I have a 4.2 Sachs in an ESM Spitfire. The carb sticks out only 1/4" thru the cowling. The choke butterfly sticks out perpendicullar to airstream when open, causing a turbulence at flying speeds at carb face. The engine runs fine on ground but in air it surges and runs inconsistently, especially at half or full throttle. Get it back on ground and it runs like a top. Good transition and smooth top end . Do ya'll think a velocity stack may help this problem? Hard to find one that will fit a larger Walbro SBD carb. Any ideas? Thanks, Paul

Have you flown it without the cowl? Dan.

Sounds more like it's changes in air pressure within the cowl that is affecting the metering diaphram in the carb and causing the engine to act up when in the air, which is a very common problem with side mounted carbs in some cowled installs. Try this fix, install a nipple into the air hole in the carb plate and fit a bit of fuel tubing and run it inside the fuse into the static air there and see if that solves your problem.

What is happening is that you have a piston ported engine and there is a fuel/air charge that is blowing out the front of the carb. On the ground some is blown away by the prop blast, but a lot more is blown away in the air by the slipstream. You can feel and see the fuel/air charge if you hold your hand between the prop and carb blocking the prop blast. If you have it at idle and pop the throttle open while blocking the prop blast you'll see a "FOG" right over the carb opening. To test if this is the problem tape a air-dam in front of the carb opening, it can be a anything about 1" high by 2" perpendicular to the airflow, a servo or a playing card folded in half at 90° then cut the base so you can fold it to give it a little strength against the wind. I think you'll find this will cure your problem, then you can figure out how to make something nicer.

Yeah it sounds like your carb metering diaphragm is reacting to pressure changes inside of the cowl. Like mentioned, you can solder on or epoxy on a tube or fitting and then route a tube from it into the fuselage someplace where the air is static and that should solve the problem.

A intake stack is not a bad thing to put on either. A side carb engine setup tends to spit fuel and the stack helps alleviate that and also stabilize the airflow into the carb. The main jet is mounted high in the carb and tends to be affected by the fast moving air blowing past the carb's intake.

reference: http://www.prme.nl/forum/viewtopic.p...b50e44b92723d0

earlwb and karolh have it nailed down.


Soar who wants to fly around with a "damn" glued to the carb........ way off

3w dealer should have stack to fit,Aircraft international,any piston port should use stack,good luck,P,S i mite have one,

I notice better fuel economy and less fuel spatter on my planes when using a velocity stack; even with the engines with reed valves that don't have as much reversion, or stand off as a piston ported engine.

AV8TOR

I do use them on all my engines though but they are all rear carbs. get them from TBM for 30 bucks

+3

It will be interesting if he reports back what he finds. 3 against 1 on this. I usually go with what Pe says, he's one of the three best on the forum, but I had exactly this problem on a Hurricane, the static tube won't hurt, but it's not the cure for the problem, stopping the slipstream blowing the fuel charge away from the front of the carb is. I always use a velocity stack on piston port engines then tie airfilter foam around that. What he really needs is a Tony Clark 90° elbow #6587 for the carb to keep it inside the cowl.

Soarrich, make it 3 against 2 now. While I agree with the need to make sure that cowl pressure isn't causing problems for the metering system and that should be ruled out as a cause and resolved if necessary. On the other hand I have seen examples where the only fix for a side intake engine was some kind or air dam. The problem is when an air intake is near a cowl, sticks through a cowl or requires an air inlet cheater hole in the cowl, you just never know quite how the carb will react.

Examples of cures I have seen when all the common cures on the metering side of the carb failed to solve the problem..... and this includes baffling, etc. The first included installing (or in a couple of cases removing) a velocity stack. The second was turning the carb 180 degrees on its mount. The third was a very small air dam in front of car air inlet on the outside of the cowl .... this was less than 1/4" high and completely solved the problem. In another case, a small air dam was installed in front of the air inlet on the inside of the cowl.

The point here is that airflow around the carb inlet can do very tricky things on these side intake engines. You just have to be willing to experiment, using all the standard fixes until you find the one or the combination that resolves the problem.

Unlike some people on here, I'm not quite so quick to condemn the suggestions of others.

If the velocity stack has an angled entry It may be important which way the angle points. With a straight ended stack, using an air dam can create as many problems as it intends to solve. Mostly due to creating a vacuum at the carb mouth.
Condemn?? Heavy words.

If you do decide to equalize your carburetor pressure, I've done a couple of write ups with photos that may help:

DLE 20 in a Pulse 125
DA-50R in an SBach 342

I agree 100% with what you are saying Pe. That is why I suggest trying as many things as possible to resolve the issue. On more than one occasion, I have seen a fix, that for all purposes, should not have worked but it did. When an engine installation is completed using all the known proper practices and there is still a problem, one gets desperate and will try anything to resolve the issue.

As far as creating a vacuum at the air intake, while perhaps off subject, I have often wondered about the rear facing air intake horn used on some 3W engines. I would believe that would create a constant low pressure at the carb inlet.

Condemn ..... heavy perhaps, but when I see people piling on a guy for making a suggestion that I have seen work I get a bit upset. Perhaps I should have said criticize rather than condemn.

I cant say i understand the whole piston ported, cowl pressure change thing, but i do have an ESM Corsair that has a G-38 in it that has the carb just barely out of the side of the cowl. My first dozen flights or so were done with no stack and i never could get it tuned. Always ran perfect on the ground but would never run right in the air. Finally decided to try a stack on the suggestion of a fellow flyer. Within 3 flights had it tuned great. Also noticed a whole lot less fuel residue on the side of the fuse. I tried the stack that came with my G-20.

Would a stack that turns 90% to face the opening forward be better?

There are many things that remedied the carb mixture problems. Very often, the rationale behind the remedies was quite obscured, which makes people jump to conclusions, which are not necessarily true. A carb "knows" only two things. Air pressure at the jets, and pressure or fuel level inside the fuel chamber. These are used to let fuel flow into the airstream, whichever amount/mass that is. In this reasoning I bypass venturi effect, and airdam effects, as well as reference pressure effects, which can be many. In the end it boils down to previously mentioned two. If an engine starts surging in flight, you have a hunch what to look for (reduced air entry, increased fuel flow, or both; all plane speed depending.). Each symptom has it's own reasons, often including engine setup problems. Now here lies the truth. Not many report all the variables that enter the picture needed to analize the problem, due to not being able to do so, or not even thinking of the things that have importance and influence the mixture.

In piston ported engines, a velocity stack is of advantage due to the increased ram air effect reducing blowback and increasing power.
Is it needed? That depends on the user's expectations.
Will it introduce solutions/problems? That depends on the surrounding conditions
How long should the stack be? Again depends on user expectations. For best power, length depends on crankcase compression ratio, induction timing and rpm. Be prepared to use at least 8" though. The carb should be at the end of the stack. If not, and using longer stacks, the mixture will go extremely rich if rpm are below tuned rpm. That is due to the fact that blowback passes the jets three times; on entry, on blowback, and next entry again.

In that case you would need to equalize carb mouth pressure and carb reference pressure. Anything that alters carb entry or fuel reference pressure is a disadvantage that needs compensating measures.

Pe, I've always thought that if we could see airflow and pressure in and around our cowls and engines, which of course we can't, fixing obscure mixture problems would be a lot easier.

And likewise, last season a friend fixed a similar problem when he removed the stack from his carb.

I use a Ram Stack on all my G62's.

It is cut at a 45 degree angle and I turn it to the front so air is RAMMED into the carb.

Don't over estimate the ramming effect. It is negligible, unless you fly real fast. You may see a 1% power increase, at the cost of risking your engine going lean.
See the calculator here! http://www.wallaceracing.com/ram-air-calc.php

PS
A 1% power increase at 100 Mph will not be noticed by a normal person. If the scoop is located in a low pressure zone which the cowl very often is, there will be no gain at all, since the pressure lowering will cancel all ram air effects.
In pylon racing, a slight cornering mistake will nullify the gain pretty quickly.

PS PS
In a two stroke, a denser crankcase filling does not automatically translate in more power, unless the muffler system can harness the extra gas flow and prevent it from going out as fast as it came into the cylinder. Four strokes do not suffer from this.

Yes, "Ram Air" is commonly misinterpreted. Many people think "Hey, it has GOT to help." Not so. The truth is as PE stated, about 1% gain at 100 mph, with concurrent possible/probable problems in carburetion.

AV8TOR

I am not saying it is a CURE all.

I should have said


"This is what I use and has worked for me on my G62's"

Sorry