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MG Midget and Sprite Technical - -76 1500 idle problems

I am a 1275 owner who is helping restore my neighbor's 1976 1500.

Emission controls were removed. The car will idle normally and then suffer a low idle condition and stalling. Once stalled, it is very difficult to restart. Problem is exacerbated by high ambient temperatures.

Facet 3 PSI fuel pump has been adapted. There is some evidence that fuel has spilled into the canister through overflow connection to the ZS carb.

Would it be preferable to replace the Facet with a new mechanical fuel pump? Is the anti-run-on valve a problem with these cars? There is usually a "pop" through the valve when the engine is shut down. If the valve were leaking, I would expect a high idle, not the low lumpy gassy idle observed before stalling.

Any suggestions would be appreciated. I have lowered the carb needle all the way and the problem has not improved. I am a S.U, 1275 guy and do not have much 1500 experience.
Glenn Mallory

Glenn -

Been there. The decaying idle and eventual stalling, in my case, could be attributed to a faulty float valve that was allowing the float chamber to gradually overfill and flood the engine.

By any chance, has your neighbor fitted a Grose Jet float valve to his ZS? I tried one of those; it didn't seal properly, and caused all kinds of grief (including ruining my catalytic converter) until I diagnosed the problem and replaced it with the normal "needle" type. And yes, a symptom of such overfilling is fuel in the forward charcoal canister. Undo the hose at the bottom of the "can" and let the fuel out.

Since he's fitted a Facet pump, he may need to use a fuel pressure regulator to bring the pressure down to a level that the ZS float valve can accommodate. My car's got the standard mechanical pump, which is calibrated to the optimum pressure. I don't recall the recommended pressure, but I think it's less than his Facet is putting out.


Gryf Ketcherside

Thank your Gryf.

I did replace the float valve with the conventional type, NO gross jets. I have had terrible experience with them on the S.Us.

I will install the mechanical fuel pump and ditch the Facet. The mechanical pump must be calibrated to the pressure anticipated by ZS.
Glenn Mallory

<< The mechanical pump must be calibrated to the pressure anticipated by ZS. >>

Be aware that the new fuel pump may be more agressive than the old. When I replaced a failed fuel pump on my 1500 with a "Rover" branded one a few years back the new pump immediately flooded the carb (Weber DGV, not ZS, but still...). I don't know what kind of pressure it was putting out, but installation of a FPR set to 2.7 lbs cured the problem.

Also, and Gryf will know this better than I, but isn't one of the screws of the auto choke mechanism casing shorter than the others and if reinstalled incorrectly the longer screws can block the float from full closure? I mention this as it sounds like you had the carb apart recently.
Richard Reeves

Pump might be a problem, but shouldn't be at 3psi.
Q is, is it now flooding, or was that a past/transient?
If not flooding now, then check the carb diaphragm for holes. This makes it run rich, and will get worse as the hole gets bigger, until it won't run at all.
Richard is correct on the float chamber screws. They are very odd screws, and a too long one where a short one is needed will jam the float.
Also, if the autochoke screws are loose, fuel will bypass the choke, again causing rich running. That is also what dumps fuel on the catalytic converter and burns the car down.

FR Millmore

>>> Richard is correct on the float chamber screws. <<<

Just to avoid confusion, I'm sure you meant Choke Body screws, the ones that hold the choke assembly to the carb. Yes, one is definitely shorter, and if you use the wrong screw in that hole, it can block the float from rising all the way and shutting the float valve.

As for the diaphragm, they can be ruined (or so I hear) by the use of starting fluid. So inspect that... they're cheap.

Gryf Ketcherside

Dat's right!

FR Millmore

I installed the mechanical fuel pump. The car does not stall any more but is still running rich, even with the needle lowered as far as the adjustment will take it.

I will remove the carb, reset the float level, check the choke body screws for possible interference with the float and generally do what is necessary to lean things out.

The car does not have a cat, which avoids the worry about rich mixture overheating that.

I notice that there is rust in the fuel filter. The car sat for a long time and the tank seems to have rust cancer. The fuel also has an odd odor, almost like turpentine. The tank has been run almost dry and refilled but even so, something is not right.
Glenn Mallory

This thread was discussed between 25/09/2011 and 05/10/2011

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