IAC Stepper on Air Cooled Engine

[sirhaxxalot]

Hi All,

I'm setting up a Microsquirt (V3) on a 1981 Yamaha XJ 750 motorcycle. The engine is AIR COOLED. I have installed a Honda CBR 600 throttle body unit to the engine to allow me to run EFI.

From what I have read the ECU controls idle speed based on RPM and Coolant Temperature. Since I will not have a Coolant Temperature sensor, will I still be able to use my 4-wire IAC stepper motor to control the idle speed? Is there some way to bypass the coolant temperature, or maybe even use Oil temperature?

Is there any other problems I might run into by not having a coolant temp sesnor?

Thanks in advance!

[R100RT]

You should observe coolant temperature for many reasons. There are a number of the fueling and ignition maps that will be wanting to "know" what temp the engine is at.
Sensing oil temp is not often easy to get calibrated correctly. Easiest solution is to utilize the stock GM temp sensor that gets listed in the set up manual pages, and mount it in the slipstream side of a cylinder. I have tested all these methods and settled on the sensor threaded into a small chunk of tapped aluminum that is fastened on the back side of one of my jugs. I took the 3 calibration values that come up in Tuner Studio and skewed the top value in order to get a more linear temp sensing behaviour (those systems assume you have water jacket temps, quite different than air temps coming off a cylinder). Regardless, its pretty simple and works very well. Warm ups used to be tenuous, now its start and ride as soon as you feel like it.
I too have a twin throttle body from a Honda donor engine (mines from the earlier CTX650 turbo bike), that has a "choke" feature of sorts with a cam plate that increases idle via another cable. I have not worried about utilizing an IAC for idle control to date. Idle on the boxer engine is pretty stable, needs the "choke" device to increase rpm's when colder.(I say that, it is not enrichening but people like to call it a choke control) Good luck.

[Tassuperkart]

Yes agreed.

Heres the thing. If you going to the effort of injecting an engine that already runs spectacularly well on carbs as an FJ does...or just about any jap bike built in the last 30 years, then cutting corners such as not incorporating a CHT warmup and trimming, AIT for air density compensation purposes, it justs makes the whole exercise pretty much a WOFTAM and you might as well stay with carbs as the bike will NOT perform, overall, as well. Simple as that.

I have a 90 degree L-twin and my CHT sensor is buried currently between the head cooling fins right next to the inlet manifold. This gives a reasonable representation of the warming characteristics of the engine and the additional mass of silid aluminium right agains the inlet port casting dampens the temp swings.

Its only ever a problem on extremley cold days where the temps there will sometimes drop back into the WUE ranges when on overrun down long hills. Again, slight enrichments at those temps are incremental anyway and dont pose a problem really.
I only add additional fuel up to about 60 degrees C iirc and its only a couple of % to keep the ARF's from swinging at idle. The bike will run perfectly well much colder than this without any enrichments.

Do it right the first time mate and there are rewards waiting. Cut corners and leave things out and you are going to spend a lot of money on an engineering exercise that will NOT improve the bike at all.