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Engine optimum Running temperature

  • 21 Sep 2019 12:03 AM
    Message # 7892076

    Can someone tell me, what the optimum running temperature would be for the 45 cubic inch 750 cc engine.  I have set up forced draught cooling on both sides, coming in from the front at 45 degree angles, so as to miss the huge front guard.  Oddly enough, my rear cylinder runs cooler than the front.  I’m aware the this may not be the case whilst riding, as the airflow around that front guard tells otherwise.  At present the temps read 110 degree celsius front, compared to 67 degees celsius for the rear.  These temps are taken at the exhaust cylinder head exhaust ports of both cylinders.  The rear cylinder has a NGK B5HS plug and the front has the same.  I’m thinking leaving the B5HS in the rear and using a B6HS colder plug for the front.  Any ideas as to what may be occurring here.  The ambient temperature during this test was 26 degree celcius

    Last modified: 21 Sep 2019 12:10 AM | Danny Marks
  • 22 Sep 2019 8:03 AM
    Reply # 7893041 on 7892076

    Firstly the  Rear cylinder gets the most oil so will be cooler   ,But given your Spark plug ?  I would suggest an air leak at the front cylinder  ,making it lean and hotter by a larger amount

    regards Phil


  • 22 Sep 2019 9:12 AM
    Reply # 7893099 on 7892076

    You really need to run on the road to see the real temps and the front/rear difference.  I suggest the purchase of a 2-channel thermocouple temp meter on ebay for ~$15.00 US which will come with 2 type K thermocouple junction probes.  The junctions can be crimped into ring terminals and used under head bolts, or much better under the spark plugs.

    I just returned from the first Cross Country Chase (themotorcyclechase.com) which ended in hot Key West, Florida.  I rode a Danish Nimbus, which has a 750cc inline 4-cyl engine with all 4 cylinders in a single casting.  The front & rear cylinders run cooler.  The plugs are normally B6HS. I set the mixture much richer than normal to run cooler in the hot south, but this fouled plugs 1 & 4, so I changed just 1 & 4 to hotter B5HS.  Now the plug tips all looked the same and the head temp was as much as 100 deg F cooler than before.  After more than 2300 miles at ~50 MPH, the bike still runs fine and oil consumption is slightly reduced.

  • 26 Oct 2019 3:17 PM
    Reply # 8078295 on 7892076

    Thanks Phil and thank you Brian.  Yes Phil, the front was a bit lean, as air was entering via the inlet manifold.  After tightening that manifold, the temperature dropped considerably.  Next process is, lowering the float bowl, as suggested, to stop plugs sooting up.  Thanks.

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