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Missing Beats at mid range RPM


lovtoofly
(@lovtoofly)
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Joined: 2 years ago
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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
(@doug-smith)
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Joined: 3 years ago
Posts: 82
 

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.

Regards,

Doug. 


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lovtoofly
(@lovtoofly)
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Joined: 2 years ago
Posts: 5
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Hi Doug,

I have been working on this problem for over a year. I did get the cobra elbow and moved the air cleaner up in the cowl, but it did the same thing so I went back to the filter on the carburetor. I installed a fitting in the back of the air cleaner for the sensed tube. I have no room to put a straight tube in about 6" between the carb inlet and the fire wall. I just reamed my 2.90 needle jet to 2.95. This made me have to turn my idle mixture screw in the whole way to get a smooth idle. I trailer the plane to fly it so I am planning a test flight next week. Maybe I can find a K&N filter that is a little bigger around and shorter. The filter I have is 3.5" Diameter by 5.5" Long.

Thanks.   Len 

Len Schmader


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lovtoofly
(@lovtoofly)
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Joined: 2 years ago
Posts: 5
Topic starter  

Hi Doug;

I finally flew the plane today with a new 290 needle jet, 35 idle jet, and 250 main jet. On climb out the fuel flow was 8.8 gph with rough running. When I pulled the throttle back a little it smoothed out. I found a shorter K&N filter and had room to put in a 1.5 inch aluminum tube between the filter and the carb. I have the sense tube in the back of the filter. At rpm's between 2200 and 2800 the motor jerks like it stopes for a fraction of a second. Once I get to 2900 and above it is smooth. At 2500 it was burning 3.3 gph, at 2700 3.8 gph, at 2900 4.7 gph, and at 3000 5.2 gph. all in cruise. I usually climb at 3200 but today I could not do it due to the rich mixture. I checked the plugs, the wires, the caps, and replaced one of the coils due to high resistance on the high voltage lead. Nothing seems to help?

Thanks.  Len Schmader

Len Schmader


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lovtoofly
(@lovtoofly)
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Joined: 2 years ago
Posts: 5
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Hi Doug;

One other thought, do you think I could have a valve sticking at those RPM's ?

Thanks Len Schmader

Len Schmader


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lovtoofly
(@lovtoofly)
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Joined: 2 years ago
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Hi Doug;

Two flights ago I took the sense tube from the float bowl and wire tied it to the engine mount, this made an immediate improvement at 2700 and above rpm's. I flew today for 96 minutes and burned 5.2 gallons of 100ll, or 3 1/4 gallons an hour. I need to get the carb to supply more fuel at lower then wide open. I burned 8.1 GPH on climb out with EGT's at 1234 and CHT's at 300. The temperatures are great at 2800 cruise the EGT's were 1294 with CHT's at 292 on an 80 degree day. I have a Hackman mixture control coming. Do you think that will help. I also installed a larger K&N filter. The float bowl sense tube is now vented inside the cowl. Thanks Len

Len Schmader


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