85 VF500 Petcock Diaphragm question

Discussion in 'Mechanics Garage' started by Alan1234, Jun 6, 2021.

  1. Alan1234

    Alan1234 New Member

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    I recently bought a running 85 VF500 that Im trying to sort out the diaphragm and vacuum hose setup on the petcock.

    First I guess is does anyone know what the diaphragm does? Is it adding pressure to the fuel line? Second the petcock has three nipples. One is the fuel line and the other two are the vacuum line to the number two cylinder and the other is an air vent?

    My problem is when the engine is warm and I hold it steady above 5-6,000 rpms it seems like its not getting fuel. It runs great but theres a hesitation, consistently, every couple seconds. Almost like I took my hand off the throttle for second then it goes back to normal.

    Right now the vacuum lines are just capped off. If I take the caps off fuel pours out like they're fuel lines. If I replace the diaphragm and get the vacuum line sorted out will that get more fuel to the carbs? Or is it more likely the float levels arent correct? I just used a ruler to set them but they should be close. Also it runs great when its warming up so thinking they should be fine.
     
  2. Captain 80s

    Captain 80s Member

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    The nipple off the back is the vacuum. The one off of the center is a drain to let you know when the diaphragm has failed (fuel drip). Either research how to eliminate the vacuum operation (recommended) or replace the diaphragm. Not saying this is THE problem, but it is A problem.
     
  3. Alan1234

    Alan1234 New Member

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    Is the diaphragm a shut off? or does it add pressure to the line?

    I'm thinking that the hesitation is from not enough fuel or line pressure but if its a shut off then that shouldnt be it. Its every couple seconds and has a consistent rhythm to it, like every second or two and only when its fully warm and cruising at higher rpm.

    Could the fuel pump relay be shorting when it gets hot? Or maybe the fuel pump is getting worn and not gone yet but getting ready to go?
     
  4. Captain 80s

    Captain 80s Member

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    When vacuum is applied to the diaphragm it allows fuel to flow, so essentially a shut off if the bike is not running or being cranked. The pump (if it still has one) is responsible for flow.

    Lots of possible causes. Get this issue sorted and move on if needed.
     
  5. Alan1234

    Alan1234 New Member

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    Thanks for the help Captain. I got a little further. Was thinking it could be the fuel pump or relay so just ran the fuel line straight from the petcock to the carb. At first it ran great, better than before, but after about 30 min in traffic it just died. It started right up but I had to keep the throttle a little open for it to stay running. It also felt like it was bogging down when I accelerated. So I pulled off and let it cool down for about 10 min and it ran ok at first but once it got warm again if the idle got below 1500 or so it would just die. It would start back up but bogg down pretty bad when I accelerated.

    So got home and was thinking it could be the diaphragm. Opened it up and turns out the diaphragm was already eliminated.

    I know fuel is getting to the carbs. The carbs were recently cleaned and it runs great when its cold so dont think its them. Also dont think its compression since it runs fine when its cold. That leaves spark. If its the CDI or coils shorting I cant figure out why keeping the rpms up would affect it though.

    Is there a way to test the CDI? Or do I have to test the coils and pulse generator and if not them just assume its the cci?
     
  6. Captain 80s

    Captain 80s Member

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    Sounds like the pulse coils under the right crankcase around the starter clutch. CLASSIC problem when bike gets hot. There's another thread going on right now about likely the same issue on a 500.
     
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  7. Alan1234

    Alan1234 New Member

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    Turns out the screws that hold the bottom coil to the bracket came loose. The coil was just hanging almost on top of the valve cover. Tied it back down and its running like a dream again
     
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