & exhaust) / re-assemble, etc.

What I want is the best idle and decent road performance for a daily driver Midget. I don’t know how much of a marginal improvement I would get with another head & valve job. Also - what might be the progressive symptoms of deferring any head / value guide work (other than saving a few dead presidents…) – this seems like something that does not have to be done – I have no other plans to strip down the engine in the next few years. As they say: “...if it ain’t broke, don’t fix it...”

Mike P.
Buffalo, NY
USA
Mike Pelone

Mike. Interesting readings. The place where you are attaching the vacuum gauge hose to is a small hole, supposedly covered, when the engine is at idle. This is why vacuum from the rear carb is called "ported vacuum"--the port is uncovered as the throttle plate is moved off the idle position. I have always found some vacuum signature at idle when attaching a gauge to this port but suspect that is because I set my idle at 1,000 rpm rather than the factory 600 to 700 rpm.

To get the most accurate vacuum reading, you need to be reading manifold vacuum, taken from a port on top of the balance bar between the two legs of the intake manifold. If you get the same readings when using such a connection, the results would be significant. Right now they are interesting, but not significant.

Les
Les Bengtson

This thread was discussed on 19/10/2009

MG Midget and Sprite Technical index

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