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Author Topic: Yamaha XV400  (Read 1293 times)

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Offline jaytee-nz

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Yamaha XV400
« on: January 04, 2020, 06:16:33 PM »
My son's XV400 (only sold in Japan, Australia and New Zealand but basically the same as the mid '80's XV500/550 Virago's etc.) is flooding the rear cylinder via the vacuum line from the fuel pump. The vacuum fuel tap (petcock) is dribbling a few drops of fuel when in the On or Reserve position but I don't think it's enough to pretty much fill the cylinder overnight, which is what happened yesterday. I'm not familiar with fuel pumps but could the pump be faulty and passing the fuel or do people think the tap is the issue ?
Any thoughts would be much appreciated.
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Offline seanbarney41

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Re: Yamaha XV400
« Reply #1 on: January 05, 2020, 12:51:39 AM »
Vacuum petcock is most certainly the cause, although I don't know anything about those Yamaha XV's or any carbed bikes with fuel pumps...I am assuming the pump is some kind of vacuum diaphragm thingie?  Probably gonna have to rebuild/replace  both as the rubber diaphragms have certainly failed.  Ethanol in the fuel he is using?
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Offline Terry in Australia

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Re: Yamaha XV400
« Reply #2 on: January 05, 2020, 02:24:26 AM »
We didn't get XV400's in Oz as far as I'm aware, or maybe we did, but I don't think I've ever seen one? It does sound like your fuel tap (petcock in the US) is the problem though, and cheaper to replace than the fuel pump. From memory vacuum taps were only brought in to produce a fuel delivery system with an automatic "off" when not in use (i.e., they require engine vacuum to suck a diaphragm "valve" open to allow fuel to pass) so if you're smart enough to remember to turn your fuel off after a ride, might be a better idea to install a manual tap, like a Pingle? I put Pingle's on several bikes with CV carbs, and never had an issue. Just remember to plug the (redundant) vacuum line to the tap. ;D
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Offline CBJoe

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Re: Yamaha XV400
« Reply #3 on: January 05, 2020, 04:15:56 AM »
For sure can be the pump as well...if it’s a vacuum operated diaphragm pump and the diaphragm is torn or leaking it can get into the crankcase



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Offline jaytee-nz

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Re: Yamaha XV400
« Reply #4 on: January 05, 2020, 01:02:19 PM »
Thanks guys - very useful.
Terry - I got hold of a service manual for this and it's for the Australian version but maybe it was never actually sold there ?
The fuel pump is definitely a diaphragm one so I'll check that as well. 
No ethanol in NZ fuel (yet).
Cheers - JT
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Offline Terry in Australia

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Re: Yamaha XV400
« Reply #5 on: January 05, 2020, 05:11:24 PM »
No worries mate, it might have been an official import that didn't sell well, I've definitely never seen one in the flesh, but back then the 250 Virago's were very popular with learners (when the biggest bike they could ride was a 250) and there wasn't much in the middle until the 750 "Yama-Harley". There was a 550 that the magazines raved about, but didn't sell well, and I've never ridden one. Yep, anything with a diaphragm is suspect. ;D   
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I was feeling sorry for myself because I couldn't afford new bike boots, until I met a man with no legs.

So I said, "Hey mate, you haven't got any bike boots you don't need, do you?"

"Crazy is a very misunderstood term, it's a fine line that some of us can lean over and still keep our balance" (thanks RB550Four)

Offline low-side

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Re: Yamaha XV400
« Reply #6 on: January 07, 2020, 05:11:13 PM »
It takes a perfect storm of conditions, but if the tap leaks or is open, the rear piston is down, and the pump is in just the right position when the engine is stopped there can be a siphon effect that can even hydrolock the cylinder.  Happens to early Wings once in a while.  I'd definitely fix or replace the fuel tap.  I'd check the pump's weephole and if it has any staining I'd replace it too.  Might be a good time to check carb float heights and ensure that the float valves are sealing properly.  I just read that there's fuel in the vacuum line from the pump - I'd definitely repair or replace the pump too.
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Offline jaytee-nz

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Re: Yamaha XV400
« Reply #7 on: January 07, 2020, 08:07:37 PM »
Thanks Low-side. I've ordered a repair kit for the tap and will see how that goes first. I was worried about it hydrolocking so I pulled the rear cylinder plug and turned it over on the starter - fuel squirted right across my garage !
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Offline Terry in Australia

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Re: Yamaha XV400
« Reply #8 on: January 07, 2020, 09:06:57 PM »
Lucky you weren't sucking on a Winnie Red mate! ;D
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I was feeling sorry for myself because I couldn't afford new bike boots, until I met a man with no legs.

So I said, "Hey mate, you haven't got any bike boots you don't need, do you?"

"Crazy is a very misunderstood term, it's a fine line that some of us can lean over and still keep our balance" (thanks RB550Four)

Offline jaytee-nz

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Re: Yamaha XV400
« Reply #9 on: January 07, 2020, 11:23:04 PM »
Haha Terry - the funny thing is the wife was walking right past the bike about 2 seconds before I hit the starter. Could have been me staying in the garage for awhile I feel !
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Offline Bankerdanny

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Re: Yamaha XV400
« Reply #10 on: January 08, 2020, 02:01:35 PM »
Here in the US those bikes were called the Vision. Once in a while I am tempted by the 2nd year Vision, which has a full sport fairing. The first year bike was naked. I saw the faired version up at Road America at Rockerbox a couple years back, it was the first one I can ever recall seeing in real life.
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Offline low-side

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Re: Yamaha XV400
« Reply #11 on: January 09, 2020, 05:24:27 PM »
Close call on the wife and the fuel!  Sounds like the bike is nearly sorted.  Looking forward to reading the conclusion.
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