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-   -   Vacuum on 2F motors... (http://www.risingsun4x4club.org/forum2/showthread.php?t=20870)

cbmontgo 02-11-2014 12:42 PM

Vacuum on 2F motors...
 
What are your vacuum readings on well-tuned 2Fs here at elevation?

SteveH 02-11-2014 01:27 PM

At 7K feet elevation, 15. maybe 16-16.5 if the barometric pressure is really high (on a nice day). This is on my '78 2F with decent compression. The lower the elevation, the higher the vacuum reading.

cbmontgo 02-11-2014 01:49 PM

Quote:

Originally Posted by SteveH (Post 246239)
At 7K feet elevation, 15. maybe 16-16.5 if the barometric pressure is really high (on a nice day). This is on my '78 2F with decent compression. The lower the elevation, the higher the vacuum reading.

That's what I was thinking. I am running at 13.5-14" on one of my 2Fs and was told that I may have an intake manifold leak that is big enough to necesitate additional carb tuning. I also live close to 7,000'. I would think that if a well tuned 2F pulled 20" hg at sea level, then 13-14" at 7,000' is not necessarily indicative of a significant vacuum leak?

kurtnkegger 02-11-2014 03:27 PM

I got this information by googling engine vacuum at elevation...I've understood there would be a 1" drop for every 1000ft of elevation.


When testing manifold vacuum, it’s important to remember that if an engine idles at 22” Hg at sea level, it will idle at about 17” Hg at 5,000’, 14” Hg at 8,000’ and 12” Hg at 10,000’ altitude. Variations from the calculated standard, of course, are the weather conditions, the engine design, and how well the engine management system adjusts spark advance and air/fuel mixture to correspond to a change in barometric pressure.

cbmontgo 02-11-2014 05:12 PM

Quote:

Originally Posted by kurtnkegger (Post 246244)
I got this information by googling engine vacuum at elevation...I've understood there would be a 1" drop for every 1000ft of elevation.


When testing manifold vacuum, it’s important to remember that if an engine idles at 22” Hg at sea level, it will idle at about 17” Hg at 5,000’, 14” Hg at 8,000’ and 12” Hg at 10,000’ altitude. Variations from the calculated standard, of course, are the weather conditions, the engine design, and how well the engine management system adjusts spark advance and air/fuel mixture to correspond to a change in barometric pressure.

Yep. That's why it seems that 14" isn't too bad at 7,000'. I just don't want to start replacing manifold gaskets if I don't need to.

subzali 02-11-2014 05:56 PM

That's about what mine gets at my parents house at 7000 feet

MDH33 02-11-2014 07:19 PM

Carson, if this is my old 2f we're talking about, then it was running really well even on the coldest winter days up here at 8000+ feet. You mentioned you took the carb off and rebuilt it due to that hesitation and then it started to run funny. I would look to the last thing you did first. Or, before doubting the manifold, check old vacuum lines, etc. :wrench::thumb:

cbmontgo 02-11-2014 08:11 PM

Quote:

Originally Posted by MDH33 (Post 246255)
Carson, if this is my old 2f we're talking about, then it was running really well even on the coldest winter days up here at 8000+ feet. You mentioned you took the carb off and rebuilt it due to that hesitation and then it started to run funny. I would look to the last thing you did first. Or, before doubting the manifold, check old vacuum lines, etc. :wrench::thumb:

Hey Martin...it is indeed. Love the rig though.

It idles fine once the engine is at full operating temp, but doesnt hold idle when cold. My stock '76 only needs choke for a minute or two for cold starts. Could it be the aftermarket headers that warm up more slowly because they are more separated from the intake manifold than OEM? I also might have created a leak at the base of the carb with the new gaskets. I also suspected getting something stuck in the idle circuit during the rebuild, but replies to my idle circuit thread on ih8mud suggested a vacuum leak. Who knows. I'll get it dialed in soon.

Anyway, it is fun wrenching nonetheless. I'm just splitting hairs at this point. I'd drive this puppy to Alaska tomorrow if I had the time!

MDH33 02-11-2014 09:33 PM

The header with no heat riser absolutely makes a difference. Choke to start then push it in half. a few minutes at least with the choke at 1/2 to 1/4 until warm. I would usually start rolling after a couple of minutes warm-up with the choke 1/4 then open it up all the way once I was down the road. We've had these Butt Cold temps lately too. That 40 spent winters hibernating in the garage. I would get it out for snow runs occasionally and it always started and ran even after sitting weeks in the garage in the cold. It only needed choke in the summer for initial start up.

cbmontgo 02-11-2014 09:42 PM

Quote:

Originally Posted by MDH33 (Post 246266)
The header with no heat riser absolutely makes a difference. Choke to start then push it in half. a few minutes at least with the choke at 1/2 to 1/4 until warm. I would usually start rolling after a couple of minutes warm-up with the choke 1/4 then open it up all the way once I was down the road. We've had these Butt Cold temps lately too. That 40 spent winters hibernating in the garage. I would get it out for snow runs occasionally and it always started and ran even after sitting weeks in the garage in the cold. It only needed choke in the summer for initial start up.

Sweet. I think my whole issue might be adjusting to the non-stock headers! The low vacuum hypothesis just didn't seem right.

And yes, it has been really cold lately. Not the best time to play with a 2F.


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