Can changing gas grades change idle speed?

I don't mean to pull the thread off topic, but how is this? How can any engine running without cooling not overheat? Even piston aircraft engines running quite rich at idle, using extra fuel for cooling, require some airflow from the prop to not overheat, right? I remember years ago I saw a guy park a harley outside a bar idling and go inside, I was working across the street and thought, hmm, I wonder how hot that thing's gonna get? No wind, hot california summer. It kept running, but after a little while there was a lot of blue smoke coming out the exhaust, I'm guessing because the oil got so thin. Oil probably doesn't work very well when the cylinder head's what, 400 degrees?

Back to the fuel thing though, all I can come up with as far as the idle speed vs fuel, ethanol has a different stoichiometric ratio than gasoline. For example, E85 requires about 30% bigger injectors/jets. So going from straight gasoline to an ethanol blend without any type of correction would make a motor run slightly lean. I wonder if 91 with ethanol would have the same effect. ?

 

I did some research on gas up here and the 87 octane that I put in(Pioneer) has ethanol and the brand of 91 octane gas (Esso) I put in is ethanol free. Don't know if that made a difference.
As for the not over heating, I don't know. But there is s video out there somewhere of an XJ being run at a biker bash full out trying to blow it up and I think they gave up after a full tank of gas. So maybe Yamaha has some good voodoo with the XJ bikes.

 

I think there is maybe one thread with pics, but I can't seem to find it.

By design.
Aircooled stationary engines have been running farm machinery since before either of our parents were born.
Aircraft engines are somewhat different in that the risk is becoming too cool at altitude.

If you do a search on Youtube you can find a video of an XJ650 being run well above idle without any oil in the sump as a "fun" event at a Harley rally. The video lasts for nearly 30 minutes, at which point the engine finally catches fire.

 

Based on my experience of using E-10 (and "unblended" gasoline) for over 30 years, it would not.

I fill up my XJ (as well as my other engines/vehicles) with E-10 about 30% of the time and she idles the same regardless.
She also idles the same regardless of octane (because octane is a measure of resistance to detonation, not of potential power production).

 

I should probably first state here that I'm not intending to be adversarial, or argumentative, I'm just genuinely curious what you're talking about. I guess it would theoretically be possible to design an air cooled motor with enough cooling fin surface area to cool adequately purely by convection, without forced airflow, but I've sure never seen it. Every stationary air cooled motor I've ever seen had a fan as part of the flywheel and ducting around the fins, like a lawnmower. Or like an old air cooled VW.

Even air compressors have fans blowing across cooling fins, although they're sometimes not very effective, since you want the oil over 212 degrees to boil out water.

I guess also then we'd have to define what exactly it means to "overheat", too, since with a liquid cooled motor it's pretty obvious, it's dumping coolant on the ground, or has blown a head gasket and is getting coolant in the oil, etc. But with an air cooled motor, I guess I would define overheating as when either the oil becomes too thin to work correctly, the head gasket fails due to different coefficients of thermal expansion, valve train going way out of spec for the same reason, but also, and importantly, octane tolerance is reduced by heat. For example, an irrigation pump on a farm, if it gets hot enough to cause knock at that power level, it's overheated, because running like that will break the ring lands, etc.

 

I understand your concerns. I had them once too.

2015, Summertime. 105ºF, Kansas City, MO. I-35. Traffic is at a near standstill, and I can't pull to the side because I have an appointment and construction means that there is no shoulder to pull onto. Creeping along as below a walking pace for 15 minutes (the engine was well-hot from the previous 4 hours of riding). No loss of power, no hiccups, no damage. (Full disclosure; I did have an oilcooler fitted for that trip, but similar situations occured before I had the cooler).

Can you make an XJ airhead overheat? Sure, almost anything can be made to happen if you want it to happen. Will it happen as part of the normal course of events? Not likely.

My definition of overheating is when any part of an engine is damaged due to excessive heat.
In aircooled engines (well watercooled engines too. A boilover is telling you that the cooling system has failed, but you still have time to avoid engine damage if you act quickly). the valves and piston are the first to tell you that the engine is too hot, and a sudden loss of power will be evident.

 

Octane of a fuel will affect timing to some extent if the engine is hot causing detonation. Higher octane fuel is less likely to combust before the compression stroke finishes. I think with the cylinder pairs it may be hard to feel random detonation due to the cyclic mass.

 

Octane has no effect on timing. Valve timing is mechanical, and spark timing is electronic (and non-adjustable with the stock TCI).

Octane is only a measurement of the resistence a fuel has to detonation. Running higher octane fuel allows an engine builder to run higher compression, and/or "aggressive" valve timing. An engine designed for low-octane fuel does not (and can not) benefit from a higher octane fuel without the engine being modified. What octane to run will vary by altitude, but for the purposes of ground-based vehicles it's always going to be a shift to running a lower octane fuel at higher altitudes.

Any detonation in a stock XJ will be the result of excessive carbon buildup in the combustion chamber (and that is extraordinary uncommon), or a dangerously lean fuel mixture.

I think what you're thinking of is modern engines that have anti-knock sensors. In that case ignition timing is retarded when a lower octane fuel is used in an engine that was designed for a higher octane fuel.

 

Spark is irrelevant when detonation happens. Ever seen a run-away diesel?

 

You mentioned that octane changes timing. You did not specify which sort of timing, so I covered both.
Detonation does not work exactly the same in a gas engine as it does in a diesel (Yes I have seen one runaway, and shut the fuel valve as soon as it happened).
The only result of detonation in a gas engine is melted aluminum, and battered bearings.
An early combustion event due to detonation is not usually refered to as a timing change (because it's confusing).


I think we've exhausted this distraction from the thread's purpose.
No problems are being solved.

 

My problem has been solved for now... probably bad gas. If the rain holds off this weekend I'll be lucky run the tank dry/reserve and fill with 87 octane. I'll let you all know what happens. Hopefully get it good and hot and it idles fine. If the idle changes when hot I'll try to keep it hot and test the coils don't think the hair dryer got it hot enough and go from there. I would like to run the 87/reg unleaded as it's about 20 cents cheaper per liter and every bit helps.

 

ride with your meter in your pocket

 

Just to finish this up I hope. Ran through 2 tanks of 87 octane and the bike runs fine it does idle a little lower 900-1000 but other than that no difference. Just a touch to idle adjust has brought it back to the idle rpm I'm happy with(1000-1100), now on to other problems..... in a new thread when I get to them.

 

Preferably in this thread. History matters. It helps us make sure we are giving the best advice for your machine.