MG MGB and MGB GT Tech Talk
Hi all,
I'm joining the list with fuel problems. I just recently swapped in a set of rebuilt HS4's. I've been battleing fuel rushing out of the rear float. I just replaced both needle valves and the rear float with new pin, but it's still pouring out! I also cleaned the chambers before putting in the new parts. So I'm at a loss as to what's causing this. Could the original fuel pump supplying the stromberg be putting out too much pressure for the SU's?
Could be your fuel filter. Was that changed, too? If so, maybe the old one acted to slow down the flow of fuel more than the new one.
What do you mean by fuel "rushing out of the float"?
An SU expert will be along any minute.
Maybe something will grab your attention from my "Seafoam" thread just below, Ron.
Paul
maybe you hooked the fuel line up the the wrong tube on the carbs?
it happens!
I hope you took time off for Gary's run.. fix the carbs and join the drive!
Deniz Wrote:
Misadjusted float?
What type float needle setup are you using?
Ron - "Could the original fuel pump supplying the stromberg be putting out too much pressure for the SU's?" When you say "original fuel pump" do you mean a SU fuel pump, or some aftermarket pump that came with the car when you purchased it? It is impossible for a SU fuel pump to put out too much pressure. The have a volute spring to push the fuel out and the spring is set for the pressure that the pump is specified for. For the MGBs, the pump pressure is 2.7 PSI. The highest pressure you can get in a SU fuel pump is 3.8 PSI, which will not cause the problem you are experiencing.
If you have an after market pump installed, then it is entirely possible that the pump is putting put excessive pressure (in excess of 4 PSI).
Cheers,
To my knowledge, it's the original pump or a factory replacement. It's in the exact same location in the trunk, covered by a black cover.
I replaced the rear float with a new one to try to fix this. As well as a new pin, new needle valve and brass washer. Octagon(our local parts dist.) only sells the brass washers, no fibre option. If anything, the front float sits a little closer to the float cover face when tipped upside down than the rear so if there was any problems with too much fuel you'd think it'd be the front one.
I'm wondering if these SU replacement needle valves have too heavy of a spring in them?
Later today, I started the car up to get it back into the parkade and once again fuel was shooting out of the rear overflow. As soon as I revved the car up and got it to settle at a decent idle the overflowing stopped. Bizarre...
post a photo of the rear carb fuel input and overflow tubes with the fuel line connected
Will try to get a pic tonight. I have the upper/longer spout connected to the fuel in, lower/shorter spout as the overflow...
Well, time to take the other sportscar for a spin :)
I think your rear carb top should only have two pipes. (Fuel in and overflow) What you have is a front top on the rear carb. (Which has fuel in, fuel out (on the same vertical level, and and overflow below. ) unfortunately your air cleaner is hiding the rest of the top of the carb. Can you reshoot it more vertically?
The 'overflow' you are seeing, I think, is the feed to the rear carb from the front carb which should not be on the rear carb top.
Here are my front/rear setups
Carbs
I think your rear carb top should only have two pipes. (Fuel in and overflow) What you have is a front top on the rear carb. (Which has fuel in, fuel out (on the same vertical level, and and overflow below. ) Unfortunately your air cleaner is hiding the rest of the top of the rear carb carb. Can you reshoot it more vertically?
The 'overflow' you are seeing, I think, is the feed to the rear carb from the front carb which should not be on the rear carb top.
Here are my carb tops - front and rear
Hi! I think you have the wrong "lid" on your rear floatchamber. Put a blocked peace of fuelhoose on the pipe and the problem is solved.
Pixbo,Sweden
Sorry, those are poor pics. Here's a couple more from top view. 1. front carb 2. rear carb:
Ron
Well that proves you have the correct tops and so now you have the same problem that I do... the rear carb is not closing when the float rises in the chamber. And for me it is intermittent. Therefore, it is either a poorly sealing needle and seat (or grosse jet), or wrongly adjusted float height. Those are the only two variables unless your pump is delivering too much pressure..
I am expecting to get new needles and seats and floats today.
Tell us this: if you remove that top and blow into the fuel delivery tube then manually push the float up does the air flow stop? Mine DOES. But under the vibration of the engine it does not always shut off (I think) and the overflow tube that I have routed to the ground (not to the canisters) gets damp and smells of raw fuel.
I personally am terrified of this condition because it is what caused my fire 4 years ago and led to a $23,000 restoration job. Don't want to go there again.
The reason why your overflow does not weep at high revs is that the fuel is being consumed by the carb faster than the pump is delivering it and the level is dropping in the bowl and not coming out of the overflow pipe.
Get some tubing on those overflows and route them to the ground below the exhaust. I 'T'-d them together and routed them with tie wraps to the left front inside of the rad.
I am sure others are facing these same conditions..... and it scares the hell out of me.
I also have a fire extinguisher on board.
Yup, exactly what it happening with mine. Very inconsistant. I had the overflows connected to the charcoal canister so this is why I don't have oveflow tubes, but I will be adding them, at least until I have this problem sorted and I can reroute them to the canister. I will try blowing through the inlet tube and see if I can close of the air supply by activating the needle valve. I'm wondering if these SU brand valves are the problem...
