We have had exactly the same problem since starting with the Iame in January. Engine seemed to 'hunt' at higher revs when not under load ie on the stand. I phoned around various builders and was told that this happened on all the engines and that Iame were aware of this and were looking into it, but that it was not a problem on the track when under load.
Since then we have gone through loads of plugs (NGK and Denso), with each time when the engine performance at higher revs dropped off, putting a new plug in seemed to help. When we had the engine serviced after 20hrs, our builder even put in a new coil to see if it made any difference - no change. The power output dropped off a cliff at greater than 10,500 rpm. Setting the carb on the dyno helped a bit, but at a slight loss of bottom end power.
Recently, my lad told me that he felt that the top end was almost hitting a 'limiter' when getting up to 12,000 rpm on the track. I also noticed that some engines sound very different to others at top end revs.
However, following a recent post on this forum, I contacted JM engineering last week about a replacement coil and was told that they have produced a new 'beefier' coil which suits the Denso (recommended) plugs better - apparently they have a lower resistance (?) than the NGK plugs and using the original coil with the Denso plugs can result in a gradual 'deterioration' of the coil.
I duly ordered a new style coil which I fitted today - the engine now revs cleanly through to 12,600 rpm on the stand......
Whether or not this results in better performance on track remains to be seen - we are going testing on Wednesday. If it only results in getting through a lot fewer plugs I shall be happy !
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Iame Coil Faulty? by 'fatsam' (18 Aug 2013 @ 22:21)
