[axman88]

I agree with @gscrivner749 and @oldguy's assessment. Sounds like the diaphragm in the vacuum valve in the petcock has gotten stiff or cracked and is no longer opening.

I would assume that the "On" position is equivalent to "Prime" and bypasses the vacuum valve, Easy enough to test. Can also test these with a hand vacuum pump, or even by sucking on the line, (if you don't mind risking a mouthful of unknown).

Can also use a big hypodermic or fuel bulb to push fuel up into the carb. bowl the back way, through the drain nipple fitting, with the drain screw open. That should at least give a minute and a half of run time, for testing purposes.

I find that aging vacuum valves frequently get stiff and develop problems, especially in cold weather. This time of year, my little GZ250 has a problem mustering up enough vacuum to run the first few blocks, so I run in "prime" position for the first 1/4 mile, then switch the petcock to "Run", while also playing with the enrichment, and keeping the revs up. It keeps things interesting, especially during rush hour, when the roads are slippery, and it's fricking cold.

I recall reading a post here on the forum, where somebody took apart a vacuum valve and added a spacer to force it always open. Personally, I'd want a mechanically closed position, but I'll do what it takes, on a given day, to not have to push.

 

[axman88]

I do remember my 800 Vulcan had Prime/ON/RES as the 3 petcock positions. On had the vacuum operated valve. prime ran straight through. Don't remember if RES went through the vacuum valve or not.

Here's what I'm finding in an online scanned 2005 Aero 750 manual:

http://owners.honda.com/assets/ownerlink/model/own_man/powersports/2005/2005_vt750c.pdf

It took a while to track down a readable image of the inscribed side of the Aero petcock, but it agrees with the manual diagram.

This info suggests that the Aero does NOT have a "prime" position for the petcock. Fuel flow lives or dies with that internal vacuum diaphragm.

My VLX had pretty much the same configuration, a single carb, gravity fed from the fuel tank, via a petcock with three positions. However, what was different, and I would say BETTER, on the VLX, was that the vacuum valve function was a separate device from the petcock. That meant in a side of the road breakdown situation, a guy would be able to bypass the Vacuum Safety Valve with a piece of hose, or just a piece of tubing.

On the Aero 750, the Vacuum Shutoff Valve is integrated into the petcock. To MacGuyver this setup on the side of the road, you'd probably have to disassemble, which would likely have fuel running here and there. I'd have a go at simply dismounting the the whole vacuum diaphragm housing, and see if that had an effect on fuel flow, or perhaps some solution presented itself. Certainly the longer term solution is to replace the vacuum diaphragm or the entire petcock assembly.

Diaphragm assy. is about $35 from Parzilla.

https://www.partzilla.com/catalog/honda/motorcycle/2009/vt750c-a-shadow-aero/fuel-tank

Entire aftermarket petcock assy. is less than half that figure.

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Don't rely on my research. Check any links to make sure it's what you really need. I'm good at finding stuff, but I make plenty of mistakes.

 

[axman88]

Maybe we need to have someone test whether the RES position bypasses the diaphragm? Having trouble with Honda not providing a way for initial prime other than cranking longer than is good for the starter.

A test won't hurt, but it looks to me like the owners manual (again from the '05 Aero) is explicit on the point.

Maybe they changed it on later models, '04 through '06 used petcock number 16950-MEG-003, which looks the same as the 16950-MEG-023 (shown above) that was used from '07-'09 to me, but maybe the two work differently. We'd need to see the manual for the later model, or even better, the test you suggested for each.

But, personally, I wouldn't expect it to be different. Extended cranking required whenever carb. bowls are empty is also a thing on my '96 1100 ACE, and any of the dual carb machines that used the fuel pump cutoff relay. Unless somebody has made changes to the electrical system, these electric fuel pumps only work when the engine is cranking, because the "relay" only sends power pulses when it's receiving ignition pulses. That "relay" #36100-MN4-008, was used on 95 different models, from '87 till '07. https://www.partzilla.com/product/honda/36100-MN4-008. It seems very likely, that if the circuitry had been changed at some point, a new part number would have been issued.

One can only think that Honda considered extended cranking to be an acceptable price for the safety cutoff feature.

If is possible that some machines have a "fuel pump pre-run" feature built into the ICU, where upon wakeup, the ICU sends out a stream of timed pulses to activate the fuel pump, reducing the need for cranking. I have not personally seen this, but it's possible. I would wonder, however, if such a trick could be accomplished without also producing sparks, which seems problematic, if not dangerous, since who knows where the engine will be in its rotation.

A couple work-arounds that comes to my mind, for the vacuum valve, would be to either:
1) Unfasten the vacuum line and apply a little suction for a half minute, or
2) Push a little fuel up through the carb drain with the drain screw open.

The 2nd method also works on models with the electric fuel pump, as does electrically bypassing the "relay" until the fuel pump clicking slows or stops.

 

[axman88]

I always replace those vaccum operated petcocks with a manual one. I never have issues with the manual ones.
Just turn it off when you leave your bike sitting.

I think that is a great idea, especially for this Aero with the integrated vacuum valve.

How do you find a substitute that will work the first time? Are all the male nipple threads the same, such that we could use any other Honda petcock?

 

[axman88]

One could also run a couple wires from the relay harness to a momentary pushbutton switch for priming after sitting or carb work. tap in to the black and black-blue wires, turn key on and hold the button down for approx 5 seconds.

Along similar lines, one could add a Tee to the vacuum line and plumb in a suction bulb to create vacuum. These things generally have a one-valve built in. I'd cap the open end when I wasn't using it, or cap the Tee, and keep this gadget, with a length of tubing attached, around.

Kind of reminds a guy of those machines from the early days, where the rider would operate a lever, to inject lubricating oil, every so often while traveling down the road. Even before that, it was accepted practice that you would shovel in more coal every so often if you wanted to keep making miles.

But workarounds like this do seem very kludgey, and to me suggests that Honda could have done more to head off the issue.

I have 3 - '02 , '05 , '07 ,, Have replaced petcocks ..

Were you replacing a petcock with integrated vacuum valve with same type, or with a manual petcock?

I wonder if one could substitute the manual petcock #16950-MBA-023, which was used on VT750C Shadow ACEs from '98 till 2003?

Or perhaps #16950-MCR-A03, which was fitted to Honda VT750DC Spirits from 2001 - 2007?

Both of these have the Reserve position with the lever facing upwards, while the original petcock assy avoided the 12 O'clock position and used 3, 6 and 9 O'clock. I've seen it happen before, where a replacement petcock didn't work so well because the lever hit the weld flange of the fuel tank in that 12 O'clock position.

From the pictures, it appears to me that all these variants use the same nipple thread size, and the main variation besides the knob positions used, is the location of the outflow nipple. I'm wondering if petcocks from other size Shadows might even fit. Here's a picture of the petcock that was used on VT1100 ACEs, ACE Tours, Spirits, Aeros and Sabres, from 1995 until 2007.

 

[axman88]

One could also run a couple wires from the relay harness to a momentary pushbutton switch for priming after sitting or carb work. tap in to the black and black-blue wires, turn key on and hold the button down for approx 5 seconds.

Along similar lines, one could add a Tee to the vacuum line and plumb in a suction bulb to create vacuum. These things generally have a one-valve built in. I'd cap the open end when I wasn't using it, or cap the Tee, and keep this gadget, with a length of tubing attached, around.

View attachment 321500

Kind of reminds a guy of those machines from the early days, where the rider would operate a lever, to inject lubricating oil, every so often while traveling down the road. Even before that, it was accepted practice that you would shovel in more coal every so often if you wanted to keep making miles.

But workarounds like this do seem very kludgey, and to me suggests that Honda could have done more to head off the issue.

I have 3 - '02 , '05 , '07 ,, Have replaced petcocks ..

Were you replacing a petcock with integrated vacuum valve with same type, or with a manual petcock?

I wonder if one could substitute the manual petcock #16950-MBA-023, which was used on VT750C Shadow ACEs from '98 till 2003?
View attachment 321495

Or perhaps #16950-MCR-A03, which was fitted to Honda VT750DC Spirits from 2001 - 2007?
View attachment 321496


Both of these have the Reserve position with the lever facing upwards, while the original petcock assy avoided the 12 O'clock position and used 3, 6 and 9 O'clock. I've seen it happen before, where a replacement petcock didn't work so well because the lever hit the weld flange of the fuel tank in that 12 O'clock position.

From the pictures, it appears to me that all these variants use the same nipple thread size, and the main variation besides the knob positions used, is the location of the outflow nipple. I'm wondering if petcocks from other size Shadows might even fit. Here's a picture of the petcock that was used on VT1100 ACEs, ACE Tours, Spirits, Aeros and Sabres, from 1995 until 2007.

View attachment 321499