Weird fuel pressure mystery!!! Flow stops. - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 11 (permalink) Old 04-08-2008, 07:34 PM Thread Starter
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Weird fuel pressure mystery!!! Flow stops.

Here's a problem I've never been able to figure out. I have a transparent fuel filter on my petcock which then splits to the front and rear carb fuel lines. Everytime I hop on the hwy (75-80mph) to work I notice the level in the fuel filter drop slowly. By the time I get off the hwy the bike's sputtering, the filter's empty. I have to pull over, remove the filter, let fuel pour freely from the tank into the filter to fill it and then plug it back onto the petcock. Then I'm set for another 10 minutes or so. Sometimes it empties again, & sometimes not. The weird thing is that it never does it @ night on the way home. In fact, I drove back one night from Daytona to Orlando & the bike was fine. At first I thought it was pressure-lock but I tried riding with the gas cap off and it still does it. When the filter empties out (whether the gas cap is on or off!) you can actually see NO gas dripping from the tank. As if there were no more gas!!! But as soon as I pull off the filter the fuel pours out rapidly from the petcock. That's the only way I can refill it too - by letting the fuel 'fall' into the filter. If I actually connect it to the petcock nipple it won't refill. BTW, it seems that once I've done it 1 or 2 times it doesn't empty anymore. She runs great. Not sure what the secretive 'variable' is. Is it a matter of warm-up time? Maybe the hose seals need to heat up or expand? Is it a driving pattern that I do while going to work but not when coming back? Is it air seeping into the system? I've thought of everything! Thanks for taking the time time to read this.
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post #2 of 11 (permalink) Old 04-08-2008, 07:57 PM
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Try removing the fuel filter from the equation and seeing if that solves the issue. Not sure if there's enough hose to remove the filter and hook the fuel line sans filter to the petcock or not... but even if ya have to replace the fuel hose it's a $1 test and hopefully solution.

If that fixes things, either run without an external filter or replace with a higher flow one (there is a filter in the petcock...)

My hypothesis is that your fuel filter isn't allowing enough flow.

Not sure I'd be looking down under the tank while I'm riding either...

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post #3 of 11 (permalink) Old 04-08-2008, 08:15 PM
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Have you tried reverse flow through the filter?? I do not recommend that you use the same filter, but another one like it.
However, curtis is right, there should be a filter in the tank.

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post #4 of 11 (permalink) Old 04-08-2008, 11:41 PM
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Most of the folks with Vulcan 750's do NOT have external filters on them and do not encounter your problems...soooo..it be hard to give you a good explaination as we do not have the experiance of your mod.

There really is no reason to use an external filter unless you suspet you have alot of crap in your gas tank...and then the solution is to clean the tank, not add a filter......


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post #5 of 11 (permalink) Old 04-09-2008, 12:08 AM Thread Starter
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Thanks for the replies guys. The reason I put the filter in is because of a really rusty tank. I mean to get a new one but in the meantime I installed the filter, just in case.

The most baffling thing, which I just confirmed again on my way home this evening is that at night (or on the way back home) it doesn't do it. I made sure to really gun it. I was doing like 80mph the whole 15 miles on the hwy and when I got off to local roads I looked at the filter and it was completely full. Not a single bubble of air. You couldn't even see a miniscus (old high school science word, lol). The only difference I could think of between going TO work and FROM work is cold air vs. hot air. I guess the ulimate test would be to ride to work one night and see how it does.

Here are 2 answers I received from Yahoo! answers. Just wanted to share others' suggestions as well. Again, thanks to all!!!!!!

--------

by Tom B.

clogged vent hose on the carbs
see those small clear plastic hoses hanging off the bottom of the carbs?
get an air compressor and blow some air thru them, they are clogged
if that don't work then your problem is low engine compression

--------

by indianjohn

You have a "small" air leak past a slide or diaphram in one or both of your carbs that is causing just enough pressure in your fuel line to prevent fuel from flowing into it a gravity feed system it dont take much
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post #6 of 11 (permalink) Old 04-09-2008, 09:52 AM
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Soo... if the filter is doing it's job and catching pieces of rust it'll clog. Clogged filters flow less...

Clean the tank (I myself am a fan of the electrolytic method), remove the filter, see if your problems continue.

Do a search here on clean tank or rusty tank and you should find a decent amount of ways to clean the tank - they range from shaking bolts in the tank to acid cleaning to the electrolytic method (electricity + washing soda as an electrolyte + sacrificial anode = clean tank)

The one major disadvantage of the electrolytic (I can't believe I'm spelling that right) method is that it IS line of sight - so you'll probably have to move the anode a time or two - through the filler hole and through the petcock hole would probably suffice. Just MAKE SURE you look up good directions or you can make things worse.

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post #7 of 11 (permalink) Old 04-09-2008, 11:13 AM
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I would check the vents for the carb. If they are not vented the fuel will not flow. If you have a clear filter on it, you should be able to see if it's clogged. You say when you disconnect the fuel line the fuel flows. Well, are you disconnecting it from the inlet or the outlet side of the filter. If you disconnect it from the outlet side and the fuel fills the filter back up, then you have a vacuum lock problem and most likely plugged vent lines (since you said the fuel won't flow when you take the fuel filler cap off, I'm assuming the vent for the tank is working properly, but you should check that also). If the fuel only flows when you unplug the inlet side of the filter, and not the outlet side, then you have a faulty filter that is somehow getting plugged up. Just replace it with a good high-flowing clear with brass filter element and you should be fine. I would recommend if you are going to fun external filters that you run two filters....one on each line, and put the t before the filters.
Also, if you don't have to run the bike with the rusty tank, then don't. It's not a good idea and it can create lots of problems for you. Get it cleaned or get a new/used tank.
Someone help me with this, but I also seem to remember reading something about the fuel not flowing without vacuum from the carb being connected to the petcock. Is this right, or am I just imagining it. Perhaps your vacuum line to the petcock is collapsing in the heat, but not so much at night in the cool.
Good luck and keep us updated.

Bobby in Lakeland, FL

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post #8 of 11 (permalink) Old 04-09-2008, 06:49 PM Thread Starter
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I THINK I GOT IT!!!!! Well, I drove to AND from work today and no problems!!!! Here's what I did.

First, I replaced the round spring-clamp on the filter with a screw-type clamp. I was thinking that a not-so-tight seal could allow air in and cause a vacuum problem. Before, I remember I could swivel the hose, even with the spring-clamp and now with the new clamp it won't budge. Nice and tight.

Second, I removed the vacuum line from the filter housing and re-routed it up and back under the tank pointing towards the rear. I read somewhere here that someone that was having some vacuum problems rerouted his hose the same way and it fixed it.

All in all, I think it was a vacuum line problem because while testing, when the filter was half-empty and wouldn't fill I sucked on the hose (let the jokes roll in) to see what happened and immediately I heard like a bubbling sound and fuel flowed into the filter and filled it up in like a second. Also, I read on line that these vacuum hoses are better off pointing up as to prevent vacuum lock. Maybe that's what was happening to me. In any case, great ride home, thanks to all you guys' suggestions. If anything changes, I'll let yall know. Thanks a million!!!!
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post #9 of 11 (permalink) Old 05-24-2008, 04:32 PM
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one more thing that can cause fuel flow problem

There is an automatic shut-off valve in the petcock valve, make sure the vacuum line going to the bottom of the valve is tight to fitting, the valve may also be sticking inside. I dont think they make them anymore, but i believe i remember seeing a fix for this somewhere on this site
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post #10 of 11 (permalink) Old 05-24-2008, 04:50 PM
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The "vacuum line" you refer to is most likely the carb vent line. If this plugs, the carb bowls won't fill since you have created a high pressure space in the bowls that the gravity feed system can't overcome. The vents let the air out so fuel can replace the air. If clogged, the air can't escape so no fuel gets in.
I think it was combination of the fuel filter connection allowing air into the fuel intake and the vent line either gettin ram air in or just being plugged.
You didn't mention whether the bike had been shaved or not but with you description, I would guess it is shaved. Otherwise the vent line goes into the right ear.

Jim W
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