Can changing gas grades change idle speed?

Took the bike out for its first good hwy ride today ran through a tank of 92 octane everything was fine. Decided to fill up with 87 octane (long weekend coming up here a the price went up 5c a litre in my 2hr ride) for the second half of the ride still had fine power but the idle got slower went from just a tic over 1000rpm where I had it set to 500rpm/dying. Can gas have that big of an effect on idle speed or should I look for something bigger.

 

Alot depends on alchohol content 10 or 15% and sometimes water is in the fuel. I usually only run 87 octane , once I got gas in the middle of no where and bike did not run so well ran tank out and filled up local and problem solved.

 

The gas I was running @ 91 octane has no ethanol at all could that make that big of a difference?

 

I put the non ethonal hi test in my bikes near the end of season to get the system purged of the ethonal gas I do not see any change in running .
I do find that my bikes run better on one brand of gas verses the other station I go to.

 

You shouldn't notice any difference at all. Idle speed has much more to do with air volume than it does fuel type, so long as fuel flow is sufficient and the fuel is of a type that matches what the engine is meant to burn (Diesel will lower your idle speed substantially).

Do you have an inline fuel filter fitted?
If so, what type is it.

Are you sure that the gas cap is venting properly?

Did you ride far enough on that second tank to need to switch to reserve?

Were you on reserve before filling up and forget to switch back?

 

No inline fuel filter gas cap seems to vent fine.
I get about 230km out of a tank before reserve and filled the tank after about 210km rode about 10-15min at 6000rpm/120kph with the lower grade gas no problem with high speed but getting back into town the idle dropped. This was the first prolonged day of high speed riding and high temperature 29c/85f .
I used the tank up of lower octane in the ride back home and had to switch to reserve about 3km from home.
Could the hot day be the problem? The bike getting too hot? I didn't idle until much until later in the day. I did hit traffic and was sitting stopped for awhile on the way home the bike was pretty hot and I had to hold the throttle open a crack to keep it from dying.

 

The engine can't overheat by design.

Something is going on, and it might not be fuel related. One sign of faulty ignition coil failure is a weakened (or no) spark when the coils get hot.

I ride in conditions that are much hotter, and for longer periods of time (in traffic and out). I always fill the tank with the lowest octane available (because that's all the engine needs).

If your trouble is fuel related I'd tend to suspect that you were sold contaminated fuel.


One more thought: Is the oil level between the marks?

 

I checked the oil when I left. The engine does have a little sweat at the head gasket so knowing I would be riding a long time at high rpm it was topped off before the ride to a few mm below the top line. Haven't checked where it is now I'll check in the morning.

 

Checked the oil this afternoon started it up and let it run for about 2-3 min couldn't take it for a ride,raining waited 4-5min after shut down and checked level. Oil is down a bit from where it was before I left yesterday that was expected. I miss spoke in my last post the sweat is from the valve cover gasket not the head gasket I have new doughnuts but not a gasket yet, don't know if there is any point in installing the doughnuts without a new gasket.

 

My oil level question had to do with it really being the only thing that might cause the engine to overheat, so that's not a concern.

Go ahead and install teh doughnuts. They are all that puts clamping force onto the gasket. It won't hurt anything, and if the old gasket is still pliable enough you might get a decent seal.

 

bad donuts are the usale cause of valvecover leaks. I change them first then if it still leaks I replace the gasket

 

Thanks for the all the input. I have some time put aside to work on the bike this weekend. I have braided brake lines to install and now doughnuts to put on. I think and oil change is going to happen too. Is there a way to check the coils while I have the tank off to see if they are bad? I know k-moe said they can fail when hot, would anything show up in a test when cold?

 

Typically coils that are failing when hot will test fien when cold. You can use a hairdryer to heat them and then check the coil resistances and compare them to the cold reading. Also inspect them closely for cracks in the casings.

 

K thanks,I'll raid the wife side of the bathroom without her seeing. I haven't used a hairdryer since the late 80's heat gun yes hairdryer no.

 

wear gloves so you do not get any grease and oil on the drier

 

Alberto V-05 hot oil treatment isn't a thing anymore is it? I could lie and say I'm trying a new look if I screw up

 

Must have been bad gas filled the tank up today with 91 octane and it went back to a smooth just over 1000rpm idle. When this tank runs out I'm going to try the 87 octane that is here in town. Coils tested fine but I don't think I got them that hot with the hair dryer. The other thing I noticed is I have a new leak in the exhaust, I'm starting to hate that crossover box. Has any one had any luck building a new one? I know there are internal passages that help in how this motor runs.

 

the xj 650 maximdoes have an exhaust with a cross over pipe instead of collection box

https://www.partzilla.com/catalog/yamaha/motorcycle/1982/xj650j/exhaust

 

The 750 collector has two 'v' shaped diverter plates that help a bit with scavenging and balancing the flow of gasses. I believe that I've seen a few threads where members have built their own with good success. If you go that route you can cut the bottom off of the OEM collector pretty easily in order to see how she's built (and maybe even scavenge the diverter plates).

 

I've been trying to find a thread about building one before I take the exhaust apart again. I know my collector doesn't have all the diverters inside because when I bought the bike I had a rattle in it and when I took the exhaust apart I got what I'm sure was a diverter out the front of the collector.

 

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.