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  #21  
Old 04-03-2021, 08:34 AM
00Dan 00Dan is offline
 
Join Date: Nov 2016
Location: Atlanta, GA
Posts: 183
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Quote:
Originally Posted by lr172 View Post
9.5 GPH at 7500' is right around or a bit higher than max power (approx 75-100 ROP). I routinely run 8 GPH at 8000', which is around 20-40 LOP. Rejetting is not your answer, at least at cruise. You should remember that many carb'ed engine won't be happy LOP. If WOT, intake leaks have a small impact on mixture. Trying pulling back the throttle just a bit at cruise as well as adding carb heat to see if it helps to get leaner. Many seem to have some success with this. Have you checked your ignition timing and insured that each plug is firing?

Shortly after T/O, nose down and accelerate to 2500 RPM, still WOT. What is the fuel flow then? You're looking for around 13.5, give or take, at low altitudes. If you're getting that, no need to rejet.

Down low and WOT or max prop RPM, compare full rich EGT to peak EGT. If you are getting a delta of 200*, your jetting is fine.
Next time I fly Iíll redo the EGT test and Iíll try a higher RPM. I will say the other night as I was returning to the airport at 3000í I set FT and full rich. With that I saw about 11 GPH at 2680 RPM; I expected higher there given the relatively low altitude.
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  #22  
Old 04-03-2021, 10:51 AM
00Dan 00Dan is offline
 
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Location: Atlanta, GA
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Originally Posted by 00Dan View Post
Resurrecting this thread because I have some new info.

I was going through the engine log the other day and I discovered the builder at one point swapped the nozzle in the carb. It originally had a 47-773 and he swapped it for a 47-828. What sort of difference would this be expected to make?
I finally found the nominal diameters for these two jet sizes.

The 47-773 is .1065Ē (#36 bit).
The 47-828 is .0935Ē (#42 bit).

Obviously nozzle size isnít the only variable but when my carb was previously re-jetted that made a big step down.
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  #23  
Old 04-04-2021, 05:40 PM
00Dan 00Dan is offline
 
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Location: Atlanta, GA
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Did some testing today. After takeoff I pitched down and accelerated to 2500 RPM. This resulted in 11 GPH.

At 8500í I performed a lean test. Engine was in steady cruise at full throttle, 21.1Ē and 2640 RPM. OAT 33 dF.

At full rich fuel flow was 10.5 GPH. Full rich EGTs:

#1: 1390 dF
#2 1390 dF
#3 1520 dF
#4: 1460 dF

When leaning #3 was the first to peak at 1570 dF, approximately 9 GPH.
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  #24  
Old 04-05-2021, 09:06 AM
lr172 lr172 is offline
 
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Location: Schaumburg, IL
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Quote:
Originally Posted by 00Dan View Post
Did some testing today. After takeoff I pitched down and accelerated to 2500 RPM. This resulted in 11 GPH.

At 8500’ I performed a lean test. Engine was in steady cruise at full throttle, 21.1” and 2640 RPM. OAT 33 dF.

At full rich fuel flow was 10.5 GPH. Full rich EGTs:

#1: 1390 dF
#2 1390 dF
#3 1520 dF
#4: 1460 dF

When leaning #3 was the first to peak at 1570 dF, approximately 9 GPH.
WOW. 50* ROP at 8000 with full rich is quite lean. seems a jet change is in order, assuming you have confirmed that the mixture cable installation is allowing the arm on the carb to go full travel. Down low, WOT and 2500 should be well north of 11 GPH.


If I recall corrctly, I drilled out my jet to 38 or 39 and it is too rich at low altitude clmbs. I manually lean to a target EGT.

Larry
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Last edited by lr172 : 04-05-2021 at 10:21 AM.
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  #25  
Old 04-05-2021, 09:31 AM
00Dan 00Dan is offline
 
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Location: Atlanta, GA
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Quote:
Originally Posted by lr172 View Post
WOW. 50* ROP at 8000 with full rich is quite lean. seems a jet change is in order, assuming you have confirmed that the mixture cable installation is allowing the arm on the carb to go full travel. Down low, WOT and 2500 should be well north of 11 GPH.

That said, I don't see how full mixture at 21" is flowing less than 29". Might be worthwhile to view the SB on the two piece venturi's to confirm that is not the issue.

If I recall corrctly, I drilled out my jet to 38 or 39 and it is too rich at low altitude clmbs. I manually lean to a target EGT.

Larry
Larry Vetterman sent me some information about the 3678 series carbs from a while back. As I understand it, once sufficient airspeed is achieved ram air is pressurizing the bowl causing it to lean out. Thatís the most likely explanation for the flow difference, seeing as both were at WOT and the MP change was from atmospheric lapse.
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  #26  
Old 04-05-2021, 10:22 AM
lr172 lr172 is offline
 
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Quote:
Originally Posted by 00Dan View Post
Larry Vetterman sent me some information about the 3678 series carbs from a while back. As I understand it, once sufficient airspeed is achieved ram air is pressurizing the bowl causing it to lean out. That’s the most likely explanation for the flow difference, seeing as both were at WOT and the MP change was from atmospheric lapse.
The bowl vent is not exposed to ram air. If it was, the excess pressure on top of the fuel in the bowl would cause a rich condition, not lean. The same way that higher float settings cause rich conditions.
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  #27  
Old 04-05-2021, 10:48 AM
00Dan 00Dan is offline
 
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Location: Atlanta, GA
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If my current jet is nominally a #42 (on paper, Iíll verify once I have it out), should I go one size at a time or jump to a #40 on first go? I have a #40 reamer if that makes it preferable to a drill bit.
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  #28  
Old 04-11-2021, 08:40 PM
00Dan 00Dan is offline
 
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I reamed my jet out from #42 to #40. I also addressed some throttle shaft play by having one of the bushings replaced - itís nice and tight now.

The two size step brought my top end fuel flow up about 0.5 GPH (low altitude climb), and my leanest cylinder is now 100* ROP at full rich (8500í, WOT).

Obviously I could stand to go a little bit richer just looking at the total fuel flow, however I wonder if it is worthwhile to try and chase mixture distribution for the rear two cylinders. Iím aware itís a common problem with carbureted Lycs and can in many cases be attributed to a limitation of the design due to the intake geometry. Is there anything worth trying here before I just accept it and pour in more fuel?
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  #29  
Old 04-12-2021, 12:17 AM
OKAV8r OKAV8r is offline
 
Join Date: Feb 2007
Location: Seattle
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I have an almost identical situation.
50 deg ROP @ 8000, MS MA4 SPA 10-5217
Anyone have a repair manual for the carb that I could buy or borrow?
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  #30  
Old 04-12-2021, 08:19 AM
lr172 lr172 is offline
 
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Location: Schaumburg, IL
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Quote:
Originally Posted by 00Dan View Post
I reamed my jet out from #42 to #40. I also addressed some throttle shaft play by having one of the bushings replaced - it’s nice and tight now.

The two size step brought my top end fuel flow up about 0.5 GPH (low altitude climb), and my leanest cylinder is now 100* ROP at full rich (8500’, WOT).

Obviously I could stand to go a little bit richer just looking at the total fuel flow, however I wonder if it is worthwhile to try and chase mixture distribution for the rear two cylinders. I’m aware it’s a common problem with carbureted Lycs and can in many cases be attributed to a limitation of the design due to the intake geometry. Is there anything worth trying here before I just accept it and pour in more fuel?
The standard is 200 ROP when full rich, WOT at sea level. It is nice to have that margin for detonation prevention on Take off. You should be more worried about WOT SL than at cruise, as that is where you will have the most problems from being lean. Did you by chance measure the float height while the carb was apart? Fuel flow is determined mostly by jet size and fuel height in the bowl for any given airflow rate. A float that is slightly low will cause lean running. Even 64ths and 32nds can make a meaningful difference. Always measure float height when the carb is apart.

Larry
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N11LR - RV-10, Flying as of 12/2019

Last edited by lr172 : 04-12-2021 at 08:24 AM.
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