Lean idle when hot

[jdeignan]

Hey all-

My AF ratio goes lean when idling hot. Car drives fine and hangs out around 14.7 now that I got the carb (eddy) tuned up.
Cold and hot starts are great, but if I let it sit and idle after I drive, I can watch the AFR progressively lean out to 17.0 over a few minutes, and my fuel pressure drops from 6 to about 4.

Blockage? Dont think I am boiling fuel (1/2in spacer works great)
Carter 4070 doesnt seem to be surging.
Any reason the fuel pressure would drop at idle when the truck is hot?

Eng temp is around 200-210 and holding.

Thanks!


Joe

 

[EricLar80]

Lean idle is not a bad thing if temps are still in control. Better emissions, and it wont cause damage because it shouldn't pre-detonate at idle. If you start getting that burning eyes feeling, then you may want to fatten it up a little.

 

[jdeignan]

Fair point- more curious to know why itnis happening.

Burning eyes- whats that about? Wouldnt that happen if is rich vs lean?

 

[EricLar80]

I'm not totally sure why, but you can tell when it's really lean because your eyes will burn and water like crazy, and your clothes will stink. Smells different than rich.

 

[DirtDonk]

How long after initial startup are you noticing this? Just a few minutes, or after quite a bit of driving?
Just wondering if it's the choke finally opening up that last tiny bit, or if it's completely unrelated because the choke has been fully open for a long time when this lean-out occurs.

Perhaps with the low demand the pump is warming up and actually loosing efficiency. Not a lot, and not to it's detriment, but a cause-n-effect thing.

Not sure why the lean mixture though, if it's not the choke changing a position, since in theory it should remain stable no matter what. But then again perhaps the low demand from the carburetor is also letting the float bowl go slightly low, or perhaps slightly high, and that all by itself is changing the dynamics inside the carb.

No idea if any of that is legit thinking or not. Just trying to throw ideas out there to see what you think about it.

Maybe monitor your ignition timing at those exact moments to see if anything is changing.
Also a vacuum leak somewhere after it's warmed up?

Paul

 

[ransil]

Maybe need a carb spacer

 

[Rightpace]

If your air / fuel ratio is 14:7 while driving and it drops to 14:0 at idle, then this is a richer fuel condition. As the 14.7:1 means 14.7 parts of air to 1 part fuel. The fuel pump pressure decreases as the float opens allowing fuel pressure release into the carburetor. The adjustment on the carburetor is only idle air adjustment. Turn the adjustment in one turn each and your air fuel ratio will increase at idle. Fuel metering is internal, performed by main jets and venturi. The initial difference in throttle opening is compensated by the accelerator pump at acceleration point.

 

[Rightpace]

I see that should have been 14.7:1!
Fat fingers and little coffee were involved.
Any input on this out there?

 

[EricLar80]

Any chance you are running a liquid filled pressure gauge? Apparently those can be heat soak and say the pressure is dropping when it is not.
https://www.hotrodders.com/threads/fuel-pressure-drops-after-runs-for-a-while.97734/

You may have 2 issues. Fuel gauge. And maybe a dirty carb causing the lean condition.

 

[jdeignan]

Paul-
It only happens when at operating temp, and then park it and let it idle.
Temp cycles from around 195-210 with my fan and tstat working.
I’m thinking you’re on to something with the low demand carb/float bowl

Eric- I’ve used two FP gauges with and without liquid- same results.

 

[gr8scott]

Is yours the Bronco running 30° idle timing (15° initial + 15° vacuum) on full manifold vacuum?

 

[jdeignan]

Yessir!

That said, this issue ONLY happens with a hot engine, idling after 5 or so minutes in the garage. Other than that, the bronco is tuned pretty close to 14.7 all around town.

 

[73azbronco]

Maybe vacuum leak when hot?

 

[WheelHorse]

4bbl carb?
Aluminum or iron intake?
Vacuum booster?

Have you checked all vacuum lines to ensure that none are starting to dry rot?

If you dial it in, once it is hot and at 17 afr...richen it up to 14.7-ish...the next time you go out and it get's up to operating temp...will it still be at 14.7-ish?

 

[jdeignan]

4bbl edelbrock 1405, 1/2” spacer
rpm performer aluminum manifold
i havent tuned the AFR rich when it idles lean- i have very little hope for that, but ill give it a try.

could be a vacuum leak when hot- but that leak should persist. for example: if it is lean/hot/idle, i can take it for a spin and the AFR is normal again.

I am thinking it is fuel pressure/bowl level at idle. maybe the bowl isnt filling as well as it sits?

 

[anoblefox]

early stages of vapor lock? When I had my carbed bronco with stock hood, I used a 3/8"phonalic spacer AND aluminum heat shield under the carb, I seem to remember they were made for Corvettes at the time (1970), thats where I purchased the spacer from a chevy dealer.

 

[jdeignan]

Thats my top candidate so far.. I just cleaned my tank, and was doing research on some new lines- decided to run a return line, and swap out my deadhead regulator.. I am thinking that the deadhead holley regulator is just letting fuel sit in the lines at idle and its increasing the fuel temps drastically.

Once I finally pull the trigger on the lines and install a return style FPR, this problem should go away if you are correct! Thanks everyone