I assume that it is an iron piston Fox 35. If so, they are still in production so parts are available. Search for Fox Manufacturing on the internet.
The ideal fuel has 5% nitro and at least 25% castor oil. This engine likes lots of oil. I have never tried using synthetic oil in mine (I have four) but if I did I would add Klotz to bring the total oil up to at least 25%. You can buy Klotz at Harley-Davidson motorcycle stores.
Do not try to use fuel with more than 10% nitro because the engine is not mechanically built for high nitro fuels and something (like the wrist pin) will probably bend or break. I am using SIG 10% with 25% castor oil because that is all that I can get where I live. This works OK because I am flying at 5000 feet altitude.
The normal needle valve setting for these engines is usually with the needle opened about six turns.
There is a chronic manufacturing problem with the needle valve assembilies on these engines. Read the folowing (sorry the table format does not copy):
From: Ken McClenahan [mailto:klmcclen@comcast.net]
Sent: Sunday, December 21, 2003 10:40 PM
To:
'Foxmotors@aol.com'
Subject: Fox 35 Stunt Needle Valves
Sometimes it takes a very long time to figure out the cause of a problem.
I have recently been breaking in a Fox 35 stunt that I bought circa 1983 (it was a spare that I had bought for backup but never needed). It was generally running okay (but not perfectly) as long as the weather was warm. When the weather turned cold, it died as soon as I put the nose of the airplane up. Someone suggested that the problem was a sloppy needle valve thread fit. Since fuel and air was coming out around the needle valve threads when I blew out the tank, I checked the outer diameter of the thread on the needle and it was 0.004” undersize. I put some castor oil on the threads and it ran better. Then I had an afterthought and checked the size of the threads of the needle valve body by dropping drills in until I got to a drill that would not go in. The thread checked out as a 66% thread. After thinking about this for a few days it occurred to me that I have had similar problems with engines many times in the past so I checked all of the Fox stunt needle valve bodies that I have. The results follow listed by quantity needle valve bodies and the size of the largest drill that will go through the thread in the body along with the corresponding percent thread size.
Number
Drill Size No
Thread in %
4
47
75
5
46
66
3
45
62
1
42
20
1
41
10
Four of the bodies came out of previously unopened packages that I bought as spares in the early 80s. The part number on the packages is 13510. Three measured #46 drill and one measured #47 drill.
Even worse, the needle setting is typically at about ten turns out so there are only three or four threads engaged. After a lot of thought on how to do it, I used my Unimat to grind the flat back on the needle for the engine that was the subject of the second paragraph above*. It should now run at about two turns out.
If I were making my own needle valve bodies (and I may have to do so), then I would use the smallest tap drill that I could make work. I can’t claim that I could run a tap through a #50 drill size hole to get a 100% thread but I am sure that I could tap through a #49 drill size hole.
As a matter of practical experience, I have one engine that I used for about 500 flights without any problems and the needle valve body has a #47 hole, but I put castor oil on the threads and never adjusted the needle valve after the first couple of flights.
Looking back on many years of flying (6-bolt head Fox stunt 35s from 1953 but excluding from 1989 through 2000 when I did not have a place to fly), I am sure that I lost two Noblers and some other airplanes due to this problem.
Have a Merry Christmas and a Happy New Year.
Ken McClenahan
AMA 1973
Lone Tree, CO
* A better solution that I now use is to drill the step in the body back about 3 mm or 1/8" using a lathe and a number 48 drill.