How-to check all vacuum lines

[MN_Scoot]

I've looked through this forum for info on vacuum issues. I see plenty of mentions to "check your vacuum, but can't seem to find info onhow.

The most I've seen is to disconnect the vacuum line from your intake manifold and suck on it. Not being able to suck apparently means you're good. Aren't there more lines to be tested than this one? If I'm missing a forum post, could someone please link to that one where it explains how to test the entire vacuum system on our bikes?

[zombie]

The easiest way is to install a "T" fitting on the vacuum line going to the fuel petcock. Start the bike, and you should see a steady reading of 20"hg.

Rev the bike to approx 3k rpm, and hold it there. The gauge should drop when you raise the rpm, and again hold steady when the rpm are steady.

If it is lower than 18ish you have a leak somewhere. same for a fluctuating reading. Vacuum leak.

[BettinANDlosing]

Sometimes I've found intake manifold cracked, not many on Taiwanese bikes but a fair number on Chinese scooters. You should be able to start your scooter and cm grab the carb and pull it up and down gently, if there are cracks in the manifold you engine will stumble or even die.

[zombie]

You know about removing the rubber from those... There are lot's of ways to get it off, use the aluminum core, and a proper coupler w/ clamps.

Cheaper/better

[MN_Scoot]

The easiest way is to install a "T" fitting on the vacuum line going to the fuel petcock. Start the bike, and you should see a steady reading of 20"hg.

Rev the bike to approx 3k rpm, and hold it there. The gauge should drop when you raise the rpm, and again hold steady when the rpm are steady.

If it is lower than 18ish you have a leak somewhere. same for a fluctuating reading. Vacuum leak.

Ok thanks, I will pick up a vacuum gauge and do this.  Do our bikes essentially just have the 3 vacuum lines to worry about (i.e. from the T, the three lines that go to: petcock, intake manifold and…not sure where that last one goes under the floor)?

Also, I see a lot of posts about upgrading to automotive grade fuel and vacuum lines. Do you use the same hose for both systems? Is there a specific brand/type anyone would recommend?

Cheers.

[BettinANDlosing]

You can remove the emissions system and run just the petcock line.

[zombie]

That third vacuum line runs to the emissions canister. Essentially it is a vacuum leak. It draws vapors off from the fuel vent system. 

I use clear fuel line for both the petcock vacuum, and the fuel feed from the tank. that way you can see if there is a problem without pulling it all apart

http://www.partsforscooters.com/search-results?keywords=fuel%20hose

[MN_Scoot]

Thanks for all the good info.

Related question: My fuel filter was empty when I sucked on the portion going to the intake (bike was off). The fuel filter did not fill from sucking on the hose. I seem to remember that that sucking on that hose should fill the filter. If the vacuum hoses are clear, is that an indicator that the petcock is bad?

[BettinANDlosing]

You have to suck the line going AWAY from the intake. The vacuum has to go to the pet cock. Take the vacuum line off the intake and suck there.

[MN_Scoot]

You have to suck the line going AWAY from the intake. The vacuum has to go to the pet cock. Take the vacuum line off the intake and suck there.

Yeah, worded that wrong. I meant I was sucking on the portion of hose that I had just disconnected from the intake. Shouldn't that have filled the empty fuel filter?

[BettinANDlosing]

Yeah it should. If the bike sat for a while, those petcocks seize up. Clean up the fuel line, then blow really hard on it in the direction of the petcock you'll hear it pop open hopefully. I use compressed air but sometimes that blows the diaphragm.

[zombie]

Agreed.

Sometimes it will not flow if the float bowl is full, and the line is full up to the filter. Try it again but this tiome pull off the fuel line as well.

It could also be a clogged up in line filter or the screen inside the tank. For 20-25 bucks I'd scrap it all, and install a whole new set up.

[herrspoot]

Yeah it should. If the bike sat for a while, those petcocks seize up. Clean up the fuel line, then blow really hard on it in the direction of the petcock you'll hear it pop open hopefully. I use compressed air but sometimes that blows the diaphragm.

Do these petcocks sometimes seize in the open position? After removing the vacuum line (so there should be no vacuum) I still have free flow from the fuel line to the carb. If they can will blowing backward in the fuel line do anything other than give me a mouthful of gasoline?

[zombie]

 have had them fail open more often than closed. Replace it.

The reason they fail open is debris from inside the tank buggers up the valve when open, and it sometimes jams in a 1/2 way open position both restricting flow, and preventing full shut off.

Parts for scooters . com has the petcocks for 10 bucks or so. I don't know which one it is exactly. there are three different one available. They all go by thread size of the tank. Every-time I have to order one I use a thread die to determine the correct one.
If it helps I do know Kymco's all use the finest thread pitch but I forget the diameter. Perhaps put a caliper on it to get the diameter.