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  #1  
Old 07-12-2021, 10:45 PM
rvbuilder2002's Avatar
rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 9,595
Default Before you drill your carb. main jet.....

........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.
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Opinions, information and comments are my own unless stated otherwise. They do not necessarily represent the direction/opinions of my employer.

Scott McDaniels
Hubbard, Oregon
Van's Aircraft Engineering Prototype Shop Manager
FAA/DAR
RV-6A (aka "Junkyard Special ")

Last edited by rvbuilder2002 : 07-13-2021 at 10:03 AM.
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  #2  
Old 07-13-2021, 02:44 AM
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Malndi Malndi is offline
 
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Location: Sydney Australia
Posts: 133
Default

Thanks Scott. Timely advice for me, having replaced the old and tired Precision 10-5217 on my 320-D1A with a new 10-5217 from Avstar, my climb fuel flow has changed from 12 to 10 gph and CHT is up a little, but cruise speed and fuel flow is unchanged. I have a standard airbox with a fixed pitch Sensi.

I don’t know if my old carby was drilled or modded, but given the difference in climb fuel flow I suspect it was. I’m now wondering if the Mooney Mod is available for my configuration and if I should be considering doing it.

Edit: it appears the kit is not available for my configuration. Any advice welcome.

Last edited by Malndi : 07-13-2021 at 02:52 AM.
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  #3  
Old 07-13-2021, 07:20 AM
lr172 lr172 is offline
 
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Location: Schaumburg, IL
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I had a similar experience, but with a -12 carb. The drilled jet worked ok, but the pepperbox jet provided much better distribution. It is an extended design with a series of small holes that was designed to better mix the fuel and air and seemed to do a much better job than the standard jet. In my case, I just did the jet, not a full kit. My carb had no economizer circuit.

Larry
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  #4  
Old 07-13-2021, 07:22 AM
lr172 lr172 is offline
 
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Location: Schaumburg, IL
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Quote:
Originally Posted by Malndi View Post
Thanks Scott. Timely advice for me, having replaced the old and tired Precision 10-5217 on my 320-D1A with a new 10-5217 from Avstar, my climb fuel flow has changed from 12 to 10 gph and CHT is up a little, but cruise speed and fuel flow is unchanged. I have a standard airbox with a fixed pitch Sensi.

I donít know if my old carby was drilled or modded, but given the difference in climb fuel flow I suspect it was. Iím now wondering if the Mooney Mod is available for my configuration and if I should be considering doing it.

Edit: it appears the kit is not available for my configuration. Any advice welcome.
I don't remember the part number, but you can buy just the pepperbox jet for around $150.
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  #5  
Old 07-13-2021, 09:14 AM
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Ironflight Ironflight is offline
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Location: Dayton, NV
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Great post Scott! I must admit that I have always looked on the “drill the jet” advice from folks with great suspicion, probably because I didn’t take a “Propulsion” track when I did my Aero Eng schooling, but I learned more than enough fluid dynamics to know that shade tree mechanizing is as likely to make things worse than better (which means half the people will have good results, half will have bad…..).

I think the jet drilling school sort of comes from the early days of homebuilding, when people put “found” engines on whatever airframe they had cobbled together, and had to adapt to make things work. Heck, Van’s isn’t Van’s in the top three of Lycoming OEM suppliers now? In that case, I’d think that their engines work pretty well with the airframes represented here in this forum…..

Anyway Scott - thanks for the info!

Paul
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  #6  
Old 07-14-2021, 09:16 AM
krw5927 krw5927 is offline
 
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Location: Wichita, KS
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Good post, Scott. Very educational for everyone running carbs.

What is your typical EGT spread at WOT and properly leaned now? Does your EGT spread vary significantly with throttle position? Also does your sump have a lip (sump opening larger than carb)?

My O-340 with pepperbox jet carb and Titan-provided conical filler for the tapered sump throat seems about the best it can be. But depending on throttle position I can vary the EGT spread so that the front two cylinders (1&2) are as much as 200 degrees richer than the rears, or vice versa. There's a sweet spot about 1" away from the firewall where mixture distribution between front and rear cylinders is about even, but economy at that throttle position isn't great.

So for best economy, usually I fly around at higher altitudes with cylinders 3&4 at peak EGT while cylinders 1&2 are 100-200 degrees rich. I'm not sure what other mods to try at this point. CHTs are fine even climbing heavy on a hot day. I'm just burning probably 1GPH more than necessary in cruise.

Very interested in your EGT spreads, FF, and throttle/mixture settings for cruise.
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  #7  
Old 07-14-2021, 10:43 AM
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Planecrazy232 Planecrazy232 is offline
 
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Default Fuel flow

Scott,

What is your fuel flow on climb? I have the "Mooney Mod" also but I think I'm still slightly lean.
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  #8  
Old 07-14-2021, 11:00 AM
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rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
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Quote:
Originally Posted by krw5927 View Post
Good post, Scott. Very educational for everyone running carbs.

What is your typical EGT spread at WOT and properly leaned now? Does your EGT spread vary significantly with throttle position? Also does your sump have a lip (sump opening larger than carb)?

My O-340 with pepperbox jet carb and Titan-provided conical filler for the tapered sump throat seems about the best it can be. But depending on throttle position I can vary the EGT spread so that the front two cylinders (1&2) are as much as 200 degrees richer than the rears, or vice versa. There's a sweet spot about 1" away from the firewall where mixture distribution between front and rear cylinders is about even, but economy at that throttle position isn't great.

So for best economy, usually I fly around at higher altitudes with cylinders 3&4 at peak EGT while cylinders 1&2 are 100-200 degrees rich. I'm not sure what other mods to try at this point. CHTs are fine even climbing heavy on a hot day. I'm just burning probably 1GPH more than necessary in cruise.

Very interested in your EGT spreads, FF, and throttle/mixture settings for cruise.
On a recent test flight at 8500' (didn't look at the DA but it would have been higher... probably in the neighborhood of 10.5K)

WOT
2660 RPM
F.F. - 8.7 GPH

EGT's
1- 1497
2- 1456
3- 1488
4- 1452
Leanest cyl. was about 25 lean of peak

TAS - 171 kts

My EGT spread is wider at lower altitudes and lower power settings, but I don't pay much attention to those values because if I am low, it means I am not really going anywhere so I typically fly at about 55% pwr burning 5.5 - 6 GPH at about 125 kts TAS
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Opinions, information and comments are my own unless stated otherwise. They do not necessarily represent the direction/opinions of my employer.

Scott McDaniels
Hubbard, Oregon
Van's Aircraft Engineering Prototype Shop Manager
FAA/DAR
RV-6A (aka "Junkyard Special ")
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  #9  
Old 07-14-2021, 11:02 AM
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rvbuilder2002 rvbuilder2002 is offline
 
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Location: Hubbard Oregon
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Quote:
Originally Posted by Planecrazy232 View Post
Scott,

What is your fuel flow on climb? I have the "Mooney Mod" also but I think I'm still slightly lean.
I haven't looked at at any of the engine data yet.
I will try and make another post after I have.
__________________
Opinions, information and comments are my own unless stated otherwise. They do not necessarily represent the direction/opinions of my employer.

Scott McDaniels
Hubbard, Oregon
Van's Aircraft Engineering Prototype Shop Manager
FAA/DAR
RV-6A (aka "Junkyard Special ")
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  #10  
Old 07-15-2021, 09:13 PM
dbaflyer dbaflyer is offline
 
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Location: Iowa USA
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Also been looking into why my EGT spread is so small on 3&4. They seem to be running lean and get leaner as I pull the red knob back. Here is a flight at 3500 70% power and 23 MAP.

The first part is full rich and all CHT dropping as I was leveling off and gaining airspeed.

The overall spread from rich to lean:
#1 #2 #3 #4
175 210 70 65

This is on an O-320 with a 10-3678-32 model carburetor.

Do they make a kit for this model that might even out fuel distribution?
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