1500 Vulcan Fuel Pump Bypass - Revisited

Well as anyone that reads this blog knows, some of the projects I document are tried and true maintenance practices and ideas, while some things are experimental. I generally always to try to show things that worked and things that didn’t. I was going to blog about a second attempt to re-do my fuel pump bypass. The first try wasn’t working all that well. Before I could write up the redo some things thing went crappy yet again. So my photos in this blog were not taken in one session and are not presented in true chronological order. This how I would do it correctly in one try. Hopefully the point of the project and the lessons shine through with this completed blog, but the latest configuration has not been fully vetted yet either.

So the fuel pump bypass was working okay until I started running out of gas near/on reserve riding in town stoplight to stoplight. I looked at the situation. I figured the combination of fuel line going over up and over the intake manifold(and thus closer to the top of the lower fuel level in the tank) and the stock multi-pleated pump style filter were slowing my fuel flow and possibly losing my siphon. This situation led to my second attempt. I used see through 5/16 blue fuel line, a gravity style glass filter, HD P/N 45092-66 cable guide, and a V-Twin P/N 35-0134 brass carburetor inlet nipple. I wanted to point the fuel down instead of up. This would all me to route the fuel line under the intake manifold for better flow without the concern about losing the siphon in a low fuel situation. The clear fuel filer and fuel line would allow me to observe the fuel flow characteristics as well to help with any troubleshooting of further issues.

The second attempt was a failure twice. The first failure(and part of the second) was the V-Twin P/N 35-0134 brass inlet. I had assumed that this was one piece elbow, because all the ones I had seen on the internet were one piece. The V-Twin P/N 35-0134 inlet was a two piece threaded design, I didn’t notice that and the first time I put everything together fuel poured out on to the ground. I took everything apart and attempted to seal the threaded fitting. I put everything back together and I decided to give a good shakedown on poker run with a friend.

The bike ran out gas on me a few times, again more of a problem during stop-and-go than WOT. I had to keep messing with the fuel line to keep it running. I could see the visible air bubble in the line growing from time to time. It was actually getting a bubble on both ends of the fuel line even while sitting still. My poker hand was terrible, so I didn’t bother getting the last card and headed home to figure out my next plan. The dang bike seemed to be defying the laws of hydrodynamics to say the least. And yes I did check out the float vent and the tank vent and both where clear and working.

I finally realized what was happening after studying the situation and burning the palm of my right hand on the exhaust pipe. There was leak on both ends, but neither leak was big enough to actually cause a noticeable drip. I could see gas running around the joints, but it evaporated before it went too far. This was causing the loss of siphon when the float needle closed off and the fuel flow slowed and especially with when parked. Soap bubbles and compressed air revealed air leaks at the brass barb and aluminum housing join on my modified petcock and still around the threaded join of the V-Twin P/N 35-0134 carb inlet that I had mostly sealed.

Here is my current configuration; JB Weld sealed joint on the petcock, Genuine Harley-Davidson brass inlet P/N # 27371-76A, ¼ inch see through fuel line on the carb inlet(for better fit), universal glass filter with a ¼ inch outlet barb, HD P/N 45092-66 cable guide, and 5/16 inch inlet barb(like a Vulcan 800 set-up), 5/16 inch see through line from the petcock to filter. If would was going to start this from scratch I would consider all ¼ inch line and ¼ inch inlet/outlet filter with a smaller filter and stretch it over the petcock. The 5/16 line is useable, but a bit loose and ¼ inch line is really tight. My experiments lead me to believe that the full volumetric flow from a 5/16 hose isn’t ever quite utilized. Talk with me about this in few tanks.

Here's the petcock leak. I believe this was contributing to my loss of flow. It seems a bit because in the original configuration the brass would occasionally pull out when I remove the fuel hose. I think that may have caused some air bubbles going into the pump and contributing to the pump failure.

If you can't fix it with JB Weld....

then you ain't using enough.


Here's the two piece inlet that should be avoided. I tried sealing it with liquid thread sealant, it didn't quite seal it. More sealant or tape might have worked, but why when the one piece is the same price and readily available.



Here's what you need. Genuine Harley-Davidson brass inlet P/N # 27371-76A about $12 at any Harley dealer. It's one piece and has a longer hose barb, too.


Now on to the fixing the problem. Here's the stock inlet position; straight up.


With the carburetor removed for the bike on the bench, it was obvious this plastic was in sorry shape. After this whole ordeal, I would venture to say that this was leaking and may slowly emptied the line from pump to the carburetor when parked. Possibly causing air to get into the pump and hurting the pump.


To replace the plastic and brass stock inlet you have to break off the all the plastic parts.



It may have pull straight with vise grips, but I opted for the drill and tap method of removal. I used grease on the drill bit to minimize the brass chips falling into the carburetor.


The grease works well.


Same on the tap.


Here's my long bolt and deep socket.


It works good, too.



I still pulled the float and float needle to make sure no chips got in.


I wasn't positive that I have enough clearance to install the inlet pointing at the 8 o'clock position. So I got the plastic elbow about the same size to mock things up.



There was a lot more clearance than I thought.


I put the inlet where I wanted it.


Then carefully pressed it in with a large c-clamp.


It looks something like this in place.


The thread hole in the front head what a perfect place to mount the HD P/N 45092-66 cable guide to keep the line and filter off the hot heads and cylinders.


I tweaked the guide and used a larger piece of hose over the fuel line to keep everything in place.

It was a trick to figure out and cut the line from the filter to the inlet.

The same set-up, but not truly the configuration on the bike now.