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Vittorazi Moster 185 misfiring - spark plugs?


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Hello,  I am hoping someone can help me with an ongoing problem driving me nuts....  the full story:

I had an engine-out at about 25hrs from new on my Moster. Turned out the ignition failed. I got a new ignition and all was ok after replacement however I noticed some reasonably bad radio interference that was not previously there. After about 6 more hours, I started to get some misfires at high rpm, during climbing. The misfiring started intermittently and got worse. It was usually from about 7400rpm or higher. It got worse a and the engine would not rev past once the misfiring started. (Strangely I noticed a carbon build up on the top (as in at the lead connection) of the spark plug. Changed plug, tried some dielectric grease at top of the plug and the problem stopped. Then about 6 hours later same issue. Checked the carb, filters, etc - no problems. Cut and re-fit the high tension lead at the plug end. No joy. Changed the plug, worked again for about 5/6 hours before the same issue again. 

I got another ignition - a third one- Radio interference was gone now and all ok for about 7 hours, then the radio interference returned and the engine started misfiring again, same situation at high rpm. 

Im using the standard BR9ES plug and Im coming up to 50 hour maintenance, engine temp has been good and have no other issues - but changing plugs every 5 hours seems crazy.

Any help would be greatly appreciated!

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Hi Kiwi K, thanks for your response.

I’m using Belray H1, synthetic oil.

The plugs I pull out don’t appear fouled,  slightly coffee coloured, no wear to electrodes and still gapped to 0.8mm....I never re-use any of them(4 now)....but I do have an iridium plug on the way...

What do you mean by starting order? How I start?  I prime the carb, pull start (reliably on 2nd pull), warm up for a few minutes, run up to clear prior to take off, then take off. 

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