In the past, I have made little paper gaskets or used small nylon washers.  I now use a small amount of High Temp Silicon gasket cement.  Works well, simple & easy.  Even that maybe a bit of over kill.

   Numerous notables have said it makes no difference, but I think they are very definitely wrong. I can tell who did it and who didn't sitting 200 feet away.
   
   But, be VERY CAREFUL to make sure the silicone goes where it belongs.

    Brett

Brett, I am very curious to know how you can tell if and NVA has been sealed or not sealed from 200 feet away. I am intrigued to say the least.

Regards, Phil

I agree that it makes a difference. A small air leak will change the mixture as the case changes temperature (particularly in the verticals and overheads when the motor starts to work and create more heat).

I found some Permatex high temperature silicon (Ultra Black) in a small tube at Discount Auto. My PA Fox style NVA came in yesterday and I installed it today. It indicated that it cures in 24 hours.

Best,       Dennis

   When it's sitting there running on the ground before launch, the engine will usually surge up and down in rpm. This includes some of the people who swear up and down that it doesn't matter.  The sealed ones are dead steady like an electric motor.  Oddly, it doesn't seem to make nearly as much difference in the air as it does on the ground.

   Brett

Brett, what would cause an engine to surge?  It would seem to me that the total airflow through the venturie  would have to change in relation to the needle setting in order for the engine to surge.
If the needle is set with a very small air leak around the spray bar as would be the case with an unseal spray bar, I don't see how the total airflow through the venturie would change unless the spray bar retaining nut is so loose that the spray bar could rattle around in the venturie.
Since the expansion rate of aluminum is greater than the expansion rate of the common spray bar materials, wouldn't the tendency be that the tightened unsealed spray bar would get even tighter?

Regards, Phil

The differential thermal expansion can occur because the spraybar has liquid fuel for coolant. The case only has air cooling and if you are cowled or have a spinner the amount of cooling air that flows over the case can be very little.  Then as the fuel is ingested into the lower part of the venturi you get almost a flash cooling so the case can move all over depending on the fuel flow and cooling airflow around it. Sealing is simple.

Which way do you guys point the spraybar hole for open uniflow system? I've gone with back or straight down.

Best,      DennisT

Lauri,
Small leaks can drive you crazy, if you are running a 4-2-4 style run much less if you are running a 2-2-2.

If the thermal expansion doesn't get you drops of oil covering and uncovering the leak will, particularly if the engine has not been run in a some time. Oil can cover the leak then as it warms up or is hit by fuel it is all or part removed or altered which can impact the needle setting and motor run. This is similar to leaks around the needle threads, many bad running stunt engines have been tamed by tightening the collar or slipping a piece of fuel tubing over the needle threads.

Fortunately, you don't have to believe any of this, simply go out and out fly everyone.

Best,        DennisT

Brett, what would cause an engine to surge?  It would seem to me that the total airflow through the venturie  would have to change in relation to the needle setting in order for the engine to surge.
If the needle is set with a very small air leak around the spray bar as would be the case with an unseal spray bar, I don't see how the total airflow through the venturie would change unless the spray bar retaining nut is so loose that the spray bar could rattle around in the venturie.

   I expect to get blasted again shortly, but it's a good question.

    We are almost certain that it is air leaking into the spraybar pass-through. It's more complicated than you suggest above. The air doesn't leak straight into the venturi (that actually makes almost no difference on "straight-through" venturis). In the sprinkler arrangement, it's prone to two additional issues - one, fuel doesn't necessarily go from the spraybar to the venturi/spigot. It can also leak around the pass-through, down between the case and the venturi, and then dribble into the port as raw drops. This was a very serious issue on the ST46, but not nearly as serious on the PA/RO-Jett.

   It does pay to choose the venturis that have the best fit in the fore/aft direction (and both the PA and RO-Jett venturis vary significanly from unit to unit, some are loose overall, some are tight overall, some are tight in one direction (like fore and aft) but loose in the other (side to side). On the PA the spraybar holds the venturi in at the rear, and it wants to spring up in the front from the tension of the o-ring you are compressing, which also tilts it so there is a gap right below the spraybar where this fuel leak from the spraybar to the crabkshaft through the boss on the case can occur. The RO-Jett is also held by the spraybar but there's little tension on it because instead of being compressed buy the insertion force, the o-ring goes into the bore in the boss. This is offset somewhat by the fact that boss is a little shorter and the spraybar diameter is larger, meaning the seal area is shorter along the bore.

    But the issue appears to be *tiny* amounts of air getting around the spraybar (threads on one end and "head" on the other). The spraybars are different, the RO-Jett has the nut on the collet side, and the PA has the nut on the inlet side, but I don't think that makes any difference. On the Jett BSE this is a machined surface, on the PA its bead-blasted where the spraybar contacts the case. At least to start with, after a few remove/replace cycles it's worn shiny, too. What we think is happening is that air gets in this way (which is under suction when running) and mixes with the fuel just like a pinhole leak in the line. This then displaces some of the fuel and makes it go slightly lean, then clears up (since the flow rate is miniscule). I also think it's a bigger issue on the PA (which is where David first noticed it) because the fuel inlet on a stock PA spraybar is extremely small so any there is significant restriction on the fuel flow. The Jett is much larger, and counter-boring the inlet on the PA makes a huge difference with the 61 and 75. Don't drill it all the way through, because that will also remove the needle seat!  Guess how we found that out....

    We have found all sorts of interesting things on the venturi/spraybar/fuel flow/flow restriction side of the system, and almost the only fiddling we have done in the past 5-10 years is in this area with sometimes very surprising results. Merely being able to open the needle far enough to get the desired setting is not the only criteria, reducing the restrictions seems to change a lot of things that you didn't even realize were happening until you fixed it.  

   We started looking at this after the 2002 NATs and to give credit where credit is due Brad Walker and his good advice on fuel tanks and other fuel discussion for 4-strokes were the precipitating factor. I think Brad would hasten to point out that he got it from RC guys, Brian Eather, and Bill Wilson. We were never going to run a 4-stroke but physics is physics.

     Brett

Brett, it is just common sense to under stand that a leak that introduces air into the incoming fuel stream as would be the case with a 'sprinkler' type arrangement would cause erratic running just as bubbles in the fuel line will cause erratic running. No one disputes that.
It seems to me that the OP was talking about a conventional venturie and spray bar arrangement as would be found on a Fox .35 S and that is the arrangement that I was referring to.
My contention is that air leaks around the spray bar make no difference because the needle is adjusted to the total air flow through the venturie.
An unsealed conventional spray bar would have such a small leak as to be insignificant in terms of total air flow through the venturis as to be meaningless anyway.
Many, many thousands of engines have been manufactured over the years with 'unsealed' spray bars, some of them, many times over national winners.

  He said he was installing a PA needle, one might presume that is in a PA. Actually I agree with you on a "through the middle" venturi, it doesn't make much difference. Sealing it makes a big difference on a conventional sprinkler setup which is by far the most common in current stunt engines. The only currently competitive engine that is "through the middle" that I know of is the 40/46VF, and it doesn't make any noticeable difference there. ST46's and 60's were sprinklers, too, that takes us back to the 70's. And in fact I was sealing those up, too, in several places. although I can't claim to have noticed or realized that it would cause surges.

   Quoting people who won stunt contests is not that impressive, I have won a few myself and the guy who discovered the value of it has won a few as well. I know plenty of people who don't agree, they are easily identified at the field, so I offer my experience for what it is worth. Several people will shortly tell you we don't know what we are talking about and it never happened to them, so you will get both sides of the story.

 You asked how I could tell, this was the answer.

   Brett

Brett, you are correct in that I was thinking ; PA needle installed in a Fox; type situation

I have several engines with the sprinkler tupe fuel delivery system and agree with you on that system.

 On the conventional spray bar set up, it makes absolutely no difference if it is sealed o not.

Regards, Phil

Brett,
You indicated that you drilled out the PA NVA to reduce the inlet drop. What # drill did you use and how far in did you drill?

Best,          DennisT

  BTW, there are non-standard "through the middle" spraybars where it does matter. OS makes a spraybar with what amounts to a right-angle fitting on the end. This is a separate piece that is held on the end of an otherwise conventional spraybar with a crown nut. Fuel enters the right-angle fitting, goes into a annular manifold then into a hole in the side of the spraybar, then out to the middle of the venturi just like a FOX or McCoy. (snip)    Brett

Around here that NVA was referred to as a banjo fitting.  I used a sharp toothpick to spread silicone sealer lightly around the edge.  Seemed to work fine, just hope you didn't have to remove it during a meet.  The silicone would have to be given time to set up again.

BIG Bear
RNMM/AMM

Dennis ,never got your muff's done, mill is still down but i have a bunch of aluminum washers that i made up  send me your address ind i will send u some   

two years ago Burt and i did some test at the VSC  with needle valves and spray bars and it did make a diff with the washers. i tried fiber but they get loose with time . we also found the the S/T type with the slits also can leak  .i file the end of the sprybar flat and put a piece of delron in the collet and and no leak  also need to be carefull as there is two diff size needles, if u put the thinner one in the bigger spraybar it will drive u crazy

Use  CAUTION drilling the spraybars, you can ruin them quickly, I also went thru that exercise many years back and did not find anything useful, my results were opposite of Dave's, They did not improve anything in my motors, The standard PA NVA will feed all the fuel a 75 can use, and you can flood it out by opening the needle, It also feeds the 85 and 88, I used a 91 and I have a 105 that the standard NVA will feed enough fuel to work perfectly, opening the hole size did nothing at all in my test.

Also about the having to seal the bar, That is not normally the case, It does not hurt anything to seal the sides of the seat at the case, but I have never done that except for once, I have had zero problems with any air leaks .
This is what I have found out is more times the problem, people remove the NVA and venturie, They put them back and either press the venturie too hard or not hard enough, this results in a misalignment of the hole for the bar and the slot in the venturiie, they slice off a tiny piece of venturie with the threads of the bar, or a piece of aluminum, that gets into the threads of the spraybar, this is pushed against the sides of the case when the nut is installed and tightened. This tiny debis is what causes the airleak on the sides of the case seal. To guard against this, use a tiny stiff brush to clean the threads, or a stiff toothbrush. I see this and fix it for people all the time.

Randy