Octane, spark, air-fuel rate
 

Hello
I ve tried some 91 octane in my 98 3TB XTE in Slovakia, and the engine seemed to run quite well, but after the second full, I saw that the motor was overheating, and that caused leaks between the cylinder and the low engine.
I m in worry whith what will happen in Turkey or Iran whith really poor octane fuel. So I checked some things.
My spark is a DPR8EA. Would a colder DPR9EA would help with poor octane fuel by limiting self-lightning?
The spark is quite white but electrods are fine.
The specification for the carb is PT010 or something like that. I don t have the reference in my head but in the technical book, they say to unscrew the mixing screw of 1 turn for this reference. But when I cleaned the carb, it was unscrewed of 2 turns, so I left it like that. If I screw one turn, it will run with less fuel, but I think I m already too poor in fuel.
What do you think I should change?

Thanks a lot for good advices and a little bit of theory would be fine also : what is this overheating, as I didn t heard the click click sound of self lightning?
Philippe

Methinks no worries:
1. You say the spark is white, do you mean the sparks themselves? If so, no problem; no way you can deduct air / fuel ratio from the colour of the spark. You should look at the colour of the ceramic tip of the plug, it should be somewhere between dirty white to dark brown, beige (sand) is fine.
2. How do you know the bike is overheating? Do you have an oil temp gauge or something? Since a couple of weeks I have an oil temp gauge on my 3AJ, that almost always is at 100 - 110 dC when driving fast or on much load (sand for instance). That is not hot, it's normal working temp and no problem for the XT.
Seems to me not logical that possible overheating causes oil leaks, is it not just some 'sweating'? If, so, perfectly normal for the XT.
3. I always use the 91 octane, no problems there.
4. The self-ignition occurs when there are spots in the cilinder that are so hot that the air/fuel mix ignites before the spark does it. This would give a hardly noticable ping ping sound. It could also be caused by bad fuel with a very low octane number, but manufacturers always recommend better fuel than really required. I never tested, but I would be prepared to use far worse fuel than 91 octane.
5. Overheating and self-ignition are not directly related, although overheating could cause self-ignition (hotspots under 4). Overheating can be caused by a far too lean mix (not enough fuel) but if you have not changed anything on your carb settings before you noticed all this it is highly unlikely that suddenly your mixture becomes far too lean.
A lean mixture burns far slower than a good mixture and more of the combustion energy is transferred as heat to the cilinder head and walls instead of being converted into mechanical energy. But you would notice a very noticable lack of power.
You could try to pull the choke out when running about 110 km/hr in fifth. If pulling the choke will make the bike running better there might be indeed something wrong.

Succes!
Auke

Philippe, what kind of air filter do you use and which exhaust pipe do you have?

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Fred, XTZ660, Holland.

Hi all
Thank you for interest.

I don t have oil temp gauge, but one hour after stop, I couldn't touch the engine, as I can do usually. That's why I think I have overheating.
The colour of the ceramique is white, and I compared whith graduated scales of colours, and I am at the limit of being too lean.
I tried to put on the Starter at 120 kmh in 5th, and nothing happen, no lack of power and no extra power.
I have a KN air filter, and the exhaust is standard.

Thanks again
Philippe

Seems a bit hot, if parked for an hour and engine still too hot to the touch. But, if you are in a hot place, no wind, the engine will cool very slowly. When I stop in winter for half an hour I sometimes have to use the choke again to start if there is also a wind blowing, the reverse case.
If your plug is nearly too hot I wouldn't worry, especially since your K&N no filter admits more air giving you a leaner mixture.
If there are no other problems and the bike runs fine, no worries and have fun on the trip.

Auke

Philippe,

This screw ONLY affects the first 1/4 throttle opening and is EXTREMELY unlikely to have any effect on normal riding, and none on the spark plug colour. However I would set it to one turn as recommended and see how it runs. With the K&N and pipe, it may need to be at 2 - try it at both!

Run whatever spark plug Yamaha recommends for what you're doing. Period....

Taking a stock bike in good condition and adding a K&N air filter often means you need to richen the mixture slightly. The K&N filter allows more air than the standard filter, so could be causing it to run a little lean. You MAY need to RAISE the needle in the slide, and one size larger main jet. Talk to an expert - e.g. a Yamaha dealer about it. And get them to adjust the mixture screw as well, they'll know how to set it correctly.

Pure bright white electrode is lean - it should have a little colour. Not much, just off-white/grey.

Choke / starter lever will have no effect at half to full-throttle if it's an "enrichening device" as opposed to a slide that covers part of the throttle bore (I have no idea what an XT has, so you have to work it out.)

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Grant Johnson

Seek, and ye shall find.
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One world, Two wheels.
www.HorizonsUnlimited.com

I would say that the K&N filter only optimizes the air/fuel ratio which is too rich standard. Especially with the standard exhaust. Try to screw out the idle mixture screw a bit because as the bike gets older you'll need to adjust it. Works setting is only for virgin bikes.
If it was my bike, I would modify the inner exhaust tube a little bit too.
When you go to Turkey you'll probably find mountains also, in that case it's only good not to have the standard [read too rich] setting.

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Fred, XTZ660, Holland.

Fred, what do you mean by 'I would modify the inner exhaust tube a little bit too.'

Well, I have the problem again with some fuel here in Romania. I ve been driving slowly for 50km, and after 50min stop, it's impossible to let the hand on the engine, and it seems that there is a kind of ping-ping sound, but I don't exactly know what this noise could sound like.

I think I'm going to change the spark for a colder one, and as I will meet a german traveller biker in a few days, I will check some things with him (air intakes, valves clearance). I don t think that I could have a problem of ignition advance on the TCI...
Or I can go fastly in the Turkish mountains... It s so rainy here that it could be a good idea ;-)

Thanks

If your engine gets so hot, try to find the fault first before you start modifying things.
There should be something wrong. My guess is that the engine gets false inlet air somewhere, probably at the rubbers between cylinder head and carb.
About the exhaust, I mean remove the end damper piece from the muffler and cut the small pipe in half, then put it back in again.
But first try to get the bike in a normal working state.

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Fred, XTZ660, Holland.

Hi
We have raised the needle of one position, put a colder spark, checked the false air intake at the rubbers, adjusted the valves, checked the temperature with an electric gauge (80 on the carters, 100 on the cylinder).
The spark has a better colour (rather sand than white) and the consumption have raised from 4.2 to 4.6 l/100km, but the bike runs well.
I ll try to get some habitudes to check if the fuel has an effect on the temperature.

Thanks a lot