MG Car Repair: 77 Midget 1500 Running Very Rich, vacuum leaks, charcoal canister

Question
QUESTION: I'll try to be as brief as possible.  I've owned and maintained MGBs for for 26 years so I have a basic understanding of these cars.  That being said, I bought a 77 Midget 1500 that had been sitting for 2 years.  PO was a machine mechanic but not versed in LBCs.  Car had emissions removed mostly no air pump or air valves etc.  I removed air pipe from exhaust manifold (CA Spec) and plugged hole in manifold since pipe was rotted away. New manifold gaskets etc.  New Petronix module for distributor.  New plugs and wires.  Engine is hard to start and runs very rich.  Didn't seem so bad until I did manifold work.  Rebuilt carb and and choke after it started smoking.   Did not replace needle.  Set choke per John Twist method on youtube.  Carb was thoroughly cleaned (dipped) and inspected before re-assembly.  Mixture adjustment appears to have no affect.  Needle is moving correctly during adjustment. Timing is correct 10BTC.  Valve lash correct at .010.  Re-torqued head to specs just in case.  Plugs gapped to .025.  Compression at 155 to 160 on all 4 cylinders.  No apparent vacuum leaks.  Car runs rough and puffs black smoke at low rpm.

Car did get hot recently before work began.  Even with good compression, could head gasket be suspect?



ANSWER: Hi Dave,
It sounds like you have most of the bases covered. Black smoke out the tail pipe is only one thing and that is excess fuel. So you either have a coolant operated choke problem or extra fuel is entering some where else. Check the vent pipe from the float chamber to see that it either is vented to the charcoal canister or vented to outside air.

The coolant operated choke on the Stromberg carb is highly suspect as that has a very high failure rate. leave the center bolt in place on the coolant cover and just remove the three edge screws used to lock down the adjustment of the choke. This allows you to pull the coolant unit with the two hoses away from the carb exposing a small lever. You can now manually operate the choke by hand. I would first warm up the engine before removing the choke unit. Then it should be easy to test if the choke is at fault by manually operating the choke.
Let me know,
Howard


---------- FOLLOW-UP ----------

QUESTION: Howard

Thanks for the quick response.  Checked the vent and it is clear and connected to the cannister.  I can blow into the cannister so it's not clogged.  Can blow back into carb and force fuel up out of bowl so I think it's ok.  

I checked the choke and got some curious results.  The action was free as I had already worked that over extensively.  If I force it to the lean position, the smoke clears up for the most part but the revs go way up.  Even with the idle setting on minimum I'd say it's running at 4000 or more rpms.  If I let it return to the rich position, revs go down and it sputters and emits black smoke.  The choke seems to be working correctly.  The cam and spring and all however, the throttle lever never pushes the pin all the way in on the cam.  At the most it will only reach about the middle step on the cam at the lowest postion of the throttle shaft with the set screws backed all the way out for fast and slow idle. I'm encouraged that I could all but eliminate the black smoke with the choke.

Any ideas on what would make the revs go up that high if I manually lean out the choke with the throttle shaft all the way down?  I'm beginning to think I made an assembly error on the carb.  Can you describe how the inner spring on the throttle shaft is supposed to work?  I guessed on how that went back on and probably mucked it up.  I picture might even help as I don't have one that's usable in the kit instructions or my manual.  I usually work with HUs and have not had much trouble with them.  This ZS is gonna be the end of me I think.  Thanks for what you do.  Your opinions and guidance would be helpful.

Answer
Dave,
The choke must be positioned in the Lean position, then see where all the air is entering the intake manifold. NO ENGINE CAN RUN AT 4K WITHOUT A LOT OF AIR GOING INTO THE INTAKE MANIFOLD. Either the throttle plate is not closing or you have a massive air leak somewhere. It is as simple as that.
The spring must be positioned so as to hold the throttle CLOSED. Also check the throttle bypass valve as when it's diaphragm gets old they get very hard and thus can prevent closing all the way. I have had to run the adjusting screw so as to keep the valve closed all the time.
Howard