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Blanking off carb pipes


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Hi,

I've changed my weber carb in my 90 and the new carb had a 'spare' inlet above where the fuel inlet was compared to the old carb.

I'll try an get pictures later on to make it clearer but I connected up the fuel line to one of the inlets and got a short piece of tubing and secured it to the other and clamps it off but I'm now wondering whether that was the best thing to do and what the other inlet is actually for.

If anyone has any info on the purpose and the best way to deal with it then please let me know?

Thanks

Steve

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Hi,

I've changed my weber carb in my 90 and the new carb had a 'spare' inlet above where the fuel inlet was compared to the old carb.

I'll try an get pictures later on to make it clearer but I connected up the fuel line to one of the inlets and got a short piece of tubing and secured it to the other and clamps it off but I'm now wondering whether that was the best thing to do and what the other inlet is actually for.

If anyone has any info on the purpose and the best way to deal with it then please let me know?

Thanks

Steve

Are you sure it's not an overflow or a return to the fuel tank?

A photo would be a help in diagnosing it. Did the new carb not come with instructions or a diagram?

Mark.

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Heres a picture of the inlets/outlets onthe carb.

The tubing on the right goes no where and is sealed off.

The carb was a second hand one so no manual but it's a weber twin carb.

3445839042_30cea12224_b.jpg

Hi Steve,

Definitely a second inlet. Are they both open with the hoses removed? Sometimes a single casting is used for different installations but they're quite often blanked off internally.

IMHO I reckon you can leave it as it is and it won't be detrimental to the operation of the carb. I can't see why you'd have or need two inlets, other than for routing of the supply hose in an installation that's short of space or something like that.

Maybe soemone else knows?

HTH

Mark.

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Cheers Mark,

Yes, they're both open. I found that out when I forgot to block it off and petrol quickly dripped out

The reason I asked is that I have a smell of petrol coming from under the bonnet and in the investigation of said smell and other performance problems I wondered if this has any effect or any general use.

but I think you're right, I'll leave it alone unless anyone has any other reason for it beng there

Cheers

Steve

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Back to the tank...

Some carbs are designed to be run with only a feed, others require a feed and return and possibly a pressure regulator too.

Usually the return drilling will be a smaller diameter, or will have a restrictor fitted to act as a crude regulator.

If you get the model of the carb, you might be able to find some technical details about it online, or from webcon directly.

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Where would a return go to from there?

The fuel tank!

I'd find the part number and have a google for a manual/spec or a vehicle it was fitted on originally, and go by the Haynes or similar, this info must be out there somewhere.

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I suppose it could be a return as Aragorn says.

Possibly, when the needle valve closes the fuel still being supplied to the carburettor gets re-routed to the fuel tank via the hose which is blanked off. It depends what type of fuel pump you have I think. IIRC the old SU type fuel pump used to stop running when it "sensed" an increase in fuel pressure. Not all fuel pumps work in that way and they run continually, hence the need for a return.

My brother in-law's 2.5 petrol 90 of '86 vintage definitely had a return to the tank. It also had a Webber carb but it wasn't like the one in the picture, his had a separate hose for the return, I think it came off the float chamber somewhere. We've recently converted it to 200 TDi and utilised the fuel tank return for the diesel return.

If it is as Aragorn says this could well be the source of the fuel smell you are experiencing.

Mark.

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The fuel tank!

I'd find the part number and have a google for a manual/spec or a vehicle it was fitted on originally, and go by the Haynes or similar, this info must be out there somewhere.

Thanks for the helpful info, I thought it just p**ssed away on the road hence the poor mpg.

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I suppose it could be a return as Aragorn says.

Possibly, when the needle valve closes the fuel still being supplied to the carburettor gets re-routed to the fuel tank via the hose which is blanked off. It depends what type of fuel pump you have I think. IIRC the old SU type fuel pump used to stop running when it "sensed" an increase in fuel pressure. Not all fuel pumps work in that way and they run continually, hence the need for a return.

My brother in-law's 2.5 petrol 90 of '86 vintage definitely had a return to the tank. It also had a Webber carb but it wasn't like the one in the picture, his had a separate hose for the return, I think it came off the float chamber somewhere. We've recently converted it to 200 TDi and utilised the fuel tank return for the diesel return.

If it is as Aragorn says this could well be the source of the fuel smell you are experiencing.

Mark.

Cheers Mark, I'll see if the in tank pump has a return on it anywhere.

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