Exciting day - rebuild of 78 coupe

[Mad4slalom]

It's been a while, but thought I'd give an update on the Air Fuel Ratio situation. My lovely wife bought me a wideband sensor kit for my birthday and I've had a bung welded into the exhaust.
View attachment 28157
The surprising result was that I found the car was running a bit rich at idle, which gradually leaned out as you increase revs up to 2000rpm and then shot up to 17.5:1
The value that was measured at the MOT station was actually in terms of lambda (it was a value of 1.22), I'd multiplied this by 14.7 to get an AFR of 18:1. I realize now that was wrong, because the value of 14.7 is for pure petrol. For the E5 that I'm using it should be 14.4 and that makes the MOT measurement an AFR of 17.6:1. Basically, the MOT station measurement was correct all along!
Taking the car for a drive it was running at 17.5:1 whilst cruising anywhere from 45 to 60mph, hopefully I haven't done the exhaust valves any damage.
It was simple to fix the idle mixture, just needed to go from 4.5 turns out to 3.5 turns out and also it was easy to fix the cruise mixture as well. On my carb you can adjust the Adjustable Part Throttle from outside of the carb and winding this up brought the cruise AFR down to 14.3.
Here are the results I ended up with.
View attachment 28158

One other little job that seems to work well was for my Air Con. The air box that houses the evaporator is very close to my headers and gets too hot to touch, which can't do much good for the cold air inside it. I've now covered the box with some self adhesive aluminised fiberglass sheet. Now the box doesn't get hot at all and the air con seems to work much better (although maybe that just because it's not very hot out now).
View attachment 28159

That is interesting johng, did you have any symptoms or just the mot readings? My 72 has a bung to access the exhaust flow, but was set up in buffalo NY and altitude my be different to cornwall.
Mine has always smelt a bit rich so maybe i should find a holley carb guy and have a tune

 

[johng]

When I fitted the headers and sidepipes I was a bit concerned that being more free flowing this might make the car run a bit lean. Also, I'd stripped down the carb previously to clean it out and found that a previous owner had made some dubious alterations (the jets were smaller than standard, the rods were from an earlier carb and were the wrong length and the rod hangers had been bent presumably to account for the wrong length). I decided to put it all back to standard and then get the AFR measured to see where I was.
Strangely enough the carb is now set up pretty much exactly as it would have come from the factory and isn't running lean at all (except for a strange blip at 1000rpm which I don't understand).

 

[Roscobbc]

Carbs really are such old tech. If that was a Holley I'd put the 'blip' down to the carb 'transitioning' from idle jetting and on to main jet.
Not 100% sure how the Rochester operates off idle jetting and on to main jets but it could simply be a characteristic of the carb?