Vacuum Taps.

[Moxey]

Simple question really, at a constant high speed of 70+ mph is it possibe for a vacuum petcock to fail provide enough fuel to a 4 cylinder bike (i.e. Bandit) and subsequently lead to loss of cylinders firing due to fuel starvation eventually resulting in the engine stalling?

If so I would like to know why and any known solutions.
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Current: 02 FZS 1000 & 91 Royal Enfield Bullet 350 Some Bike Pics...
'I was too busy paying attention to pay attention'

[Bezzer]

Should be fine there are plenty of bored 1216 Bandits that still run the standard tap, let alone for a standardish 600 motor running at a conservative 70mph (same tap). You've got to have a pretty radical motor to have to uprate to a Pingel tap or similar for the extra flow.
One problem could be a partially blocked breather not letting the tap flow to it's maximum , be worth checking/cleaning the breather and overflow pipes to make sure.
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I used to be indecisive but now I'm not quite so sure.

[temeluchus]

or the diphragm has holed, that will starve the motor. This happened to meon my old gixxer 1100 and on a GPz1100S.
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Some shite cruiser. Now with guns and FREEDOM!

[Moxey]

Well renewed the diaphragm and other components of the tap assembly just over a week ago, tank breather has been cleared out (previous owner had painted the cap and clogged it) but even so opening the fuel cap doesn't lead to a hiss or any sign of decompression.

Bike runs fine up to 60mph but any speed over that for a prolonged period (motorway) and the bike suffers from what seems like fuel starvation (had the plugs out they looked pretty lean), I'm lost off as to what can cause the problem.
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Current: 02 FZS 1000 & 91 Royal Enfield Bullet 350 Some Bike Pics...
'I was too busy paying attention to pay attention'

[Bezzer]

Float heights all OK??
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I used to be indecisive but now I'm not quite so sure.

[Casper]

Are you sure you cleaned out the cap vent fully. If you have a spare key stick it in the cap and go for run, open cap when bike is starting to run out. If it picks up again then air vent is your problem.

[philoldsmobil...]

[P.addy]

If there a prime option on the tap?
If so my GS500 had a dodgy tap... which turned out to be both the vacuum pipe and the diaphragm and only screwed up after 60ish on the motorway.

PRI on the fuel tap eliminated it...but did like to piss petrol away when left standing

[Moxey]

Float heights have been checked quite recently (had the carbs off more than a few times) they're at 16mm atm which would be slightly richer than recommended but still acceptable, also set the pilot screws to 2 and half out which is a half turn more than recommended again would make the mixture richer but instead i'm experiencing the opposite (don't know how they would have any effect on the bike beyond tickover but worth mentioning).

Fuel tap has prime setting but then I get the opposite problem at low speed the cylinders sometimes flood and the plugs fouled after running it for a week like that, already replaced most of the components of the fuel tap that can be replaced (also cleaned the filters which somebody mentioned) so thinking it can be ruled out and something on the carbs I don't know about is the cause but what?

Another thing to mention is after the bike died on me recently going along the A66 after cruising about 75, I whipped the plugs out, looked like it was running lean and there was still a spark, yet put the tap on prime for about 30 sec just to get some fuel in the bowls which I thought had run dry then tried to start it and it just would not fire up.

Eventually me and the mate I was with had to bump and electric start it to get it going (considered block jets but they've been checked recently and fuel tank isn't rusty, filters clean on the tap etc so doubtful of a blockage developing since I checked).

Bike has got me genuinely

*Recommended float heights and such taken from Haynes Manual.
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Current: 02 FZS 1000 & 91 Royal Enfield Bullet 350 Some Bike Pics...
'I was too busy paying attention to pay attention'

[hmmmnz]

as you are riding and the promlem occurs, flick over to prime on the tap,
if it starts running sweet again, then i'd be looking at you vacum line from the carbs to the tap,
if it stays the same and doesnt change anything, then i'd be looking at main jets,
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the humans are dead
I kick arse for the lord
Wiring Diagrams BIDNIP it bitches

[Bezzer]

You're going to think me stupid, but hear me out

You seem to have checked everything out that may be causing a problem but reading the previous ...

[Pete.]

He ain't getting carb icing at 70mph coz I'm not getting it at night going much faster

If you want to test your tap, just turn it to PRI as hmmmnz said, which by-passes the vacuum part. Sounds like either a blocked gauze filter or blocked tank breather, as others have said.
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a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[Bezzer]

[Moxey]

Well I have got my carb heaters dis-connected (due to bad wiring and didn't think they'd be needed this time of year), I'd be inclined to look into that though, think I might get some isopropanol or rewire the heaters up when I get chance.

Going to test the bike today though, I've adjusted the pilot screws again to richer mixture and going to see if any effects from that area.
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Current: 02 FZS 1000 & 91 Royal Enfield Bullet 350 Some Bike Pics...
'I was too busy paying attention to pay attention'

[philoldsmobil...]

[Moxey]

^ Yes the Carbs shouldn't flood (only on Prime mind) but checked float heights and the valves are all in working order so bit exhausted on ideas in that area but the problem i'm getting now is opposite so looking elsewhere, also speaking of the GPZ series my dad reminded me of a friend of his who had the either the 750 or 900 (not familiar with them) and bent a con rod due to the mentioned problem makes sense to have the carb heaters I guess.

Ran the bike today and it's been fine barring didn't take it above 70 for extended periods and it was abit of a git starting up again when I pulled up to talk to somebody for barley 5 min (started on 4th or 5th attempt so not as bad as before when I had to bump it).

Managed to get in touch with somebody else who has a Bandit 6 who has his carb heater disconnected as well, his runs spot on he's referred me to somebody who's supposedly shit hot when it comes to carb's and fuelling so might as well let him have a look.
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Current: 02 FZS 1000 & 91 Royal Enfield Bullet 350 Some Bike Pics...
'I was too busy paying attention to pay attention'