Carb problem

[Tony Griffiths]

Zenith 30VM downdraft carb (similar to that on a Nippy). The Ulster was running really well when it slowed and would not rev, only tick over. Checked that the fuel pump was working and fuel getting through. Removed the float bowl and the three jets, which were all clear. Blew the jets out anyway - and the holes in the bowl. Reassembled - and it revved cleanly. Has anyone had a similar problem or knows what the cause of this sudden-tickover situation might be?

[Tiger]

My advice would be check the ignition system

[Tony Griffiths]

My advice would be check the ignition system

Ran over that, too (especially with the previous problems) but a good fat spark at the plugs and from the coil, etc.

["Slack Alice" Simon]

Condenser starting to fail / earth intermittent to same?

[Howard Wright]

Hi Tony

I had a similar problem some time ago. It turned out to be a ‘floater’ in the fuel line. Tickover was fine but any draw on fuel would pull the offending bit of grit up the pipe and block the filter on the carb. The problem was that it was intermittent and happened just after I got a distance from home. Advice… blow through all the fuel lines.

Cheers

Howard

[Tony Griffiths]

Hi Tony

I had a similar problem some time ago. It turned out to be a ‘floater’ in the fuel line.  Tickover was fine but any draw on fuel would pull the offending bit of grit up the pipe and block the filter on the carb. The problem was that it was intermittent and happened just after I got a distance from home.  Advice… blow through all the fuel lines.

Cheers

Howard

Thanks Howard - that's a very good idea.

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Condenser starting to fail / earth intermittent to same?

Thanks. I've had lots of interesting condenser problems, so I carry a spare with wires fitted with clips, ready to try (plus, of course, a complete distributor and a bag of electrical bits and pieces).

[dickie65]

Hi Tony,
Were you running at high speed then got a sudden loss of power and misfire ?
I have had the above symptons which cleared after ticking over for a short while.
In my case I found out that it was the carb icing up.
I strapped a digital thermometer probe to the carb and went for a drive and all became clear as the temperature plummeted, I actually managed to stop and open the bonnet to find the outside of the carb covered in frost.
It might be worth trying a carb with a slightly larger choke and jets.
The Nippy manifold has a hotspot and it still got frosted up.
My car would repeat its misfire trick at the same distance along the road from home give or take 100 yards.
Now running a semi downdraft SU with no problems

I hope this might give you additional ideas.

Cheers Richard

[andrew34ruby]

Hi Tony,
Were you running at high speed then got a sudden loss of power and misfire ?
I have had the above symptoms which cleared after ticking over for a short while.
In my case I found out that it was the carb icing up.

Carb icing can be quite amazing, has to be seen to be believed. It happens on cold days but especially on cold damp days. Never mind a bit of frost on the outside of the carb, I have seen cars where the intake of the carb is a large ball of ice.

Not ice this time, but a cortina that a friend was foxed by had a dead fly floating around in the float chamber.

[Tony Griffiths]

Thanks dickie65 and andrew34ruby - that's a good idea but not the case here. The next day, with a stone-cold engine, the situation was the same - tickover but no revs. I have a Chummy with a SU carb and 4-branch exhaust - an arrangement that lacks a hot spot - and that suffers from carb icing. The previous owner thought that the misfire might be because the petrol was boiling, but when I looked during a run, there it was, a carb covered in ice. The solution was to wrap copper wire around two of the exhaust pipes, and from there around the 'inlet manifold'. Problem solved. As a flight instructor once told me when we were discussing carb icing in light aircraft - it's always winter inside a carb.  Even at temperatures exceeding 30 °C on humid days, the drop in temperature between the inlet and the venturi can lead to icing.
Incidentally, many pilots have lost the plot when, suffering from icing, they've switched to hot air and despaired as the engine ran even more roughly as the ice melted. In a panic, they turn the hot air off, only for the engine to stop and refuse to restart. At least we can park at the side of the road....

[A G Wood]

Not ice this time, but a cortina that a friend was foxed by had a dead fly floating around in the float chamber.

My all-time favourite was a playing-card sized piece of torn clear plastic bag floating in a truck petrol tank (it was a while ago) intermittently blocking the pick up pipe in the tank when the fuel level was less than 1/4 full, then helpfully floating away again once the engine had stopped...