Petcock knock

[TomB]

Question for the petcock experts

I love the look of the European Sport with its small tank but its crap for touring so fitted a 2H9 tank, I stripped the vacuum petcocks and cleaned them out.

Recently I fitted new vac pipes as the originals were hard as hell, I made them really long to make tank removal easier, tank can lift up a LOT with the pipes still connected to make it easier to disconnect them.

Some time after fitting the new longer vac pipes I noticed a really bad knocking from the top end of the bike, funny though coz it sounded like a bottom end knock like a big end on its way out, putting my ear on the tank seemed to amplify the knock. When I say a knock, I mean I'd pull up at traffic lights and mates I'm riding with would notice the knock! Then it dawned on me, it wasn't the engine knocking at all, it was the vacuum petcock opening and shutting with each pulse of No.3 cylinder, I confirmed this by putting my ear on the tank and nipping the vac pipe, the knocking would suddenly stop.

Well fantastic news, my engine isn't wrecked ......I shortened the vac pipe thinking having it long may create a bigger pulse, it knocks less, I'm assuming its opening a closing all the time which won't do it any good over a period of time.

I've been told that there are little non-return valves in the brass spigots in the inlet manifolds, but I've never noticed any. Is this correct?? coz it would sort out the problem if I could renew them.

To me the fact it is knocking means the tap is losing its vac, which is being re-vacuumed up with each pulse of No.3 cylinder hence the knocking. So have I an air leak on my tap?? which will have an effect on No.3 cylinders mixture??.
Does this sound right? could a holed diaphragm cause this?

[donebysunday]

Just a thought, could it be the tank bumping the frame ?

Question for the petcock experts

I love the look of the European Sport with its small tank but its crap for touring so fitted a 2H9 tank, I stripped the vacuum petcocks and cleaned them out.

Recently I fitted new vac pipes as the originals were hard as hell, I made them really long to make tank removal easier, tank can lift up a LOT with the pipes still connected to make it easier to disconnect them.

Some time after fitting the new longer vac pipes I noticed a really bad knocking from the top end of the bike, funny though coz it sounded like a bottom end knock like a big end on its way out, putting my ear on the tank seemed to amplify the knock. When I say a knock, I mean I'd pull up at traffic lights and mates I'm riding with would notice the knock! Then it dawned on me, it wasn't the engine knocking at all, it was the vacuum petcock opening and shutting with each pulse of No.3 cylinder, I confirmed this by putting my ear on the tank and nipping the vac pipe, the knocking would suddenly stop.

Well fantastic news, my engine isn't wrecked ......I shortened the vac pipe thinking having it long may create a bigger pulse, it knocks less, I'm assuming its opening a closing all the time which won't do it any good over a period of time.

I've been told that there are little non-return valves in the brass spigots in the inlet manifolds, but I've never noticed any. Is this correct?? coz it would sort out the problem if I could renew them.

To me the fact it is knocking means the tap is losing its vac, which is being re-vacuumed up with each pulse of No.3 cylinder hence the knocking. So have I an air leak on my tap?? which will have an effect on No.3 cylinders mixture??.
Does this sound right? could a holed diaphragm cause this?

[DAVINCI]

The diaphragms are open to atmosphere on the other side of the vac nipple, otherwise they wouldn't open at all. Restrictors, like the ones used in vac gauge lines may help reduce the knock considerably.

Another consideration is that your carbs are out of synch, that can cause a wised vac swing at the source nipple.

[DAVINCI]

That should read WIDE vac swing, damned timeouts

[bikerphil]

Yep, I had a knock in my Pingel vacuum shutoff valve, the unit was pulsing at low engine RPM and the valve ended up destroying itself. I had it rebuilt and installed a restrictor in the vacuum line and all is well. It sounds like the same thing is happening with your vacuum shutoff fuel tap. Try a type of restrictor like Randy mentioned. GL

[TomB]

Thanks lads, I have a spare old pilot jet I could put in the vac pipe.

I'm going to rebuild it this weekend, I want to find out if the diaphragm is holed, I don't want to be pulling in air into No.3 cylinder, I have Germany trip coming up soon.

[TomB]

I checked out the petcock, all was good, working Ok.

I used a small length of hose and a surgical syringe fitted to the vac spigot and operated the tap by pulling the syringe, held it there for a while then let go, the vac pulled the syringe back to the same position when started, so no air being dragged into the inlet through leaking diaphragm.

So I fitted an old pilot jet into the vac pipe and the knocking has gone and the petcock works fine.

Thanks for the tip lads.