Author Topic: The fox35 "fix" (Read 4814 times)

Bob Disharoon · « **on:** February 04, 2007, 03:42:42 PM »

I recall seeing a wooden venturi fix for the engine somewhere around here..now cant find it...need help..also any pics would be helpful..thnx, Bob

Ken Deboy · « **Reply #1 on:** February 04, 2007, 03:59:49 PM »

The mod you're asking about is for the transfer port, not the venturi. If you do a search for "Fox Burp" on Stuka Stunt you'll find a thread called "Fox .35 guys" that has detailed instructions and photos. It's actually pretty simple. Just get a piece of wood the length of the transfer port, about half as wide, and thickness to fill the gap between the cylinder liner and case. It cures a misfire on side mounted Stunt 35s. Apparently not needed if mounted upright or inverted. Hope this helps.

cheers,
Ken

Dennis Vander Kuur · « **Reply #2 on:** February 05, 2007, 01:04:23 PM »

Bob,
If your question pertains to partially blocking the transfer passage to eliminate the "Fox Burp" then these pictures may help. The first two pictures show the "before" appearance of the transfer passage. The third picture shows the wooden plug (this example is spruce wood but any hardwood should work). The pictures four through six show the "after" appearance with the plug installed. The plug should be trimmed to be a slight press fit between the upper and lower lip of the passage depression. Also note the bottom edge is shaped to give clearance for the crankshaft.

If your question concerns restricting the venturi, I don't have any pictures but can relate some first hand experience. Normally the Fox .35 Stunt motor does not require restricting the size of the venturi opening. Some people, myself included, have restricted the venturi opening on combat-type engines (i.e. Fox Combat Specials) to allow them to use normal suction fuel feed. Some have also restriceted the venturi opening size to improve fuel suction, reduce power and/or fuel consumption. This can be done by installing a smaller ID (inside diameter) venturi or by restricting the opening of the existing venturi. I have made some restriction plugs from spruce and soaked them in CA to harden them. Some flyers merely jam a whittled-down popsicle stick or broken wooden prop remnant into the venturi opening (between the spraybar and the venturi wall). See the last picture sketch.
Hope this helps.

DennisV

Clint Ormosen · « **Reply #3 on:** February 16, 2007, 01:00:26 AM »

Another option is to use JB Weld in the bypass. It's a little more work, but a more permanant solution. I have Fox .35's that I put JB Weld plugs into 12 yrs ago and still no problems. Check out my write-up of this mod in the July/August '95 issue of Stunt News. (I know, I know. A long time ago) I did it after reading Frank Williams powertrain article the previous month and then calling him for further info. He encouraged me to test further.
In fact, I only have ONE Fox .35 that does NOT have the JB mod. It's NIB and I haven't needed it yet.

Clint
Master of the Figure 9

Jimmy R. Jacobs · « **Reply #4 on:** February 16, 2007, 04:29:47 PM »

For blocking ports I have been using plumder epoxie , it's like clay and very nice to use and cheap. Also apermanant solution, it's worked for years for me. I have also used it in foxs instead of wood.

Jimmy R. Jacobs

Tom Perry · « **Reply #5 on:** February 17, 2007, 01:49:38 AM »

The "stick" works just as well as the epoxy and weighs less. Also easily reversible and takes less time to install. Some of mine worked OK after 30 years of sitting in the attic.

Clint Ormosen · « **Reply #6 on:** February 17, 2007, 11:35:58 AM »

The stick method does work just fine. But after a time, the wood gets soft and starts to move around some. At least it did in one of mine. I had installed as tight as I dare in the case too. I just don't like the idea of having loose wood in my engines. True, it is reversible. I'm not sure why you would need to reverse it, though.

Alan Hahn · « **Reply #7 on:** February 17, 2007, 03:59:20 PM »

Well you know, Fox always recommends wood over plastic.

Clint Ormosen · « **Reply #8 on:** February 17, 2007, 05:34:01 PM »

Quote from: Alan Hahn on February 17, 2007, 03:59:20 PM

Well you know, Fox always recommends wood over plastic.

HA! I can't argue with that one!

Allen Eshleman · « **Reply #9 on:** September 25, 2010, 11:24:11 AM »

I recently had a bypass plug put in a Fox 35. Every thing seemed to go together well and it flipped or turned over well before we put fuel in it. Then it tightened was almost impossible to turn over. Eventually it did flip but it was not free. Something seemed to be binding. We first did not have the needle valve set in far enought so a lot of fuel came in and it was very flooded. Any thoughts on what went wrong?

john e. holliday · « **Reply #10 on:** September 27, 2010, 09:18:58 AM »

Did you seallthe wood plug. A like water, fuel will soak into the wood and make it expand. Probably warped your cylinder. Take the plug out and see if it loosens up.

Marvin Denny · « **Reply #11 on:** September 27, 2010, 10:57:35 AM »

I have been using a clothes pin as a source of wood as it is a VERY tight grain and doesn't swell as much. After I get it where it fits well I also rub thinned epoxy glue into the surface to further assure that it doesn't swell.
Bigiron

Allen Eshleman · « **Reply #12 on:** September 27, 2010, 06:45:47 PM »

In looking at the plug, I am wondering if it is too wide. I believe we made it a bit wider than was recommended. Could it be that the port is blocked off too much? Is the idea to let the port partially open or sort of like a a double port with the plug in the middle. If the liner or piston is warped, would I need a new piston and liner? If so, would the ceramic option be better?

Thanks again for your advice.

Rob Killick · « **Reply #13 on:** September 27, 2010, 09:23:12 PM »

A diagram to go with the photos ...

W.D. Roland · « **Reply #14 on:** October 02, 2010, 07:36:17 AM »

On Foxes website I say the Fox burp fixer glow plug.

Anyone know anything about it?

Looks like and R/C idle bar plug in the picture.

Claims it "cures the burp"

David

Chris Wilson · « **Reply #15 on:** October 04, 2010, 04:08:28 PM »

I recall someone years ago saying that the wooden plug may function better if it is tapered, small end of the wedge at the bottom large edge at the top, as it will accelerate the gas flow all the better.

Cheers.

Brett Buck · « **Reply #16 on:** October 05, 2010, 10:37:38 AM »

Quote from: W.D. Roland on October 02, 2010, 07:36:17 AM

On Foxes website I say the Fox burp fixer glow plug.

Anyone know anything about it?

Looks like and R/C idle bar plug in the picture.

Claims it "cures the burp"

David

They are wrong. This is the same sort of nonsense we have gone through for 30 years - the problem is not the plug, the tank, the fuel, the prop, etc. It's caused by the excessively wide bypass and the cure is to make the bypass smaller, period.

Brett

Allen Eshleman · « **Reply #17 on:** October 08, 2010, 08:21:15 PM »

Just wanted to report that we've gotten my Fox 35 with the plug in it to run. We had made the plug too wide, constricting the flow of fuel and exhaust or whatever goes through there. We made another to the correct size. I just had my first two flights today. It ran great but fast the first run. I did some high flying and dives with it and there was "no burp". The next flight I opened it up too much and it was too slow and didn't finish the flight. I opened it a half turn on a Randy Smith needle valve assembly. That's the first flights with the valve. I need to fine tune the adjustment - 4 oz. Hayes tank with pressure.

W.D. Roland · « **Reply #18 on:** October 08, 2010, 09:48:29 PM »

Quote from: Brett Buck on October 05, 2010, 10:37:38 AM

They are wrong. This is the same sort of nonsense we have gone through for 30 years - the problem is not the plug, the tank, the fuel, the prop, etc. It's caused by the excessively wide bypass and the cure is to make the bypass smaller, period.

Brett

Yea to me the plug looks like an idle bar R/C plug.

The (burp)cause is low boost velocity and apparently the stick in the bypass helps at least most from what I have read.
Low crank case compression ratio and possibly to big a by pass.
The back plate is the worst match the engine I have ever seen.

I fitted up 3 nice Fox .35s (pre 74?)with stick, as shown in reply #13.
So far have broken in and used one of them.
The stick did not fix.

After making a stuffer back plate it is now 99% better but will still burp and with a big one during hard transition to outsides.
Only thing more I know to do would be to cork the piston, but is it worth the time and rebalance?

The tank wound up being around 1/4 inch above CL of engine to make it this good and toward end of flight the outside of overhead 8 wants to go rich. The uniflow vent restricter is size of smallest pin drill.

I think the real cure for the Fox .35s burp on profiles is an OS .35s.

david beazley · « **Reply #19 on:** November 02, 2010, 04:15:40 PM »

would Delrin be a suitable material for the bypass plug?

Tim Wescott · « **Reply #20 on:** November 02, 2010, 05:28:02 PM »

Quote from: Allen Eshleman on October 08, 2010, 08:21:15 PM

constricting the flow of fuel and exhaust or whatever goes through there

Fuel-air mix.

As the piston goes up it draws fuel/air/oil mix into the crankcase. As it comes down it compresses this mix. When the transfer opens, some of that mix 'puffs' into the cylinder, providing the charge for the next power stroke and helping to push the last bit of exhaust out of the engine.

kenneth cook · « **Reply #21 on:** November 03, 2010, 03:47:23 AM »

I know when I tried the bass stick, I wasn't happy with it. I didn't like the way it ran and I switched it back immediately. I then discovered that Thunderbolt plugs cured the problem more than anything for me. One of my Fox's , I smoothed all the steps inside of the case hoping this would give a slight edge on performance but all it did was to help throw more fuel onto the problem. Initially I was convinced that running the high oil content fuel was the accepted norm for running these engines. I've since switched to 5-22 which puts a bit of kick back into the engine while keeping it free. I've run one of these engines as lean as you could possibly run it in my Ringmaster for the last 6 years over and over to the point of just about every nice weekend we could have. If that engine was going to let go or seize due to insufficient oil it would have surely went by now. Running the 25%-29% oil will surely bring on the burp as you need a little fuel with your oil content. It just drowns out the plug. This doesn't mean I'm saying for those using it currently to not use it. I'm just stating this is what works for me. I run my older Fox's on the higher oil content fuels. Ken

L0U CRANE · « **Reply #22 on:** November 08, 2010, 06:05:39 PM »

Odd!

I've been mod-ing Fox 35s for over 30 years for my own use, and never found the 'burp'. That includes Foxes produced from the early '70s to the 40th Anniversary series. Haven't worked on the latest ones, with the added metal for a decent muffler mounting.

One of my standard mods is to remove the 'step' at the bottom of the crankcase/bypass junction. (Reducing the bypass channel volume with a wooden 'stick' becomes less possible.) Basically, that 'flows' the bypass more cleanly...

Another standard mod is to either use an OEM 'stuffer backplate' or to file down the back end of the crankcase so that there is no more than 0.010" clearance between it and the crankpin. This sems to be effective in hyping bypass flow velocity.

Combined with my most essential mod - retiming the sleeve ports to 130° exhaust / 120° bypass durations - the 'dreaded burp' has never been a problem.

I have other 'standard mods,' but they aren't relevant to this topic.

david beazley · « **Reply #23 on:** November 13, 2010, 07:03:48 AM »

Lou,
I am interested in the other "non-standard" Fox fixes along with how you re-time the engine. I for one, would like to find out how you do it.

DanielGelinas · « **Reply #24 on:** October 03, 2011, 02:18:54 PM »

Hi Guys,

Before I try the FOX burp Fix, I have a question.

Because the liner and piston will be removed from the engine, does that not mean a re-alignment is impossible...which would also mean a new, possibly shorter, break-in?

Just wondering what i'm getting into before I start.

Thanks,

-Dan

kenneth cook · « **Reply #25 on:** October 03, 2011, 06:00:58 PM »

Dan, before removing your liner take a #11 blade and scratch across the top of the liner onto the case so you can line it up again. I would try a hotter plug before doing the mod. I just didn't like the way it worked as I described above. Our club runs on the average 10-15 Fox's a weekend. These aren't just sport pilots but top expert flyers. Some would say why use a Fox. Well, they like them. I know that none of their Fox's have this mod done to them. I'm not saying not to try it, its just that simpler things can be changed before attempting it. It is nice that it is a reversible modification that can be easily switched back. As for the guys that never heard of the dreaded burp. They may be running the engine in the vertical or inverted position. The engine doesn't burp unless it is mounted profile. Getting the crankpin off of the crank can be a bit of a challenge if you've never done it before. Usually you can get the rod off due to it just falling off into your hand. Putting it back on takes a bit of the wiggle jiggle rotation all in one shot. Putting the scratch on the liner also prevents you from putting it in 180 deg backwards. Usually, if I go as far to take a Fox that far apart I also clean up a few other things such as the crankcase. I lap the back of the case where the backplate attaches on a piece of glass using 400 grit and wd-40. I do this to the top of the liner as well where the head seats. I do this enough to where I barely can see the scratch I made. Ken

DanielGelinas · « **Reply #26 on:** October 04, 2011, 02:35:23 PM »

Thanks Ken,

I'm actually running the fox with a thunderbolt RC Long plug. THe engine runs better with this plug but still burps. I tried an OS LA.25 on the plane to see the difference. No burps from the OS! The problem is I love fox 35s.

I love the 4-2 break... but I hate the burp!

I'll give the fix a try and see if that fixes things.

Regards,

-Dan

DanielGelinas · « **Reply #27 on:** April 16, 2012, 01:39:14 PM »

Fox 35 Nirvana....

I got the Randy Smith crank, hemi head and NV. Added the hardwood in the bypass.
Finally got to fly yesterday with my HEAVY 4 times crashed, super clown.
WOW, what a difference...
Way less vibration and a nice steady stunt run...WITH NO BURP.

Can 't wait to try this engine in a light plane.

I'm in FOX 35 nirvana!

-Daniel