what is the proper nva, and where do you get it?
The proper needle valve/spraybar came with the engine, unless someone tossed it. There is an Enya replacement of similar dimensions :
https://www.ebay.com/itm/OS-FP-LA-20-46-NEEDLE-VALVE-ASSY-DIRECT-REPLACEMENT-NIP/293139719059?hash=item44407d2393:g:0-8AAOSwJRFcolWt:sc:USPSFirstClass!94086!US!-1 This was made specifically to fit as a replacement part. I haven't tried it, but it does avoid you having to destroy another OS20/25FP or 25LA crankcase by drilling holes in it to fit an incorrect part.
The correct venturi size for the correct spraybar assembly is .256"/"6.5mm" for the 20/25FP or the 25LA. This works with the recommended setup.
Based on painful experience, check the diameter of the holes for the spraybar in your engine. On an undamaged crankcase, the hole in the case to pass the spraybar is 0.1459" in diameter +-.0002". That's difficult to measure without the proper equipment, but it's 3.705mm, which is about what you will get with a 3.7mm drill, which is probably what they used. As a check, a #26/.147 drill will not go, and a #27/.144 drill will fit pretty easily. This intended to fit the "3.5 mm" spraybar, which measures around 3.506mm, or .1380"+- a few tenths.
If it has been crankcase has been damaged to accept an ST needle valve, it will fit at least a #22/.157 drill, or larger (even 11/64"/.171, which is way oversized, but easily available at Home Depot in a fractional drill set). Or, someone took the tang of a file, or a Dremel 1/8 burr, or a 6 penny nail, or something like that, to hack it out without a drill. And if you think I exaggerate, I have seen this done with the tang end of a small file, at the field, stick it in the hole and twist back and forth until an ST spraybar goes through it.
It's practically impossible to actually fix the crankcase and fit the correct parts with no modifications. But you can put sleeves on the ends of the spraybar that pass through the crankcase to make it fit the bigger hole, without narrowing the part that goes through the center of the venturi. It ends up looking like a dogbone (or like a Fox 35 spraybar), thicker where it goes through the crankcase and venturi, and narrow (.138 ) in the middle. These sleeves have to be secured in place somehow, solder or JBWeld, so they can't slide into the airstream, or fall off and into the engine.
An alternative is to replace the venturi as well, with a much larger one, to make up for the much larger (.1575"/4.0xx mm) ST spraybar and associated flow restriction. The correct replacement ID is about .274", and the closest US drill size is a Letter I drill (.272) which will make about a .274 hole. However, if the engine has already been modified, you can probably expect either 17/64 (.265x) or 9/32 (.281x), since letter I drills are rare and many people doing these sorts of modifications probably consider either metric or the decimal system to be a socialist conspiracy, when in fact the metric system is merely a Europhile affectation.
A .265 is too small and will reduce the performance and invalidate the recommendations on prop and fuel, and require lower-performance props like a 9-5. A .281 is far, far, too big, and will not needle correctly on any usable stunt prop. If it has a .281 venturi, you probably have to start looking for extraneous head gaskets as well, since the previous owners may have tried to solve their incorrect venturi size by staking head gaskets in. This just makes it worse, but that never stops them. To restore the engine, any extraneous head gaskets must be removed and only the factory stock parts should be used.
I offer this from extremely painful experience. Since I put people on to the 20FP and "new" 25LA in the first place, I have had people complain on the internet and in person about how it didn't work the way I said it did. In all of these cases, they had *modified the engine*, usually with a f*cking ST spraybar (sometimes in the crudest way possible) left the venturi, and then it won't put out any power - since the venturi choke area is reduced by something like 30%. Tossing the muffler "to save weight" is also a very popular improvement that absolutely destroys the stability of the run.
I have spares, but you can't fix it, because the hole in the crankcase is damaged by drilling it out and the muffler is in the landfill. A secondary modification, by the real "experts", is to drill the venturi 9/32" (usually while holding it with vice-grips). Then, power is not a problem, it's got too much, can can't be needled, and "runs away". This is invariably "cured" by stacking in a handful of head gaskets, which just makes it run worse. Then you sell it or give it away with a bunch of complaints about idiots from California "who don't know nothin'". Again, you might think I exaggerate, but I have seen this on multiple occasions.
Another popular improvement is attempting to run a "Real Stunt Prop" like a 9-6, which works with an ST spraybar and stock venturi, but is gutless and no improvement, so you go back to a Fox 35.
Brett
Topic: 25 sized stunter (Read 2798 times)
