4th Gen Fuel Pump Question

I was putting a heat sink on the OEM R/r/ today. (I think it failed yesterday from overheating, but seems fine. I plan to buy a new finned version ASAP) while I was waiting for the bike to heat up (so I could take heat measurements, I was listening to various parts of the bike with my mechanics stethoscope. I kept hearing a weird, soft thumps every few seconds. I noticed the fuel in the filter "swishing" with each thump. Anyway, long story short...the fuel pump was making a "thump" about every four/five seconds at idle, and about every two seconds with throttle at around 3K.

So, is the pump going out? (I.e.-binding up every few seconds, and releasing) or is the pump doing what it's supposed to do? (i.e.-loading up and then pumping fuel?) I would imagine the fuel pump should be providing a steady, constant flow with no thump. I would gues it's going out. But what is the real answer?

I think it should thump. The pump is operated via pressure, so as soon as the pressure falls under a certain value, the pump cycles, pumps some gas in, and stops. Think of it more like an air compressor. As soon as the pressure gets too low, the compressor kicks on, gets the pressure back up, and stops.

It's a diaphragm style pump, so it makes noise unlike what you would expect.

In the carbs, there are floats that lift up when the float bowls fill, and block the place where the fuel feeds in. The pump cycles until the fuel stops moving, and then it stops. When the floats drop enough to open the feed holes again, the pump comes back on and fills them up again. It usually only cycles once, then sleeps until it's time to cycle again.

The pump makes a pulsing sound as it cycles. This might be the thump you are hearing.

From my experience, this is normal. From what I've read, the bike will run without a pump but it won't drain the entire tank.

On my highly modified RCBVFR, the pump is located above petcock and it works well with a pulse type pumping action.

you only need to worry when it won't "thump"; it works by pulsing, not a continuous flow like a fuel injection pump.

This is all good news. Thanks to all. -Mark

Okay. Seems my bike is now one of those I read about from time to time that just has a slew of gremlins. The R/R went out, so I finally bought another (the updated kind with fins) as well as a VFRness. It was good to finally get back on the road (for two days). Anyway, while I had the plastic off, I noticed a pin hole in the elbow shaped fuel line the comes out of the petcock. Bought a new one from Bike Bandit and installed it.

Now, I noticed the fuel pump is leaking. There is a small, rectangular tube that comes out of the bottom of the pump, and when I rev to about 4K-5K, it starts leaking pretty well. (Enough so I shut it down and thought, ...'This could catch on fire'). I wonder what that rectangular tube is? Some kind of weep hole that lets you know the pump is going? I removed the black plastic cap on the back of the pump, and didn't see anything amiss.

I think this is the original pump, so I probably need to replace all of the fuel lines because they are probably all original too. Anyway, about this pump...time to get a new one? I'm getting tired of spending more time on the internet looking for parts than riding the bike. Thoughts?

p.s.More data to add to the mystery. I put my mechanics stethoscope on the pump, and at first it sounded like it did months ago when I first started this thread, which is: at idle, it sounded like a heart beating around 80 BPM, then increased in speed as I gave it throttle, then back down. After a minute, however, I tried it again, and it started slow, then went really fast (almost like a constant fluttering) at 5k, then did not slow as I let off the throttle.

If the pump runs fast and long it usually means that fuel is not getting to pump from tank.......

[image: View Post]squirrelman, on Aug 23 2009, 01:17 PM, said:

If the pump runs fast and long it usually means that fuel is not getting to pump from tank.......

Yes. I had the petcock closed, which explains the fast running pump. (i.e.-no fuel from tank.) But I really need a new pump now, since I took mine apart. Had to see what was inside. (Luckily, I could see the rubber pump diaphragm was cracked, which probably explains the leaking, and the reason for the weep hole).

NOTE: Once you take these pumps apart, they would be nearly impossible to put back together. Does not look fixable unless you can find a way to unbend all the tabs, re-do all the rubber/gaskets inside, then put the pump halves together and re-bend the tabs. ( I cut the tabs off to get it apart...since I figured needed a new one anyway.)

Well, off to search for a new pump. Anyone have one for sale? Let me know. Thanks. -Mark

Camarkim-at-yahoo-dot-com