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NCtoy View Drop Down
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    Posted: 02 July 2018 at 2:06am
After reading the other thread that talked about vacuum tuning an engine I did some reading on scoutpilots website and decided to give it a try. 
Here’s my issues:
1) The engine is a 225. At a low idle, the vacuum gauge flutters pretty bad. Bouncing between 18-20 inches of vacuum. If I speed the idle up some, probably 900-1000 rpms, it smooths out and holds steady vacuum. Is the fluttering because of the odd fire engine? I tried it on my v6 cj5 with the same results. 
2) After turning up the idle and setting the timing for maximum vacuum, I double checked the timing with a light and found that it was way advanced. It’s off the timing marks on the engine, I would estimate over 20 degrees btdc. This seems excessive, although it seems to run good, at least in the driveway. I was kind of afraid to test drive it like that. I had the vacuum advance blocked off for this, and turning the idle back down didn’t seem to affect anything. Should I go with the gauge?
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I would think leaving it timed at 20+degrees BTDC (measured with vacuum advance blocked off) would cause issues at higher load,   I imagine it would "ping" like crazy. 

What vacuum "source" do you connect the advance unit to?   ie.  "port" (taken above the throttle plate),  or "manifold" (unaffected by the throttle plate)  . .   I think the default is "port",  possibly for emissions reasons,   but I always felt that the engine has more torque at idle,   when connected to "manifold". 

Thinking about this more . .  if your vacuum advance is connected to "port" source,  it might explain the fluttering.   At 700 rpm,  the advance unit is not seeing vacuum,  so is running unadjusted timing,  too low for what the engine wants :-(  . . .   once it hits 900-1000 rpm,  the throttle plate is opened to the point that the port now sees the vacuum and advances the timing to what the engine likes. Smile

No expert here,  just theorizing.  LOL
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Originally posted by JeepFever JeepFever wrote:

.... What vacuum "source" do you connect the advance unit to?   ie.  "port" (taken above the throttle plate),  or "manifold" (unaffected by the throttle plate)  ....
 
Just to clarify, at least as I understand "ported vacuum" is read from a port that is directly above the throttle plate when the throttle is at idle, but when the throttle is cracked open the plate moves above the port.  So ported vacuum is only above the throttle plate when it's at idle.  It's below the throttle plate the rest of the time.
 
(If it was always above the throttle plate it would only be measuring the pressure drop across the air filter.)
 
What JeepFever said a little farther down in that post makes me sure he understands it.  Just trying to clarify for others.
 
And no expert here either!


Edited by Nothing Special - 02 July 2018 at 5:54pm
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Originally posted by Nothing Special Nothing Special wrote:

Just to clarify, at least as I understand "ported vacuum" is read from a port that is directly above the throttle plate when the throttle is at idle, 

Bob,  thanks for adding that clarification,  I did not think to mention,  and it is important to know. 
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I don't know anything about the Odd-Fire or where one would connect a vacuum gauge on this style engine.   I do know that a bouncing vacuum is generally not a good thing...in the world of L-134 engines when connected to the intake manifold port.
  
 If you study the written explanation on ScoutPilot's website,  he says that you advance the timing to achieve the best vacuum, but then you must reduce the advance until the RPMs drop 100 or 200 RPMs.  (I can't remember which)  At this point, you start to adjust the carb to achieve  600-700 idle RPMs.
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  The "time" or "space" between cylinder firings are even, or the same, in an inline 4 or 6 (or even a 5), or a V-8. 180* of crankshaft revolution in a four, 144* in a five, 120* in an inline six, and 90* in a V-8. The odd-fire V-6 is not that way. I don't recall the exact numbers, but the cylinder spacing is not equal. It's long-short-long-short-long-short. That's why it's called odd-fire. Would this not show up on a vacuum gauge, especially at a slow idle?

  If you set your timing with a vacuum gauge to achieve the best vacuum and do not back it off as Scoutpilot suggests, I can garan-goll-darn-tee you, and be willing to bet the wife's egg money on it, that you will leave it set 'way too fast. If you back it off by "about X# of rpms" or "about X# of vacuum inches" you're throwing guesswork into the process. I prefer to use a timing light and the marks and set the timing at about 10*, which is about 5* above what the book says. So guess what? There's that guesswork again. Wink  But I have a benchmark to start from if I want to advance or retard it a bit more.  BW
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the vacuum advance plugs into a port on the carb. It doesn’t pull vacuum at idle, but does once the throttle is pressed. 

The gauge was connected to a fitting on the intake manifold. I tried it on a vacuum fitting on the manifold, then thought maybe it was too close to an intake valve (it was off to one side) and that could be causing the flutter in the gauge. So I hooked it to the egr port just below the carb, but with no change. 
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I must have been typing at the same time as you Bruce. The uneven timing of the cylinders is exactly what I was thinking about causing the flutter, especially at lower rpms where there is more time between each cylinder firing. 

I’ve always set the timing with a light to between 5-8 degrees. Reading about setting it with a vacuum gauge interested me, so I thought I’d try it. Maybe I’ll write that one up to another failed experiment. Maybe it would work better on a 134. 
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  All of the years that I worked for Chevrolet, there was always some new guy that thought he could set timing with a vacuum gauge or a tachometer, but Chevrolet's advice was: "Don't even think about it!" They wanted that guesswork left out of it. We did set timing 5* above the spec, but that was not guesswork on our part, Chevrolet said we could bump it up 1* per 1,000ft of elevation.  BW
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I'm happy to see this thread, it's at just the right time for me. I just got the Henry J vacuum chamber today, now I have to find a suitable return spring (internal) for it. At that point I'll see what I can do to time it. I read a lot about vacuum timing from jeeproger, and that was very interesting. Learning a lot by reading but the real learning will come as I try it.
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Originally posted by Bruce W Bruce W wrote:

  The "time" or "space" between cylinder firings are even, or the same, in an inline 4 or 6 (or even a 5), or a V-8. 180* of crankshaft revolution in a four, 144* in a five, 120* in an inline six, and 90* in a V-8. The odd-fire V-6 is not that way. I don't recall the exact numbers, but the cylinder spacing is not equal. It's long-short-long-short-long-short. That's why it's called odd-fire. Would this not show up on a vacuum gauge, especially at a slow idle?
 
I think these engines are even more "odd"  . . .  my understanding is that spacing is  - > short-short-LONG-short-short-LONG  . . a 90 degree V8 with 2 cylinders lopped off. Smile   
 
Is the fluctuation on the vacuum gauge = really quick pulses, or something slower?   I don't have a gauge,  or would try on my 225. 
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firing order
1,6,5,4,3,2.
150-90-150-90-150-90=720 degrees crank rotation.
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