problem setting idle

[n/a]

Joe,
Your original post said that if you reved the engine to 1700 rpms, the CO dropped to 4%.  My understanding is that CO drops naturally at higher revs.  This is why they check it at idle, when the CO is higher.
Cheers,
Don

quote:Originally posted by Joe

My new 71 280SL won't pass emissions. When I tried to turn the thumbscrew on the back of the IP, I couldn't feel any detent. I turned the thumbscrew all the way CCW, and it jams up and stops turning. I suspect things are not well inside the rear of the pump. Any advice is appreciated.
Incidentally, I managed to get the CO down to about 4% with the Gunson meter by turning the air screw CCW, but the idle speed is about 1700. The emissions place said it has to be below 1350 RPM for them to certify it.
Joe

[Ben]

quote:This is very fascinating. Can someone explain how changing the timing can affect the fuel richness mixture

Hey Don you're the only one that realised the mistake with the BTDC and ATDC !  Let me try to explain the whole timing thing. Basically fuel needs air to allow it to burn. The process takes time and the amount of time increases as the amount of fuel and air increases. So as you accelerate you add both fuel and air so you need to allow more time for this to burn. Therefore you need to advance the timing, i.e start the spark a little earlier.

Now the timing doesn't actually affect the mixture at all but basically if you dont allow it the correct time then you dont have a complete burn (retarded timing) or you will get pre-ignition (advanced). Retarded timing will basically produce higher emmissions since unburnt fuel is pasing into the exhaust system. Over advanced ignition can cause hot spots on the piston crowns which can lead to engine failure.

The CO% will drop at higher RPM's as the whole process is completed quicker and the engine does'nt require as much fuel once its job is done, i.e it has increased the RPM of the motor already !  Whilst idle CO% is the main one people set it is also very important to ensure the car isn't running to lean at cruising RPM's. It really unlikely for a 113 to run too lean at cruising speed, more of a problem in a modern car which runs lean anyway and if a sensor goes down you could burn a whole in a piston !! :evil:

Regards,
Ben in Ireland.
'64 230SL 4sp.
'03 CLK Kompressor

[n/a]

Excellent post, Ben.  Thanks for taking the time to explain.  Is it correct to say that whenever we adjust the timing to a slightly higher number BTDC, we are "advancing" it?  Adjusting to a lower number BTDC, i.e. moving closer to 0/0 would be "retarding?"  And, vice versa for ATDC adjustments?  I just want to make sure I got it straight.
Best regards,
Don

quote:Originally posted by Ben

quote:This is very fascinating. Can someone explain how changing the timing can affect the fuel richness mixture

Hey Don you're the only one that realised the mistake with the BTDC and ATDC !  Let me try to explain the whole timing thing. Basically fuel needs air to allow it to burn. The process takes time and the amount of time increases as the amount of fuel and air increases. So as you accelerate you add both fuel and air so you need to allow more time for this to burn. Therefore you need to advance the timing, i.e start the spark a little earlier.

Now the timing doesn't actually affect the mixture at all but basically if you dont allow it the correct time then you dont have a complete burn (retarded timing) or you will get pre-ignition (advanced). Retarded timing will basically produce higher emmissions since unburnt fuel is pasing into the exhaust system. Over advanced ignition can cause hot spots on the piston crowns which can lead to engine failure.

The CO% will drop at higher RPM's as the whole process is completed quicker and the engine does'nt require as much fuel once its job is done, i.e it has increased the RPM of the motor already !  Whilst idle CO% is the main one people set it is also very important to ensure the car isn't running to lean at cruising RPM's. It really unlikely for a 113 to run too lean at cruising speed, more of a problem in a modern car which runs lean anyway and if a sensor goes down you could burn a whole in a piston !! :evil:

Regards,
Ben in Ireland.
'64 230SL 4sp.
'03 CLK Kompressor

[n/a]

This topic has captured my interest.

As Ben in Ireland enlightened me, retarding the timing will produce higher emissions because unburnt fuel enters the exhaust system.  Assuming that Joe meant to report that he adjusted the BTDC timing, not ATDC timing, from 50 down to 24 degrees, he was retarding the timing.
Joe reports that his adjustment had the effect of bringing his CO reading down.  Do I have the facts straight here?  If so, I don't see how the CO was lowered.
Best regards to all,
Don

[Ben]

quote: Is it correct to say that whenever we adjust the timing to a slightly higher number BTDC, we are "advancing" it? Adjusting to a lower number BTDC, i.e. moving closer to 0/0 would be "retarding?"

Hi Don yes you are correct here !

 

quote:Joe reports that his adjustment had the effect of bringing his CO reading down. Do I have the facts straight here

Obviously there is a sweet spot where the spark timing is just right. If you try to light the fuel too early you will ALSO end up with an incomplete burn. Therefore emissions will be high and general running will be poor.


Regards,
Ben in Ireland.
'64 230SL 4sp.
'03 CLK Kompressor