Date Posted:08:05:22 10/24/09 Sat Author: roger Author Host/IP: 82.152.192.107 Subject: Re: Cylinder Nuts In reply to:
Howard
's message, "Cylinder Nuts" on 19:45:39 10/23/09 Fri
Howard, since you mention the 'roger' check, let me give you a brief run down on this issue.
Yes, you are right. There is little information regarding the the 'tight engine syndrome' which have plagued jabiru since day one. Apart from the loss of top end power (up to 100 rpm max static) there are other more subtle effects which include:
Jabiru's base nut modification only tackles half the problem. An un-chamfered nut puts point pressure on the radius of the barrel wall and this pressure (30 ft lbs torque) can distort the barrel when it gets hot. You have discovered this with your 'roger check'.
However, some of the nuts do not run true on the thread of the through studs and will 'wobble' as they are torqued down. These nut need to be machined as well as chamfered so that they exert even pressure on the base of the barrels.
When I do a top end, I also hone the barrels to the max allowable limit to get better clearances. The latest engines from Jabiru now have increased barrel IDs. They explain that this is to reduce 'carbon build up' whatever that means, but the real reason is that they got it too tight in the first place.
David Person, who owns a machine shop recently did a top end overhaul on his Jabiru which included the barrel base nut modifications. To give him credit, he discovered the 'wobble'. He will be happy to modify and machine the base nuts for you and also give you a heads up on how to tackle the job.
Remember that a tight engine will effect the carb mixture. This may explain why there are so many 'solutions' to jetting issues.
