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Engine basics => Engine set up tips => Topic started by: Allen Eshleman on November 15, 2016, 07:52:21 PM
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My brother is working on my FP 40. He is trying to remove the original metal venturi for control line to put in a smaller one made by Jim Lee. How can the old one be taken off? Or can it?
Thanks ahead of time. It is an original CL engine, not a converted RC engine.
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well it probably does not need to come out actually, but if you need to, heating the case in the oven, then cooling the venturi whild the case is hot would be one plan...probably look at about 300 degrees for 10 or 15 minutes,, oh and wear gloves,, He does kinow to pull the needle valve assembly first right?
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Unscrew needle.
(Note or mark the position of the spraybar for reassembly)
Slightly bend open the springy metal needle retainer and use needle nose pliers or a small socket to loosen the nut on the spraybar assembly.
Remove spraybar assembly.
Remove the venturi.
Reassemble in reverse order.
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Don't use pliers if you can avoid it.
The nut on a factory OS NVA is 5,5mm AF. 17/32" is close. Get a long series socket and use that rather than attack the spring.
The venturi should just slip out.
When you reassemble, ensure that the spraybar is located so that the hole is just below the nine o'clock position in relation to the centreline of the venturi.
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Venturis can get stuck. If it won't come out after you remove the spraybar, you can clamp the venturi in a vise and pull on the engine while you rock it back and forth a little.
MM
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I've seen way too many good engines with plier marks all over. The safe method is to drill a slightly oversize hole in a piece of hardwood. Then cut a slot to the hole. The "slot" can be clamped, which draws the hole smaller. But first put the hole over the venturi, or similar part. Clamping the "slot" makes the hole smaller to grab onto the metal part. Then, apply your agressive pliers to the wood, and pull. Works with things like cylinder heads, cylinder fins, etc.
Floyd
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Thanks for all the suggestions. My brother got it to come out. It took one person to hold the venturi with pliers and my brother moved it around as I understand it. The new one will soon be installed. It will be interesting to see if that cures my runaway problem. I do have an extra head gasket. I'm try to decide if I put that in also.
Thanks again.
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I just found an older post I started where it was highly suggested to not put in an extra shim. I am going to run with this small venturi and see how that works.
Thanks
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Venturis can get stuck. If it won't come out after you remove the spraybar, you can clamp the venturi in a vise and pull on the engine while you rock it back and forth a little.
MM
Nooooooooooooooooo! Never, never, never clamp it in a vice unless you want to f*** it up!
Jerry
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Jerry
Agreed in a flat faced vice
but some of us are crafty enough to make a round adapter grip from wood--- or have the molded inserts with various round and "V" openings made from semi hard rubber to grip the round part with and hold firm....Jest sayin
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Assuming that the spraybar comes out ok and there is an o-ring under the venturi insert, I'd give the venturi insert a good whack with a rubber mallet or using a piece of soft wood (end grain) and a steel hammer. There should be a little bit of space available to "shock" the insert loose from the engine casting. No harm should befall the insert or engine case by doing this. Might need 4 hands to get 'er done.
Be sure to put a new o-ring under the new venturi insert. Brett says to use silicone seal around the spraybar also. I haven't found that necessary, but maybe I'm not as fussy or observant. :-[ Steve
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Jerry
Agreed in a flat faced vice
but some of us are crafty enough to make a round adapter grip from wood--- or have the molded inserts with various round and "V" openings made from semi hard rubber to grip the round part with and hold firm....Jest sayin
Hi Fred,
Obviously I was thinking of a flat vise.... And yes, a round adapter would be perfectly fine.
Cheers, Jerry
PS: When I was a dumb kid (now I'm a dumb adult!) I put an engine in the vise to run it and when I took it out I noticed that I'd oblonged the CC...........oops!
So much for that engine!
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A hair dryer or heat gun works wonders for disassembly.
Phil
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Nooooooooooooooooo! Never, never, never clamp it in a vice unless you want to f*** it up!
Jerry
Depends on the vise and the operator I guess, he said he's replacing it.
MM
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I have never had a problem with one coming loose for removal, but I think it is best to start with the least amount of effort and progress from there. I would start by putting raw fuel on it. Melting any gummed castor. Then I suppose the tools would come out after that.
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If it's all stock parts, and it's not gummed up with castor oil, it should just come out (once the spraybar is out). I'd use Rich's raw fuel (or some denatured alcohol) first, then I'd try heat (i.e., a Monocoat gun). If you have to put pliers on it, something is wrong.
(And, next time, you DO NOT have to change to a smaller venturi -- just pile squares of pantyhose material over the opening until you've got it adjusted the way you want. It acts like a smaller venturi AND it filter's the air going in -- what's not to like?)
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(And, next time, you DO NOT have to change to a smaller venturi -- just pile squares of pantyhose material over the opening until you've got it adjusted the way you want. It acts like a smaller venturi AND it filter's the air going in -- what's not to like?)
Always wondered about this Tim.
Traditionally you suffered a throttled response in exchange for, obviously the engine speed you desire and a far better fuel draw because venturi only restriction gives you that.
The point of highest speed and lowest pressure is always at the jet.
But with a diffuser placed well before the jet ( as would be the case with gauze stretched over a bell mouth), the incoming air is slowed prematurely thus losing draw at the jet.
I get that any intake restriction would throttle an engine but fuel draw must suffer somewhat.
As a fine tuning aid I like the idea but really it should be no sustitute for a correct sized venturi.
Case in point, double the current working diameter of any venturi and try and figure out how much gauze will correct this error. I know that the above is a wild example but it aptly illustrates my point.
Thanks.
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Always wondered about this Tim.
Traditionally you suffered a throttled response in exchange for, obviously the engine speed you desire and a far better fuel draw because venturi only restriction gives you that.
The point of highest speed and lowest pressure is always at the jet.
But with a diffuser placed well before the jet ( as would be the case with gauze stretched over a bell mouth), the incoming air is slowed prematurely thus losing draw at the jet.
I get that any intake restriction would throttle an engine but fuel draw must suffer somewhat.
As a fine tuning aid I like the idea but really it should be no sustitute for a correct sized venturi.
OY! What happens to the fuel draw if you restrict it completely, like, say, with your finger? In that case essentially no air flows through the venturi, but it sucks fuel like gangbusters, in fact, it might suck up to about .40 cubic inches (or whatever the swept displacement of the engine might be), depending on the port timing.
In the typical venturi, the draw is a function of both the Bernoulli flow effect, and the upstream restriction. The restriction generally *helps* with overall fuel draw, otherwise choking it wouldn't
work. It does reduce the power, but power is irrelevant as long as you can use any sized engine you want, like a piped 60 in an airplane that we once would have flown with an ST46.
Moreover, given that power is not an issue, the diffuser diffuses the flow so it is much less sensitive than an "ideal" venturi to variations to the inlet air direction, which seems to be a much more important problem than loss of power. So-called "True Venturis" optimized to the extent possible for flow efficiency, tend to have *much* more variation in flight from, apparently varying angles of attack, and the blockiest and turbulence- and restriction- generating configurations seem to be the least prone to in-flight variations.
Brett
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Traditionally you suffered a throttled response in exchange for, obviously the engine speed you desire and a far better fuel draw because venturi only restriction gives you that.
The point of highest speed and lowest pressure is always at the jet.
But with a diffuser placed well before the jet ( as would be the case with gauze stretched over a bell mouth), the incoming air is slowed prematurely thus losing draw at the jet.
That would be true if you vented the tank to the portion of the venturi above the jet. But you're venting it to the atmosphere, so what Brett said applies.
What you lose, AFAIK, is good metering with variations in the amount of restriction below the jet, and possibly with engine speed variation. That's why typical US automotive carburetors put the venturis above the throttle butterflies*, and why typical RC carbs more or less wrap the throttle barrel around the needle valve.
* I am NOT going to try to explain Solex carbs here -- in large part because I don't understand 'em!
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Solex carbs basically are just a convenient means to distribute fuel to the ground,, LL~
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Amal forever. You ALWAYS have something to tinker with.
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Hi Brett and Tim,
I see fuel draw as the cause and atomisation as the symptom, you can't have one without the other in a carburetted run situation.
And I believe that placing those two at the jet will always give the best control.
The issue that I see with restricting the flow (as you rightly say the adjunct for draw) before jet is that it desensities the control you have over needling.
Any time that you take flow control away from the jet, say as in sub piston induction that remotely enhances air flow after the jet or as here with a remote denial of air flow, the amount of control or rate of change at the needle lessens. And indeed an air leak somewhere else will also erode control.
I also believe that the results you are finding with diffusers in regards to desensitising the flow during manevours may not be due to eliminating the changing angle of the incoming air but a general effect that remotely placing a restriction gives. A diffuser dampens everything.
I wonder if a model engine that is not subject to air flow angles would react the same as one that suffers them, and I am thinking of Lauries rear induction masterpiece as a test bed here.
And I still contend that a correctly sized and angled venturi that has the restriction or smallest bore diameter at or very close to the jet is the best starting point before reaching for system dampeners. Saying that using a too large a diameter venturi bore can be exactly compensated for with a dampener is in error - the effect might be desireable but not the same.
Thanks.
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I don't know if this is usefull, but if you compare the scavenging speed (including in air intake) with any airspeed that's happening outside the engine, you'll see that outside air movement doesn't have much effect. Another question is how stable the airstream near the propeller root is. Not very.
But what clearly helps is a longer venturi tube (before spraybar). In some classes they take the benefit of maximum volume between spraybar and shaft opening but that's something else.
Longer venturi for sure stabilizes and straightens airflow but according to my tests, more important stabilizing factor is that less fuel splashes out from venturi.
The intake duration and -timing must be matched to carter volume and operating rpm. So it is not possible to have it perfect in a classic stunt run. But it sure helps the stability if all fuel that has been metered by needle valve goes through the engine.
As it has been noted allready several times, an air filter clearly stabilizes the run. Propably not by increasing the suction from spraybar but by adding turbulence. In that part of engine, Chaos is a good way to get some order. L
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I had forgotten about flow reversion and fuel splash, thanks Lauri.
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We're quite far from FP.40 venturi, but but there is one funny benefit in rear intake concept; it stabilizes and when done properly (engine cooling), even self-regulates the fuel mixture according to the engine temperature. L
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I don't know if this is usefull, but if you compare the scavenging speed (including in air intake) with any airspeed that's happening outside the engine, you'll see that outside air movement doesn't have much effect. Another question is how stable the airstream near the propeller root is. Not very.
That's what I thought, too, until we found that you couldn't get through an outside corner with a "true venturi" without some engines going screeching lean.
Brett
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So what was your solution? Another type of venturi?
I can think of at least 3 other phenomens that can cause similar behaviour in many engines, and I am able to reverse the problem by changing engine position or direction of rotation.
But I was a little unclear. A badly designed venturi can be disturbed by outside airstream, it can be too short, the edges od shroud can be too sharp etc.