5 replies to this topic

[Purple Tom]

Hey

Have been thinking, is there any reason why the vacuum gauge on my old Cooper (MG Metro engine) sat bang in the green 'idle' section at idle, yet my 1293 wavers somewhere around the 'drive' section (i.e lower vacuum?)

Is it something to do with the modded head and lumpy cam at idle, due to overlap of inlet/exhaust valves etc? I know its not an airleak between the inlet manifold and head, thats all brand new....so just wondered if there was any other reason?



Cheers!

Tom

[Sprocket]

Where have you taken the vacuum feed from?

[dklawson]

The question about where you connected the tubing to the manifold is a good point. You need to be between the carb and the head on the manifold. Do not connect to the carb itself if it has a vacuum port. The carb ports are for the dizzy and won't indicate your manifold vacuum.

That said, lumpy cams do often deliver "higher" manifold pressures. A good stock engine properly tuned will idle with a steady vacuum reading around 20" of mercury. I can't read the face of your gauge but obviously it's not the same reading you had before. Check your connection point, adjust your valves, set all the ignition components (plugs, points, and timing), and go through a carb adjustment. If that doesn't improve the situation and your car is running as you want it to... blame it on the cam... particularly if it's a hot one.

[Purple Tom]

The vacuum take off is from the inlet manifold, on the small outlet opposite the servo take off (which is blanked).

Timing, tappets, fuelling, everything is bang on.

The MG Metro was tuned perfectly and showed the reading (in the green basically).

Its a hot cam in the 1293, comes on cam at roughly 2000 rpm, so I think thats the reason for it. It runs perfectly even pulling well below the 2k, so I think i'm just worrying too much.

Thanks

[ian]

ive got a manifold with the hole for servo take off, and just got a blanking plug from minispares, but can i fit a adaptor there instead to run a vacum gauge?
if so any idea where i can get one?

sorry to hijack the thread but it gave me the idea

[dklawson]

My Triumph has a hot cam in it that my father-in-law installed 30 years ago when he raced the car. He has no idea what it is and I haven't rebuilt the engine yet so I don't know either. My idle vacuum readings are like yours. By comparison, my 1275 had been bored out +.060 and I'm running the SW5 cam. My idle drifts a bit but averages about 900 RPM with about 20-21" of vacuum. Since your car is running well I wouldn't worry about the idle readings.

Ian, you're not hijacking the thread at all ! Since you already have the plug from Mini Spares that begs a question. Do you still have the old hose nipple that would have been on the manifold? If so, you can buy plastic step-up/step-down adapters for different diameter hoses. With a short piece of 1/2" vacuum hose and such an adapter you could plumb your gauge. If you don't have the nipple, the easiest thing to do is go to a plumbing shop and buy a nipple that has small pipe threads on one end and the appropriate small hose barb on the other. Drill and tap your plug to accept the nipple and plumb your gauge. (Drill and tap the plug OFF the car so chips don't get in the manifold). A third option for you is to go to an automotive parts store and look for a vacuum "T" fitting. These frequently have a large tube going "through" and a smaller diameter barb "T-ing" off the side. You can cut the servo vacuum hose and insert the "T" fitting there to plumb your gauge.