Venturi oddness

Hi all,
I recently took the plunge and bought a proper OS venturi and CL NVA for my 10FP and swapped it for the R/C carb that I normally fly with. I thought this would be more "legitimate" as well as more robust.

However, it hasn't exactly been painless. I found that the NVA leaked air around the point where the nut squashes the ratchet spring into the crankcase. In fact it leaked so badly it wouldn't draw fuel when priming!

Anyway, I fixed that, and then when I tried flying I found it leaned out *hugely* in the air, and would almost quit instantly at the slightest hint of an upwards turn. Most disconcerting.

I tried running it so rich on the ground that it would hardly take off, but once airborne it was screaming lean. Not good.

I ended up deciding that it must have too large a choke area for suction feed (great!) so I put it on muffler pressure and turned the NVA so that the single spray hole was pointing back rather than straight down the venturi. Then it flew much better with no cut-outs on manouevres.

So, can anyone fill me in on whether this is the correct setup for the venturi? It's a spraybar through the venturi job, not a spigot. Where should the hole be, and should I really need muffler pressure with a venturi when the RC carb was fine?

Cheers
Jon

PS, the new tank seems to be a big improvement over the old tall one.

sounds like the venturi diameter is too big. Measure it and compare to the RC carb. It should be measurably smaller than the 25 venturi. OS might have cut corners and shipped the same part for both engines.

Hi Ferocious,

The venturi bore is (as near as I can measure) 7/32" diameter.
The NVA is 3/32" diameter.

The bore is larger than the R/C carb, but then the spraybar takes up a lot of that, so it should be a little smaller I'd say.

>The venturi bore is (as near
>as I can measure) 7/32" diameter.
>The NVA is 3/32" diameter....
>
>Jon

Jon, you confused me a little here. The smallest OS needle valve assembly that I am aware of is .125, or 4/32 inch diameter.

However, your 7/32 diameter venturi is .15 size, and this would be with a .135 diameter OS needle. Just because a manufacturer supplys a venturi for CL use, doesn't mean it is sized properly for stunt. The larger the venturi they use, the more horsepower they can claim for the engine. When one manufacturer's claimed power out does another manufacturer's claimed power, well, that sells engines.

For a "normal" stunt venturi on your .10 size engine with (supposed) .125 needle, I would suggest a .193 or even a .191 diameter venturi. If you have a .135 needle assembly, go to a .201 or .204. You could either go to a larger needle assembly (like the .155 ST needle) which would mean you would have to drill out your case, or go to a smaller venturi to get the proper fuel draw. (or maybe go to a pressure tank like the combat boys do.)

As to the direction that the hole points in the spray bar, I have tried all the way from pointed forward, to pointed backwards to straight down, and haven't noticed any difference on a properly sized venturi. Some people "notice" differences. I have not.

sounds like they are using the same venturi for 10 and 15. It will be oversize for the 10, which is why you need the muffler pressure. Like Len says, it will let the motor put out more power, but maybe not on a desireable prop size, esp. if you are looking for a stunt type run.

I haven't run the 10, so follow Len's recommendations on venturi and NVA size.

Leonard, et al

Some interesting stuff here and the following is taken from actual component measurements with a vernier and research at Tower.

Jon, you should have:

Venturi P/N 2111 2003 with a throat diameter of 6.0mm, and
NVA P/N 2111 1006. This has a step down across the throat of the venturi with a diameter of 3.5mm. There are step ups on both sides that are invisible with the spraybar installed. The larger diameter is 4.0mm.

These dimensions are taken from my OS 10 FP-S which was purchased about 18 months ago.

For reference, my NVA for the 15 FP-S, P/N 2171 1000 measures 0.124" dia (1/8" or 4/32 as Leonard points out) and has no steps.

P/N 2138 1980 is the venturi nozzle fo the Remote Needle setup for FP 20--40 and is 3.5mm dia, again no step and I assume that this is a straight bolt-in exchange for the NVA P/N 2230 1000 listed for the FP 20--40.

For reference for those unfamiliar with mixed units:
3.5mm = .1378"
4.0mm = .1575"
6.0mm = .2362"
1/8" = .125"
7/32" = .21875"

Jon, my 10 FP is a really strong little unit and everything was a very tight fit when I swapped it from RN to std NVA configuration. Have you got the right parts? Is it possible that the carb has different sized fittings? I rather doubt it.

Cheers

Geoff

LAST EDITED ON Aug-13-02 AT 11:33 PM (CDT)

ignore what I said (I removed it). I was looking at a 25LA venturi--doh!

According to the Tower Tech notes, the venturis for the 10 and the 15 are very similar--6mm ID, 7mm OD--but the spraybar on my 10 is 3.5 mm, while the spraybar I have for a 15 is 3.175 mm.

That seems to make sense to me but then I checked the FP 20/25 parts and the venturi is 6mm ID while the spraybar is 3.5mm OD.

Now I'm confused!

Cheers, Geoff

PS Another number among the millimetres.

Leonard's 7/32" mentioned above is 5.55625mm.

So all the numbers mentioned are pretty close if measured with a rule.

Easy FP venturi re-bore:

Plug unit solid with JB Weld epoxy, drill open NVA hole, redrill intake hole to smallest diameter opening you believe might work. This is adjustable in the field with a tapered hand reamer and can be completely redone if you go to wide. Worked for me. (Old timers used to shove a piece of balsa into the intake.)

Makes you wonder why I bothered paying for a genuine one...

I don't think those Japanese engineers actually fly control line.

Jon

What parts did you actually get?

Were they the numbers that I listed. From the dimensions you quoted, it sounds a bit like you have a 15 NVA.

Hi Geoff,
well the packet says 10FP-S venturi, code no. 21112003.

I don't have a part number for the NVA but it didn't have a step cut in it, but that's probably going to help if the venturi is too large.

Jon

The problem looks like the NVA.

The FP 10 NVA is 4mm dia with the centre necked down to 3.5mm dia so the holes in the crankcase and the venturi are 4mm dia. The plain NVAs are 0.125" dia if it's from a .15 and 3.5mm dia if it's from the .20 or bigger. If you thought it was 3/32" dia and is genuine OS, the odds are you have a .15 NVA. That's exactly the reason why I have one unused in my kit. I bought a 10 FP-S Rear Needle and bought a .15 NVA so I could change from the RN setup to standard if I wanted. It looked like the right diameter and worked in the good old days of OS Pets and Max 15 Mk IIIs--but not today.

Whichever you have, the odds are that you have air leaking around the sides of the NVA and maybe the O-ring is not sealing properly as there is not much pinch on it to make the seal with the crankcase when you have the 4mm dia spraybar.

Get the OS 10 NVA or borrow one from a Super Tigre and see how it goes. The ST unit should slip in without modification to the crankcase or venturi.

Good luck