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Dirty exhaust normal at startup ?

Stanley
Stanley
in Carburetor Central

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Posted
comment_481308

Did the white paper towel test at the tailpipe, when first starting the car for the day. It immediately got the black spatters that show overly rich mix. Took a short drive to the grocery store, about 4 blocks away. When I got back, I did the paper test again at idle, it was perfectly clean, not a single spatter.

 

Normal, or something that needs to be tuned out ?

 

I don't use the choke when starting. Using SM needles; had to set the mix fairly rich to get it to run at low RPM's. Guess that's because the SM's (like other British needles) are .099 at station 1 whereas N-27's etc. are .095. That makes them lean at idle (stations 1 & 2 according to SU publications). So when you adjust the mix for idle they get extra rich at mid range and top end. But on my car, they lean out if too much throttle is applied, too fast. I can't set them any leaner or it fouls the plugs. Mix adjustment is tricky. Top end miss is gone with the SM's, and I'm getting some nice low end dig at part throttle, but I feel like I've still got a lot of tuning to do. If I polish down the idle sector, then the idle settings will change and top end might get too lean again.

 

Stock displacement 240Z, K&N's, velocity stacks and 2.25" exhaust, non-stock JDM  head, so maybe that's why stock N-27's were way too lean. According to official SU literature, just putting K&N's often requires a needle change.

 

Rambling here, but recently read a (translated) thread on a Japanese site about drilling out the nozzles to use British needles. Read similar on Australian sites.

good idea?


comment_481316

Did the white paper towel test at the tailpipe, when first starting the car for the day. It immediately got the black spatters that show overly rich mix. Took a short drive to the grocery store, about 4 blocks away. When I got back, I did the paper test again at idle, it was perfectly clean, not a single spatter.

 

Normal, or something that needs to be tuned out ?

Cold operation is a transient phase, a region to get through as quickly as possible.  That's why the thermostat stays closed and there are temperature operated choke flappers, fast idle mechanisms, AAR's, and timing advance schemes on carb'ed and EFI systems keep the engine running while it's cold.  I wouldn't do any "tuning" to change a cold engine observation.

 

Just me.  You'd probably change the hot end by working on the cold end.

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