86/87 Petcock Operation

[MarioR]

Guys,

Sorry if this is an obvious question, but I want to understand how the petcock operates on an '86/'87 model.

The petcock is vacuum-operated, and the electrical connection is only for the simulated reserve.

So if the tank is removed from the bike and the wire for the reserve valve is disconnected, should fuel leak from the petcock or not?

From what I understand, until vacuum is applied, no fuel should flow. Right?

Thanks.
 

[RPM - Robert]

Correct on the 86/87 model it should not flow any fuel until vacuum is applied. If it is flowing fuel the petcock is bad.

The 84-85 has a prime function which will flow fuel when the selector switch is in the prime position but in the on position it should not flow fuel until vacuum is applied.

[MarioR]

Correct on the 86/87 model it should not flow any fuel until vacuum is applied. If it is flowing fuel the petcock is bad.

The 84-85 has a prime function which will flow fuel when the selector switch is in the prime position but in the on position it should not flow fuel until vacuum is applied.

Thank you Robert.

[Warp84]

Correct on the 86/87 model it should not flow any fuel until vacuum is applied. If it is flowing fuel the petcock is bad.

The 84-85 has a prime function which will flow fuel when the selector switch is in the prime position but in the on position it should not flow fuel until vacuum is applied.

Thank you Robert.

I recently had to get a rebuild kit from RPM for me 84 because my tank started marking its territory when I took it off the bike. Put on the rebuild kit and it seals it right back up.

[MarioR]

Correct on the 86/87 model it should not flow any fuel until vacuum is applied. If it is flowing fuel the petcock is bad.

The 84-85 has a prime function which will flow fuel when the selector switch is in the prime position but in the on position it should not flow fuel until vacuum is applied.

So looks like I still have some issues with gas supply.
Petcock does not leak so I assume membrane is still good/or not??? However, starving for fuel - means issue with vacuum??? Vacuum hose looks good so solenoid doesn't work???

Thanks

Thanks

[T Legg]

It could be that your fuel line isnt routed correctly and is getting pinched off.

[Pat Conlon]

It could be that your fuel line isnt routed correctly and is getting pinched off.

I agree with Travis...pinched fuel line would be my first choice.
It happens often,  that's why I have it in my signature line.

If it's not the fuel line, then it's that wonky electric reserve 86/87 petcock.
 I would throw it away and put in a new oem 84/85 petcock from RPM. It's plug and play.

[red]

MarioR,
Mouth suction on the vacuum line should be enough to make the petcock flow. Block the vacuum line, holding the vacuum, and the petcock should continue to flow. Otherwise, the diaphragm (or the vacuum line) is leaking. Releasing the vacuum should stop the flow of fuel.
Personally, I would replace a malfunctioning petcock, rather than repairing it. It has done enough years for you already. A new run of vacuum line to the new petcock would not hurt, either.
YMMV.

[MarioR]

It could be that your fuel line isnt routed correctly and is getting pinched off.

Fuel line is new and direct to Flat carbs, Mikuni's 38 so no pinch line.

[MarioR]

MarioR,
Mouth suction on the vacuum line should be enough to make the petcock flow. Block the vacuum line, holding the vacuum, and the petcock should continue to flow. Otherwise, the diaphragm (or the vacuum line) is leaking. Releasing the vacuum should stop the flow of fuel.
Personally, I would replace a malfunctioning petcock, rather than repairing it. It has done enough years for you already. A new run of vacuum line to the new petcock would not hurt, either.
YMMV.

Will play with it today and if no good result, as Pat said, will order new manual petcock 84/85.

Thanks

[RPM - Robert]

Is it starving all the time or only when hot? If when hot only have you done the flappectomy?

[MarioR]

Is it starving all the time or only when hot? If when hot only have you done the flappectomy?

Hi Robert,

Starving all the time. Basically engine trying to start but starving on fuel so dies immidiatelly. Petcock doesn't leak.
If I feed fuel directly, runs like a champ. So petcock issue or suction to petcook.