MG Car Repair: Emissions, colorado emissions testing, rich mixture

Question
QUESTION: Hi Howard, the '76 MGB I've been reviving from long storage is up and running.  Thanks for your help in getting to this point.  I took it to the Colorado emissions testing station today and failed with flying colors.  Here are the results:

         Allowed   2500 RPM   Idle
Hydrocarbons  (PPM)    600       900      1400
Carbon monoxide (%)    3.5       6.5       9.2

Regarding the CO, as I understand it that means too rich a mixture?  I have the 45M high-altitude needle and I thought the car was running too lean already; I can try leaning it out further and see what happens.  Any other suggestions?

For the hydrocarbons, I have no idea where to start.  Any suggestions would be awesome.


Thanks as always,
Billy


ANSWER: Hi Billy,
When HC is high it is usually a mechanical and/or an electrical problem dumping raw fuel into the exhaust and when CO is high it is normally a rich mixture. However when you have these excessively high readings on both, you need to test several items.

First check to see that the choke is off to see if it is dumping excess fuel into the engine. (also noted by black smoke from the exhaust) Also with that much raw fuel in the exhaust the CAT has to be bright red or it has been removed or gutted.

Next you need to do a "cylinder kill test". This is done with the engine warmed up and set at idle and you short out one plug at a time with a grounded sharp probe. This is to see that you get about the same RPM drop on each cylinder that is killed. (If you have pointed ignition you can pull a plug wire for each cylinder for the test but don't do it if you have electronic ignition)
This will tell if you have a dead cylinder or not. No use continuing any more tests if you have a dead cylinder until that is corrected.

Take a 3mm Allen wrench and remove the top cap from the carb and turn the adjusting screw counter clockwise to adjust the needle downward. (your leaner high altitude needle may be needed later but that is not likely the cause because of the excessive readings. confirm that the air filter is not clogged up. And confirm that the fuel pressure is not too high. (1.5 to 3 PSI max.)

let me know,
Howard
PS, Are you required to do the L-1 emission test?

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

QUESTION: Thanks!  I calibrated the choke by boiling the heat mass when I rebuilt the carb, but how do I verify it's off?

Here's the test I have to pass:
http://www.aircarecolorado.com/idletest.htm


ANSWER: There were two types of auto chokes used on the Stromberg carbs. One had a wax filled bulb with a pin that fitted into the piston of the bulb unit. The wax expanded when heated and would push the pin to disengage the choke. The other type had a bimetal strip in a coil like an old an older domestic car had. This is the type your car should have and to be sure it is off you may have to remove the three pinch screws on the edge of the coolant unit and pull the unit away with the hoses still attached. You then will see a short lever sticking out toward the front of the car. This lever is what the bimetal strip is suppose to move to apply and remove choke. You can operate the lever when the engine is running and quickly see which way is on and off. Temporarily wire it in the off position to see if the engine runs cleaner. (no black smoke out of the tail pipe)
The test readings you had must have given you black smoke out of the tail pipe and flat black powder on the spark plugs.

Even though unleaded gas is more difficult to read the plug color for mixture it is still a good indication.

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

QUESTION: Verified the choke is off.  Cylinder kill test passed, all cylinders firing.
Leaned the needle all the way, I'll see what that gets me.
Air filter is brand new, and fuel pump is low pressure; float chamber is not flooding.
Black smoke no, black powder on plugs yes.

I'll replace the points and check the ignition timing.  I'll also check out the ignition wires.  Is there an upper limit on their resistance at which I should replace them?
I'll also pull the cat and see if it's gutted.  If it's not gutted, what can I do to determine if it's still functional?

I have a strange, unrelated? issue: the engine starts and runs well - unless the clutch is pushed in.  If you're stopped and hold in the clutch, the engine slows and eventually dies.  It will not start at all with the clutch in.  Any idea what's up with that?

Thanks,
Billy

Answer
You have some clear facts like, flat black plugs comes ONLY from a overly rich mixture which is confirmed by the high CO readings. You also have very high HC readings which is raw fuel going into the exhaust.
Under normal conditions this rich mixture and raw fuel in the exhaust will have to fuel a very hot fire in the CAT making the CAT glow bright red. These are plain facts, not anyones theory or guess. So you need to find out where all the fuel is entering. Put in your high altitude needles and check it by lifting the test pin on the side of the carburetor and make a cruise speed run and kill the engine and pull a couple of plugs to see if they are still flat black. Then find someone with CO/HC test equipment to test the emissions again.
Howard