912UL Engine vibration on throttle back

Re: 912UL Engine vibration on throttle back

by Alan Arnold » 13 years ago

Alan Arnold

20 191 170

Kevin.

I am looking at the picture of your starboard carb. Your throttle is fully advanced and the throttle cable is bowed from the pressure of the throttle control. When you pull the throttle control back the cable will unbow. I can't imagine your carbs being syncronized at other than full throttle because each cable will unbow at a slightly different rate causing a significantly different throttle setting. The cable should be adjusted so that when the throttle control is fully forward (WOT) the carb throttle arm is just touching the stop. Also, since the throttle cables are so flexible it will be difficult to sync the carbs anyway. The springs will help with some of this but not all.

Re: 912UL Engine vibration on throttle back

by Kevin Stewart » 13 years ago

Kevin Stewart

5 184 0

Hi Alan,

The installation in the Eurostar is such that the idle speed is set by the cable length when the hand lever is against its end stop. The idle limits screws are not to be used otherwise the throttle levers will be bent. There is no limit adjustment to the travel of the throttle lever in the cockpit, that means that when the carb throttle arms reach their wide open position the cable slack will have no alternative other than to bow.

I don't see how this could cause a problem with synchronisation as the cables are adjusted when they are under tension. My vacuum gauges show accurate synchronisation right throughout the rev range.

Kevin

Re: 912UL Engine vibration on throttle back

by Alan Arnold » 13 years ago

Alan Arnold

20 191 170

Greetings Kevin.

I see what you are saying. So the throttle arm is actually advanced by the springs only and the cockpit throttle control just allows the throttle arm to be pulled forward by the springs to the limit set by the throttle control. OK, my bad. As long as the cables are flexible enough to straighten out with the spring pull all should remain in sync.

Good luck with the vibration.

Alan

Re: 912UL Engine vibration on throttle back

by Kevin Stewart » 13 years ago

Kevin Stewart

5 184 0

Alan you are correct. I think that it only works well if the cable isn't too stiff and the bow doesn't put a kink into the cable. It appears to work surprisingly well. If only I can fix my rough running I'll be in business :)

Re: 912UL Engine vibration on throttle back

by Kevin Stewart » 13 years ago

Kevin Stewart

5 184 0

I managed to get to the airport today and continued the investigation. Firstly I noted that the idle mixture was set to 2 turns out so I set it to 1.5 turns out. I then went for a short flight (oil temperature stabilised), landed and immediately stopped the engine and pulled 2 of the plugs. The porcelain insulator was bright white on both plugs. See this first picture:



I still had some old plugs that I removed a couple of years ago and they look brown on the insulator as you would expect. See this picture:



So it would appear that for some reason my mixture has become very lean. I have double checked for air leaks and the float level appears correct.

If anyone has any suggestions I would be most grateful.

Kevin

RotationofIMG012.jpg (You do not have access to download this file.)

RotationofIMG013.jpg (You do not have access to download this file.)