Hi,

I haven't seen any responses for a while so I'm guessing we've narrowed the problem down to fuel draw at the carburettor input. I'll try and summarize what we have found in this post.

The problem seems to be fuel turbulance when fuel leaves the fuel intake nipple and takes a sharp turn when it enters the needle valve area (see drawing attached). This can create foaming, and in worst cases, a fuel air lock. This makes carburettor adustment difficult and may even cause the engine to quit while in flight. To give credit where credit is due, this problem was identified by another modler, Andy, in an earlier post and you should read it.

We have tried many things to fix this, such as, more muffler pressure, enlargening fuel/exhaust nipples and drilling the carburettor brass insert. Drilling the hole larger in the brass insert seems to have done the trick, I have just drilled three and all work well after the modification. Drilling the nipples does not seem to affect the performance, but doesn't hurt either.

I'm going to try and upload a picture to help those who have not done the modification yet. The modification can be done with the brass insert in place (although you have to be very careful not to damage the nipple threads) or by taking out the brass insert and drilling (preferred).

The hole in the brass insert should be as close as possible to the "shoulder" of the brass insert and directly under the fuel intake nipple.

If you chose to try this modification, please make sure you do not have a carburettor air leak (where carburettor enters engine body) or an air leak at the engine back plate. If you do, you must seal first. Seal with Ultra Copper Hi-Temp RTV by Permatex (available from auto motive stores) or replace "O" rings as necessary. Testing for air leaks is described earlier in this thread.

Please be aware that any modification will void your warrantee.

See instructions below and refer to the attached picture for part location. I've included two sets of instructions, one for drilling with brass insert in place and the second to remove brass insert for drilling.

If you chose to drill the brass insert in the carburettor:
1. First remove the needle valve retaining nut, needle valve and the intake fuel nipple to avoid damage to the needle valve. Drill the fuel intake nipple one (or two) size larger.
2. Reinsert the fuel intake nipple and then use it as a drill guide, this will protect the nipple threads.
3. Very carefully drill through the fuel intake nipple and into the brass insert.
3. Remove all brass filings from inside the brass insert and re assemble the carburettor.

If you wish to remove the brass insert to drill (prefered method):
1. Remove the intake fuel nipple
2. Remove the needle valve assembly by taking off the needle valve retaining nut retaining nut to avoid damage to the needle valve.
3. Remove the retaining screw
4. Remove throttle barrel, be careful, there is a spring behind it. Note: small end of spring goes into carburettor first.
5. Using a hardwood dowel or brass puch, carefully tap the threaded end of the brass insert until the brass insert comes out. Note: be careful not to damage the threads.
6. Drill the hole into the brass insert at a slight angle towards the carburettor throat (same angle as fuel intake nipple).
7. Clean out all brass filing out of brass insert and remove any drill burrs.
8. VERY IMPORTANT! Reinstall brass insert so the enlarged drill hole is directly underneath fuel intake nipple. To reinstall brass insert, you will need a hollow punch and a backup block with a hole (preferably made out of a hardwood block). Set the brass insert in place so the newly drilled hole will line up under the carburettor nipple. Put the carburettor over the hole in the wooden back up block and gently tap the brass insert in place using the hollow punch.
9. Reassemble carburettor.

After the above, you should adjust your engine as described in earlier posts.

I have just done this with three engines and they all work well.

If anyone else has any input, please let us know.

Thanks for everyone's help in solving this problem. If anyone is going to Toledo on April 2nd, let me know by PM and perhaps we can meet.

Fly4Fun,

Wayne Miller