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Austin Big Seven carburreter
#1
My carburreter is a Zenith downdraft 30 vm-4 and the car is a pig to start. We thought we had traced this to a worn out mechanical fuel pump as the flow was slow. After replacing it with a new one the flow was really good and it seemed better but if left for a few days it is still very hard to start. I have found that if I remove the float chamber and fill it it starts immediately. I have also replaced the flow valve in the top of the float chamber. Any ideas?
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#2
(17-12-2017, 11:51 AM)Thowdfella Wrote: My carburreter is a Zenith downdraft 30 vm-4 and the car is a pig to start. We thought we had traced this to a worn out mechanical fuel pump as the flow was slow. After replacing it with a new one the flow was really good and it seemed better but if left for a few days it is still very hard to start. I have found that if I remove the float chamber and fill it it starts immediately. I have also replaced the flow valve in the top of the float chamber. Any ideas?

Porous float chamber?.Does the pump have a primer lever, if so does priming prior to start make a difference?
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#3
(17-12-2017, 02:32 PM)Zetomagneto Wrote:
(17-12-2017, 11:51 AM)Thowdfella Wrote: My carburreter is a Zenith downdraft 30 vm-4 and the car is a pig to start. We thought we had traced this to a worn out mechanical fuel pump as the flow was slow. After replacing it with a new one the flow was really good and it seemed better but if left for a few days it is still very hard to start. I have found that if I remove the float chamber and fill it it starts immediately. I have also replaced the flow valve in the top of the float chamber. Any ideas?

Porous float chamber?.Does the pump have a primer lever, if so does priming prior to start make a difference?

No the float chamber is fine - no fuel inside. Yes there is a priming lever and next time it's cold I will try it. However, today I took the float chamber off and turned the engine over and the new pump is gushing it out. I then took the carb off and checked the throttle spindle for wear but none there. It's as if it needs a higher level of fuel but there's no adjustment for that and if I leave the washer off the fuel cut off valve it doesn't seat very well.
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#4
Have you made sure that the choke is fully closed especially this time of year? I had the same problem with a zenith downdraught. I missed the fact that it wasa tiny bit open even when the choke button on dash was pulled fully out. The cable had slipped slightly on the lever connection.
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#5
With 30VM's, in fact any high level carb and mechanical pump, I find a few strokes of the fuel pump priming lever usually gives an instant cold start, after that no issues. As Dave says a fully closed choke helps and it is also worth checking the idle circuit is clear, they can sometimes be blocked and cause problems. If you lack a priming lever a couple of turns on the handle prior to turning on the ignition will have the same result.
Black Art Enthusiast 
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#6
What about the old favourite, stale petrol? It doesn't take long for it to go 'off' these days. A drain and refill with fresh might do the trick.
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#7
I have the filter off so I can see the choke and it's definitely closed. Definitely not stale Petrol as I have just thrown the old away and put fresh in. I am interested in the "idle circuit" as I don't know what that is. Could you explain please? I am sur the clue is that if I fill the float chamber it starts immediately.
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#8
(17-12-2017, 09:32 PM)Thowdfella Wrote: I have the filter off so I can see the choke and it's definitely closed. Definitely not stale Petrol as I have just thrown the old away and put fresh in. I am interested in the "idle circuit" as I don't know what that is. Could you explain please? I am sur the clue is that if I fill the float chamber it starts immediately.

It is a small drilling in the carb body that exits just below the throttle butterfly it bleeds fuel air mixture when the butterfly is closed and is connected to the mixture screw, they can become blocked and may be linked to your issue. It may simply be that the fuel drink from pipe to the carb, and small amount of fuel evaporates from the float chamber, all then need to be recharged in order for a cold start mixture to be rich enough. if after priming with the pump lever it starts instantly I suggest the latter.
Black Art Enthusiast 
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