Cylinder head temperature on air cooled engines

Discussion in 'Thumpers' started by Modron, Apr 11, 2016.

  1. Modron

    Modron Adventurer

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    Hi,

    Me and my wee bike might have a problem. Im leaving on a 6month trip from China to Europe in about 8-10 days and today I discovered my engine might be running hot.
    I bought myself a TTO spark plug temperature meter and put it on. It says that in the city my cylinder is about 160-170 degrees C which is ok, unfortunately on cruising speeds it runs hot. After about 10min at about 7000rpm (red line 10000rpm) its gone to over 200 C.
    I heard that the danger zone is above 200 C but Im not sure. Id appreciate any input.
    I might add that Ive changed the air filter about 500-800km ago and nothing really blocks the air flow.

    I did install a cylinder fan for those long, bad roads in Central Asia where one cant go over 25km/h but its on the side so i dont think it obstructs the air flow
    [​IMG]
    IMG_20160410_124644.jpg

    What might be the problem?
    I did a search but most of threads are for liquid cooled engines. I did a search but most of threads are for liquid cooled engines.

    p.s. Its a Yamaha YBR250
    #1
  2. yokesman

    yokesman Long timer

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    Have you ever checked n adjusted the valves, check the compression also .
    #2
  3. Beezer

    Beezer Long timer Supporter

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    200C is 392F. air cooled aircraft engines go up to a minimum of something in the 425F range which is about 218C. some are even a bit over 500F.... 275C. the trick is the cool down... got to get under 300F before you shut down. thats about 150C. cooler is better
    #3
  4. Bultaco74

    Bultaco74 Been here awhile

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    Sounds like it's running lean. What does the spark plug look like. I use a infrared thermometer pointed near the spark plug, not at the spark plug.
    #4
  5. Modron

    Modron Adventurer

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    Checked and adjusted valves about 10k km ago. Never checked compression though

    No difference between aircrafts and wee 250cc bikes?

    I did check spark plug few days ago and it was quite clean with white coating on it which mean its prolly running lean. Its fuel injected though and I have no idea how to sort it out on FI bikes. Any ideas?
    #5
  6. NJ-Brett

    NJ-Brett Brett

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    That fan might be a problem if its blowing air onto the motor.
    Maybe try a run without it.

    Bikes go lean if you open up the intake or exhaust, a dirty air filter might help...
    You can add some resistance to the engine temp sensor to get the computer to add more fuel, but if the bike is stock, you should be fine.
    #6
  7. Modron

    Modron Adventurer

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    I didnt turn that fan on. I put it on there for low speeds through hot areas.
    I used to have an aftermarket exhaust but it was too loud for long trips so I put stock on. There is no coolant and no temp sensor whatsoever hahah
    Tomorrow I will put some more oil on that air filter, see if it helps. Thanks
    #7
  8. aguim

    aguim Been here awhile

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    The 250 Fi (same engine as my XT250) has an Engine Temp. sensor screwed to the back
    of the cyl.head, right next to the intake 'rubber'. Adding a small Resistance (in my case 300 ohms) to one of its wires will let the ECU open the injector just a tiny bit longer every time it is activated, giving a richer mixture all through the operating range.

    The engine temperature can be read at any time, right on the METER (at least on the XT), by
    just selecting the DIAGNOSTIC MODE (push and maintain both 'select' and 'reset', turn switch
    ON, wait 8 secs....DIAG will appear. Then select d:11 and the meter will show actual cyl. head
    temp. (lower than spark plug area temp, but a better indicator of engine operation).

    Then, of course, the indicated temp. will no more be true after adding that 300 ohms...

    UPDATE: Error here in the suggested DIAG procedure, see later post. The S+R
    buttons need to be pushed/maintained a SECOND time for 2 secs at one point.
    #8
  9. NJ-Brett

    NJ-Brett Brett

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    Not sure why I said coolant temp sensor, I corrected that.
    You can also run a cooler spark plug.

    The fan is a good idea, but if it blows onto the motor it might offset air flow from moving, or block some flow at speed.
    It might do better to blow hot air away from the motor the way its mounted...




    #9
  10. yokesman

    yokesman Long timer

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    How many miles, my buell rear cylinder sensor delays the spark At about 420f, the temp of the head to maintain spec depends upon material n its density. Heat is mostly transferredtomthe head thru the valve seats n guides if these are defective it creates a hot spot leading to uncontrolled heat transferr n overhrating unless you have something blocking the airpath to the engine , even the designers made these errors , by raising fenders, certain bodywork, inadded a tank bag on my buell cyclone it slipped into the air stresm below the tank n it overheated, didnt help it was over a 100'f.
    #10
  11. adam728

    adam728 Long timer

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    200 degC on a spark plug is no problem. Generally the higher the load, the higher the plug seat temperature, even with more cooling air. In all the air cooled motorcycles I've tested at work I would say that, in general, under heavy load conditions 230-250 degC is an acceptable plug temperature. I think the worst we had was one engine that would start detonating at a constant load condition once the plug reached 240 degC. A different engine went over 260 degC with no issues.
    #11
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  12. Modron

    Modron Adventurer

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    Hmmm, I measured this with empty panniers and I expect it to go hotter when loaded. Tomorrow or day after Im doing a test run with my buddy. I will load the bike with all the gear for my trip. Will see how hot it gets
    #12
  13. aguim

    aguim Been here awhile

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    I had not paid attention to that fan before... boy is it ugly...plus I dont think it is of any help
    the way it is installed. Uh, why not add another one to cool the now overtaxed R/R...

    Maybe just the ETS mod, and a DR8EA would do just fine?
    #13
  14. Modron

    Modron Adventurer

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    Thats great info, thanks. I might do that as last resort.

    Honestly I dont care if its ugly or not and Im sure it will help on them horrible roads in Central Asia with temp avove 45 degrees and speed 15km/h hahah

    ETS mod?
    #14
  15. aguim

    aguim Been here awhile

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    Engine Temp. Sensor.

    'Ugly' was, of course, a rather subjective appreciation. Ex: I find the XT's tail light
    utterly 'ugly'. But I also dont give a damn. hahah!
    #15
  16. JensEskildsen

    JensEskildsen Long timer

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    Youre overthinking it, the bike is most likely fine.
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  17. baldman1

    baldman1 Long timer

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    +1, I had an old Honda XR500R that I rode for many years and one day I decided to install a trail tech speedo. It included a cylinder head temp gauge. I was constantly seeing 450 plus degrees, scared the shit out of me so I simply disconnected the temp gauge, problem solved. It's air cooled it will always run hot and get rid of that fan it will not help at all.
    #17
  18. Mobil1

    Mobil1 Long timer

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    It's normal. Doing the temp sensor mod should slightly help. A cooler spark plug only makes the plug cooler , not the motor.
    #18
  19. adam728

    adam728 Long timer

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    As said, you are over thinking this. 200C is not that high.

    Generally higher revs will run hotter, as there is less time for cooling between power strokes. I glanced back at some testing earlier, we tortured one engine to plug temps over 300 degC. That was extreme, but like I said, 240 degC is about the lowest temperature I've ever seen an engine get unhappy with. Many will run thru hundreds of hours of durability in the mid 200's without issue. You are worrying a lot about nothing.
    #19
  20. aguim

    aguim Been here awhile

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    I can remember an argument over hot- vs-cold spark plugs' effect on engine temperature
    that was settled with a few sets of plugs and a temp gauge. There was a noticable difference,
    mostly under high loads. Looks like a real hot plug can disturb the normal combustion
    process. Just going from a grade 7 to a grade 8 NGK ...hard to tell, but I'd do it and have
    a little more peace of mind.
    #20