Fox .35 Burp

What mechanism is behind the infamous burp of the venerable Fox .35? I've tried different tank positions but the problem persists. I'm sure this has been discussed here in past posts, but I'd like to be enlightened as to what causes it while engines of similar design are free of this malady. Thanks in advance.

>What mechanism is behind the infamous burp of the venerable
>Fox .35?

I would take a guess and say port timing.

The engine has too much crankcase/transfer passage volume. When you do an outside loop it all sloggs up into the cylinder and puts the glow plug out.

There are ways to compensate by using JB weld and/or wood pieces in the transfer passage to restrict the flow.

Wood? Inside the engine? Surely you jest....Perhaps polishing the passage walls would help. This works in larger engines. It reduces the suface friction and all but eliminates the ability of the fuel droplets to adhere to the passage walls....Works on my "Big Block" ...

>Wood? Inside the engine? Surely you jest....Perhaps
>polishing the passage walls would help. This works in larger
>engines. It reduces the suface friction and all but
>eliminates the ability of the fuel droplets to adhere to the
>passage walls....Works on my "Big Block" ...

Hi Mike,
Actually, a wood plug has been used by a lot of people for quite a while to fix the burp.
IIRC, Frank Williams was the first I remember talking about it "in print" some time ago. Basswood, or spruce (??), about 3/16ths wide, tight fit between the case and sleeve in the middle of the port from top to bottom.
Bill <><

Maybe it was something it ate!

Giffy gave us the mechanical cause.

"The engine has too much crankcase/transfer passage volume. When you do an outside loop it all sloggs up into the cylinder and puts the glow plug out.
There are ways to compensate by using JB weld and/or wood pieces in the transfer passage to restrict the flow."

Speaking to the effect:

The burp (effect) happens because velocity of the fuel charge is low due to the large transfer passage. The larger the cross sectional area the slower the charge travels thru the passage. Some condensation of the fuel charge occurs on the walls of this passage because the velocity isn't high enough to sweep the walls clean. I think its' primarly oil condensating, have a hard time believing that alcohol will condensate on the walls of this passage.

Mounted on a profile, the transfer passage is up (usually). Giffy described perfectly what happens in an outside loop.

SO why does it go away with the stuffer plate on?????

SO why does it go away with the stuffer plate on?????

Because, the stuffer reduces the crankcase volume.

>SO why does it go away with the stuffer plate on?????
>
>Because, the stuffer reduces the crankcase volume.
>
>
>Giffy

I've tried the stuffer plate and it did not stop the dreaded burp. I have, however reduced the by-pass cross section with a piece of wood (as suggested by Brett by the way). The mixture volume decreases but the velocity of the mixture increases The higher velocity keeps fuel in suspension thereby keeping it from creating droplets along the by-pass walls, combustion chamber and plug.
Old Harley Man, big buck tuners have found that polishing intake ports is not the way to go. They will polish the exhaust ports like a mirror, but leave a honed-like finish on the intake ports. Seems that the smoother surfaces promote fuel puddeling along the intake runners.

Larry....Well....Maybe...Using a flow bench, I've found that cleaning up the intake path enhances flow...and Not putting on a mirror finish ehnhances atomization of the fuel mixture during it's travel in the intake path...I thought that a little polishing might help with the "burp" as it might be the oil that is the culprit....No oil in the "big block" intake....I hope!

It doesn't stop it on all of them. It does on some of them. I have tried to polish the bypass and also scuff it up roughly in attemps to reduce the BURP. Neither worked to complete satisfaction, but roughing the area in the bypass seemed to work better than polishing it. I have been able to cure the burp in in almost every engine by changing tanks, venting systems, props , fuel, or plugs in various combinations, but it is a real hassle to riddle around rot flight after flight when what you did to the last engine doesn't work with the next one. The filled or "stick in the engine" method seems to work in almost every instance. Don't ask why the factory doesn't fix it!!! myself and others have TRIED AND TRIED. No luck.

Here are a couple of past discussions on the subject. I tried to answer last night but lost power.

http://www.clstunt.com/htdocs/dcforum/DCForumID1/10011.html

http://www.clstunt.com/cgi-bin/dcforum/dcboard.cgi?az=read_count&om=7871&forum=DCForumID1

Tight lines,

Marvin's stuffer back doors should never be used to eliminate the "Fox Burp." I will take him at his word--not even a close call--when he says they fix some engines, but I do not use stuffer back doors, and all of Fox 35s have them, for this reason at all. And in fact, I very much doubt if any of my engines have improved over stock when mounted sideways on a profile.

If you really must use a Fox 35 on a profile, and this is getting more and more difficult to justify, the only sure thing is to clog up the bypass port. I've never seen this trick fail.

All stock Foxes do not burp when flown in a profile; there are a few that don't. Some of them burp worse than others. The burp is worse when the engine is run richer than normal.