Revving problems

[GrahamFountain]

I once had a problem with some rust in the fuel line between the carbs causing a problem a lot like you describe. However, I had found that when the engine was refusing to pull or rev under load, pulling the choke out made it better for a short time.

I’m not absolutely sure that the problem was as described by the garage. But they reckoned that a substantial flake of rust was lifting up and partly blocking the feed to the front (I think) carb, but only when there was enough flow. This wouldn’t stop the engine revving with no load, but made it effectively a two cylinder engine when there was. I suppose that, had I been able to stop the engine immediately, when the problem was there, I might have seen the effect on the plug colours. But since I only pulled the plugs a fair bit after, when it had been running at low load for a while, I never did.

Whether their analysis was right or not, cleaning out the steel line under the carbs got rid of it entirely.

Graham

[red&black]

I dont see any mechanism for pressurising a Dolomite fuel tank. Only sucking petrol out if the vent is
blocked will cause a loss of pressure, not an excess.

Jonners

It happened on my old 1500 to such an extent that the tank actually ballooned outwards slightly.Fuel was pushed out through the float chamber overflows until the filler cap was removed to release the pressure.
Fixed by replacing the fuel filler cap.

James

[AlanH]

Would the inside diameter of the rubber fuel pipes make any difference?

I have fitted 8mm up to the metal pipe and 6mm on each of the branches into the carb. float bowls.

[Jon Tilson]

That will be fine.

Have a read of this...

https://www.dollywiki.co.uk/wiki/SU_Carburetor_Tuning

Have you identified your fuel pump as the normal one?


Jonners

[AlanH]

The fuel pump is a replacement mechanical one that was on the car when I got it. It looks like the correct Rimmer's part. They only do the early one so I assume that this is OK.

Given Graham's comments I will try replacing the metal tube with some sort of temporary set up. I have also ordered a set of gorse valves for the bowls.

The MOT issued a warning about the fuel lines being rusty so, since I intended to get under and check for any corrosion, I will replace the fuel pipe and then carefully set the carbs up.

[Jon Tilson]

Can I suggest you put an inline cheap fuel filter just on the inlet to the pump.
This will soon tell you if you have muck in the fuel system.

Does the pump have a removeable top cover held on by a screw? If so take this off and you'll see a small
filter disc which you can clean. If its gunged up that will also tell you a bit about the state of the fuel.

Grose jets are IMHO a waste of time. I put them on my Stag and they made no difference.

Just tell if your own float valve is working. Take off the top cover of the SU and just blow down the tube as you raise the valve float. You can easilly tell if the valve shuts off as you are blowing. If it does its doing its job. Also worth a look...is your float actually floating? They do get pinholes and sink.

The fuel rail between the carbs is only 18 inches or so. There is no way this will be so rusty on the inside as to crud up your float valves after you've given it a good clean out. If it was it would fall to bits in your hand.

Jonners