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Using Vacuum Gauge to Set Timing (and adjust the carbs)

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5452 Riley L
Princeton, USA   USA
Thinking of using a vacuum gauge to set timing - and seeking advice on

1> attachment ( eg. disconnect brake servo to manifold hose at servo - and attach gauge);
2> process (loosen and rotate distributor and lock at max vacuum - or perhaps a couple of lbs less); and
3> is this done at idle and some higher rpm (eg. 2500 or thereabouts)?

Lastly - does this process eliminate the need to use a timing light - or is the timing light the starting point (in my case a base setting of 10 degrees BTDC).

Thanks

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poolboy Ken D
Sandy Hook, MS, USA   USA
If you need to get the engine started by using the book static timing spec then by all means do so..then once the engine has warmed up check the manifold vacuum reading at idle with a vacuum gauge....You may want to adjust the ignition timing after you see that reading...or not.



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kencorsaw@aol.com

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Uberxy Avatar
Uberxy Steve Fox
Va, Charlottesville, USA   USA
My max vacuum was 19. I set timing to 17 and car starts/runs great.



SR
73 TR6
86 930

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AADTR6 Anthony D
Purchase, NY, USA   USA
Riley,
Follow Ken’s instructions, then you may want to adjust the timing so that it is 1-2 Hg below the max. I can’t lay my hands on older posts (probably all from Ken) describing the process but found an old bookmark that has some step by step instructions that may be of help. https://72triumphtr6.wordpress.com/2013/06/09/how-to-fine-tune-timing-on-a-triumph-tr6/

If you get down to the carb section, and decide you need the carbs rebuilt, contact Ken. He rebuilt mine and I followed his instructions on setting the mixture based on spark plug readings—perfecto

Anthony



Anthony
‘76 TR6



Edited 2 time(s). Last edit at 2019-03-06 08:14 PM by AADTR6.

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poolboy Ken D
Sandy Hook, MS, USA   USA
Thanks for your endorsement, Anthony.
Yeah, you don't want to advance the timing for the maximum vacuum...There is no one shoe fits all number for your engine..That's the point, you'll be looking for YOUR engine's sweet spot, but if you live at sea level and have a stock cam for the US market and your engine is in good shape, no vacuum leaks, you should find the spot between 17 and 21 in-Hg
My engine in it's current state pulls 19+ nearly 20 at an idle of 900 rpms.
Here's a few links:
http://automotivemileposts.com/garage/v2n8.html
https://therangerstation.com/Magazine/Summer2003/VacuumLeaks.htm
http://www.secondchancegarage.com/public/186.cfm
And a Moss Motors Video:




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kencorsaw@aol.com

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Fogspawn, CA, USA   USA
1974 Triumph TR6 "Machine"
this is the truth. listening to Ken and traipsing off to FLAPS for the vac gauge...
it reveals a lot....$20/30. bucks... and multi use if you pay attention... most
useful device... helps to prove/disprove things....
w

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POW Peter Wirth
HEBRON, NH - New Hampshire, USA   USA
In reply to # 1600473 by poolboy If you need to get the engine started by using the book static timing spec then by all means do so..then once the engine has warmed up check the manifold vacuum reading at idle with a vacuum gauge....You may want to adjust the ignition timing after you see that reading...or not.

So Ken, what do you typically end up with degree wise after timing using a vacuum gauge? - Pete

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5452 Riley L
Princeton, USA   USA
Great stuff! - Thanks again

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poolboy Avatar
poolboy Ken D
Sandy Hook, MS, USA   USA
In reply to # 1600535 by POW
In reply to # 1600473 by poolboy If you need to get the engine started by using the book static timing spec then by all means do so..then once the engine has warmed up check the manifold vacuum reading at idle with a vacuum gauge....You may want to adjust the ignition timing after you see that reading...or not.

So Ken, what do you typically end up with degree wise after timing using a vacuum gauge? - Pete

I don't have a timing light, Pete, to compare.
I borrowed one in 2008 at the TRials when I first learned about Vacuum. At 17 in-Hg on that 74's old engine it was off the scale on the damper.
FWIW I had already had AD set the curve; since then I had Dale mfg rebuild the damper when I reconditioned the engine with advanced cam timing and put it in my 73 and a few years later when I had Rob at BVU install the Vacuum Advance module, he checked the advance curve again.
I'm pretty sure @ the current 19 in-Hg there be some conflict between the reference marks.



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kencorsaw@aol.com

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barry s Avatar
barry s Silver Member Barry Stoll
Alexandria, VA, USA   USA
1972 MG MGB GT
1974 MG MGB
1976 Triumph TR6
1980 MG MGB
In post #5, Ken refers to an engine's "sweet spot". Not surprisingly, tuning using a vacuum gauge is utilized for fine tuning of all engines, not just LBCs. I recently researched the "sweet spot" concept for setting
optimal ignition timing. What I found was a lack of agreement in its definition. Some define sweet spot as maximum steady manifold gauge vacuum. Some seem to define as vacuum at maximum idle, assuming there is a difference in idle at maximum manifold vacuum reading. Lastly, as indicated in the link in post #4, optimal ignition timing is a setting at a specified amount (i.e. 1 1/2 - 2 " Hg) less than maximum measured vacuum.

My current approach is the maximum vacuum that does not result in 'pinging' under rapid acceleration. [Interestingly, to me, the post #4 link addresses pinging but under gentle acceleration rather than rapid acceleration.]

As we are talking about a couple of inches of Hg differences, I'm uncertain whether it makes a lot of difference (excepting serious pinging) which definition one uses. Still, it would be nice if all mean the same thing when referring to "sweet spot".

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POW Peter Wirth
HEBRON, NH - New Hampshire, USA   USA
Ken, it sounds like you've covered all of your own bases with timing by vacuum. Presumably advanced cam timing (stock cam?), and a re-curved/reworked distributor all work well for you. My concern for others who will try this, particularly using the standard distributor will be way too much total advance. The stock distributor is what, 12° (24° crankshaft)? So with the book setting of initial @10° BTDC + 24°, total is a reasonable 34° BTDC. I'm guessing timing by vacuum is resulting in an advance of around 5°- 7° beyond the book's 10° giving an initial of 15°- 17° for a total 39°-41°, yikes! - but then the recommendation is to back off until pinging stops, which brought me to my original query, what is the actual timing of the crank?

Just as the carbs you rebuild are sent with a statement regarding all pollution controls should be in good working order or suitably defeated for the carbs to perform as designed, I think ignition timing by vacuum will only be satisfactory if the distributor is re-curved and it's total advance is reduced an appropriate amount. - Pete

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poolboy Avatar
poolboy Ken D
Sandy Hook, MS, USA   USA
Time the ignition any way you want, would be my advice..then hook up a vacuum gauge to observe the interaction of the ignition timing with the valve timing and piston position.
Contemplate those relationships as opposed to a mark on a damper and the one shoe fits all approach that represents...It's an old technique, really OLD.
I experienced the same kind of doubt nearly a decade ago over on the 6-Pack.org Forum...All but a few thought I was an alien from "Planet X" (which I can neither confirm or deny)....but go there now and ask those who gave this method a chance what they think and if any have blown up pistons as a result.
The manifold vacuum is just a real time indication of how combustion, valve timing and piston position in the cylinder are getting along...that's it, just an tool providing that information...DO NOT BE AFRAID to try it and then IGNORE IT if you will...no problem



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kencorsaw@aol.com



Edited 2 time(s). Last edit at 2019-03-07 11:18 AM by poolboy.

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poolboy Avatar
poolboy Ken D
Sandy Hook, MS, USA   USA
Barry, I just read your post...As far as I'm concerned, the "sweet spot" is where a particular engine seems to run best or at least where the individual driving feels such.
Just find a reference number on the dial of a gauge or the rim of the damper to represent the sweet spot once it's found....I have one now, but 20,000 miles from now I may have to look for another number that represents the engine's sweet spot in it's current condition.



ZS carb repairs
kencorsaw@aol.com

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POW Peter Wirth
HEBRON, NH - New Hampshire, USA   USA
Ken, I'm not skeptical as much as I'm trying to glean more info. What about the issue of potentially too much total advance? Why after timing with vacuum does it seem not many care what crankshaft degrees are? Seems that would be important to know so one could correctly limit total advance at the distributor. Even if you sent it out to BVU , Rob would have to know degree wise where you intended to set initial to properly re-curve and set a total limit. After all is said and done, even with a re-curved or optimally modified distributor, does timing at idle with the vacuum method improve anything other than idle? Or is there in fact a benefit throughout the power curve? Additionally, would not a correctly re-curved but otherwise stock distributor set up with a stock 10° BTDC in mind, control the ignition just as well? - Pete

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poolboy Ken D
Sandy Hook, MS, USA   USA
I guess my best answer, Pete, would be to set the timing anyway you feel comfortable with and just keep in mind that you have the option to see what the manifold vacuum indicates at that setting.



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kencorsaw@aol.com

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