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TR6 TR6: Intake Vacuum & Timing

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Webb

Webb

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What do you all get as a manifold vacuum reading? Bentley says 7-8 in. Hg at idle (800rpm), and my car is doing about 14 (timing's off), which is short of 18-22 in. Hg, which I hear is what a healthy, well-tuned car should be at. Is the 7-8 in. Hg a minimum reading, or is that what it should be?

My timing light hasn't come in yet so this is theoretical, but what happens if you turn the distributor clockwise (advanced) as far as it will go, and it's still not enough to get the timing right?
 
The 18-22 is a reasonable range with a stock cam. The 7-8 is awful low for idle conditions; it sounds more like manifold vacuum at wide-open throttle.
 
This was my reading at idle, with worn valve guides causing the fluctuation.
 
Paul,
That's about like my reading, except that it holds steady at 16 in. Hg. Is 16 ok? My gauge says "Ign. Timing Late" in the 15-18 region, which I take to mean the ignition timing needs to be retarted some? I've turned the distributor clockwise as far as it will go and it still isn't 18-22 in. Hg.
 
Don't forget that a 72 US-spec TR6 is supposed to have vacuum retard, which means it's supposed to idle with the timing substantially retarded (and hence have low manifold vacuum at idle). "Emissions" engines also are typically detuned in other ways (lean mixture, low compression, etc.), so the idle vacuum may be "low" even without the retard.

And while particular vacuum readings may point at particular problems (like late timing, etc.), it's not an appropriate way to adjust those things. IOW, set your timing with a timing light, not with a vacuum gauge.

I don't have a TR6 Bentley, but if it's like most Triumph manuals, it's written for the UK version (which would mean fuel injection for a TR6). Some information for emission controlled versions will be in a supplement, but other information is simply lacking. I do have the emissions supplement for 73 models, and it doesn't mention manifold vacuum at all.
 
I disconnected the vacuum line to the vacuum retard and it showed no change on the gauge. Bad retard unit?

As a purely hypothetical question, what happens if I put the timing light on this (if it ever comes in!) and it turns out that it needs more advance to be set correctly than I can get by rotating the distributor?

FWIW, the Bentley manual for TR6s, and I would assume other Triumphs, does make specific references to both US and UK-spec cars.
 
Webb said:
I disconnected the vacuum line to the vacuum retard and it showed no change on the gauge. Bad retard unit?
Click to expand...
Most likely, although it might be a problem with the vacuum port or line.<div class="ubbcode-block"><div class="ubbcode-header">Quote:]As a purely hypothetical question, what happens if I put the timing light on this (if it ever comes in!) and it turns out that it needs more advance to be set correctly than I can get by rotating the distributor?[/QUOTE]The general solution to that is to remove the distributor and change how it engages with the camshaft. Sometimes, though, it can mean the wires are on the wrong towers.
Oh yeah, sometimes it also means that the rubber in the harmonic dampener has shifted, and the timing marks no longer match the actual crankshaft position.<div class="ubbcode-block"><div class="ubbcode-header">Quote:]FWIW, the Bentley manual for TR6s, and I would assume other Triumphs, does make specific references to both US and UK-spec cars. [/QUOTE]So, does it give you different manifold vacuum readings for the various engines ?
 
The harmonic damper was bought new from Moss when the engine was rebuilt two years ago. When you say remove the distributor, do you mean pull the distributor driveshaft out and jump it forward a tooth? Is that possible what with the oil pump being driven from it?

Also, besides doing good things for emissions at idle, what does the vacuum retard do? My unit is bad and I really don't want to blow $75 on a new one if all it's doing is keeping a hydrocarbon or two out of the air while I'm idling.
 
Webb...
You are correct; that's all the retard unit is for.
When I have my timing light hooked up and pull off the vacuum hose to the unit I see a large movement in the timing marks so I know mine works and have left it alone.
When I tried to time the engine without the retard unit hooked up I ended up having trouble finding a place where the engine sounded "proper". Mine's a '73
 
Only problem with not having the retard working is that you may have a hard time getting idle rpm smooth and down where the book says.

Timing without the retard should be the 'static' value given in the book, which was 10 BTDC for 73 US-spec TR6. (Other years might be different, ISTR it was 12 BTDC at some point.)

Of course, you may have to retard a bit if you have trouble with pinging on the witches brew they sell for gasoline these days.
 
Webb said:
When you say remove the distributor, do you mean pull the distributor driveshaft out and jump it forward a tooth? Is that possible what with the oil pump being driven from it?
Click to expand...
Yeah, that's the idea. I don't recall the details for a TR6 offhand ... you may have to turn the engine a little to get the oil pump shaft to engage.
 
Aside from the possibility of a defective vacuum diaphragm or other vacuum leak, your advance retard mechanism may be frozen. Make sure it is free and working correctly. Lack of oil on the big screw that hold down the dizzy cam results with the mechanism binding and consequently inoperative.
 
So without the retard unit working I can't time the engine dynamically? (ie. with it running). I have actually had trouble getting a smooth, slow idle. It's fine as long as I keep it up around 1000rpm, but below 800 revs, forget it.
 
No, what I meant is that the value the book identifies as the "static" value is the one you should use when timing with a light (engine running with the retard disconnected/non-functional).
 
Hello Webb,

the manifold depression depends a lot on what camshaft you have installed. (Which is why the information with your gauge is not to be relied on)
My injected car idles at 7 to 8"Hg which is what it should be for a P.I. engine, and that is due to the camshaft I have. Later P.I. engines ran at 12" Hg.

I wouldn't expect disconnecting the vacuum line to the distributor to have any significant effect on the manifold depression as the port is a very small diameter.

Alec
 
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