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carb rejet

Frank N821BF

Active Member
I have a Precision model MA-4SPA on a O-320-D3G in my 9A with dual Lightspeed ign. Can any one give me some feed back on rejeting.
 
Drill it

Frank N821BF said:
I have a Precision model MA-4SPA on a O-320-D3G in my 9A with dual Lightspeed ign. Can any one give me some feed back on rejetting.
Yes you can drill the main jet one or two drill bit size larger (or more if needed). You will need some new tab washers and gasket. You need to take the bowl off and main jet out of the body. The tricky part is the MA-4SPA jet is I recall it has a radius in the orifice, and when you drill it will make a sharp notch. How that affect the fuel flow who knows, but your may want to go in an carefully break that shape edge. Treat is like jewelery.

Look its a updraft crab like a tractor and about as sophisticated an old fashion bug spray gun. The main jet is really just a tube. No big deal. You can't hurt it much by drilling the jet, except by over doing it. I would only go two sizes up at most at a time. I did mine long ago and forgot but I needed to go back twice to open it up to get it right. I forgot the exact drill number, I think it started out as #42 and went #40, than #39 or #38, but measure the jet diameter and go up two drill sizes from that equivalent to start.

Before drilling get a magnifier out and look at the details. When you drill use high speed and low pressure with some dry drill lube (like Boelube). Some have gone to bigger (#37, #36 I think?), but you might like to go incremental. Some have gone too big (rich). You can solder the orifice with silver solder and re-drill. Again pay attention to the detail, at the inlet and exit.

It made a huge difference in my O320, and the engine run much better. Better and when I leaned I got EGT rise. Before it would just start to run rough.

The other solution is to call Precision Airmotive and buy a new rich carb part number. I don't have a part number, but recall 10-5217 or 10-3678-32, CARB ASSY MA-4SPA. It's a bigger buck route to replace the carb. There is no guarantee it will be perfect. You did not give the part number, there are several MA-4SPA's, some richer than others. In fact I suggest you call them and get the parts you will need to replace. They may be able to sell you a "kit" with a richer jet but you are looking at $200 to $300 and no guarantee it will be perfect, but you have to call. They know the story. Depending on who you get at Precision, they may say don't drill the jet, or its OK. If it where me I would look into getting the proper jet first, however many have drilled the jet with good results. The trick is not making the geometry ODD, scores in the bore by drilling poorly or going too large.

THERE IS ANOTHER ISSUE. Some MA4-5's have single piece Venturi. It was required to replace the two piece. However the replacement one piece where made of poor quality castings, which caused the carb to run lean. This will cause lean running regardless of the jet. Actually a two piece is better from a flow stand point. I guess some two piece venturi's came separated. Unfortunately the early single piece units where worse. There is some history there to research. Get the carb part number and "mod" status of your carb, and I might be able to look it up. You can called Precision Airmotive yourself and get help. Just give them a call. Make sure you have all the data on your engine and Carb (off the data tag).

How do you know you are too lean? When you shut down do you get a 50 RPM rise when you lean slowly to cutoff? Do you get at least 150EGT rise when you lean from rich at 75% power? Some people have problems and some don't. If your plugs are light grey and you can't lean at all you need to do something. To recap: Drill Jet, get bigger jet or get new richer carb model. G
 
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Carb

Thanks for the info. I have part #10-5217 ser# BL198084, when I was breaking in the engine it seemed like I could lean the engine, but as the oil cunsumtion became less so did the leaning. My problem is at 75% when I try to lean even a small amount(1-2 turns) the egt goes up 50-80deg, thenit drops off(LOP?) I did put a call into Precision and should get a call back tomorrow.
Frank
RV-9A 150+hrs
 
Good one

Frank N821BF said:
Thanks for the info. I have part #10-5217 ser# BL198084, when I was breaking in the engine it seemed like I could lean the engine, but as the oil consumption became less so did the leaning. My problem is at 75% when I try to lean even a small amount(1-2 turns) the egt goes up 50-80deg, then it drops off(LOP?) I did put a call into Precision and should get a call back tomorrow.
Frank
RV-9A 150+hrs
Yea that is the GOOD part number #10-5217 (I think), and 50-80deg rise is a bit too lean but not horrible.

Good Luck with it. I found a refrence about that carb having a 828 jet and recommendation of going to a #773, OR drilling out the existing jet two # sizes. Also do you have any stamps in the metal data tag on the carb. You will see letters stamped in it signifying if it has metal float or one piece venturi. Make sure you let Precision know those letters.

If I can help anymore let me know. G
 
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Frank,
At what altitude are you getting the 80-degree egt rise, from rich to peak, when leaning? Also at what power setting?
Good Luck,
Mahlon
"The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at you own risk."
 
Not trying to hijack the post but i'll add my experience and ask a question about mine too. I have a Precision #10-5217 MA-4SPA that was running lean. I drilled from #42 to #38. I can now get I would guess about a 200 degree rise out of 2 and 4 but only 75ish out of 1 and 3 (both right side cylinders which have always been the leanest and run slightly hotter CHT). I say guess on 2 and 4 because it starts running rough and losing RPM before I can get them to really top out. I have been thinking about going to #36 or #37. Question is, should I? I think yes because I still cannot get a very big rise out of 1 and 3. This is leaning at 2400-2500 rpm and 4500 feet (FP prop). I do have 9:1 compression (H2AD pistons) and dual electronic ignition so "should be" in the upper 160's for horsepower.

Thanks,
Scott
#90598 - N598SD Flying - 23 hrs
 
Scott, If you can get 75 degrees out of the leanest cylinder at altitudes at or below 2000 ASL then to me, when you are full rich in cruise, you are running the leanest cylinder at least 75 ROP when full rich. The other cylinders are richer and as you climb they all get richer. Here at the shop, my cutoff is the leanest cylinder must be 65-75 ROP when full rich at 2000 feet or below on days that have an ambient surface temp of 65 degrees or less. To me you are rich enough when full rich at that point. It sounds to me as if you meet the criteria, or are very close to the criteria I use to say its OK here in the shop. Here at the shop, we swap carbs to get richer or leaner, we don?t re-jet. Re-jetting is not recommended by Precision or Lycoming.
Good Luck,
Mahlon
"The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at you own risk."
 
Well they ahve mellowed

mahlon_r said:
Re-jetting is not recommended by Precision or Lycoming.
Good Luck,Mahlon
Depends on who you talk to. They are thinking liability. I remember 6, 7 or more years ago I called Precision about a MA-4SPA, and I thought the guys head would explode as he told me how terriable that idea was. That is the company line. However I am happy I did it.

Now after all these years they know RV's sometimes a carb needs to be re-jet or drill. My last and recent conversation with them was about my MA-4-5 (for a O-360) was reasonable and helpful. They recommended I drill the jet and told be even the Mooney jet kit may not be enough.

The fact is RV's are fast and have free flow induction and exhaust and can run lean. YOU GOT TO DO SOMETHING despite what Lyc and PA say. I like Mahlon's idea of swapping Carbs to get a richer one, but that is not an option for many of us. IT does show that between carbs of the same type/model are richer or leaner than others.

G

Scott DellAngelo said:
I have been thinking about going to #36 or #37. Question is, should I? I think yes because I still cannot get a very big rise out of 1 and 3. This is leaning at 2400-2500 rpm and 4500 feet (FP prop). I do have 9:1 compression (H2AD pistons) and dual electronic ignition so "should be" in the upper 160's for horsepower. Thanks,
Scott #90598 - N598SD Flying - 23 hrs
With all those mods I think going to #37 is a reasonable thing to do. I think you sound just a tad lean but Mahlon says about right, and he knows more than I do. I would just say you may want to take the carb out and have it checked over. Make sure the venturi is in properly (correct hight), float and over all health is double checked. However with you HC pistons, EI, more fuel is not a bad idea. I have heard of several people needing to go over #38. So its a reasonable thing to do. You may need #36, but I would go #37 and make sure you have a radius/break sharp edges (subtle) in the jet orifice exit / entrance. I had to go into my O320 Carb twice myself. The results where I was very happy and cold get 150 rise in 75% cruise and the engine RPM went up 50 RPM when leaning during shut down. Disclaimer: Do it at your own risk. :D G
 
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Drilling out the jet?

It made a huge difference in my O320, and the engine run much better. Better and when I leaned I got EGT rise. Before it would just start to run rough.


My O-320 E3D with MA4-SPA 10-5009 has had a rough idle and I have had the problem up here on the forum. I've done everything possible to find the problem but it is still running a bit rough at 600-700 rpm. I saw you post here and one line got my attention: "It made a huge difference in my O320, and the engine run much better. Better and when I leaned I got EGT rise. Before it would just start to run rough". When leaning my engine I don't get the usual 50 rpm rise when leaning from 1000 rpm to cut-off. I've tried to turn the idle mixture screw out five turns and there is no change so I was thinking, maybe I should drill out the jet?
 
My O-320 E3D with MA4-SPA 10-5009 has had a rough idle and I have had the problem up here on the forum. I've done everything possible to find the problem but it is still running a bit rough at 600-700 rpm. I saw you post here and one line got my attention: "It made a huge difference in my O320, and the engine run much better. Better and when I leaned I got EGT rise. Before it would just start to run rough". When leaning my engine I don't get the usual 50 rpm rise when leaning from 1000 rpm to cut-off. I've tried to turn the idle mixture screw out five turns and there is no change so I was thinking, maybe I should drill out the jet?

The main jet has little to do with idle mixture other than allowing fuel to pass through it to get to the idle circuit. It’s generally accepted that the main jet starts to come online at 1300-1500 RPM.

Have you looked at fuel float level? Low fuel level will make a very lean idle mixture, or the accelerator pump check valve if it’s dirty may be allowing fuel to bypass and make your mixture very rich.
A couple of things to look at......
 
Lycon

Lycon in Visalia ca, will wet flow your carb and based on your info of your motor and so on and will re jet and wet flow after and when I did it the costs was all of 213.00 bucks with shipping.. knocked 20 degrees the cht all four of them. Motor ran perfect just bolting on carb . Lycon I recommend
 
Lycon in Visalia ca, will wet flow your carb and based on your info of your motor and so on and will re jet and wet flow after and when I did it the costs was all of 213.00 bucks with shipping.. knocked 20 degrees the cht all four of them. Motor ran perfect just bolting on carb . Lycon I recommend

It is not the motor it is the very high efficiency inlet (and ram rise of a fast plane) and high efficiency header/exhaust. If they guarantee their re-jet is good and will redo it for no charge if they miss the mark, that is $213 well spent. I am pretty sure they know what RV's need. You can do it your self. You can tell if it is running lean and can drill the main jet incrementally....

As far as the idle circuit, you can run lean at idle. Lean is good at idle. You need rich mixture for cooling and detonation prevention at higher power settings where the main jet comes into play.
 
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