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rough running 912ul

 
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kerrjohna(at)comcast.net
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PostPosted: Mon Feb 05, 2007 9:15 am    Post subject: rough running 912ul Reply with quote

my 912 engine began running roughly a few weeks ago and I attibuted it to cold weather. adjustments to mixture did not alter the rough running.

in checking the plugs those in the front two cylinders were wet, sooty black and the those from the rear cylinders were toasty brown. (Pictures available upon request).

any suggestions about the possible cause of the obvious uneven burning of the fuel/air mixture between the front and rear cylinders?

John Kerr
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pulsair(at)mindspring.com
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PostPosted: Mon Feb 05, 2007 7:15 pm    Post subject: rough running 912ul Reply with quote

John,
Your rear plugs will burn from a brown to gray. Your front plugs will burn jet black. Stand back and look at your manifold. The long straight unobstructed distance gives you that look to the front cyclinders. The rear gets a 90 degree bend to the rear cyclinder. The color of the plugs is normal. Jeff Classic IV 912UL
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jleathem(at)cogeco.ca
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PostPosted: Tue Feb 06, 2007 3:54 pm    Post subject: rough running 912ul Reply with quote

Hi John, we have been struggling with this same problem on our 912 on and off for a few years. We did notice the same effect on the plugs. It has been explained to us as the effect of the radius on the intake pipes which are curved just before they enter the cylinders. Since the front cylinders are on the “outside” of that curve so to speak and since the centrifugal force will tend to push the fuel droplets towards the outside of the bend, then the front cylinders get a richer fuel/air mixture than the back cylinders. That has always been our experience and even though we have resolved the rough running problem, we still have sooty front plugs and nice brown rear plugs. We have never been able to get them the same colour – but our engine runs smoothly now after many, many other adjustments and trials.
I suspect that this is what you are experiencing and may not have anything to do with the rough running. Hope this helps.
Do not archive.
J. Trevor Leathem
Ontario
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