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MG MGA - Couple of issues

1) the car's idle is going to hell as it warms up, the hotter the worse, as a matter of fact after being in traffic it gets particularly bad and improves somewhat after driving on clear roads. Increasing the choke prevents the car from dying at idle during these times. I know that this points to air leaks around the carbs, but when I spray carb cleaner around the carbs there is no jump in idle speed.

Thoughts?

2)the car continues to make a metalic gargling noise between 2200 and 3500 rpm in each gear. I've been searching down the noise. Is it possible for something in either the water pump OR the generator to be makeing this noise in only this band width of RPM's?

Thanks
T McCarthy

Stumble with a hot engine can be a vapor lock condition. When fuel boils in the base of the carburetor around the main jet it induces bubbles into the flow which makes it run lean. Pulling the choke helps. This situation is aggrevated by alcohol content in the fuel. By now our midwestern fuel is almost all 10% gasohol, and it is hell with a hot engine.

Additional heat shielding between carbs and exhaust manifold can help. An in-line fuel cooler could help. Using premium fuel might help, as premium fuel may have a lower vapor pressure (slower to boil or evaporate).

The light metalic gargling noise between 2200 and 3500 rpm under load is most likely spark knock (a mild form of pre-ignition). If it is very loud it may be harmful to the engine, eating spark plugs, burning exhaust valves, or even burning holes in the pistons. If it is barely audible and only under full throttle it should be harmless and may even be the best setup for good fuel economy.

Anything more than a soft tinkle should be avoided. Try premium fuel first. If that doesn't work try retarding the spark timing a few degrees. If necessary, do both. If you do both and you still get pinking, then you most likely have very high compression. In that case you may be in for using avaition fuel, or otherwise some expensive mechanical work to lower the compression ratio.

There are other possible sources of rattling noises, like loose rocker arms or valve to block interference. Such mechanical sources will commonly happen most of the time, not just in the mid rev range, and not load sensitive.
Barney Gaylord

The fan or generator pulley can also be a source of a mechanical rattling sound that happens in a particular RPM range. Look for cracks, or loose rivets or just loose bolts.
Jeff Schultz

Barney, I've readjusted the fuel mixture and I've retarded the spark.

I also added octane booster.

None of which improved matters.
T McCarthy

Since you have tried the octane and timing tests and also tried spraying around the carbs, have you tried checking for air leaks at the intake manifold to the block? Or between the carb bodies and the intake manufold? This might be the source of air leaks. Maybe a gasket or just loose nuts.

Is the noise coming from the waterpump / generator area? As a LAST resort, after you have eliminated ALL the others, it might be a chain tensioner gone bad?



Chuck
Chuck Schaefer

Look for resonant sheetmetal - carb heat shields, loose internal baffle in the exhaust (probably not on A as the muffler is in back), inner fender panels, etc. Most common on A would be heatshields and the pulleys Jeff mentioned - also the crank pulleys are known to disintegrate - check this.
FRM
FR Millmore

Buy a mechanic's stethescope, cheap and useful diagnostic tool.
Barney Gaylord

This thread was discussed between 12/05/2007 and 13/05/2007

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