........consider this.
I want to start by saying that I know there are people that have wanted to increase fuel flow in order to reduce their takeoff / climb CHT's, and have had success in doing so by drilling the main jet to a larger I.D.
Because of that, I finally decided to give it a try.
Back story...
I have an O-360 powered RV-6A with a fixed pitch Sensenich prop. It is equipped with the 10-3878 carb and a mostly standard FAB.
Based on Lycomings recommended fuel flow charts, even with the lower climb RPM because of the fixed pitch prop., I felt that my fuel flow was just a bit on the low side for our low airport elevations that we have locally (150-200 MSL).
Because of that, and because my airplane has always had CHT's higher than I would like, I decided to give drilling the jet a try. (Note - My airplane has baffling and seals that are probably as good as any, and I have done mods to improve flow on cyl. 2 and 3, but my #3 cyl was always the hottest and would go to 430+ in a climb on a hot day if I didn't cruise climb immediately after take-off.
So I drilled the jet. It was at #31, and I planned to just make a very small incremental change but I accidentally drilled it to #30 instead of 1/8 (mix up of drill bits).
Did it increase the fuel flow? Yes. Did it help with CHT's? No
What it did was change the fuel distribution among the 4 cylinders so that #4 was now the hottest, #3 was second hottest and #2 was way cooler than the rest.
This was the case in high power climb and in cruise. Prior to drilling, the CHT's were within a 10-12 degree spread in cruise.
Now the difference between #2 and #4 was 30+ degrees and the mixture was now 2/3rds of the way out when running at ~peak EGT (at the best I could manage with the level of EGT imbalance).
This was totally un-acceptable, so since I needed to get a replacement main jet anyway, I decided to try the "Mooney Mod kit" (part # 666-660-F) which among other parts, includes a different main jet.
Installing the kit results in a readjusted economizer valve as well. This converts the carb. to a 10-3878M model #.
The result is remarkable.
I can now do a somewhat normal climb on a 85-90 degree day and have cyl. 3 & 4 top out at just under 400 F (a faster cruise climb keeps them even lower).
In cruise at WOT my CHT spread is less than 10 degrees for all cyl., and the mixture control is back to being in a normal position with the EGT's peaking within a spread range that is quite good for a carbed engine.
I haven't done enough detailed testing to confirm how much yet, but the airplane seems to be a bit faster as well. Possibly from a slight power increase with a better mixture balance among the 4 cyl.
My takeaway...
The stuff that has been written cautioning about drilling the main jet, because even the shape of the opening at the exit can have an effect on the fuel flow pattern and subsequent distribution seems valid. Or maybe I just did a poor job of drilling the jet compared to what others have done.
Bottom line is that in my opinion, the $235 cost of the kit is totally worth it. You might get lucky drilling your jet, but considering the effort involved (not to mention you are buying some of the same parts the kit has, just to take apart the carb. to drill the jet, it makes sense to just go with the carb kit right from the start.
I want to start by saying that I know there are people that have wanted to increase fuel flow in order to reduce their takeoff / climb CHT's, and have had success in doing so by drilling the main jet to a larger I.D.
Because of that, I finally decided to give it a try.
Back story...
I have an O-360 powered RV-6A with a fixed pitch Sensenich prop. It is equipped with the 10-3878 carb and a mostly standard FAB.
Based on Lycomings recommended fuel flow charts, even with the lower climb RPM because of the fixed pitch prop., I felt that my fuel flow was just a bit on the low side for our low airport elevations that we have locally (150-200 MSL).
Because of that, and because my airplane has always had CHT's higher than I would like, I decided to give drilling the jet a try. (Note - My airplane has baffling and seals that are probably as good as any, and I have done mods to improve flow on cyl. 2 and 3, but my #3 cyl was always the hottest and would go to 430+ in a climb on a hot day if I didn't cruise climb immediately after take-off.
So I drilled the jet. It was at #31, and I planned to just make a very small incremental change but I accidentally drilled it to #30 instead of 1/8 (mix up of drill bits).
Did it increase the fuel flow? Yes. Did it help with CHT's? No
What it did was change the fuel distribution among the 4 cylinders so that #4 was now the hottest, #3 was second hottest and #2 was way cooler than the rest.
This was the case in high power climb and in cruise. Prior to drilling, the CHT's were within a 10-12 degree spread in cruise.
Now the difference between #2 and #4 was 30+ degrees and the mixture was now 2/3rds of the way out when running at ~peak EGT (at the best I could manage with the level of EGT imbalance).
This was totally un-acceptable, so since I needed to get a replacement main jet anyway, I decided to try the "Mooney Mod kit" (part # 666-660-F) which among other parts, includes a different main jet.
Installing the kit results in a readjusted economizer valve as well. This converts the carb. to a 10-3878M model #.
The result is remarkable.
I can now do a somewhat normal climb on a 85-90 degree day and have cyl. 3 & 4 top out at just under 400 F (a faster cruise climb keeps them even lower).
In cruise at WOT my CHT spread is less than 10 degrees for all cyl., and the mixture control is back to being in a normal position with the EGT's peaking within a spread range that is quite good for a carbed engine.
I haven't done enough detailed testing to confirm how much yet, but the airplane seems to be a bit faster as well. Possibly from a slight power increase with a better mixture balance among the 4 cyl.
My takeaway...
The stuff that has been written cautioning about drilling the main jet, because even the shape of the opening at the exit can have an effect on the fuel flow pattern and subsequent distribution seems valid. Or maybe I just did a poor job of drilling the jet compared to what others have done.
Bottom line is that in my opinion, the $235 cost of the kit is totally worth it. You might get lucky drilling your jet, but considering the effort involved (not to mention you are buying some of the same parts the kit has, just to take apart the carb. to drill the jet, it makes sense to just go with the carb kit right from the start.
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