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MGA Idleness and MGAs

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Daimlerdb18

Daimlerdb18

Senior Member
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Here's an odd problem...the rpms at idle on the engine of my 1956 (completely rebuilt) MGA drop below 900 rpm on hard braking and the engine stops. It restarts immediately. The idle is fine when braking normally, say coming up to a traffic light. The carbs were rebuilt with the Moss kit. The shafts checked out OK. I've checked the manifold nuts and the carb attachments and all seem in order.
Any ideas what's happening? I've adjusted the fast idle screw and balanced the carbs. I can prevent the stalling by having the choke about half way out, but it results in an idle speed of 1,500 rpm.
Thanks for any thoughts on this interesting phenomenon.
 
I think that I would take the car on a back road where there isn't any traffic and try a couple of panic stops to see if I could recreate the problem. When the problem recreates, shut the ignition off immediately and remove the lids from the float chambers and check for low fuel level in one or both. If both have a low fuel level, then yu might want to see my fuel delivery troubleshooting guide at: https://www.mgexperience.net/article/fuel-tsg.html
Cheers,
 
yep, sounds like the float levels are incorect. Think I would just pull the tops and check em and adjust as necessary for starters.
 
Have you checked the oil level in the dampers? A low oil level will cause the dampers to drop too far and too quickly when you suddenly back off the throttle, resulting in exactly the symptom you are experiencing. Rolling off the throttle slowly allows the damper to drop slowly, and you don't see the problem then.
 
That is really interesting to me, because my B has A similar problem.

It does the exact same thing if I coast downhill.
Otherwise it idles right at 1000 rpm. Only when I put it in neutral and coast downhill, it slowly drops rpm until it dies (if I let it).
 
Here's an update. Checked the oil in the dampers and found that they were still full to the mark with Moss's finest damper oil. Pulled out the repair manual and checked each float lever for proper 7/16" clearance using a piece of aluminum bar stock. They were right on the mark. Checked the pistons for free movement and found them to be free. Checked the fuel filter (new), check the fuel pump (new) delivery and found it to be well over the .6 liters/min shown in the manual. Sooo...
Went ahead and changed the float clearance to 1/2" on the front float and tried it out. Runs great. Idles fine and doesn't die at idle on hard braking.
Which leads me to wonder if there may be some crud in the passageway from the float bowl to the jet. The raised float position effectively raises the head of gasoline in relation to the jet orifice, overcoming any small restriction in flow. Plus, the car had been behaving well up until a few weeks ago.
Thanks very much for the advice!!!
 
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