Missing Beats at mi...

Missing Beats at mid range RPM  


New Member
Joined: 5 months ago
Posts: 1
15/07/2019 2:17 pm  

At wide open throttle on climb at 3200 rpm at 112 mph at 8.4 gph runs fine and strong cht's 298 egt's 1209 average.  At 2500 rpm at 3.1 gph and 100 mph in cruise the motor jerks from time to time, turning off either mag makes it much worse. At 2700 rpm and 3.5 gph it still jerks. At 3000 rpm and 4.2 gph the jerking stops. My carb is a Bing 94-40-148 with a straight K&B filter on the carb. I have a 35 idle jet, 250 main jet, and a 290 needle jet. Does anyone think I am running to lean at these lower rpm's. At 2800 cruise the cht's are 293 egt's 1308, 3.9 fuel flow. If I cruise at 2500 the cht's are 271 and egt's are 1227 average. These readings are on a 80 degree day, I have electric carb heat and it was on high the whole flight, oil temp about 185. I have tried every thing.


Len Schmader

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Doug Smith
Member Admin
Joined: 2 years ago
Posts: 82
27/07/2019 4:10 am  

Hi Len,

Those fuel flow numbers are a little low, so yes, it's possible you're running a bit lean on at least 1 cylinder.  I have a vague memory that the pod filters can be a little troublesome with the carby sense pipe (external to the carby) or with the sense ports on the carby inlet.  Tweaking the jetting would be the logical place to start, but if there's room you could also try putting a little straight section between the filter and the carby - a short length of suitable sized straight radiator hose or similar - to give the airflow a little more time to settle and smooth out between the filter and the carby.




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