Fox Eagle IV .74 Engine - RC Groups
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Oct 18, 2017, 08:36 PM
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kablake's Avatar
Help!

Fox Eagle IV .74 Engine


Hi All,

I bought 2 Fox Eagle IV .74 engines from here recently. They look and are different. Attached are photos of the two I have with the differences exposed. One looks "newer". they have different head buttons, carburetors and cylinder fins.

One engine has a carb with a single needle valve, a larger head cavity and more fins.

The other has a carb with 2 needle valves, a small head cavity and less fins.

Any ideas on the reason they differ and what I should use as a basis for rebuilding at least one of them. I have read some about the differences FOX did to this engine but I am not 100% sure which is the best design.

Any help will be appreciated..

Thanks

Kevin

PS you can match the motors in the photos by the screwdriver type of the bolts holding the cylinder to the crankcase. One has plain ones, the other has philips ones... Anyway, the 2 left photos are one engine and the 2 right ones are the other engine...
Last edited by kablake; Oct 18, 2017 at 08:48 PM.
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Oct 18, 2017, 08:40 PM
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kablake's Avatar
I bought this motor because I want to duplicate the Kaos 60 I had in the early 90s. It had a Fox 74 in it, it took me forever to break it in and get it tuned but once I was able to do that it ran well. I have the kit, now I have these 2 motors. And the question which one to use. If you magnify the photo of the Kaos I think it has the one with the 2 needle carb.
Oct 18, 2017, 09:31 PM
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earlwb's Avatar
The one engine with less fins is a early version. Later they changed it to have more fins to reinforce the muffler mounting flange more. Basically the molds wear out after so many castings, so they have to make new molds. When that happens they make changes and/or improvements to the engines too. The head buttons, with the combustion chamber shape in it, was changed numerous times over the years. So it isn't a surprise to see different combustion chamber types.

The two needle MK-X carb is the earlier version that was used for many years on their engines. The newer engine has the EZ-Carb on it. The EZ-Carb was a effort to make the carb easier to adjust.

People over the years, had trouble with the two needle Mk-X carbs. They had a tendency to use the low speed needle instead of the high speed when making adjustments. Fox changed the low speed needle to be smaller in hopes people would ignore it more. Basically once you set the low speed needle then you rarely, if ever, need to touch it again. But people seem to have an incessant need to tweak it.

Well, as to which engine to use. I would bench run them and see which one you like more. if both engines run good, flip a coin. But if one seems a bit more picky then use the happier less picky engine.
Last edited by earlwb; Oct 18, 2017 at 09:34 PM. Reason: add more information
Oct 18, 2017, 10:38 PM
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kablake's Avatar
Thanks for the response earlwb...

I would even welcome advice to change the carb or head button if advised... In other words, put the 2 needle carb on the one with the bigger combustion chamber or the opposite....

I have also seen one website give a new design for the head button. A drawing and dimensions to have it fabricated. Anyone know anything about this. Although I did not measure it, the one with the larger cavity in the head looks a lot like the "Custom" head button. The site does say it is unique from anything Fox produced at the time...

One other thing, neither engine had a head gasket. I cannot say if that was from the factory or they were removed by the previous owners..
Oct 19, 2017, 06:07 AM
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I TOBOR's Avatar
The EZ Just carb is the better of the two, way better in fact. Have you measured the bore of the earlier one that just says Eagle, it may be a .60. The head button with the more open combustion chamber is a later lower compression button. The .74 has a 1" bore and the .60 has a .907 bore, both have a .937 stroke.

Do your engine a favor and toss those vice grips as far as you can into your neighbors field.

http://sceptreflight.com/Model%20Eng...gle%20III.html
Oct 19, 2017, 09:49 AM
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earlwb's Avatar
I would suggest trying the EZ0Carb out, you ought to like it. But the older Mk-X two needle carbs are great too.

Fox probably made a 100 or more different head button designs and shapes. The larger size combustion chamber is for lower compression and more nitromethane. So if you like 10%, 15% or more nitro in your glow fuel, then the lower compression head button is the one to use.

For a number of years Fox went with high compression and little to no nitromethane in the glow fuel. I tended to like that as I wanted to use cheaper glow fuel of course. But more nitro tends to widen the needle valve adjustments making them less sensitive. So a lot of people liked that of course.

Fox was sort of terrible at identifying the different engine displacements, so double checking the bore size is a good idea.

I think that Fox dispensed with head gaskets on these engine designs. I do not remember seeing or using head gaskets in these engines.
Oct 19, 2017, 12:01 PM
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kablake's Avatar
Quote:
Originally Posted by I TOBOR
The EZ Just carb is the better of the two, way better in fact. Have you measured the bore of the earlier one that just says Eagle, it may be a .60. The head button with the more open combustion chamber is a later lower compression button. The .74 has a 1" bore and the .60 has a .907 bore, both have a .937 stroke.

Do your engine a favor and toss those vice grips as far as you can into your neighbors field.

http://sceptreflight.com/Model%20Eng...gle%20III.html

They are both 1 inch bore, one is .996 and the other .997 actually.. And the vice grips were for something I had done before. I like many others tend to get one thing done and move onto the next without organizing the bench.. My engines are not of the type shown in that article which says the one in the article is an Eagle III, I am sure both of the ones I have are Eagle IV, just different mods, biggest being the carb and the head button. . Both mine have a one piece crankcase with a traditional rear crankcase cover. But the article does have some discussion on the two needle carb and that engine has the smaller head button cavity.. Thanks for the advice.. LOL
Last edited by kablake; Oct 19, 2017 at 12:19 PM.
Oct 19, 2017, 12:19 PM
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I TOBOR's Avatar
They're both .74's then, I worry when I see pliers near an engine.
Oct 19, 2017, 12:21 PM
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I TOBOR's Avatar
Their address

Physical Address:
7124 Richter Road
Elmendorf, Texas 78112
Oct 19, 2017, 12:27 PM
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kablake's Avatar
Hmm, the more I look at the picture of my Kaos the more that engine looks like an Eagle III. The cylinder head looks like the article as well as the front of the cylinder. A flat surface.. the photos I posted above, one has a curved front and the other has small fins on it.. Although I cannot remember 100% what I bought it was purchased around 1994 I believe. Give or take a year... Just looked at the engine article again and it shows 1981 so the one I had is probably a Eagle IV...


Hmmm, I think the above is wrong. The photo magnified ALOT I really cant tell the exact look of the front of the engine and my POOR memory doesn't believe it had the separated crank case..
Last edited by kablake; Oct 19, 2017 at 12:55 PM.
Oct 19, 2017, 12:29 PM
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kablake's Avatar
Quote:
Originally Posted by I TOBOR
They're both .74's then, I worry when I see pliers near an engine.
No need for the vice grips, the engines came apart relatively easy, at least as far as I have them now which is what the photos are showing. What is the address for?? Is it for FOX, I thought they have been out of business quite a while..


http://sceptreflight.net/Model%20Eng...20IV%2074.html

This is the one I have, article dated June 1990.
Last edited by kablake; Oct 19, 2017 at 12:36 PM.
Oct 19, 2017, 02:36 PM
Engine Frankensteinerô
1QwkSport2.5r's Avatar
Mecoa now owns Fox engines.
Oct 19, 2017, 04:54 PM
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I TOBOR's Avatar
That address is for Power Master fuel, that post was meant for another discussion. The laugh is on me.
Oct 19, 2017, 10:18 PM
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earlwb's Avatar
The Eagle IV's had a one piece bottom crankcase with the bolt on cylinder and backplate. The Eagle II's and III's had a two piece bottom crankcase with no bolt on backplate. The Eagle II's and III's have a smaller diameter crankshaft than what the Eagle IV's have. If I remember correctly, the Eagle IV's were the only engines to have a .74 cu in version. The Eagle II's and III's were all .60 cu in displacement engines. Fox was still selling the venerable big .78 cu in non-Schnuerle engine at the time. The Eagle I was also a non-Schnuerle engine too.

I do not think that MECOA has much if any parts to sell, as Fox had pretty much used up everything before they finally quit. So MECOA would have to make the parts themselves. But maybe I am wrong.
Last edited by earlwb; Oct 19, 2017 at 10:23 PM. Reason: add more information
Oct 20, 2017, 08:49 AM
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Cougar429's Avatar
Hopefully they have a stock of the newer head button design. Otherwise that high comp head may limit the nitro content.


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