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Considering Vacuum Operated fuel valve

Randall

Veteran Member
Joined
Jun 26, 2021
Total Posts
635
Total likes
35
Location
Cape Coral, FL, USA
Does anyone have any experienced advice on whether this is good idea or not? Or what to look at, and what to stay away from. It might have helped me with gas problems developed after letting sit for some months. My Harley has one, and I like it. One thing that comes to mind right off is vacuum source. Need it strong enough to operate the diaphragm, but not so much as to damage it.

Or is this just more trouble than it's worth?
 
Randall,I'm interested to learn if the stock OEM Honda vacuum operated fuel valve/petcock which fits the 1986' CB450SC Nighthawk part#16950-MC9-830 will fit into your fuel tank outlet threads.
The stock Honda valve I just mentioned is not available new from Honda anymore,but I think you could source a used one and rebuild it,or search for an NOS item..
The main question I have is:will it fit the threads in your 82' CM450E tank :unsure:
 
Does anyone have any experienced advice on whether this is good idea or not? Or what to look at, and what to stay away from. It might have helped me with gas problems developed after letting sit for some months. My Harley has one, and I like it. One thing that comes to mind right off is vacuum source. Need it strong enough to operate the diaphragm, but not so much as to damage it.

Or is this just more trouble than it's worth?
Downside is, it still leaves the carbs full of gas to mess up the jets etc. My Suzuki has one that I'd love to get rid of but due to clearance issues, that'll be more involved. If I don't start it once a month, I could face the 4 banger carb pull punishment.
Mark your calendar or set a monthly phone reminder to shut off the petcock if you can't train yourself to just do it every time you ride.

If your head has vacuum gauge attachment fittings, one of those could conceivably be used.
 
My Hornet has a vacuum operated fuel tap, and it has proved completely reliable so far, 25 years old. I leave the tap in the 'on' position.

I drain the carbs when the bike is not in use, and that is the only downside of the vacuum tap, takes a lot of cranking on the electric start to get enough fuel into the carbs for it to start up. No 'prime' function on mine. That said, I also remove and charge the battery, so there is plenty of electrical juice to crank that first startup. ( Which still hasn't happened yet this year, half way through already :rolleyes: )
 
My Hornet has a vacuum operated fuel tap, and it has proved completely reliable so far, 25 years old. I leave the tap in the 'on' position.

I drain the carbs when the bike is not in use, and that is the only downside of the vacuum tap, takes a lot of cranking on the electric start to get enough fuel into the carbs for it to start up. No 'prime' function on mine. That said, I also remove and charge the battery, so there is plenty of electrical juice to crank that first startup. ( Which still hasn't happened yet this year, half way through already :rolleyes: )
I have used a small vacuum source(Mityvac)with low Hg/vacuum to open the valves on certain bikes I was working on in the past;I've also manually primed the carbs. direct using a small aux. tank.
The cranking does take a long time.
 
I have opted to ditch the idea of a vacuum petcock on this bike. Much easier to just remember to shut down the fuel flow with the petcock after a ride. And if I'm not going to ride for a while, then burn off the bowl fuel by running it with the Petcock switched off until it stalls.

In my own defense, I never really cared about the petcock until I got ill for the last year and just didn't have my mind on proper bike maintenance, which resulted in both of my bikes succumbing to nasty fuel problems. I got this one back to normal, but my HD still won't run after about 9 months sitting with a bowlful of gas that went bad, very very bad, To say I was surprised at how funky the gas in the bowl was would be putting it mildly. It was greenish and thick and looked like summer pond water, leaving a stain at the bottom of the bowl. After 9 months in my garage mostly during the dry season.

Appreciate the help, as always.
 
Yeah, today's fuel composition degrades quite quickly. If we get a season of non-use (3 months) you'll be Okay. but 3-months + 1 wk = forget it (only slight sarcasm there)! Drain and reload is the advice. Also, while in the draining mode, don't forget to drain the carby float bowl(s) as well and start with as much fresh as possible everywhere. On the 450/500 there is a sediment trap/screen in the system that occasionally plugs up.

We like Sta-Bil and Seafoam products to name a couple, I'm sure there are others as well.
 
I have opted to ditch the idea of a vacuum petcock on this bike. Much easier to just remember to shut down the fuel flow with the petcock after a ride. And if I'm not going to ride for a while, then burn off the bowl fuel by running it with the Petcock switched off until it stalls.

In my own defense, I never really cared about the petcock until I got ill for the last year and just didn't have my mind on proper bike maintenance, which resulted in both of my bikes succumbing to nasty fuel problems. I got this one back to normal, but my HD still won't run after about 9 months sitting with a bowlful of gas that went bad, very very bad, To say I was surprised at how funky the gas in the bowl was would be putting it mildly. It was greenish and thick and looked like summer pond water, leaving a stain at the bottom of the bowl. After 9 months in my garage mostly during the dry season.

Appreciate the help, as always.
Given that it’s a newer HD, you might be able to find someone to do a reasonably priced full carb clean for you and just do the disconnect/reconnect work yourself - although at least carbs can be cleaned indoors, I know first hand shop weather in the south gets rough.
 
if your float bowls have overflow tubes, no harm in having one but if they don't and the vacuum fails, you'll get gas in your oil. happened to my 83 cb650sc. i changed to manual. the clearance was tight but i rerouted the line.
 
"On the 450/500 there is a sediment trap/screen in the system that occasionally plugs up."

I am not familiar with this trap/screen. Where is it located?

"Given that it’s a newer HD, you might be able to find someone to do a reasonably priced full carb clean for you and just do the disconnect/reconnect work yourself"

The Harley is 29 years old, I wouldn't call that newer. It's a dino compared to what they put out these days. I have already done a full carb clean/removal, that's not a big deal.
 
"On the 450/500 there is a sediment trap/screen in the system that occasionally plugs up."

I am not familiar with this trap/screen. Where is it located?

"Given that it’s a newer HD, you might be able to find someone to do a reasonably priced full carb clean for you and just do the disconnect/reconnect work yourself"

The Harley is 29 years old, I wouldn't call that newer. It's a dino compared to what they put out these days. I have already done a full carb clean/removal, that's not a big deal.
I guess when the bike model year starts with 199 it feels new around here! Glad you got the carbs cleaned out. Hopefully it’ll fire for you and you can burn out the gunk.
 
"On the 450/500 there is a sediment trap/screen in the system that occasionally plugs up."

I am not familiar with this trap/screen. Where is it located?
He's talking about the other 450, the DOHC. It has a removable sediment bowl on the petcock with a screen above it.
 
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