Jetting

[phaetn]

I had my head machined earlier this summer, with a multi-angle valve cut. I didn’t play at all with jetting, but did add a 14mm spacer under my 32/30 DFEV carb.

Now that the temperature is going down, performance is going up. I know this is because there’s a lot more oxygen by volume in the air charge.

It’s about 55°F (12°C) here this evening and the car is SOOO much more responsive than on hotter summer days. Does that mean I have an overly rich mix during warmer temperatures? The car sure loves to run leaner in the cold air...

Cheers,
phaetn

[sptcoupe]

Carbs are a compromise throughout their operating range (throttle position) at a given ambient temperature and altitude. They will rarely, if ever, provide the perfect AFR under any circumstances, and if jetted correctly, they will rarely be very wrong, either.

Unless you have definite indicators that it is running rich in warmer weather (black smoke, stumbling, gas odor, badly fouled plugs, etc) just enjoy the power boost you get when the temperature drops and drive on.

[phaetn]

Wiill do, thanks.

I guess the only way to really perfect jetting is to get the car on a dynometer and test with various jets, given that all other variables remain constant. That's not going to happen for me.

I'll just enjoy the autumnal power.

Thanks and cheers,
phaetn

[michaelj]

I play with jetting a bit in my big block chevy, its not too hard really. I would recommend you get the next size BIGGER jets for winter. Maybe even two sizes bigger. You need more fuel with the denser cold air. Takes me about 5 minutes to change my jets. And i leave them for 5 months, then go back to smaller jets when the weather heats up. Makes a big difference.

I just welded a bung on my Spider and bought an AFR meter so I dont know exactly how far I can push the fiat, but Im sure it will need two diff sizes for best performance year round

[phaetn]

Good idea! I sense another "project" on the horizon.

If anything, given the better performance now with the cooler air, I'd go a bit leaner in the summer. Maybe I've been too rich all along.

I don't feel like doing plug chops at WOT or mid-throttle like I used to do back in my bike days. An AFR sensor sounds like a good plan. How far along the pipe do you install it? Is anywhere after the downtube fine?
Once you install yours make sure to share some pics!

Cheers,
phaetn