Lean/Rich Carb

All this talk lately about hight CHT's associated with an overly lean carb on climbout, I started wondering about my CHTs. I have alleviated some of my high CHTs but would like to see them a little lower in climb. I have been climbing out at about 124MPH or 108 kts. On any day over say, 70 I'll see my CHT's hit 410-420 and level off at cruise low to mid 370's. I am using the Dynon insert CHT probes in the bottom holes of my ECI.....Group A BTW, cyls.

The carb on my O-360 is a MA-4-5 10-5193 It is listed as used on an AIA, A4M and A4A. I have spent considerable time this morning cruising the internet looking for technical data to no avail. I am seeing others talk of the reaming the jet to a number 37 on other 4-5 carbs. Anyone know anything special about this carb? I have it set so I get an RPM increase just prior to shutdown. Everything seems fine as I can successfully use the Dynon leaning page to lean ROP or LOP and really get the fuel flows down nicely.

10-5193....any quirks on this baby?

those temps are good.. I wouldn't mess with anything I believe that's a "good" (rich) carb like the 4164...

Dana, try this
http://www.vansairforce.com/community/showthread.php?t=15485
I've seen similar temps with my 10-3878, and also thought they were a little hot. Perhaps we both just have the wrong carb.

Dana
I think your temps are in line with the majority. I had the 3878 on my O360 RV6 and the 4164 on my latest RV8. Those are the same numbers that I have seen on the last 3 180 hp RV's.
I believe that an occasional posting of " I've never seen over 350 degree's", is leading people to believe that, that is the norm.
If you talk to Lycoming and tell them your temps they'll say "whats your problem? Redline is 500, max continuous recommended is 400 or less.

I just bought a 3878 today on this forum to go on my new ECI O360 for my soon to be flying RV8.

Good Luck

Jon

CHT's

I have been climbing out at about 124MPH or 108 kts.

Hi Dana, these are travelling machines pick that climb speed up to 120 Kts.

Dana,
There are lots of opinions about this topic.
I found John Deakin's column on AV Web to help me.
Lots of good reading on the subject.
link here gets right to the point:
http://www.avweb.com/news/pelican/182176-1.html?type=pf

Quoting:
"I will also repeat from the previous column, YOU MUST BE GETTING FULL REDLINE FUEL FLOW at full takeoff power AT SEA LEVEL! In truth, the factory redlines are often a bit on the low side, and flows should be tweaked up just a bit. Even a half-gallon per hour makes a BIG difference in CHTs during climb.

In general, and speaking very roughly, if you see EGTs anywhere over about 1,300?F (lower will not hurt a thing and is probably ?better?) during a sea-level takeoff, or CHTs above about 360?F right after takeoff, YOUR FUEL FLOW IS TOO LOW. Having a good understanding of the proper relationship between the EGTs and the fuel flow at very rich mixture settings will always give you a good cross-check on whether or not you are getting adequate fuel flow -- even if your fuel flow needle breaks off and falls to the bottom of the instrument!

If your fuel flow is too low on takeoff, don't let your mechanic talk you out of setting it up a bit, and if necessary, find a mechanic who will do it. VERY few mechanics understand the importance of this. Far too many mechanics consider the fuel flow redline a maximum, and they consider a little bit less as "good," or "better for the engine," thinking they are being "conservative." After all, we all like to stay a little bit (or a lot) away from all sorts of redlines, right? That fuel flow redline is a MINIMUM, not a maximum. Treat it as such. Quote.

I was told to drill the main jet out 3 to 4 thousands at a time until you get the temps you desire. This is assuming you have addressed any other airflow cooling issue you could.
It worked for me. Good Luck.

My vote also is to climb around 120 to 130 knots - you will see notable drops in cht. Another way to look at it is that you will have 130/108 = 1.2 times as much cooling air. I'll climb at 80 or 90 knots during the winter - because it is fun! But, not when OAT's are above around 40 or so F.

Main Jet

Bob Martin said:

I was told to drill the main jet out 3 to 4 thousands at a time until you get the temps you desire. This is assuming you have addressed any other airflow cooling issue you could.
It worked for me. Good Luck.

Click to expand...

I haven't looked at the internals of the carb yet, but it it possible to get an extra main jet to drill or is it kind of built in?

I would rather drill a spare so I could go back if I wanted.

Main jet spare

bhester said:

I haven't looked at the internals of the carb yet, but it it possible to get an extra main jet to drill or is it kind of built in?

I would rather drill a spare so I could go back if I wanted.

Click to expand...

Yes, that is exactly what I did. The enrichment kit (part #666-660)comes with a new main jet as well as several other items. That give you a spare in case something goes wrong. This is assuming you are starting with the 10-3878. If you already have the 10-1464, then yes again, you can order a new jet alone. I don't have the parts book here but I believe they only make 3 different size jets for MA4-5, but you can asked when ordering.
The main jet is not built in......it actually look like a small flute about 3 inches long.

Main jet

Here are pictures of my carb. Picture 1 showes the jet.

http://picasaweb.google.com/tc1234c/8222007CarbMod

I believe the 10-5193 is the richest of the O-360 carbs. I would not be concerned with the temps you quoted. 420F in prolonged climb and 380 in cruise are rather normal temps in my book.
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 your own risk."

Thanks Mahlon

Its usually the second takeoff that gets outta reasonable limits.

Or

After you stop on a crosscountry and get fuel and leave before the engine has cooled any.

CHT's will go over 500 deg real quick.

mahlon_r said:

I believe the 10-5193 is the richest of the O-360 carbs. I would not be concerned with the temps you quoted. 420F in prolonged climb and 380 in cruise are rather normal temps in my book.
Good Luck,
Mahlon

Click to expand...

Coming from you, I am feeling better with the numbers, thanks.

Cary, I am anxious to see if the cowling/baffle mod works for you. Please keep us posted.
Good Luck,
Mahlon

caryr said:

Thanks Mahlon

Its usually the second takeoff that gets outta reasonable limits.

Or

After you stop on a crosscountry and get fuel and leave before the engine has cooled any.

CHT's will go over 500 deg real quick.

Click to expand...

Main Jet Spare

Bob Martin said:

Yes, that is exactly what I did. The enrichment kit (part #666-660)comes with a new main jet as well as several other items. That give you a spare in case something goes wrong. This is assuming you are starting with the 10-3878. If you already have the 10-1464, then yes again, you can order a new jet alone. I don't have the parts book here but I believe they only make 3 different size jets for MA4-5, but you can asked when ordering.
The main jet is not built in......it actually look like a small flute about 3 inches long.

Click to expand...

Where did you order the enrichment kit and how much was it? Do you remember how much of a difference it made?
Thanks!

Where from and how much

bhester said:

Where did you order the enrichment kit and how much was it? Do you remember how much of a difference it made?
Thanks!

Click to expand...

I ordered mine some time ago from Penn Yann Aero, but I bet the dealers have changed now since Kelly has taken over the carb lineup. A previous poster talked about them. I thought the kit "was" about 250. But I saw a previous post mention almost 300.
From what I remember I got about 25 degree reduction from the kit and another 25 degrees from one drilling. The kit alone isn't enough or wasn't enough for my installation. I highly recommmed the kit as a starting point.
The new jet that comes with the "kit" is almost the same size as the original jet in the 10-3878. Even though the jets had different part numbers. The kit come with a tiny bushing that I believe chokes off some air, therefore enrichening the mixture. So it keeps you from having to drill an even bigger jet hole.
I started with temps at 450 and they dropped to 425 with the kit and now they are at 400 with one drilling. I will soon drilling again with this condition inspection.
These temps happen when I climb hard, I agree with others that you can keep it cooler with shallower climbs and I don't normal climb hard, but if I have to or want to and the plane can do it, then I don't want to overheat things if it can be avoided. From what I know now, I feel this should be part of the original airplane set up.
Just my 2 cents. Good Luck.

RV7Aflyer said:

Hi Dana:

Now, with over 100 hrs, I can climb out at 90 Knots with the CHT's only approaching 400 degrees. CHT's top 360 degrees in cruise. Also, OT runs 170-180 degrees during the summer months.

Do you have electronic ignition? Maybe that is part of reason for CHT's being somewhat higher.

All the Best

Click to expand...

No, I have twin mags. I'll know more this weekend as the weather is supposed to be much warmer. I'll adjust my climbout and see what happens but it appears my numbers are well within the satisfactory range.

This is what I found on my 10-3878 carb

After reading all the posts about the 10-3878 carb possibly being too lean I decided to take mine apart and drill the main jet to a #38.

I found my main jet to be a #32
That's about .0145 larger than a #38

It has a nice radius leading into the hole and under magnification shows no signs of being drilled.

I have the service manual and the main jet is stamped 47-739 which is correct for a 10-3878 carb. The carb came with the Mooney engine (0360-A1D) I bought salvage. I have the yellow tag for the carb from when it was overhauled in the late 90's.

I take it from some earlier posts that others have found the #47-739 main jet to be a #41. That would indicate that mine has been modified at some time.
I wonder if an overhaul shop would really check the size of the hole before reusing the jet, I'd bet not.


Mark

Dana,

I started with a new carb, p/n 10-3878. My temps were also high and my EGT's peaked with only a small change in the mixture. Here's my post from the RV list...


When I first flew my RV-6 (O-360-A1A, Sensenich, carbureted), the CHT's
were very high, and the EGT's peaked as soon as the mixture was pulled back
at all. My engine was overhauled and the carb was new, the same part number
as the original, and as specified on the TC for the engine (I forget what
that was). When I mentioned it to the accessories guy at our local engine
shop, he said to put a service kit in it to move the fuel schedule one step
richer. I had that done and that fixed the problem. I was told this mod was
meant for carbureted Mooneys, some of which had the same problem. I'm sorry
but I don't know the kit number. The shop was Bolduc Specialized Aviation
Services in Minnesota.

It sounds as if you have a richer carb already, so as some posters have said, other factors may be at work. By the way, idle mixture in no way affects mixture above 1700 rpm or so.

Data Point

Mark Burns said:

After reading all the posts about the 10-3878 carb possibly being too lean I decided to take mine apart and drill the main jet to a #38.

I found my main jet to be a #32
That's about .0145 larger than a #38

It has a nice radius leading into the hole and under magnification shows no signs of being drilled.

I have the service manual and the main jet is stamped 47-739 which is correct for a 10-3878 carb. The carb came with the Mooney engine (0360-A1D) I bought salvage. I have the yellow tag for the carb from when it was overhauled in the late 90's.

I take it from some earlier posts that others have found the #47-739 main jet to be a #41. That would indicate that mine has been modified at some time.
I wonder if an overhaul shop would really check the size of the hole before reusing the jet, I'd bet not.


Mark

Click to expand...

I ordered a new nozzle, P/N 47-739 which is the correct part number for the 10-3878 MA4-5 carb. It arrived yesterday and it's main jet (bottom hole) is a #32, just like my old one.

Data Point

Mark,
I just drilled my jet to #29......hopefully the last drilling I need to do.
I hope to flight test this weekend.....Last weekend was unbearably hot. 101 degrees. The ground run-up checking for leaks went fine with no on ground leaning required. so I'm hopeful the temps will be good. I'll report back after flight tests.

Thanks

Bob Martin said:

Mark,
I just drilled my jet to #29......hopefully the last drilling I need to do.
I hope to flight test this weekend.....Last weekend was unbearably hot. 101 degrees. The ground run-up checking for leaks went fine with no on ground leaning required. so I'm hopeful the temps will be good. I'll report back after flight tests.

Click to expand...

Bob,
Thanks for the information!
I may drill mine to a #31 or #30 before first flight.
I don't want to be lean.

I'm going to post the new nozzle (main jet) I bought in the classified section.
API charges a $25 restocking fee so I thought I would just pass a $25 savings on to someone on VAF that might want it.

Mark

I have made the plunge to drill out the main jet also.

I was running way too hot on CHT's.

I won't rehash the old stuff.

The nozzle was a # 32 and I drilled to a # 28. And I may have overshot it.

Ground observers said I was trailing black smoke on climbout.

Is my only fix a new jet??

The upside is that I shaved 100 degrees off the hotest CHT and about 40 degrees off the oil temp.

I never saw above 340 on #4 cht.

Mahlon

can I change the 'stop' position on the mixture lever/control to cut back the flow on the upper limit??

Jet Drilling

caryr said:

I have made the plunge to drill out the main jet also.

I was running way too hot on CHT's.

I won't rehash the old stuff.

The nozzle was a # 32 and I drilled to a # 28. And I may have overshot it.

Ground observers said I was trailing black smoke on climbout.

Is my only fix a new jet??

The upside is that I shaved 100 degrees off the hotest CHT and about 40 degrees off the oil temp.

I never saw above 340 on #4 cht.

Click to expand...

Cary,
Sounds like you just may have overshot it a bit.
Remember each larger drill size allows a larger change because of the diameter
of the hole or maybe it is better to say the circumference is larger.
Thats why it is better to go slow and drill a couple times.

I drilled to #29 as posted earlier with good results, but.
I said I didn't have to lean on the ground, which is true, but I did have to lean
a little to get a good run up......my field elevation is 640 and the DA lately has been around 2400.
But I love that I can climb hard (if I want to) and only see 1300 degrees EGT's and chts under 400.....and most way under.

I have heard of guys soldering up the hole and redrilling it. But I have no experiance doing that. If it were me, I'd order a new jet.
I understand there are different size jets available so maybe you can find out which replacement jet is closest to #29 or #30 drill size and you wouldn't have to re drill it and you would start off with a factory machined hole.

FYI.......I bought the enrichment kit # 666-660 for about 240 dollars and it comes with a new main jet just a tiny bit larger than the original jet in the 10-3878 carb. but it also has a new bowl gasket and a bushing to reduce the size of an air passage to enrichen the mixture without upsizing the jet so much. (I think) So if you are planning to buy a new jet, and you had the 10-3878 carb, you may want to get the kit and start from there.

As a temporary fix.......try leaning before take off to stop the black smoke.
If you looked at the parts when you removed the main jet to drill it.
You will see the mixture lever opens and closes a shutter type assembly
that controls the amount of fuel the main jet can suck from, so it is a back door approach to leaning the main jet.
Good Luck

caryr said:

Mahlon

can I change the 'stop' position on the mixture lever/control to cut back the flow on the upper limit??

Click to expand...

Moving the mixture control leaner will lean out the engine but it might not be a perfect amount for different power settings. Never experimented to see if you get the same amount of leaning at different power settings for the same amount of control movement.
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 your own risk."

I have got a little better data today on the lean carb issue.

I had screwed up some reassembly on the carb parts. Had to rework it all yesterday.

So,

I'm real happy with the #28 orifice right now.

It was 86 OAT and I was seeing 375 on CHT on climbout and 160 on oil temp.

I could run fulll throttle as long as I wanted and not see all those out-of-range flashing lights everywhere.

If anybody is doing a carb setup in a RV, I would highly recommend going ahead and setting it up richer than stock to start off.

Been there done that

First find out if your jet is too lean (you probably are)
http://www.vansairforce.com/community/showthread.php?t=20875&highlight=high+egt&page=2
http://www.vansairforce.com/community/showthread.php?t=18000&highlight=jet

Don't waste your money on the 666 jet kit (for 1960's Mooney's). Yes richer but often still not enough, it cost a lot of money and drilling your jet is easy and fun (and cheaper) and works. I know folks who bought the +$200 kit and ended up still drilling it out. You don't don't need a spare jet; they cost too much and they don't wear out. http://www.vansairforce.com/community/showthread.php?t=30111&highlight=drill+jet

http://www.vansairforce.com/community/showthread.php?t=6246&highlight=lean

How To:
Pull the main jet out (remove bowl, pry jet lock tab down and unscrew). The jet is right there, a brass straw angled down between the floats. Hand fit the largest # drill bit you can, than drill one or two # size larger (we are talking thousands of an inch). When you drill the jet use some dry drill lube and high RPM. You can use a hand electric drill. It does not have to be air-drill fast. Don't go crazy going back and forth. You don't need a drill Jig since you are really reaming up an existing tube. Drill one size up at a time. Don't go to final size, which as I say is usually 1 to 3 sizes higher than the stock size.

Clean-up burr or sharp corner (don't go crazy radius-ing or chamfering). You may or will needs some parts. Bowl lock washers, main jet lock washer, main bowl gasket. Some times you can get by with reusing parts like gasket if the carb is newer. If an older carb plan on replacing parts. Sometime a lock tab is good for one bend, but I would not do it, especially the jet lock tab washer. The jet probably will not fall out but you don't want a piece of metal floating around / sitting on the bottom of the bowl. Clean the jet and reassemble. Don't forget to not where the cables attached to what hole. Its easy to put them in the wrong hole.

It usually takes two iterations to get it right. Its easier to drill up in small steps than go too big at first. BTW people have repaired their jet by brazing the hole closed and than re drilling. I would just say go in small steps, even if it means taking your jet out twice.

This is not an exact science but not rocket science either. The jet is just a brass fitting with a whole in it, albeit a calibrated hole. Once you drill it out, put it together and fly the "test" and note EGT. You are looking for the ability to have at least 150F ROP at WOT at 8000ft. 175-200F ROP is OK or actually good. However more than that is too rich.

You first iteration oversizing the jet ALL DEPENDS ON HOW LEAN YOU ARE.

My old O-320 RV-4 was at almost Peak EGT at WOT, 8,000' (+75% power). Any movement of the mixture control caused the EGT to almost drop immediately. That is typical of RV's. Also my plugs where very light. I knew I was very lean. Drilling allowed me to lean to peak at 8k WOT and set econ or max power (100-150F ROP). With full rich in the WOT 8K condition you want about 175F ROP. I was able to only get about 150-160F with full rich, still a tad lean but not bad, so I stopped there. Remember at full power you need that extra rich mixture you see at 75% cruise. So ROP with a margin of 175F ROP is pretty good number.

Part of the lean issue at 8k is the RAM air affect in cruise. I rationalized that my 150F ROP (should be 175-200F) with full rich was OK, since on takeoff there is no RAM air (or a lot less). At sea-level and cool temps I was probably still a little lean on initital takeoff with 100% power. However I throttle back by 1000ft and all is well. The issue with 100% power and not having sufficent FF is you lose a little power but also some detination margin. If running AUTO gas you don't don't want to be lean since it has lower octan.

On my RV-4 I did notice a slight increase in performance. I had a CS prop so didn't notice increase in RPM of course, but for fixed prop RV's you'll no doubt get a little more RPM due to increase in power from richer mixture on takeoff. I did notice lower CHT's in summer and the plugs looked normal with a light carmal color.

You can get TOO rich. If mildly over rich, you'll just use a tad more gas at full power (a big deal now). Your gas milage depends on your judicious use of those little black and red knobs, lean or rich jet. Of course if you go crazy and over oversize the jet, you can loose power and cause engine grief from engine deposits on valves, pistons and fouled plugs. Not good either. Too lean can cause engine damage in the form of burned valves, loss of power or worse, detonation, which can cause serious damage to piston, crank and lead to engine failure.

Don't be afraid its a neccesary evil of the RV's. Factory planes that are slower, have more restiricve intake and exhaust just don't need a jet as rich as we do. If you look at all the installation loss of a typical C-172, say with a claimed 160HP, it might only get 140HP max! Than add 20% loss for the prop efficency, the C-172 might only has about 112HP at the prop. Aftermarket exhaust makers and cowl makers can improve slower factory planes. One after market manufacture claims an increase of 15HP for their exahust. That does not make a 160 HP engine 175HP, it just gets back all the restrictive loss of the stock pipes closer to the rated 160HP (on the test stand).