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
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 isnāt 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 shouldnāt do anything when the starting carb is off.
~Mike
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 doesnāt 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 itās jet.
If thereās one thing the starting carb does well when itās working (other than make noise and smell of petrol) itās starting the engine. If it is not physically damaged thereās not much to it that can go wrong. They easily clog with disuse though.
~Mike
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
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.
Iām 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 70ās. The chapter was called āCarburetor is a French word for āLeave it aloneāā.
Hi George. No oil in the dashpots wouldnāt 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 itās solenoid was off. You should really look at that. A constant very rich condition will be very bad for your ringsā 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