Two petcock tanks vs. single

[GeorgeOC]

Hello, might anyone have a good idea if one could easily swap away from a tank with dual petcocks, to a tank with a single?

For example, my original tank on my 76' is a loss. Ive seen several later models both Standard and Special that feature only a single petcock... Any good reason why one could not go to a single PC system?

Thanks!

 

[NONclow]

No good reason. One petcock can supply both carbs. 'Tis what I did.

 

[weaselbeak]

One petcock drops plenty of fuel. The reason I see for two is that some gas can be stranded and unusable on the off side if only one petcock. How much fuel depends on the tank. I have a Moto Guzzi that would leave a lot of gas stranded on the wrong side of the backbone without two petcocks.

 

[GeorgeOC]

Great feedback guys, thank you. I was worried that I'd have to change out carbs as well if the fuel delivery was changed to that extent...

Thanks!

 

[grizld1]

Yep, flow is sufficient with one tap. Lost capacity is also not much of an issue. Before I installed a single Pingel petcock on my D model tank I checked for loss of effective capacity by draining the tank through the left tap, then opening the right and measuring remaining fuel. Only 4 oz. remained on the right side of the tank; obviously not enough to cause a significant loss of cruising range.

 

[XSLeo]

If your carbs are set up with two petcocks and you swap to a one petcock tank, be sure to plug off the fuel inlet on the off side carb. Or use a tee in the fuel line to hook to both carbs.
Leo

 

[mrtwowheel]

There is one good advantage to having one petcock on a deep tunnel tank. When you run empty you just have to lean the bike over, let the fuel run to the other side, then start the bike and ride to the gas station. Two reserves!!!!

Scott

 

[gggGary]

There is one good advantage to having one petcock on a deep tunnel tank. When you run empty you just have to lean the bike over, let the fuel run to the other side, then start the bike and ride to the gas station. Two reserves!!!!

Scott

I remember tipping my out of gas 305 over on it's side in a freeway ditch one night and making it to a gas station on the gas I poured "over the hump" Scraped together 40 cents and got it home.

 

[mrtwowheel]

I remember tipping my out of gas 305 over on it's side in a freeway ditch one night and making it to a gas station on the gas I poured "over the hump" Scraped together 40 cents and got it home.

I bet 40 cents bought some gas back then. My cousin had a 305 with high rise bars from a 20" bicycle. That was a nice bike, he put a new set of bars on it about every two weeks, they bent from just pushing it around.

Scott

 

[fredintoon]

There is one good advantage to having one petcock on a deep tunnel tank. When you run empty you just have to lean the bike over, let the fuel run to the other side, then start the bike and ride to the gas station. Two reserves!!!!
Scott

Hi Scott,
or you can go from 2 reserves to none.
It's not common but it happened to me, the combo filter/reserve towers on my XS650 gas tank taps fell out and lay in the bottom of the tank.
This is a completely symptomless phenomenon until you run out of gas 'way out in the country, switch to reserve and find there ain't one.
Luckily my reserve jerrycan got me to a gas station but ever since then I've always zero'd the odometer at every fill-up and filled up again at ~75% of my estimated range.

 

[mrtwowheel]

Yep, happened to me too on a CB750 Custom. Easy fix on that bike, a piece of brake tube pressed right into the petcock to give me a new main pickup tube.

Scott

 

[zeroxs650]

Good information, I recently purchased an 81 fuel tank for my 77 to install. I do have a question on the fuel line, does it matter whether I use clear or black fuel line? And should I use the existing petcock from the 81 or could I use a rebuilt petcock from my 77?

 

[gggGary]

77 and earlier (non vacuum) petcocks use a narrower bolt spacing than the later vacuum units.

 

[fredintoon]

Good information, I recently purchased an 81 fuel tank for my 77 to install. I do have a question on the fuel line, does it matter whether I use clear or black fuel line? And should I use the existing petcock from the 81 or could I use a rebuilt petcock from my 77?

Hi zeroxs650,
funny, I went t'other way; put a Standard tank on my Heritage Special to carry that extra gallon of gas.
Like Gary sez, the petcocks don't swap so you are stuck with the '81 vacuum tap.
Which can be modified to manual operation if you want.
(See the search function for details.)
Another thing that don't swap is the gas filler cap. They look the same but they ain't.
I use Tygon fuel line. Semi-transparent yellow, not cheap, but fuelproof and stays flexible.

 

[zeroxs650]

Is there any benefit to having a vacuum style petcock? How do I modify it?

 

[Brassneck]

I finally swapped my vacuum petcocks for manual ones (from Mikes), they work great so far. I actually got two, and only turn one set on (Left side)...leaving me with the right side as a reserve if I happen to need it...no tipping required. Granted, this isn't an exact science and could potentially be an issue should I happen to not have much gas left on the right side for whatever reason. Therefore I always fill up at about 120 miles on the odo when zero'd out.

As to benefits for the vacuum style petcocks...the best one is that there's no need to turn on/off gas, it is on when the bike starts and off when it stops. It also means you have vacuum ports on the intake manifolds for the carbs, and that's nice for balancing the carbs as well.

The biggest issue is if it is working correctly and not leaking. I had mine work great for a year or so after I rebuilt it/cleaned it...then it would leak...re-cleaned it, and worked for a few months, then leaked...and after the third rebuild/clean, I simply gave up. Easier to turn on/off than deal with a puddle of gas every once in a while, and usually at the most in-opportune time.

 

[fredintoon]

Is there any benefit to having a vacuum style petcock? How do I modify it?

Hi zeroxs650,
if your vacuum style petcock works OK but you want the added assurance of being able to turn it off manually,
file off the little nib on the petcock's faceplate so the lever can turn in a full circle
and shorten the lever a bit so it'll clear the gas tank.
This lets you turn the lever so it points straight up.
The straight up position cuts off the gas flow.

 

[zeroxs650]

Thanks Fredintoon, I will try it.

 

[zeroxs650]

I have been riding my 77 lately and twice it stopped as if the bike needed fuel. After looking inside the tank, the petcock side seemed empty but the right side had fuel due to the tank tunnel. I leaned the bike over and moved some of the fuel the the left side and got to a gas station. I installed an 81 fuel tank and use one petcock, non vacuum. Would the tank benefit by have a crossover tube from one side of the tank to the other to ensure the usability of the entire tank of fuel?

 

[fredintoon]

I have been riding my 77 lately and twice it stopped as if the bike needed fuel. After looking inside the tank, the petcock side seemed empty but the right side had fuel due to the tank tunnel. I leaned the bike over and moved some of the fuel the the left side and got to a gas station. I installed an 81 fuel tank and use one petcock, non vacuum. Would the tank benefit by have a crossover tube from one side of the tank to the other to ensure the usability of the entire tank of fuel?

Hi zero,
nah, the ol' "lean the bike over" trick is to remind you to believe you need to buy gas immediately you run onto reserve.