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  #31  
Old 09-16-2019, 02:16 PM
Discus2b Discus2b is offline
 
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This is a new anomaly.
You smelled it quickly.
Pax did not.
I vote back pressure from leaning creates a slight seepage? Fuel selector/pump/filter area.
Does it happen using both tanks?

Use a white cotton glove from Home Depot to pick up trace.

When doesn’t it do it?

R
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  #32  
Old 09-16-2019, 02:35 PM
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bkervaski bkervaski is offline
 
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Thanks for all the feedback!

Went to the hangar at lunch ... a few notes:

1. No blue stain around vent line
2. No blue stain around any of the injectors
3. I have yet to re-sealed the two firewall wire penetrations from my pmag install (on purpose)

Assuming #3 is where the fumes are coming from, the question still remains from where the fuel originated.

If the vent line isn't an overflow line, and no visible signs of leaks, then just fuel overflow/excess blow back from the servo?

At this point assuming it's just leaning too far, too fast. As soon as the temp drops below 10 million degrees I'll go test some more
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Last edited by bkervaski : 09-16-2019 at 02:39 PM.
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  #33  
Old 09-16-2019, 04:44 PM
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DanH DanH is offline
 
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Previously I wrote:
Quote:
The detailed record shows one very short period of pressure reversal, and the amplitude is quite small. I very much doubt there is any significant fuel escaping from the nozzle bleed.
The data is on my laptop, which is at the office today. I'll try to post a plot later.
So much for memory. Here's a data sample from 10500 ft @ WOT, a likely setup for LOP cruise. I have not yet grasped a connection to fuel flow, but it describes the pressures.



You're looking at the difference between bleed air supply pressure (here in a turbo rail and shroud) and the intake port at the primer fitting. Green is bleed pressure exceeding port pressure, while red is port pressure exceed bleed pressure. Put another way, red has the potential to blow fuel out the nozzle bleed hole.

The 23 BTDC notation is where I used a second channel to put a timing marker on the plot...it's #1 plug firing. There are 720 crank degrees between 23 BTDC markers; at 2400 RPM each division represents 0.00133 seconds.

With the throttle wide open, there are three periods of pressure reversal, not one as I remembered this AM. The periods are short. The longest averages about 3.5 divisions (0.00466 sec, 67 crank degrees), just after the intake valve closes on the compression stroke. All three added together come to 0.0117 seconds, or about 168 degrees of crank rotation. 168/720 means there is the potential for reverse flow about 23% of the time. Hmmm.

Here is the effect of throttling at 6500 feet. Assuming the same forward airspeed, bleed pressure would remain the same, while the partially closed throttle plate reduces pressure in the intake port. DeltaP shifts about 1" Hg in the desired direction, but the time periods (in seconds or crank degrees) don't change much.

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Last edited by DanH : 09-18-2019 at 12:25 PM.
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  #34  
Old 09-16-2019, 04:49 PM
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DanH DanH is offline
 
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Quote:
Originally Posted by bkervaski View Post
1. No blue stain around vent line
2. No blue stain around any of the injectors
3. I have yet to re-sealed the two firewall wire penetrations from my pmag install (on purpose)
Assuming #3 is where the fumes are coming from, the question still remains from where the fuel originated.
If the vent line isn't an overflow line, and no visible signs of leaks, then just fuel overflow/excess blow back from the servo?
You may not have spent enough time in the fuel smell condition to stain significantly.

There is no "overflow/excess blow back" from the servo. Line pressure between the engine driven pump and the servo remains stable, regardless of mixture knob position, a fact you can see on the cockpit fuel pressure gauge. The only pressure change due to leaning is detailed in the IO-390 manual...nozzle pressure.



Note that a shift from 11.5 GPH (a typical best power mixture at 65%) to 8 GPH (about 50 LOP) changes nozzle pressure by about 2 psi.
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Last edited by DanH : 09-17-2019 at 12:00 PM.
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  #35  
Old 09-16-2019, 08:26 PM
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RVbySDI RVbySDI is offline
 
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Quote:
Originally Posted by DanH View Post
Several pilots here report they only get the fuel smell momentarily when leaning is described as too quickly, or aggressively, or a similar description. Question for you...Does this rapid leaning with fuel smell go so far as to cause the engine to stumble, or more specifically, include a loss of RPM?

I'm looking for some phenomenon which would explain a momentary reversal of nozzle deltaP due to rapid leaning.
Yes, the engine will stumble in that scenario and yes the RPM drops off rather dramatically, although I do not recall how much. It has been several years since I experienced this phenomenon.
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  #36  
Old 09-17-2019, 04:19 AM
RV6-KPTW RV6-KPTW is offline
 
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In my case, I had over leaned and there was a reduction in power when I got a whiff of fuel. The engine ran a little rougher as well. Once I enriched to the point of smoothness, still lean of peak, the smell went away. It was reproducible.
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  #37  
Old 09-17-2019, 06:32 PM
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DanH DanH is offline
 
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Thanks gents. Bill will run a little experiment, as will I.

In the meantime, some nozzle detail.





These things work by shooting a pencil stream of fuel down the center of a concentric, larger bore, which entrains air from a bleed supply. In theory, the fuel and air mix in the larger bore and enter the intake port as a frothy emulsion.

More later.
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