Prefered ST needle for OS 40 LA?

I have a friend who has both a ST 51 and a ST 34 needle valve. Which one
is preferred to be used on the OS 40 LA?

They are identical except for a lock washer supplied with the G34 unit. They are both too short for the LA.

Ok, thanks for the reply. Someone said that was true but I wasn't sure.

>Ok, thanks for the reply. Someone said that was true but I
>wasn't sure.

You have to drill the cross hole larger on both the engine case and the venturi in order to use the ST needle. Having done that, if you want to use the short one, you can then file the sides of the case on both sides where the needle mounts. Otherwise I have ST 46 length needles that will fit without the extra filing.

Be aware that you are choking off the venturi more with the ST needle. Some of these venturis may be too large already (I know of at least five different sizes of venturi that were supplied by OS on these engines). But if your goal is to maintain the same power and suction as with the OS needle, then you will need to bore out the venturi more or seek an appropriate replacement.

> . . .
>You have to drill the cross hole larger on both the engine
>case and the venturi in order to use the ST needle. Having
>done that, if you want to use the short one, you can then file
>the sides of the case on both sides where the needle mounts.
>Otherwise I have ST 46 length needles that will fit without
>the extra filing.
>
>Be aware that you are choking off the venturi more with the ST
>needle. Some of these venturis may be too large already (I
>know of at least five different sizes of venturi that were
>supplied by OS on these engines). But if your goal is to
>maintain the same power and suction as with the OS needle,
>then you will need to bore out the venturi more or seek an
>appropriate replacement.

Leonard --

Very timely comment! I have a used LA 40 which I bought as "stock", but I thought it odd, as the NVA looked like what I remember ST NVAs looking like back in the 60s. Definitely not an OS FP style at all. I had to put the needle on the other side, so while I had it out I miked the spraybar and venturi:

Spraybar: .154 (maybe a fuzz strong, .155 with a harder squeeze)
Venturi: .281

That's not right, I'll bet. A standard OS FP spraybar (I thought the LAs were the same) rattles around in the hole.

I got a bad combo, right?

Larry Fulwider

(snip)
>Spraybar: .154 (maybe a fuzz strong, .155 with a harder
>squeeze)
>Venturi: .281
>
>That's not right, I'll bet. A standard OS FP spraybar (I
>thought the LAs were the same) rattles around in the hole.
>
>I got a bad combo, right?
>
>Larry Fulwider
>
>
>

If it has the ST needle, it has been drilled out. That needle makes your venturi equivilent to about a .261 with the OS needle.

I have compared the spray bar holes between the standard OS and the ST 34 needle bar. The ST 34 hole is much larger. How will this affect the
engine performance?

I have found the ST G51 NVA works real well. Like Mr. Neumann said you have some drilling to do. Said that, It is a perfect fit. I use a 280 venturi size and have great engine runs. If reworking the three port sleave, lowering the timing, This is one of the best running engines for stunt out there. And why people spend big bucks for some of these other 40 size engines out there I will never know.

>I got a bad combo, right?

>Larry Fulwider

Wrong Larry. This is about the right ID using the ST needle valve assembly in the LA 40. As Leonard says, it is pretty much the same as using a .265 ID venturi with the OS needle.
If you go much larger with the venturi the needle will become pretty critical. Much smaller and you are leaving power on the table.
I'll bet it runs well. If it tries to run away, add a head shim.
Jim

OK, guys, suppose someone has put in a ST NVA, and you want to go back to the standard .135 OS NVA.

Do you think it is OK to just fill the gap with Permatex or some other Gasket Maker product? A machinist would probably build-up the oversize venturi-crankase hole up with JB weld or some such such, and re-machine a new hole to the OS specs. I'm not good enough to do that.

I hate these problems that are like trying to run a sausage grinder in reverse to make a pig.

Larry Fulwider

A machinist would make 2 flanged bushings. If you want to go back to the OS nva just use a couple of thin washers on each side and a thicker O-ring so the venturi doesn't rattle around.

I drilled mine 5/32" and used a PA nva with a (machinist made) spacer to center the fuel hole.


Giffy

Larry
Why would you want to go back to the OS needle valve assembly?
Jim