O. S. LA .40 venturi

Stuka Stunt Main Forum · 6 of 6 known posts recovered

bighow · Oct 12, 2000 04:21 PM

Leonard,
What would be a good starting point for a venturi on a LA.40?
I made one that is .250 using a K&B NVA. Looking at some of the other posts, .25 seems too small. I'm new at modifying R/C engines for stunt, trying to learn as I go, to gain a better understanding of what makes a stunt engine work.

thanks,

Howard

LNeumann · Oct 12, 2000 04:51 PM

#1 source

If you use the OS needle and it has the boost port open, then go to a .260 or .265. The OS 35/40 FP engines were equipped with a .265 at the end. With a larger diameter needle (the OS is about .135 diameter and I think the K&B is about .155. If you had to drill out the cross hole to put it in, it is the larger size) then go to a .280 or .285.

If the boost port is blocked (some of the early ones were cut in the case, and some of the later ones, apparently aren't) then go to a .275 to .280 with the OS size needle, and a .295-.300 with the larger diameter needle.)

If the K&B needle assembly is of the larger size as I think it is, then your present venturi is quite small to what you could (and should) use.

Leonard Neumann

Joe Meyer · Oct 20, 2000 02:22 PM

#2 source

Leonard,
How can you tell which la40 has boost ports and which do not? Visually, without taking the engine apart, is there a physical difference that is noticeable? The idea is that if one comes up on e-bay would you, could you, tell the difference between the two types of la40's? it would be great if you could.

LNeumann · Oct 20, 2000 04:50 PM

#3 source

I have an early LA 40 that was sent to me by another modeler for repair and rework and it was beyond that (shot is a good word for it.) This one had the boost port cut out of the case and the sleeve just like the FP. When the 46 appeared, however, several people reported that the boost port was not cut out, and now there are reports that the 40 is the same way. I have never personally seen one without the port cut, but since it is a machining operation that they are evidently skipping (one more example of being cheap), you can't tell without looking through the exhaust or opening one up.

What gets me is I don't understand OS in their approach to this engine. This is made as an RC engine. The RC crowd rates an engine by how fast it can turn a certain propeller. Since not cutting the boost port will limit the high rpm on the smaller props and the RC crowd doesn't care about turning larger props at slower rpms, it just adds to the overall feeling that this is an enferior engine.

To be truthful, I would suggest you pass when you find one of these for sale. There is a good chance that it could be totally destroyed by the time you get it, and, if not, it soon could be. These engines are not going to hold up if they are run on low oil content. And you can get a new Tower 40 for less money than an LA. True, if you mod it the way I do you will have to block the port and drop the sleeve, (plus add a venturi and needle valve), but these weigh less, are much better in quality, and will last a long time.

Leonard Neumann

carigotti · Oct 20, 2000 09:38 PM

#4 source

LAST EDITED ON Oct-20-00 AT 09:40 PM (EST)

My buddy had a used OS 46 LA and I got to take it apart. Sure enough, no boost port. The liner is VERY thin. The wall thickness is only .039 or 1mm! I was afraid I could deform it by just squeezing it in my hand! It had the same stroke as the 35/40 FP engines. Len, I think that OS targeted the low price group with this engines and eliminated the boost port to save money and went with the fact that it can swing a prop that will most likely be on a trainer. Remember how well your 35's do without a boost port. This way they save some in manufacturing and the buyer gets some performance. Though they are cheaply made. If you ever get a chance check out that THIN liner! WOW not much there.
Crist

LNeumann · Oct 20, 2000 10:03 PM

#5 source

You are right about the thin liner. It is the thinest I have ever seen on an engine--and this is the 40 I am talking about. It has the same bore/stroke as the FP, but parts won't interchange because of that liner.

Yes, they made this cheap. Brass costs more than aluminum, so they eliminated as much brass as they could. And I agree with you. It is one less operation to not cut the boost port. They use a cast crankshaft and only two machining operations on it. That is why I have never even bothered to take a 46 apart. I just don't like the quality (or lack thereof.)

Leonard Neumann