Bike won't stay running below half tank

All the work you do in the workshop. Maintenance, repair, troubleshooting, customizing, etc.
Secarider4
New to XJR
New to XJR
Posts: 3
Joined: Thu Aug 13, 2015 11:12 am
Region or State: Minnesota
Motorcycles Owned Currently: 97 yamaha seca II

Re: Bike won't stay running below half tank

Post by Secarider4 » Wed Jun 07, 2017 4:12 pm

Fuel filter should not be a problem. I have an after market clear one. Petcock should be fina as well. I replaced both about a year ago and have put done a decent amount of miles. The tank is in great condition so rust is not an issue. I doubt water is the problem as the issue only arises at half a tank. On a side note I own carterivy's old bike. He won xj of the month at some point 😉 I'll inspect a few more points and come back later

cicatriz63
Lookin' Around
Lookin' Around
Posts: 27
Joined: Tue Oct 04, 2016 7:26 am
Region or State: PA
Motorcycles Owned Currently: 1996 XJ600S

Re: Bike won't stay running below half tank

Post by cicatriz63 » Fri Jun 23, 2017 9:31 pm

A fuel pump is like $12 on amazon. I was having the same issue and this fixed it completely.

https://www.amazon.com/gp/product/B00M5 ... UTF8&psc=1

User avatar
radare
Site Founder
Site Founder
Posts: 8259
Joined: Mon Nov 07, 2011 12:42 pm
Region or State: Rocky Mountains
Motorcycles Owned Currently: '92 XJR600, '92 Seca II
Location: Denver, CO
Contact:

Re: Bike won't stay running below half tank

Post by radare » Sun Jun 25, 2017 9:43 am

From a post I made a few years ago:
radare wrote:
Sat Feb 28, 2015 10:21 am
ride92 wrote:So does the check valve have any effect on the fuel flow when in prime? If you need more then 2 gallons to have enough pressure to push past the check valve there is no way switching to reserve would have enough fuel to bypass it. So how does that work, if you run out of "on" fuel I've always put it to prime to refill the carbs and then to reserve. Doesn't it still flow thru the pump on prime and res?
The check valve inside the fuel pump allows fuel to flow from the tank to the carburetors and offers no resistance to flow in that direction. Its function is to prevent backflow from the carbs to the tank.

First, consider the simple case of a tube with a bit of fluid in it. If we bend that tube in a U-shape and hold it such that the ends of each tube are at the same height, atmospheric pressure will push down on the fluid on both ends, equally, and the fluid will be at the same height.

Image

Now, if you move one end of the tube below the fuel height, fuel will spill out until the amount of fuel is balance again by atmospheric pressure. The fluid moves because atmospheric pressure is exerted on it; that's the driving force:

Image

Since the fuel tank and the float bowls on the Seca II are both open to atmosphere, the tubing and fuel will behave similarly.

Image

Take a look at the preceding illustration. Note the two lines: The top one is where the fuel level is inside the carburetor float bowl. The second line is where the fuel comes out of the fuel tank at the petcock.

Consider the instance with the bike off and the petcock in the prime position. In this instance, the fuel pump is not working because the bike is off and the fuel pump is vacuum actuated. The petcock is also open because you've bypassed the vacuum diaphragm, so fuel can flow freely from the tank to the carbs. The only driver for pushing fuel from the tank into the carburetors is the force pushing on the fuel from atmospheric pressure:
  • If the fuel level in the tank is above the top line, atmospheric pressure exerted on the fuel in the tank is more than atmospheric pressure exerted on the fuel in the carburetor float bowl and fuel flows from the tank into the carburetor.

    If the fuel level in the tank is below the top line, atmospheric pressure exerted on the fuel in the tank is less than the atmospheric pressure exerted on the fuel in the carburetor float bowl and fuel wants to backflow from the carburetor to the tank.
Now consider the instance where the bike is running and the fuel pump is working. In this instance, the pump is the driving force. If the pump is in good working condition, it will push fuel into the carburetor regardless of the height in the tank. This is what happens when you the bike is running and you are below the top line. This is also why the Seca II has a fuel pump.

I made a generalization on the fuel volume in the tank between the top and bottom line. In my experience, its actually somewhere between one and two gallons. If you have two gallons in the tank, you should have enough fuel to be above the top line and prime the carbs without the pump.
If your fuel level gets below the top line, you won't have enough atmospheric pressure to fill the carb bowls.

Image

It could be your fuel pump.

Post Reply

Who is online

Users browsing this forum: NekkedBike