Fox bypass blocking

OK, I am going to try it... I am going to try the JB weld method of blocking the bypass on a Fox .35... What I need to know is how much do I fill it in? I searched the archives and did not find what I was hoping for. The closest was Brett saying something about 1/3... Is this the magic number? What do you say Brett? Bigiron 13? Larry? Dan? Currell? Hello?? Anyone??

Thanks guys, I've been wanting to try this for a while.

Not that I can answer your question, but I have read of others blocking the port with a carefuly shaped hardwood plug. The idea being you can always undo the change if its not to your liking. What I haven't read is dimentions or where the port is on the engine(my ignorance in not knowing already) I read that the blocking cures the "fox burp" but have no experience. I also egar to read the replies from the guys that Know.

Use a wood block, Bass wood. 1/3 is a good. It is much easier to do and just as effective. I really didn't notice much change in the performance. I think the hemi-head and stuffer back plate is a better way to go. Also too many expect too much from the venerable Fox. If you can keep the weight of the plane to 30-36 oz and use 57.5 ft lines this seems to be a wonderful combination. Carving a wooden block takes about ten minutes, this info can be found in one of the back issues of Stunt News, I think from 1995-97. I have it some where in my back issues. Jim J.

Thanks Jim,

That would be great to find it writing... Let me know what issue(s) it appeared in and I will hunt it down. I am not looking to do a major mod, just seeing if I can eliminate the burp. This engine will find it's way on to a Ringmaster... I already have a L&J .35 but I don't want to dismantel to see what Larry does to them. I have not had much luck flying Ringmasters so I don't want to risk my best Fox to a re-kitted RM... Saving the L&J for a Barnstormer.

I must have missed this one. Never heard of by-pass mods to the Fox in this manner. Many have done the Boost Port blockage on engines, as our forum host does. The boost port is directly inline with the casing exhaust port. To either side of the boost port are the transfer ports. On the Fox 35 this is not the case as there is only a single intake port. Reducing the area in the engine case casting to reduce volume before the cylinder liners port may increase velocity of the charge,..is this the idea? My very first Fox 35 cylinder liner pictured, circa 1971 era.

Tigre:

The theory here is that that fuel will pool in the Fox bypass due to a relatively low mixture velocity due to engines lower RPM operating range. Then when entering manuevers that require transition from upright to inverted flight, the pooled fuel is forced into the combustion chamber, creating a rich condition (the burp). Reducing the volume in the bypass accelerates the mixture velocity and eliminates the pooling. Since the engine does not operate in RPM ranges that would require the full volume of the bypass, power is not affected.

UC,
IC . Kinda like I thought. During the years that the pictured Fox 35 was in service(towing Ringmasters,Sterling Mustangs and Yaks, CG-Shoestring, etc) the "Burp" was never a problem. We ran it/them screaming lean for max speed. 4-2-4 never even entred the equation. We didn't know any better. Even after all those lean runs using K&B 100 and 1000 fuel(the last one on a Stang that finally seized/stopped this motor) this motor still runs. (Had to hot tank it to free it up doncha know) That said I'm sure it's not putting out like it use to. Fuel and oil ratio's were also unknowns to us. We run them fast regardless of the castor content or fuel brand. Burp?...what's that? Something you do after dinner.

As George Aldrich pointed out, either here or at Isky's old site, tracing 50+ years of Foxes reveals some variation in the bypass size, at least as measured by the casting, looking outside. So the amount that you need to block may vary some, depending whether it is a Sandcast, Ist Gen diecast, 40th Anniv., etc.

I'm not sure, but I recall some folks who actually have played around with intake blockages recommending even more than 1/3rd. You might, then, make a couple or three blockage pieces of differing sizes. Do a little experimenting. Sure, it will be a pain to have to dismount the engine, put the new piece in, etc. But you might get the best solution this way.

And if you do this, please report it here. I have not tried this yet. I have tried a variety of heads, though! And the stuffer plate, of course. (And the ABC version, and, etc, etc). Currell

Tony: It is a funny thing, this Dreaded Fox Burp. You get so used to it, you don't notice. Kind of like hitting your space bar on the keyboard, or breathing, or blinking your eye, or...Especially if you fly Foxes a lot.

And occasionally, even going with a 4-2-4 run, you stumble (pun intended) across a Fox that doesn't do it. Maybe something about the fuel, or the particular needle setting (and I'm not claiming a screaming two cycle here) of the moment, or maybe the peculiar way the pilot is inputting commands. Or your tank,

Or sunspots.

Hi Guy, I've done this and it works! Use two popsicle sticks and shape down to the size which will just let your sleeve slide down the block! JB Weld will do the trick of holding the wood in place. If you'd rather not shim up by Popsicle sticks use a solid piece of bass wood which would be thicker than one frozen stick and just yield a cozy slip fit! Cheaper than a stuffer back plate and hemi head but almost as effective for a tiny fraction of the prices!

The popsicle trick is the one we used in the 70's. I guess it worked, but by then I had switched to K&B green head .35's. They just set the plane on fire, but no burps!

The Stuffer backplate and Hemi-Heads were NOT designed to stop the dreaded "Fox Burp" . In some cases the Backplate did alleviate it or completely eliminate it. But, It was not designed to do so.
I fly a lot of Fox 35s and have never used any bypass blocking so I cannot help you on how to properly do it. I manage to get most of my fox, profile mounted engines to run without burping bu thorough breakin and fuel, tank, prop, and needle settings. Other engines also burp about as bad, there just aren't as many of them out there to be complained about.

> The Stuffer backplate and Hemi-Heads were NOT designed to
>stop the dreaded "Fox Burp" . In some cases the Backplate
>did alleviate it or completely eliminate it. But, It was
>not designed to do so.

I didn't say or mean to imply this, Big. I was just rambling some about my Fox experimentaion in general, so sorry for the confusion.

I know Currell. I was just so stating so that those who didn't know wouldn't get the wrong idea.
Sorry if it sounded as if I were jumping on you.
I got a first in classic at the Wichita contest yesterday. The old Fox/Novi IV really came through. I took the Brodak 40 out and reinstalled the seven year old , well used Fox stunt 35 back in . Ran the B-40 less than a year before getting off it.

Took third in Foxberg race with a time of 10:14 and had to make five pit stops instead of the required four. A tactical error on my part. I went to a smaller tank to avoid getting an overrun and went too far. Top place was over 10 min, so the race was really close. We had a LOT of fun as the wind AND tempreatures were perfect.

Hi Phil,
I have only experienced the burp on profile mountings, but as the others have said, the filling in of part of the bypass does seem to help...... sometimes. I like Currell's mention of sun spots! hahaha
I know Larry Foster blocks the bypass, uses a stuffer backplate, and a hemi head on his top line rework. But, I haven't really needed the blocking since I only use that engine in invereted full fuselage set ups, and haven't experienced the burp in those without the blocking.
And I don't use the Fox 35 on profiles, anymore........
Soooooo........ use the wood and experiment!

Bill <><

Stuffed the bypass on my Mustang, no more burp. Doesn't protect them from running out of fuel in odd flight attitudes though.

OK here's what I did...

I did use JB Weld and filled in the case about 1/3... I then took my $kote heat gun and hit it with full temp until the JB Weld flowed liquid like and it smoothed out and kicked off right now (Kinda like spraying Zip kicker on CYA) I then added a Fox Hemi head (no stuffer back plate) and polished up this shiney case with muffler lugs (Like a 40th anniv.) and cleaned, then cleaned again then reassembled and it looks fantastic... Now to see how it performs! I am thinking that the polish job will reduce wind resistance and improve flight characteristics for a stellar first flight since making crash repairs...

I should note that before I added JB Weld I used my Dremel and a course grinding stone and scuffed up the bypass, then cleaned, cleaned, cleaned...

I will be using the stock Fox NVA until I believe that it is run-in... then I might add a Super Tigre NVA, but if it is performing like I hope that it will I probably won't mess with it any further.

I will let all know how it turns out... I hope to get out and fly next weekend.

>OK here's what I did...
>
> I did use JB Weld and filled in the case about 1/3... I
>then took my $kote heat gun and hit it with full temp until
>the JB Weld flowed liquid like and it smoothed out and
>kicked off right now (Kinda like spraying Zip kicker on CYA)
>I then added a Fox Hemi head (no stuffer back plate) and
>polished up this shiney case with muffler lugs (Like a 40th
>anniv.) and cleaned, then cleaned again then reassembled and
>it looks fantastic... Now to see how it performs! I am
>thinking that the polish job will reduce wind resistance and
>improve flight characteristics for a stellar first flight
>since making crash repairs...
>
> I should note that before I added JB Weld I used my
>Dremel and a course grinding stone and scuffed up the
>bypass, then cleaned, cleaned, cleaned...
>
> I will be using the stock Fox NVA until I believe that it
>is run-in... then I might add a Super Tigre NVA, but if it
>is performing like I hope that it will I probably won't mess
>with it any further.
>
> I will let all know how it turns out... I hope to get out
>and fly next weekend.

I have never done a partial block of a Fox 35, so that is my disclaimer. However, I have done a few other engines, so, in following this up...

I would probably recommend the wood plug method for the Fox 35, as it is simple to do for a partial block and also easily removeable. If you are trying to fully block a boost port, the wood block is not a good idea. But for narrowing up the port, it is simple and people report it works well. And the sleeve will hold it in place. You don't have to worry about it going anywhere.

I don't think the amount of blocking is critical. Most suggest blocking about 1/3 and that this doesn't hurt performance. It probably won't if you keep it about 1/3. There is more than adequate porting in the case of the Fox 35.

Experimenting with the Fox 40 engine shows that narrow transfer ports are very efficient. If I am going to block off the port, therefore, I would suggest narrowing it in width, not in depth. If you keep the same width, but merely pour some JB weld in to make it shallower, this might hurt efficiency (hence, the wood block recommendation sealing off part of the port in width.

And, as for the JB weld, you don't have to rough up the area. Just clean it good with alcohol first. Once it is in and hard, it is in and hard. It is not going anywhere, and it is not coming out again without a lot of hard grinding (or a lot of heat--hitting it with a torch or something). What you do with JB Weld, therefore, is pretty much what you are going to end up with. It is workable (cutable, grindable) about 6 hours after putting it in, but when fully cured it is THERE!.

Just a bit of advice and a bit of warning.

Phil,
I wrote about this in SN about '97 or so. I used a piece of basswood carved to shape. The piece was 5/16 inch wide and placed right in the middle of the port. This left two ~1/8 inch bypasses on each side of the port. The botton of the wood piece rested on the casting step in the case at the bottom of the port. A little shaping was required to make sure it cleared the crank. I haven'r tried the JB Weld. The idea is to increase the velocity of the gasses in the bypass not to fill excess fuel spill volumes. Two 1/8 ports probably create a higher velocity than one shallow epoxy reconfigured port. The 5/15 inch width probably takes up more than 1/3 of the port, more like 1/2. 1/3 was never mentioned as a magic number. The total power of the engine is not decreased, if anything increased, I've measured it. The "profile burp" or "poop out" is cured with this mod. The tank no longer needs to be positioned extremely high on the fuselage to counteract the burp.