Done with vaccum petcocks! Found a cheap fix!

[blueboy5000]

So Vacuum Petcocks failing is EXTREMELY common, on my own personal Mc-23-150 I have replaced the stock petcock twice, then upgraded to a "Mikuni style" and that failed, as well as the next one.

On this bike (and about 75% of other scoots) the fuel tank is located HIGHER than the engine, which, for some reason, can cause fuel on the tank-side of the petcock to rot away the "rubber" diaphragm inside the vacuum petcock, causing a "constant on" problem. This really is quite an issue, if your bike is left sitting with fuel pouring through the petcock, it will flood out your carb, and then sit in the combustion chamber, possibly ruining the engine as fuel seeps past the rings and then mixes with the crankcase oil.

I discovered this while doing a monthly cleaning to my carb because my bike was bogging, and when I took off the fuel line from the carb (POST petcock) all the fuel in the tank poured out into my drain pan. I then check then oil level and found the oil to be way too high and thin and REEKING of gasoline. I immediately drained the oil and found it to have almost 2 quarts of fluid! Luckily I did not drive it, or I probably would have ruined the engine.

So confirming that the petcock did not hold vacuum with a hand-pump, I was not very enthusiastic about replacing another 30 dollar petcock. Luckily, as I was messing around with the bike, my landscapers came to mow my building's lawn, and my bud saw what I was doing, and asked why I did not simply install a manual cut-off switch, like he had on his gy6 power lawn-mowers. I felt kinda stupid for never considering such a switch, and he showed me the switch on one of the mowers, and gave me a part number. Turns out, the switch is a common part, readily available at power equipment shops;

Petcock on amazon: https://amzn.to/41HIUvj

Cost about 7 bucks (with new Tygon fuel hoses ).

So now, I have the switch instead of the vacuum petcock, and what a difference! The first thing I noticed was much improved throttle response, and a large boost in top-end power, which I surmise is do to no vacuum being expended to operate a petcock.

If your bike is NOT fuel injected, and has a tank that sits HIGHER than the engine, I strongly recommend this mod, it's cheap and easy, and fixes a critical engineering failure in these stock-style petcocks.

I know some people will be quick to poo-poo this idea, but I propose:

2 weeks and no problems (300+ miles)

Much improved throttle response because of no wasted vacuum

And many many motorcycles, scooters, marine engines and power equipment use a manual petcock, and have done so for 100+ years.

Thanks for reading!

[techie610]

Awesome post.
Thanks for the word.

[Admin]

Great post!

[djbh2009]

First post! (Next will be in the welcome section). Sorry to dig up an old thread but do you ever have to adjust the manual valve, as in turn it on/off with use of the scooter? My peacock is leaking (not into the engine but onto the ground) and I do plan to replace the peacock but this is a quick solution while I wait for it to come in. (Unless this works better). Thank you,
Dj

[blueboy5000]

Nope, just turn it off when the bike isn't running, and turn it on when it is.

[kz1000st]

I'm not discounting your problem but two petcocks rotting seems very odd to me. I have the same petcock as the scoot came with, six years later, and it works fine. Are you running an additive in your fuel? Also, given the picture of your manual petcock, where is it located on the bike. Mine is on the bottom of the tank. It doesn't surprise me that Chinese plastic would give grief, but twice? They usually either work or not, restricting flow is usually not part of their issues.

[djbh2009]

blueboy5000:
Thanks! I waited to give this a try and post my results and it's working great! Will eventually replace it with a proper petcock. Maybe...
kz1000st:
I think my leaking started after adding alot of seafoam to try to fix the bogging during acceleration (Which I still haven't figured out. Maybe a carb problem).

[dwint51]

so where do you install this?the fuel coming from the tank to carb?or on the vacuum line eliminating the vacuum?

[blueboy5000]

Kz100st: In Connecticut, ALL gass is 10 to 15% Ethanol, this tends to dissolve some of the cheaper Chinese 'rubber', like diaphrams, hoses and vacuum petcocks. My petcock is located in-line between the fuel tank and the carb.

dwint51; Fuel switches generally go on the fuel line. Mine is located in the fuel line between the tank and carb. No vacuum is required as it is a manual switch.

[kz1000st]

In New York all gas is between 10 to 15% ethanol. My W650 Kawasaki was with me for 10 years and its diaphragm actually wedged open once from "Glop" accumulation, but didn't fail. I fixed it with some STP Fuel System Cleaner and a good run. My wife's Honda Rebel hasn't given me a lick of trouble in 6 years running ethanol and my six year old Bashan, running 10 to 15% ethanol, is still on the original petcock. What grade of gas are you using and are you running fuel system cleaner? I run 87 octane regular in everything and only put Seafoam in before the winter. I've never studied it but possibly higher octane fuels are running more ethanol to slow down the burn process. Two petcocks going up in smoke says something is at work. Finding its cause might help others to avoid the pitfall.

[dwint51]

all I have from my tank to carb is a fuel filter.but the other line is the vacuum going to a little canister.a guy at the shop disconnected all the pollution stuff and removed the canisters.

[blueboy5000]

Originally Posted by kz1000st

In New York all gas is between 10 to 15% ethanol. My W650 Kawasaki was with me for 10 years and its diaphragm actually wedged open once from "Glop" accumulation, but didn't fail. I fixed it with some STP Fuel System Cleaner and a good run. My wife's Honda Rebel hasn't given me a lick of trouble in 6 years running ethanol and my six year old Bashan, running 10 to 15% ethanol, is still on the original petcock. What grade of gas are you using and are you running fuel system cleaner? I run 87 octane regular in everything and only put Seafoam in before the winter. I've never studied it but possibly higher octane fuels are running more ethanol to slow down the burn process. Two petcocks going up in smoke says something is at work. Finding its cause might help others to avoid the pitfall.

The problem I had was that after sitting for a few weeks, the diaphragm inside the vacuum petcocks dissolved, twice. My bike uses a gravity-fed fuel system with an outside the tank petcock. When the bike would sit, fuel would rest on the diaphragm and the diaphragm would melt/dissolve.

I use 91 or better octane pump-gas.

I switched to the manual switch some 1400 miles ago, and have had no further problems.

[dwint51]

mines vacuum fed so the shut off valve wouldn't help me.if I could remove the fuel valve and find one like a motorcycle or lawn mower that would work for me

[jocko7777]

there is an additive for elimiating or changing ethanol as to not caudse any problems in scooter

[jocko7777]

manual petcock on with gas hoses https://amzn.to/41HIUvj