Target AFR at Idle/Cruise with carbs Options

[yeahmag]

I was reading a thread on carb tuning that John Connolly of aircooled.net was on and he suggests a much leaner idle/cruise AFR than what I've been running. I've set a target of 12.5-13.5 in that range, but he suggests you can go much leaner than that.

This may also explain why Jake thought I was nuts when I went from a 60, to a 62, and then finally a 65 idle jet on my 40 Dellorto's. Does anyone have any real world insight on this?

I've loaded in my 60's for a ride tomorrow...

-Aaron

[Mike Bellis]

That target AFR is good. You need a head temp gauge to prevent melt down. Most water cooled engines can run leaner, down onto the 16's but they are not at risk of melting a head. A lean condition will make the engine run hotter. If you do not have a way to monitor head temps, do not go leaner.

[yeahmag]

I run both an LM-1 for AFR and a temp compensated head temp sensor. I know that 12.5 - 13.1 is great for WOT, especially considering we have some ethanol in our fuel.

What I'm interested in is cruise and idle... Look around for "Dellorto idle AFR" and you will probably start to see what I'm seeing.

-Aaron

[Ductech]

Aircooled motors like a little extra gas to cool with also. Almost like a turbo water cooled car would want. If emissions are your concern then tune to stoich, 14.7

Personal experience of somewhat unrelated nature yet related ... Aircooled Ducati's Love cruising around in the mid 13.0's

I would worry about maybe loading up plugs if you run to rich a mix at low throttle openings.

I have no exp with those carbs so I am unsure of the architecture and how complicated the cruising / low end circuit is.

oh and (IMG:style_emoticons/default/agree.gif) with the head temp sensor. This will tell you what afr the motor likes!!
Thats how the big boys tune. they watch the sensors as they make changes and compare the data! Use your calibrated butt dyno with a headtemp sensor and you could really find the smoothest / coolest running combo.

But dont start thinking about other variables like ignition timing or else you could get lost!!!

[r_towle]

if you have the gauges, start testing.
Seems these cars get really hot at 60-75 mph "cruising" on the highway and you could be using just the idle circuit at that speed with the dellortos.

5th gear, long haul on the highway at normal speed 55-70 will heat it up quick.

Rich

[70_914]

Your car will run faster and faster, making more power the leaner you run. Then it will melt a piston (IMG:style_emoticons/default/headbang.gif) . Just watch your AF gauge, and right before your engine is garbage it was running at the ratio it liked best, so tune your next one for that ratio.

Of course, then you need to worry that if you drive up a hill and the air gets thin your new engine may do another Chernobl reinactment...

14.7 is the best bet for a carbureted car you drive. Current cars are running direct fuel injection with baro sensors, lambda sensors, timing controls, alcohol injection to cool the combustion chambers and other tricks to make them survive at incredibly lean condiitons.

"I know that 12.5 - 13.1 is great for WOT, especially considering we have some ethanol in our fuel." This is a little scary due to the variable amounts of ethanol you are likely to find- super unleaded isn't always regulated and may not have any ethanol, winter gas has different amounts of ethanol in most areas where it gets cold in the winter. It only takes one tank without the 5-10% mix of ethanol and you are losing your cooling effect and risking damage.

[yeahmag]

I think there are some mis-statements here...

Of course, then you need to worry that if you drive up a hill and the air gets thin your new engine may do another Chernobl reinactment...

As the air thins out you need less fuel, so I would actually be safer.

14.7 is the best bet for a carbureted car you drive. Current cars are running direct fuel injection with baro sensors, lambda sensors, timing controls, alcohol injection to cool the combustion chambers and other tricks to make them survive at incredibly lean condiitons.

Not 100% sure about that. 14.7:1 is the theoretical best for pure gas. We have a mix of gas, ethanol, and additives. Ethanol needs to run richer than pure gas.

"I know that 12.5 - 13.1 is great for WOT, especially considering we have some ethanol in our fuel." This is a little scary due to the variable amounts of ethanol you are likely to find- super unleaded isn't always regulated and may not have any ethanol, winter gas has different amounts of ethanol in most areas where it gets cold in the winter. It only takes one tank without the 5-10% mix of ethanol and you are losing your cooling effect and risking damage.

Again, take the ethanol away and I would be running too rich (on paper anyway). When I was drag racing and only tuning WOT I would shoot for 13.5:1. Now with the ethanol in the fuel I'm shooting more for 13.1:1 at WOT.

YMMV - Let me know if I'm wrong. Trying to learn all I can. I did run the 60's around town this morning and it felt fine. Head temps were fine. Gonna get it on the highway at lunch.

-Aaron

[brant]

Aaron,

I know I'm just one more opinion and that doesn't say much
but I don't think you need to worry as much about the AFR at partial load
the motor has a lot more safety margin at partial load than at full load
I think the AFR is critical at WOT-redline when the margins are thin

as long as your not fouling plugs I don't think partial throttle is as important. If you were pushing for fuel mileage or emissions (a new car manuafacturer) it would matter more.

I agree that 13.0 (my motor guy actually likes 12.7) is a good place to be at WOT redline and all the way to that redline.

[yeahmag]

I love opinions! That's why I ask on a forum!

That's where I am right now at WOT - I am trying to get a bit more fuel economy to and from the track. I'm also a tinkerer...

-Aaron

[brant]

if you lean out the idle a bit and don't get hot you should pick up that fuel economy
(I know that sentence is only restating the obvious)

but I think there is a larger thermal safety margin at 2500rpm than there is at 5700rpm so you should be very safe in giving it a try.

I only use an innovate myself, but I think an EGR would be the real way to evaluate what your looking for. Still just keep trying... tinker away

[ChrisFoley]

One should set the idle jets for a good mix when accelerating from a standstill. The main jets then need to be sized for 12.8+/- at WOT.
The usual result is that part throttle running will seem a bit rich, however a little extra fuel will help cool the heads.
If one tries to lean out part throttle running the engine will balk on takeoff.

[jmill]

One should set the idle jets for a good mix when accelerating from a standstill. The main jets then need to be sized for 12.8+/- at WOT.
The usual result is that part throttle running will seem a bit rich, however a little extra fuel will help cool the heads.
If one tries to lean out part throttle running the engine will balk on takeoff.

Carb sizing has some bearing. I was over carbed and the leanest I could get was in the high 12's before my engine stumbled. I've heard guys out there say 13.5 is ideal. I could get that at WOT with bigger AC jets but never at part throttle.

[yeahmag]

Just got back from a small drive. Maybe 5-6 miles up a grade in 5th gear. Metrics were:

180F oil (not fair though, I have a cooler and a fan)
350F #3 head (temp compensated aircraft grade gauge)
14-15:1 AFR at cruise uphill

The car felt great both just off idle and at cruise. No hesitation at all. What I am seeing is that it exacerbates my leaky slipjoint collector and doesn't like to idle until all the temps even out. Tends to pop and snap out the tail pipe like a Harley.

[Ductech]

Just got back from a small drive. Maybe 5-6 miles up a grade in 5th gear. Metrics were:

180F oil (not fair though, I have a cooler and a fan)
350F #3 head (temp compensated aircraft grade gauge)
14-15:1 AFR at cruise uphill

The car felt great both just off idle and at cruise. No hesitation at all. What I am seeing is that it exacerbates my leaky slipjoint collector and doesn't like to idle until all the temps even out. Tends to pop and snap out the tail pipe like a Harley.

If you have a leaky slipjoint on your exhaust you are most likely getting false information on your wide band gauge. Even if the sensor is a foot in front of the leak it could still back feed fresh o2 into the exhaust and cause it to look leaner. I would suggest getting rid of any exhaust leaks before you trust those numbers.

[yeahmag]

I'm looking at Permatex 1372:

http://www.drillspot.com/products/567176/p..._gasket_sealant

...to help seal the slip joints. I have used a tail pipe expander as far as I dare, so I may dare to try a little bit further too!

[Ductech]

I'm looking at Permatex 1372:

http://www.drillspot.com/products/567176/p..._gasket_sealant

...to help seal the slip joints. I have used a tail pipe expander as far as I dare, so I may dare to try a little bit further too!

(IMG:style_emoticons/default/confused24.gif) I'm not really sure how well that stuff works....it might be one of those it works for awhile so that might be the ticket so you can take a bit to get it tuned and then who cares if it starts leaking again. try a little trick, Take a rag whilst the motor is running, and cover the exhaust outlet on the exhaust can. can you hear the "tsst -tsst -tsst" of an exhaust leak? look for carbon buildup around any slipjoints. These are ok tests to find leaks if they are there