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MA-4-5 PN 10-3878 Jet size?

maus92

Well Known Member
My airplane has the "certified" version of the Lycoming O-360 that was new in 1999, and AFAIK it has the carb that was included with the engine (see PN in the header.) I have been investigating high 420+ CHTs on takeoff, and relatively high ~360-380 in cruise during hot and humid conditions (aka East Coast summers) at relatively low ~3K-ish altitudes. I've replaced cowl seals, plugged gaps with Fireseal and red RTV, and even added a cowl flap - all with little change in temps. I did a pressure differential test with a manometer several years ago, and it *seemed* OK - but will run the tests again.

Anyway, Alan from ASA suggested I check my FF at takeoff power. Luckily I've retained the data from the G3X system, and examining flights over the past 2 years reveals the FF at takeoff ranges from 14.9 - 15.9 GPH, which I understand is low. I've also read that there are "lean" carbs that came stock with O-360s engines. How can one determine if their carb is a "lean" model, or inappropriate for a Vans RV-8A installation? I'm beginning to think re-jetting might be applicable in my situation.

Thanks in advance for helpful replies!
 
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If it helps I had the same problem with this carb on an O-360. Need to up the jet size.
 
Give us a clue, Lou?

My airplane has the "certified" version of the Lycoming O-360 that was new in 1999, and AFAIK it has the carb that was included with the engine (see PN !

There are dozens of O-360 variants. If you want help, it helps if you give us detailed information. What variant of the O- 360 do you have? Try walking into your local auto parts store. When the guy at the counter asks for year make and model you reply my car is blue! What you just told us is the equivalent. Have you checked for possible intake vacuum leaks?

Charlie
 
There are dozens of O-360 variants. If you want help, it helps if you give us detailed information. What variant of the O- 360 do you have? Try walking into your local auto parts store. When the guy at the counter asks for year make and model you reply my car is blue! What you just told us is the equivalent. Have you checked for possible intake vacuum leaks?

Charlie

It's a O-360-A1A. I have not checked for an induction leak other than inspecting the engine for stains. Seems to idle smoothly, but I'm pretty sure the original seals / hoses have never been replaced.
 
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Useful documents

Charlie,
Two documents that I think you'll find useful. First is Lycoming service letter L 254. Second Lycoming service instruction SI 1523 G. I believe you can download both of these off of Lycoming's technical publication site. If not send me a private message with your email address and I'll send them to you. Do you know what year your engine was produced? Another thing to check for, is a possible restriction in the fuel system somewhere. I would check the fuel filters for possible restriction as a first step.
The parts catalog shows two different part numbers for the carburetor for your engine. P/N 71098 & 71710.

Charlie
 
Charlie,
10-3878 is the part number 71098. That number's been superseded again according to SSP 499C (page 118). the new number is 61C26046.

Charlie
 
Charlie,
10-3878 is the part number 71098. That number's been superseded again according to SSP 499C (page 118). the new number is 61C26046.

Charlie

I dug up the original paperwork for the engine, and the P/N at the time was indeed 71098 as built in 1999.

UPDATE:

I found a relevant thread on VAF, and it confirms that I have a "lean" carb. Two routes: order a "Mooney mod" kit, or drill / ream the jet. Having never worked on these carbs before, I'll probably get the shop on my field to do the drilling - already talked to the owner earlier today. He just needs to know the size I want, and I'll most likely go with #30, or *maybe* #29.
 
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Charlie,
Two documents that I think you'll find useful. First is Lycoming service letter L 254. Second Lycoming service instruction SI 1523 G. I believe you can download both of these off of Lycoming's technical publication site. If not send me a private message with your email address and I'll send them to you. Do you know what year your engine was produced? Another thing to check for, is a possible restriction in the fuel system somewhere. I would check the fuel filters for possible restriction as a first step.
The parts catalog shows two different part numbers for the carburetor for your engine. P/N 71098 & 71710.

Charlie

Fuel filters are clear.
 
Hi Dan,

Do you remember what size jet you went with? A new part, or did you ream with a drill bit?
I would also like the answer to this question. I am also having one hot CHT and have ruled out baffle issues. I also have an O-360.
Thank you
 
Educated guess

I'm curious as to why there are two different Carburetor part numbers listed for the 0 - 360 - A1A engine listed in the Lycoming parts catalog. My thinking is it's possible that the carburetors have different jetting depending on the exhaust system. Specifically, I'm referring to whether the exhaust system on a certified airframe utilizes a muffler or not. Most of our RVs do not incorporate a muffler in the exhaust system.
My past experience with cars trucks and motorcycles tells me that whenever you install a free-flowing exhaust on a vehicle, you generally have to jet the carburetor a bit richer.
I would suggest not fearing drilling out the main jet in your carburetor. That said, you need to know that the drilling should be done by hand, using a device known as a pin vise. These are available at McMaster-Carr or MSC. They are relatively inexpensive. The drilling is done by spinning the pin Vise holding the drill bit between your thumb and forefinger. In other words it's done at an extremely slow speed.

Charlie
 
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