Insights in fuel temp and effect on tune & performance

[Norzilla]

Hi All,

Rode my Aprilia RSV into work today on a 35 degC 75% humidity day in Sydney and had to wrestle the throttle all the way home to stop the bike stalling at idle.

Anyway I discovered a few things when I got home that you might find interesting if you did not already know.

Over the last week spent some time tuning the injection in the garage without riding the bike I was getting a fuel setting for the region near idle of a VE value of 36-37 giving a pulse width (PW) of about 1.95ms to give me the required 13:1 AFR at happy idle condition for this engine.

BUT

after riding the bike the settings required for idle were different at around VE 40 at a pulse width (PW) of around 2.1ms. (which I thought what little odd and counter intuative, ie high intake air temp = less dense = less fuel required) I was thining the idle problem in the heat was mixture going rich due to high intake air temp

Until today I had no obvious conclusions as to why the difference in required fuel. However today when I got home after riding in the heat and slow traffic, I noticed when I got off the bike that the fuel tank was hot and when I measured I found the tank temp to be around 37 deg C, hence 37 degC fuel temp. When the bike was cold in the garage the bike & fuel were at 22 deg C, I know this as I had put the infra red gun on it as was checking a cold start and cross checking the reading from the coolant temp sensor.

Basically what I discovered was happening was that the fuel was absorbing the heat from the engine via the Alloy tank ( no inulation ) and expanding in volume ie getting less dense, which in turn meant that the engine required more vol of fuel to hit the target AFR of 13:1 taking the VE from 36 up to 40.

SO

what I have found is that to keep the bike idling ok ( ie not stalling in the hot traffic ) I need to run the idle mixture at a richer setting than expected, so that as fuel heats up the AFR leans out to the target 13:1 AFR where the engine is happiest at idle in the traffic.

Note that the I had the EGO control is set to act only above 1400 rpm but the bike idles at 1100.

My question to the forum is, is it ok to run the EGO down to 1000rpm just below idle so that the EGO manages that changes in required fuel at that rpm in these conditions where the fuel is heating up?

Thanks in advance for your thoughts.
Cheers!

[R100RT]

Two different beasts, but on my BMW (airhead boxer twin) I have never noted either elevated fuel temps even in desert hot temps, nor a change in behaviour between fully hot, and mildly warmed up. However, my components are most likely routed and fitted in areas and ways vastly different. The tank has twin suction and return on it as it has "reserve" zones on the bottom that flank the top frame strut that I decided to just use as the bottom most point. I do note heat soak to the IAT sensor if its fully hot, parked for a short while, then restarted again - but the settings have been suitably numbed to avoid issues.
I know on diesel equipment, elevated fuel temps can be an issue but that's usually exceeding 100f (38).
If it is undeniable on your unit, have you considered adding a fuel cooler core?
The other thing I have found over and again is that tuning I perform parked in the garage almost always needs validation with road testing and allowing everything to come into play.