SU carb overfilling

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ndevans
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SU carb overfilling

Postby ndevans » Sun May 24, 2020 5:33 pm

Hi all.
I'm fitting an HIF44 to my 1275, in place of the HIF38. I have rebuilt it with new seals, jet needle, float chamber needle valve and seat.
The problem I have is that I can't stop the carb overfilling, despite following the Burlen website guide on setting the float height (1mm +-0.5 below the bowl rim measured in the centre of the float).
I have tried swapping the new needle valve for the old one, and it still overfills.
I am using a standard SU type L fuel pump which has given good service for over 20 years, and with the ignition on, it clicks away for several seconds, then clicks once every 15 seconds or so, until eventually it overflows out of the overflow pipe. I have had the float out and am happy that it is not leaking and filling with fuel.

Any ideas anyone? I daren't risk starting the engine if fuel is going to spill over a hot exhaust!

Cheers N
cheers N
'69 Traveller, 1275, discs.

newagetraveller
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Re: SU carb overfilling

Postby newagetraveller » Sun May 24, 2020 6:55 pm

I would start by taking the top off the float chamber, disconnecting the pipe that goes to the fuel pump and making sure that if you blow down the tube on the top of the float chamber you can stop the flow of air by pushing up the little lever inside the top of the float chamber.

ndevans
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Re: SU carb overfilling

Postby ndevans » Sun May 24, 2020 8:47 pm

Thanks. Good tip. It's an HIF, so the float chamber is integral to the carb body.
I got a piece of rubber pipe, connected it to the fuel inlet, and tried sucking on it. I couldn't maintain a vacuum in the pipe, either with the new needle valve, or the old. I tried the same trick on the HIF 38, and had no problem maintaining a vacuum.
I then tried the same thing on both carbs, but with the needle valve removed and a finger plugging the hole. Again, I could maintain a vacuum in the HIF38, but not in the 44.
I then removed the valve seat, checked it for dirt and machining irregularities, found none, and reseated it. This time I had no problem maintaining a vacuum. I was also able to maintain a vacuum with a needle valve in the seat.
I think the problem was that the valve seat was not screwed tight enough into the carb body. Haven't tried it with a float and connected to the fuel pump yet, that's a job for tomorrow. 🍺 o'clock now!

Cheers N
cheers N
'69 Traveller, 1275, discs.

ndevans
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Posts: 820
Joined: Fri Mar 03, 2006 11:56 am
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Re: SU carb overfilling

Postby ndevans » Thu May 28, 2020 3:43 pm

Success, it no longer overfills when connected to the fuel pump.

I have now got it running, but it's a bit rough and lumpy, not idling smoothly. I've only run it for a few minutes, more or less with the carb at the initial settings recommended in the SU tuning guide (mixture control adjusted so that the jet is level with the carb bridge, then 2 turns clockwise, idle screw wound 1½ turns further from just touching the throttle cam). The carb needle is a BFY, as recommended by Burlen.
It idles too fast without an air filter on, and sounds rough and lumpy. I have checked the points gap, and although I haven't checked the static timing, it hasn't been changed from when it had the HIF38 on. I have also checked and adjusted the valve rocker clearances. I have a water leak to attend to before I can run it much more, but I'd appreciate some advice on where to start in getting it running smoothly please!
My engine is a 1275cc with big valve head, Metro inlet, 3 branch maniflow exhaust and HIF44 carb.

Cheers N
cheers N
'69 Traveller, 1275, discs.


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