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Discussion Starter #1
PLEASE HELP ME!!!!!
I have a 1978 XS400 6500 miles on the clock. The bike will not run for long as it keeps filling up the crank case with petrol and pumping it up to the air filters. I was told that strange as it may seem that this is a problem with the petrol tap. I replaced the diaphram and this seemed to cure the problem, but taking it to the local garage for an MOT it has started the same problem,
Can anyone help?????????????
 

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sounds like you have a ton of problems going on all at once.

Ok, How clean is your tank inside? Spotless or rusty. Petcock problems are mostly caused by junk from the tank getting in the fuel cock and jamming it open or causing small leaks.

If you have fuel into the airbox and crankcase then the carbs on your bike are also shot. A full rebuild with new seals and a good cleaning is probably in order. Usually when you have an overflow of fuel it is caused by the float needle sticking and allowing fuel to spill into the cylinders.

Also I would check the timing just to be sure - sometimes ignition timing can cause a carb backfire and spit raw fuel into the airbox.



Edited by - Geeto67 on Oct 14 2006 09:12:32 AM
 

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Discussion Starter #3
Tank and petcock is fine
as you say it could be the carbs, I have had them apart and they all seem fine, I think the next thing is to replace the float valves just in case they are sticking

Thanks

Any other idea's ????????????????????????????????????
 

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Wait......you have fuel in the CRANKCASE? Wow....well, that points to a few things as far as I can see....

1) Rebuild your petcocks
2) Rebuild your carbs
3) Check/replace the rings on your pistons

Ouch....

Lead, follow or get out of the way!
 

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Discussion Starter #5
I think the carbs are the next thing to try! Thanks

Any more suggestions ?????????????

I will keep you posted, just in case any others have problems
 

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Hold on a second here. I think the XS400 uses a vacuum operated petcock. If the diaphram is leaking or installed improperly it will dump fuel straight into the intake, bypassing the carbs entirely. The first thing to do is disconnect all the lines from the petcock and see if thre is fuel leaking out of any of the connectors. If not then apply a vacuum to the vacuum fitting and see if fuel flows or if it flows into the vacuum line. If the petcock checks out ok then look to the carbs. Yamaha carbs have a fitting on the floatbowl drains that will take a piece of lar fuel line so youcan check the flaot level without pulling the carbs.Yamaha had a special tool part umber for it, but I have used a piece of the appropiate sized clear fuel line with a mark on it to match up to the gasket mating surface. Check both carbs and see if the float level is the same and lower than the gasket mating line. If one is higher than the other check that carb first. A little logic and diagnosis goes a lot further than throwing time effort and parts at it.
Of course that is just my opinion and I might be wrong.

Ken

AHRMA 412
Vintage racing - old guys on old bikes
 

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you can suck on the line to "apply a vacume" it should push gas out of the tank through the undone line. if you suck on the incorrect one, it should do nothing. (i think im correct on this but its been a while). if youre getting a fair amount of gas into the cases, its got to be getting alot of gas into the cylinders. so for some reason there is excess gas getting in there. id look for the vacume issue first.

jc
 
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