Fuel - starting issue

[MickC]

Bike: 1981 CB750 Custom
Engine: 1979 CB900Fz (with CB1000 cams)
Carbs: VB51cv.
6 month/2000mile NOS barrels and pistons/reconditioned head.

The bike has started to refuse to start unless the fuel tap is turned to reserve first. It will then start and the tap can be returned to On (after about two minutes or the bike will 'stutter' at low revs). This only happens when the engine is stone cold.

Carbs are clean and well balanced.
Charging system is spot on (coil relay mod for the coils)
Iridium plugs (6 months old and about 2000 miles)
Inlet and outlet valve clearances are all good.

Have dismantled the tap and there appears to be nothing wrong with it. The screen is clean. It also has an inline fuel filter which is also clean.

Any Ideas?
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Previous: CG125,XS250 (converted to 400),Z250A, GPZ550, Norton Commando, XS500, 78 Laverda Jota, GPZ1100,Harris bonneville, XV750 (mono shock),TR1

[nightshaddow]

maybee im stupid but i personally have had nothing but problems with irridium plugs,

for the case of £10 for 4 i would switch them out for standard nkg units

in 7 different bikes (3 of my own and 4 of other peoples) ranging in size from 125cc to 800cc all i did was switch out the plugs and the problem was solved they all had similar issues which seemed to be fueling,

all of them seem to last between 1000 and 3000 mile aprox though when i had it the skyjet one lasted 500 miles befor it gave me problems
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"WHAT DO YOU MEAN 100 MPH THATS IMPOSSIBLE Its chinese OFFICER."
Current rides CB 125 TDE(the next project) NS125RK ( now on the road ) CB125TDC (soon to be project 17) melody ( the nephews )

[Vincent]

Have to agree about the plugs. They weren't around when the bike was made and I doubt it ever had a plug problem. I wonder if the 750 tap is smaller than the 900 one? Does it just misbehave when choke is "on"?
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Space Is Deep

[MickC]

[Vincent]

You'd have thought that it has to be something to do with the tap. Maybe the reserve is a bigger bore, although I can't see how
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Space Is Deep

[Vincent]

Are all the carb vacuum take offs sealing properly?
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Space Is Deep

[YBR Ric]

The reserve will have a greater head of fuel than the main tap, supplying a slightly higher fuel pressure down stream.

Possibly just enough to dislodge float valves which have stuck in their seats perhaps?

[Fizzer Thou]

Is the tank breather in the fuel cap free flowing?

Try starting the bike with the tank cap open instead of starting from reserve.

I have NGK Irridium plugs in my KTM enduro and my R1 with no problems at all,even after several thousand miles on the bigger bike.But I have had problems with the internal resistor of the plug caps breaking down.These serve to keep electrical noise to a minimum but when hot they do cause all sorts of difficult to diagnose problems with the engine if they are in the process of breaking down.
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Just talk bikes.What else is there?

Always have a 'Plan B'

[MickC]

[Vincent]

Nice one
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Space Is Deep

[finpos]

The problem is that the diaphragm in the tap has become stiff with age. It no longer opens the fuel tap under vacuum as readily as it used to do. You'll probably need to replace it (if you can get one)

f.

[temeluchus]