Engine surge at idle (Mk2)

I am having a problem of engine surge at idle in my MK2
1 the Auxiliary Enrichment Carb (AEC) will switch on to start, however, I usually need a shot of either to have the engine start initially. Engine fires up immediately
2 Once started the engine will surge to 2K RPM, then back down to 800 RPM then back up to 2K RPM,
3 I can turn off the AEC (totally isolate from the battery) but the engine will continue to surge up and down until the engine finally dies at the lower RPM
4 Even with the AEC disconnected, the needle valve on the AEC will rise and fall in sync with the engine surge.

Any suggestions as to where to start to troubleshoot?
Thanks in advance

george leicht
Cincinnati OH
1965 Mk2

If you can isolate the cutoff to the electric choke and it still does it…Look for vacuum leaks.

Thanks

Thanks. The AEC is isolated. So, I should be looking at:

  1. The vacuum line from the intake manifold to the brake booster?
  2. The plumbing maze under the carbs from the intake manifold to the AEC?

any where else? and can you use the propane trick under the joints?

george

george

I would insert a vacuum gauge - the diagnostic variety that tells what the various levels and movements mean. Can provide a picture. I would also check that the throttle return springs are where they should be and of proper strength. Paul

Hi George. If the needle on the starting carb is moving around at all with the power supply disconnected than the solenoid operated valve that opens allowing the starting carb to be exposed to manifold vacuum isn’t seating or returning to closed position properly.

If you remove the solenoid there is a spring loaded brass disc on the bottom of it that closes off the starting carb passage. Take a look at that area. The needle shouldn’t do anything when the starting carb is off.
~Mike

Thanks Mike. I did disassemble the solenoid coil and disk etc. when removing the carbs last year. So that might be the culprit.

George

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Very much interested in the finding. Our MkII starting carb does not. The solenoid works but no joy.
Engine runs great once running with the help of Ether.

Hi @jagkeeper, if the starting carb solenoid is lifting the disc properly and it doesn’t start easily suspect the passage from the front carb bowl to the bottom of the starting carb is clogged up. Fuel just lays in there and I often find them completely blocked with rust/gunk. There and the area under it’s jet.

If there’s one thing the starting carb does well when it’s working (other than make noise and smell of petrol) it’s starting the engine. If it is not physically damaged there’s not much to it that can go wrong. They easily clog with disuse though.
~Mike

Thank you, will take a look at that.

I have encountered that twice before, and my fix is to run a wound guitar string ~ .040" thru the passage.

In my case, I had the parts out, but I believe it could be tried with the carb in place, from memory the passage leads into the carb float bowl, which can have a lot of scum sitting on the bottom

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Finally got back to the engine that would start easily, race to 3K rpm, slow to 800 rpm, race to 3K rpm, slow to 800 rpm on and on and on.

I’m stupid, stupid, stupid - assumed there was oil in the dashpots - and there was none. Topped them off and we are fine.

Makes me recall a Chapter in a book about British cars written in the 70’s. The chapter was called “Carburetor is a French word for ‘Leave it alone’”.

george
1965 Mk2
Cincinnati OH

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Hi George. No oil in the dashpots wouldn’t cause a 2200 rpm surge at idle. You might see a tiny variation at idle with no oil due to vibration at most.

You mentioned in an earlier post you saw the starting carb needle valve moving around when it’s solenoid was off. You should really look at that. A constant very rich condition will be very bad for your rings’ longevity among other things. Possibly whatever was happening with the starting carb was intermittent and is now okay but that huge variation in idle is not lack of dashpot oil.
~Mike