Question about 1985 Yamaha XJ 700 X Maxim

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Vacuum hose that attaches to the fuel petcock behind fuel line

I see in the pics in my Haynes manual showing a hose running from it. My bike is a 1982 MAXIM with a XJ750YICS motor. My bike idles @ about 1000 rpm but you can tell by the sound that its idling weak. I just changed the motor for one that with 7000 miles but I kept the carbs which I cleaned. I also have the timing parts and ignition system. I can get it running periodically after considerable coaxing but when it runs the power output fluctuates. any help is greatly appreciated

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  • arcticwolf51 May 26, 2009

    They were sychroed last summer to the old engine. When I removed them I did not touch the synch screws. When it idles the sound is quite smooth so I did not think of this. I think I could find a synch tool but I still find myself tying to find the proper connection point for that vacuum line. I looked at an older yamaha 750 and that one went to a vacuum nipple on the head. I took a cue and ran a vacuum line to the number one carbutetor horns vacuum nipple. It still idles weak. A friend sait it could be a faulty fuel tap. I haven't explored this deeper yet but i need to know if there is a rebuild kit for the tap.



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My 81 xj750 vacumm line attaches to a pip on the boot that attches it to the head. Have you investigated to diaphram in your fuel pet ****. I know mine had a small pin hole which made my idle weak at times. i even went as far as resynching the carbs and changing the plugs before i found it . just enough to bleed off vacuum . Hope that helps

Posted on Jun 10, 2009

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  • Yamaha Master
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After cleaning the carbs , did you synchro them?
Do you have a YICS tool?
Please visit this site
Good luck.

Posted on May 24, 2009

  • Murtaza Akberali  Versi
    Murtaza Akberali Versi May 26, 2009

    Since you have changed the engine you ought to set the carburators to this engine's performance.

    A total tune up would be best to do.

    Theres plenty of info at please visit tht site .

    Good luck.



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Fuel tank petcock vacuum operated

What exactly is the problem with the petcock? If it is a vacuum operated petcock and it is no longer allowing fuel to the engine check the following:

First, did you work on the bike prior to the problem occuring, and was the bike running properly before this? On bikes I've owned with vacuum petcocks, the petcock has two rubber hoses attached to it. One is the vacuum line the other is the fuel line. On one of my bikes these hoses were nearly identical in size and I managed to swap them a couple times after having the tank off the bike. Simply double check that the hoses are connected properly. If they are not, the bike will run until the fuel is run out of the float bowls then die.

Second, do you have an inline fuel filter on the fuel line between the petcock and carb? If so check it to make sure it is not clogged.

Third, remove the tank and check the entire length of the vacuum line for the petcock. Is it kinked, pinched, cracked, cut, detached, etc.? Look for anything which would prevent the line from providing vacuum to the petcock.

Fouth, test the petcock to see if it is working properly. To do this you will need a handheld vacuum pump, such as a Mityvac (you may be able to get a free loaner from some autoparts stores). remove the vacuum line from your petcock and attach the vacuum pump to it. It should only take one or two pumps to create enough vacuum to open the petcock valve. If fuel begins to flow the petcock is good, if it doesn't you could have a clogged filter screen in the tank, but most likely the petcock's vacuum diaphram is damaged. The Baha is a Chinese made bike and it's possible the rubber parts in the fuel system are not compatible with the ethenal fuel blends here in the U.S. The dealers who sell these bikes are really hit and miss when it comes to being able to provide replacement parts. Even with the internet, it may take some work to locate a correct replacement petcock for the bike. On the bright side, if you are able to get a new petcock, they are very easy to replace.

Good luck!

Nov 29, 2013 | 2011 Bajaj Pulsar 150

1 Answer

95 softail classic started and was warming up and just died and wont start, will fire with starter fluid but thats it, will not keep running, has appr. 30,000 miles

If the bike will fire with starter fluid but won't run on it's own, I'd say you have a fuel problem. In 1996, Harley introduced a vacuum operated fuel petcock. I see that you say your bike is a '95 model but Harley has always been known for crossing over year models with certain parts. Say you bike was built in late '95 and they ran out of the "old" style petcock. They just switched to the "new" style at that time instead of making more "old" style parts. That's where the term "transition model year" came from. So, check your fuel petcock and the vacuum line running to it. Pull the fuel hose off the petcock and spin the engine. Watch for the fuel flow and determine whether it is sufficient. If not, replace the petcock. Personally, I went to a Pingle manual fuel petcock. Of course, you have to plug the vacuum hose as it's no longer used. But, that was the end of petcock problems on my bike. Just turn your fuel on and off manually, "old school" style.

Good Luck

Sep 13, 2011 | 2000 Harley Davidson FLSTC Heritage...

1 Answer

Harley turns over but does not fire i checked fuses they are good i drained the fuel tank to make sure there was not any thing in the filter but i dont know if it is getting gas gas does not come out of...

You didn't say what year or model your bike was so I'll just take a guess here. In 1996 and later, Harley started putting vacuum operated petcocks on their bikes that are equipped with a carburetor. It should have a small vacuum line running to the backside of it unless someone has changed the petcock to a manual operated one. The vacuum line comes off the backside of the carburetor and supplies vacuum to the VOES switch and the petcock.

If it is a vacuum operated petcock, it will have a square plate on the backside with four small screws holding it on. Take the screws out and carefully remove the plate. Notice how the stuff in there comes out. There is a spring, a needle, and a diaphragm in the petcock. Hold the diaphragm up to the light and gently stretch it while looking for a hole. If you find one, put a new diaphragm in the petcock. Upon reassembly, connect the vacuum hose and turn the petcock to the normal run position. Leave the fuel hose off and spin the engine. Fuel should flow from the petcock. If you have a hand operated vacuum pump, you can connect it to the petcock and apply vacuum to get the fuel to flow.

Personally, I suggest changing the petcock to a high quality manually operated petcock and turn your fuel on and off manually. Old School style. Pingel makes a very high quality petcock. They aren't cheap but you'll never have another petcock problem.

Good Luck

May 16, 2011 | Harley Davidson FLSTF Fat boy Motorcycles

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1986 VT 500 C SHADOW fuel line routing

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1 Answer

Working on a friends 81 yamaha 650xs. I removed and cleaned the carbs and put them back on but can t remember how the T hose from the petcock to the carbs hook up. Can someone help me

The petcock on these bikes is vacuum operated, which means that there are two hoses that go to it. The thicker hose is the fuel line and it runs from the thick port on the petcock to the fuel input on the carburetor rack. The fuel input on the carburetors is located between the number two and number three carburetor bodies. If you aren't sure which is the fuel connection, just turn the petcock switch to prime for a second and you will see gas come out of the fuel connection.

The thinner hose on the petcock is a vacuum line. It runs from the vacuum port on the #3 cylinder (between the carburetor body and the engine) up to the smaller connection on your petcock. When the engine is turning vacuum is applied to this thinner line and allows fuel to run from the tank into the carburetors.

There are the only two lines that should run to your petcock, and the fuel line should not be split anywhere between the petcock and the carburetors.

Good Luck,

Feb 24, 2011 | 1981 Yamaha XJ 650

1 Answer

What is the proper way to hook up (4 fuel line) the return and send fuel line hoses for a 1982 Yamaha -XZ-550 Vision Motorcycle.

The '82 1/4" fuel line goes from the petcock to the carb....(install an in-line fuel filter between them)...with a vacuum line going from the petcock to the vacuum off take, located on the front carb rubber manifold, (the smaller of the two brass tubes!!).
You should also go to A web site for Yamaha Vision owners...


Oct 01, 2010 | 1982 Yamaha XZ 550

1 Answer

Bike runs for a mile then stops

You didn't tell me what year Sportster we're dealing with. So, I'll cover a few general things to check.

If your bike is running for about a mile and then stopping and it does this repeatedly, It sounds like a fuel problem. Most likely it's either the petcock or the fuel tank vent system.

First, If your bike is made after 1996, it probably has the OEM vacuum operated petcock. These petcocks are designed to open when the engine is running and close when it stops. There is a small vacuum line that plugs into the backside of the petcock. When the engine is running, this line supplies the vacuum. Check the line and make sure that it hasn't come loose or cracked. It originates behind the carb and goes upwards underneath the tank to the VOES switch and it has a "Y" in the line that reduces the size and then runs to the backside of your petcock.

If it is intact, you'll need a hand vacuum pump to test the petcock for proper operation. Connect the vacuum pump to the rear of the petcock. Remove the fuel line from the petcock. Now, apply vacuum to the petcock with the pump. Fuel should flow from the petcock. If not, the petcock is either bad or the strainer in the tank is stopped up.

Remove the petcock from the tank. Place a large funnel under the petcock and direct the funnel into an empty gasoline can large enough to hold all the fuel in the tank. Loosen the petcock enough so that the fuel runs out at a rate that you can control. When empty, finish removing the petcock from the tank. Inspect and clean the screen. Now, on the back side of the petcock is a square plate held on by four screws. Remove the screws and remove the plate. Underneath the plate you'll find a diaphragm. Hold the diaphragm up to the a light and gently stretch it and look for holes. If you find a hole, you'll have to replace the diaphragm.

Look inside the petcock where the needle is for trash. At this point you should be able to blow back up through the petcock to check for obstructions. Reassemble petcock and reinstall in the tank.

I've seen these petcocks give a lot of trouble exactly like you're experiencing. I recommend that you replace the petcock with a manually operated petcock. The Pingle petcocks are very high quality units but a bit pricey. If you'd rather stick to an OEM type unit, purchase a petcock from the Harley shop for a 1995 or earlier model. This will be a manually operated petcock. Plug the vacuum hose that you'll no longer need. You'll have to get into the habit of turning the gas on and off whenever you get on or off the bike like the old timers did. But, they'll never let you down.

Now, remove the fuel cap. Wipe it down real well with a clean cloth. Now, make sure your friends are NOT around for this test or you'll never hear the end of it. Put the threaded end of the fuel cap up to your lips and try to **** and blow through the vent. If you cannot, the vent is bad and you're not getting gasoline to your carb due to a buildup of vacuum in the tank.

Good Luck

Jul 15, 2010 | 2000 Harley Davidson XL 1200 C Sportster...

1 Answer

Wont stay running,like its out of gas. but its not

It may indeed be out of gas. You can have a full tank of fuel and be "out of gas" in the carb. The culprit could be the vacuum operated petcock. The petcock has a diaphragm in it that is opened by engine vacuum. If that diaphragm is not functioning as it should, no gasoline gets to the carb.

To test the petcock, reach around behind it and feel for a small vacuum hose. Unplug this hose and replace it with a hose you can **** on. Using your mouth, **** on the hose with the fuel line disconnected. You should see fuel flow out of the petcock. If not, you must drain the tank by loosening the petcock and allowing the fuel to run out of the loose petcock. Use a large funnel and an empty fuel container to catch the fuel. Once the fuel tank is empty, remove the petcock. Take the plate on the back of the petcock off by removing the four small screws. You'll find a diaphragm, a small spring, and a needle. Hold the diaphragm up to the light and stretch it gently. If you find a hole in the diaphragm, replace it. Check the petcock for obstructions by blowing through it. Reassemble the petcock and test it again.

Check the hose that supplies the vacuum to the petcock. It comes from the backside of the carb and runs upwards to the VOES switch and then reduces in size and goes to the petcock. Make sure it's connected and has no holes or cracks in it.

If everything looks alright, replace the petcock, connect the hose and try to start your bike again.

These petcocks are notorious for giving problems. I always recommend that the vacuum operated petcock be replaced with a high quality manually operated petcock such as a Pingle. They're expensive and require you to manually turn the gas off like the "old school bikers" do but they won't leave you stranded out of gas with a full tank of fuel.

Good Luck

Jun 17, 2010 | 2003 Harley Davidson XLH Sportster 883...

2 Answers

I have two loose hoses coming off my petcock? where do they go

On your silverwing you have a "vacuum operated petcock" which means that the carbs draw a vacuum and allow the fuel to flow from the petcock. The front hose is the vacuum hose that should run to the center of the carbs. The second is a drain hose - it's advised that you do have a small hose connect here, so that dirt does not find its way into the petcock. This hose will simply run down past the swing arm to the ground. This would prevent any gas from dumping itself onto the engine if your petcock fails. If your bike runs without the vacuum hose attached, your petcock has failed and will need to be rebuilt.

May 09, 2009 | Honda Silverwing 400 Motorcycles

1 Answer

Kawasaki ninja ex250 won't start

the petcock is vacuum operated, there should be a small hose just behind the larger fuel hose thats attached to the petcock. What it does is when the bike is cranking over it sends vacuum though this hose and operates a diaphram in the petcock causing fuel to be released in the fuel line. This is for people that don't turn the fuel off as far as I know. Anyway if you take the small line off and get another piece that fits there and turn the petcock on and **** on the small hose it should send fuel out the bigger fuel hose if it works correctly.

Hope this helps

May 16, 2008 | 2005 Jaguar XK8

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