earlwb

I never needed to make any mods to the Fox carbs. They all worked fine for me as is. Thus I have not needed to make changes to the Fox carbs. I just use them. But anyway the mods Fox talks about works for all of the engines. I think that Fox went overboard with too much information by including the mods instructions. It might confuse people too much. But it is nice to know it is there though.

Sport_Pilot

I agree. A lot of people got impatient with the break in and started filing the needles only to get bad results, simply because the engine was still too tight! It causes a lot of the posts trashing Fox.

Glowgeek

Ok, thanks fellas. Due to the unknown run hours on this .40 and the 8 yrs of storage I'll run a few tanks through it before making any mods.
I bought a Sig LT-40 ARF RTF and all the seller told me is that he held flight instruction classes with it when it was new. He had a dozen or so pupils but only one guy stayed with it long enough to solo. He said it sat in his hanger after that.

hsukaria

I had an engine sit for 11 years and it fired right up when I went back to it. It only had used partial castor oil fuel. In the case of your Fox 40, it might have been run with full castor oil fuel. In that case, just soak it with fresh fuel for a day or so and it will free up. The carb especially needs soaking in case it got clogged up. But if it was good before, it should be good still. I can't vouch for the bearings though, hopefully they did not rust out. You'll find out soon enough though.

1QwkSport2.5r

If any Fox guys are interested, I have a Fox .46 ABC I'm trying to sell. Has an EZ adjust carb on it; not run outside the Fox factory. Has reproduced paperwork, spinner backplate (no spinner), box, and muffler. Still has the "OK to sell" sticker also.

Glowgeek

How much difference in power are we talking about ie 40 vs. 45? 46? Will it take my LT-40 vert?

hsukaria

Glowgeek, I may have missed it, but is your Fox 40 the small-case or the large case version? Small case has a square base for the carb, the large case has the flat mounting flange for the carb. Obviously, the 46 is the most powerful of the bunch, not just because of displacment, but also it is true ABC instead of ringed or lapped iron piston. BTW, I love the small case 40s. Not massive power, but great performance for their small size. I have a couple of 45 also, and they run very nice, but I have not flown them yet. I accumulated 5 used Fox engines in the past 2 years simply because people were dumping them at incredibly low prices, $20 or $25 for a good running engine. Can't beat that!!

If you happen to have the small case 40, I doubt you will be able to do much vertical on the LT-40. I can do vertical with my small 40 on a 3.5 lb plane. I'm guessing the LT-40 weighs around 4.5 lbs? I'm sure the big case 40 can pull it up though.

Sport_Pilot

As I recall the .46 has the same power as the .50 but maybe more of a revver not a torquer.

1QwkSport2.5r

I've seen mixed reviews comparing the .46 to the .50; but the .46 is said to be on-par power-wise to most sport .46 engines. I have not run a Fox .46, but I have run a .50. The .50 underwhelmed me when comparing to say a SuperTigre G51. I would think a Fox .46 would be a good fit in an LT-40. Power should be the same or similar to a Fox .45 and its ABC which IMHO is far better than a ringed piston.

Sport_Pilot

The .50 needs a big prop to shine. Mine seemed so so on the bench, but very well when unloaded in the air.

1QwkSport2.5r

Yes, they don't like to rev. That makes the most sense when you consider the .50 basically has the stroke of a .46 and bore of a .61 in a .40/.46 sized case. Those transfer ports are pretty small and due to the liner thickness (or rather, thinness), have very poor flow direction. My .50 on diesel is about 1,000rpm lower in power compared to me ST .45 and .51 diesel conversions.

My .50 with an MDS .40 ABC P/L in it runs like a typical sport .40 - around 13,000rpm on a 10x6. With a .50 P/L in it, I don't think it did any better than 14k; less IIRC. I think the carb is too big on it (.330" choke). A .312" choke would be better.

hsukaria

Yes, the .330 choke might be a little big for a 40. I ran the .312 choke on the 74 and 45 and noticed an increase in needle valve sensitivity on the 45. Regarding the diesel setup, I would have thought that the port sizing wouldn't matter much since is doesn't require that much air/fuel flow? I thought some people even blocked one of the ports for diesel sometimes. Anyway, is there a trick to machine the ports to increase their size on the 50 just a little? (I don't own a 50, just an academic question. I have an inquiring mind, you know).

1QwkSport2.5r

I meant the .330" carb is too big for the .50 even. The crankcase ports are rather shallow due to the huge bore, whether there's any room to make them larger or not remains to be seen. Because the liner is so thin, there isn't much room to direct the charge into the cylinder. Not much can be done about that other than to use a different piston/liner with a thicker cylinder wall. If a guy wants to spin a smaller prop faster, a .40 or .46 would be the better option than trying to make a .50 rev better. The .50 is better suited for big props on glow or bigger props on diesel IMO.

Sport_Pilot

I don't know the bore of the carb on the .50 but it does not seem to big. I think I ran a 11x5 prop on a 40 sized Big Stick and was slightly less power than a .46 Thunder Tiger. But with an 11-6 it did better than the TT. And when the prop got cut down to a 10-5/8 (estimated) diameter it performed much better. BTW the pavement cut down the prop, not me. The RPM as I recall is 13 something K, but unwinds a lot in the air so breathing didn't seem as bad as you say.

1QwkSport2.5r

Well, .330" doesn't look that big until you actually measure it and compare it to an engine of similar displacement. The TT Pro .46 has a .270" carb FWIW. I tried a .270" Perry carb on the .50 and it turned the same rpm as the .330" Fox carb. That tells me it does no good to have a big carb on it. It's clear to me that my .50 is a bit of a lemon (and has been since I got it) even though it has a good fitting piston/liner/ring. It runs pretty good, just isn't a powerhouse by any means.

hsukaria

Probably built on a Monday morning or Friday afternoon. Anyway, my Fox engines, except for the 74, have been very easy to use and have reasonable performance (but I don't own a 50 though). The 74 I traded off not knowing that the previous owner had monkyed with the liner porting to increase flow. I think maybe he went too far and I kept getting overheating problems despite trying different tricks people recommended. The compression ratio on the 74 was insane, another reason for overheating. I put a new head button and 2 shims to help with the overheating problem.

1QwkSport2.5r

You can never be too sure of anything acquired second hand. You can never be too sure of advice you get on Internet forums either. Some of us are complete dopes.

hsukaria

Dope is ok, dopehead is not.

Sport_Pilot

Speaking of which, the Fox .50 I own is quite worn. Compression is a bit soft cold, better when warm, much worse when really hot. But still performs well. LOL maybe it breath's better that way.

1QwkSport2.5r

All ringed engines feel soft to me, new ring or old ring. It's how they run that matters - the well run engines are the ones that seem to run the best. The few ringed diesel conversions I have require a little more finesse to hand start than the ABC/AAC ones.

earlwb

Although there is nothing wrong with the Fox .50 engines, I feel that they went over the sweet spot with that engine design. The Fox .45 or .46 engines are on the sweet spot for power and performance. The .40 is a little under and the .50 went over the top a little too much. Now there isn't anything wrong with the 50's though. I found they seem to run better with a 11x6 to 11x7 propeller. The 11x7 seemed to work the best on my engines. My .45's liked the 11x6 props more. The .40's tended to work well with the 10x6 prop size.

Now one may need to have a lot of patience with some of the engines though. I have had some break in fairly quickly and others that took like forever to break in. I used to break in the engines on a trainer plane that I could fly slow so I could run the engine at 1/2 throttle and change engine speeds back and forth in the air. The engines would start out rich, sometimes really rich too. Then as the engine got worn in some it would act like it was suddenly way too rich. So I would lean it out a little. Then repeat until it acted too rich and lean it out some more, and so on. Eventually it got to where it would hold a more optimum needle setting and run full throttle for longer periods of time. Then I could move it to the plane I wanted to fly it on. But sometimes even then it still needed more run in time.

Eventually the engine would get to where it ran great. It would idle low for long periods of time, go full throttle, the works. Then I could wear out several airplanes using it on them. Due to their long wearing characteristics, they tend to last for many many hours of runtime. For my Fox engines, I haven't had to refurbish or rebuild very many of them, maybe 3 or 4 over the years. Now I have rebuilt or refurbished a number of engines that other people gave me who had problems with them over the years though. It is usually running the engines too lean, not giving them enough time to get broken in good. But probably their biggest downfall was needing to be broken in and sometimes taking longer to break in. People just didn't seem to have the patience to stay the course. They want instant gratification.

Glowgeek

I currently have a 10-5 and a 10-6 prop for my big case .40. I was under the impression that the engine should barely unload on the bench for peak performance in the air. Maybe not?

aspeed

Just thinking out loud here, but I don't think it will unload until it is in the air? A high pitch prop will normally unload more in the air.

Sport_Pilot

The only to unload it on the bench is to put a high speed fan in front of it.

hsukaria

Glowgeek,
I didn't think about it before, but your choice of propeller will determine how much vertical and unloading you will get in flight. If you want vertical but not much unloading, go with a 12x4. If you want more unloading but sacrificing vertical, go with a 9x7 or 9x8. My guess is for a trainer like the LT-40, the 12x4 would give nice performance, and it will slow down the plane more for landing.