ORIGINAL: Truckracer
ORIGINAL: pe reivers
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.
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.
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.