Fox 35 Bypass mod - can you go to far?

[Dennis Toth]

Can you fill the bypass on the Fox 35 too much and cure the burp but lose the 4-2-4? I modified the case on a 40th anniversary model with JB Weld, filling it maybe about 2/3. The motor is upright mount and does not burp - ever, never.

However, it also keeps jumping from either a solid 4 cycle that is to slow to a wet 2 cycle which works but not what I want. I've done the fuel switch, plug switch, head gaskets, stuffer backplate, hemi head. It always runs the same. The only thing is the bypass mod. Could I have filled the bypass too much? Could this cause the charge to blow through to fast and mess up the loop flow in the cylinder?

Best,       DennisT

[Jim Kraft]

I don't know Dennis. I block mine solid between the sleeve and the case and leave about 1/8" or a little more on the front and back of the bypass. Seems to work OK. The last ones I did with bass wood about the same as the JB Weld.

[Dennis Toth]

I'm going to swap out the filled bypass Fox for one that is stock. It is an upright engine so the burp shouldn't be to much of a problem. We will see if by filling the whole bypass 2/3 has any negative impact on the run. It seems looking back at my notes that before I did the mod it ran pretty good on the 9x6 old Top Flite launching at around 9200. I thought by doing the mod it might get a little stronger on the transitions. Hope to try it in the air over the holidays.

If that works then I will go back to the filled engine and grid out some of the JB Weld to open it up to kinda match what the stick fills.

Happy Holidays to all of you and your families.

DennisT

[Steve Helmick]

Since you're not side-mounting it...I would expect that if the bypass is too blocked up, your fuel economy would be really good...and that's a bad thing. Less fuel, less oil, less power, same prop load, and quite possibly running too lean to get the power needed. Something to look for, anyway.  Steve

[Chris Brainard]

I always filled mine completely and then used a ball shaped burr on my Dremel to grind out neat even channels along each edge of the bypass...leaving the center filled. I also ground out the lip at the bottom of the bypass to smooth out the flow. It takes longer to disassemble and re-assemble the engine then it does to remove a bit of the J-B weld.
Chris

[Chris Wilson]

Chris, ever tried tapered twin ports?

Something that you 'creep' up on by grinding it tapered firstly and then going to parallel if you weren't happy with it.

Not sure that grinding off the lower ledge of the bypass is going to improve things as the velocities in model engines is MUCH lower than full size ones (something like 26 kmh in a sport 2.5cc engine) and will be lower still with a low pumping ratio engine like the old Fox.

Cheers.

[Dennis Toth]

Happy New Year to all.

Today I finally got a chance to do a test with a "stock" Fox 35 in the El D to see if there was a difference against the one with the bypass JB Well fill. Well based on the flights today the answer is YES you can fill the bypass to much.

The stock Fox 35 that I flew today was set up the same way as the modified one, lapped P/C, stuffer backplate, Fox muffler. The unmodified Fox was mounted in the El D, with a 9x6 old Top Flite prop, 7%N/27% oil (50/50), Enya N03 plug. The first flight was set in a deep 4 cycle and it held that setting for the whole flight. We then when in on the needle to a 4-2-4 point. This is where the modified unit would break away into a hard 2 cycle shortly after launch. The stock unit held strong at the 4-2-4 setting.

The next step will be to take the "mod engine" and grind out as half the JB Weld and do the test again. I'll get to this after the KOI in a couple weeks.

Best,          DennisT

[Dennis Toth]

OK, flew the El D with the "stock" Fox 35, Fox muffler (opened to 3/8"ish). I checked the 9x6 old Top Flite pitch that I flew last week and found that one blade was 5.8 the other was 4.9. I heat tweaked them to 6 (at the 70% diameter station). I also set up an APC 9x7.

Flew several flights first was with the trued up APC 9x7, first flight was a little quick (4. but the engine was in a rich 2cycle and had no crackling. Next, put the trued up TF 9x6, this one was a bit slow a little rich. Next flight a few clicks leaner and it was better than before the true-up last week and in light winds would be good (wind was 12 -15 mph). Last flight was with the APC 9x7 richened a few clicks from the first flight - this gave a solid run right at the break point, 4.95 lap and pulled through the vertical solidly. For now this seems to be a good point to work from and will use it at KOI next week.

Best,    DennisT 

[Brett Buck]

OK, flew the El D with the "stock" Fox 35, Fox muffler (opened to 3/8"ish). I checked the 9x6 old Top Flite pitch that I flew last week and found that one blade was 5.8 the other was 4.9. I heat tweaked them to 6 (at the 70% diameter station). I also set up an APC 9x7.

Flew several flights first was with the trued up APC 9x7, first flight was a little quick (4. but the engine was in a rich 2cycle and had no crackling. Next, put the trued up TF 9x6, this one was a bit slow a little rich. Next flight a few clicks leaner and it was better than before the true-up last week and in light winds would be good (wind was 12 -15 mph). Last flight was with the APC 9x7 richened a few clicks from the first flight - this gave a solid run right at the break point, 4.95 lap and pulled through the vertical solidly. For now this seems to be a good point to work from and will use it at KOI next week.

Best,    DennisT 

   Running that slow definitely minimizes any issue you might have with flow restriction. Both Frank Williams (who invented it) and I found at least no power degradation, and in Frank's case, he found a slight power increase in his dyno test, at least at the revs he ran it. Running a 9-7 definitely reduced the required HP, but I am 100% sure it gives up performance over a 10-6 assuming you can get that to work (like a stock engine).

    I did mine as described above - filled the bypass and then ground it out very crudely with a Dremel ball-end burr. So crudely I wouldn't even want to show it. The net cross-section was so small I didn't even think it would run. But I got more-or-less the same power, steadier runs, and no burp at all.

    If yours doesn't do that, I would look at the wear pattern in the cylnder and see if you might be distorting the liner a bit by having the "filler" not ground off completely flush. That's a big drawback of the JB-Weld method, which I don't recommend any more. A good old spruce stick just like Frank tried works fine and is much less likely to cause any sort of problem like that.


       Brett

[Dennis Toth]

OK, got through the KOI, ship worked pretty good, medium winds, got 1st in OTS. Now time to see what can be refined. One change I made was to swap out the ST NVA with a Randy Smith NVA for the Fox. This has the spraybar machined down like the original Fox NVA. I installed this in the "stock" Fox and flew it today. Power was up a little over the ST NVA and it was very needle-able so either NVA appears to work fine no run-a-way.

My conclusion so far is still that you can go to far with the bypass filling.

The next mod will be to take the "filled" bypass Fox and machine out some groves in the JB Weld fill and see where it allows it to needle smoothly for 4 cycle to 4-2-4.

Best,    DennisT