Pod Filter Problem

[jeremydde]

I definately experienced a lean backfire out the muffler, not the carb. It was lean because I down jetted to see if it solved the problem. It did not, so I put the original jets back in. Modern 4-stroke fuel injected engines turn off the injectors during decel to prevent lean backfires and a popping exhaust. Feedback carbs sometimes have over-run fuel cut valves in them too.

[Arnadanoob]

Good examples of lean conditions but let me reinforce that 4 stroke analogies most often times does not apply to 2 stroke engines. 4 stroke engine valvetrains are much more efficient in controlling intake charge delivery and exhaust gas evacuation. On a 4 stroke engine you can completely eliminate overlap conditions between the intake and exhaust valves but it is impossible to accomplish on any 2 stroke engine found on a moped. Cannot accurately relate fuel injection systems to moped carbs as the latter is less flexible, less accurate and not adaptive as a fuel injection system with ECU.

There is another contributor to the popping sounds you hear in a exhaust system in a car called reversion. That's a condition that starts with the exhaust system being pressurized with more exhaust gas velocity and volume (from pushing on the throttle) and the driver suddenly lifts off the throttle during decel abruptly reducing the exhaust gas velocity and volume causing a pressure differential and turbulence in the pipes and silencer, if it has one.

Modern fuel injection systems shut the injectors (or at the very least reduce their duty cycles) during decel for emission control purposes also, primarily to reduce unburned hydrocarbon emission levels.

A backfire that comes out of the exhaust on a moped can be caused by a situation caused by excessive EGT (exhaust gas temps) when raw fuel is dumped into the pipe which causes it to ignite in the silencer or in very rare situations, the J portion of the pipe right next to the exhaust port on the bore. On poorly maintained stock engines this can happen but on a modded DIO, I've never seen any lean condition backfires out of the exhaust. The only way that can happen is if you were completely off on the jetting (i.e. using a 35 pilot when you should be using a 55) or your jets clog suddenly. What happens is that when it's that lean, the ignition tries to fire off the intake charge but some of the fuel goes unburned which dumps into the exhaust. The overly lean condition will definitely have soaring EGT (in excess of 1400F) which can ignite that fuel in the exhaust system.

Another simple problem some bikes encounter is the green wire from the stator which I believe Bakaracer pointed out nicely. This is evident when the bike is rode daily, just went through an engine swap, or some bottom end word like crank/bore changes which involves removing it.

I think only 2 of my customers have EGT gauges on their bikes. A moped exhaust will have EGTs in the range of 800-1250F at full power.

[Lunytune]

Arnadanoob,
Something else to factor into that is the exhaust being tuned to the mods. If you've modded the cylinder and jetting, you're "front loaded". The exhaust is going to be resonating exhaust back to the cylinder for "refire?" or reburn? I'm over my head now, but seems there would be less muffler explosion with a stock exhaust and modded fuel and cylinder. Now comes the manner of playing with the baffles.

Now cypher all that and spit it back out the way it's supposed to be.

[jeremydde]

You are correct on the part about being "off" on the jetting. I went from a 45 pilot to a 39 and immediately had backfires. Changing the sparkplug has fixed the problem. I rode it around for an hour today and there were no problems at all.

[Bear45-70]

.............
There is another contributor to the popping sounds you hear in a exhaust system in a car called reversion. That's a condition that starts with the exhaust system being pressurized with more exhaust gas velocity and volume (from pushing on the throttle) and the driver suddenly lifts off the throttle during decel abruptly reducing the exhaust gas velocity and volume causing a pressure differential and turbulence in the pipes and silencer, if it has one................

All exhaust systems have reversion or better yet reversion pulses. *, Johnson and Evinrude V-4 outbards run a modified exhaust so that each bank sees a reversion pulse from the other bank, making the exhaust think there are 3 cylinders in each bank. This is because 2 stroke exhaust tuning works best with 3 cylinder on each pipe.

[Arnadanoob]

Arnadanoob,
Something else to factor into that is the exhaust being tuned to the mods. If you've modded the cylinder and jetting, you're "front loaded". The exhaust is going to be resonating exhaust back to the cylinder for "refire?" or reburn? I'm over my head now, but seems there would be less muffler explosion with a stock exhaust and modded fuel and cylinder. Now comes the manner of playing with the baffles.

Now cypher all that and spit it back out the way it's supposed to be.

Technically, it's the mods being tuned to the exhaust (pipe) not the other way around. All experienced 2 stroke tuners will always ask you "what pipe are you using or what pipe do you plan to use" because that is a lot more important than what bore, crank, carb, manifold, reeds, trans or anything else for that matter. The entire point of tuning a 2 stroke engine is to match it perfectly to the characteristics/advantages of the pipe you plan to use.

"Pipes" as we call it for 2 stroke applications (for 4 stroke applications we call them "exhaust system", "header", "downpipe", etc.) come in generally 2 fashions. Expansion chamber types and those that are not. Expansion or simply "chamber pipes" are those diamond shaped ones. Those types use acoustic waves produced by the engine in the exhaust to help force some of the intake charge that was blown out through the exhaust during the intake/exhaust port overlap period back into the cylinder, which accomplishes a supercharging-like effect. The non-expansion chamber pipes like the PG Long use pressurized air volume to achieve a point in which turbulence is non-existent within the pipe and thus pressurizes "fills up" to prevent excess intake charge leakage during the overlap period (when the piston is near bottom dead center) where both intake and exhaust ports are exposed.

2 stroke tuning is a completely different theory from 4 stroke tuning. Since 2 stroke engines don't have valves, cams or lifters, the port size and shape plays a huge role in how your cylinder deals with breathing. When you make the ports taller, you are effectively increasing the duration much like how a cam's shape would tend to keep the respective valve open longer. Making the ports wider in a 2 stroke bore is effectively like increasing the lift profile on a cam lobe. I've trashed many bores during my early days since no matter how you port your bore, you cannot remove intake to exhaust overlap, it's absolutely impossible. This is why tuning a 4 stroke is infinitely easier than dealing with a 2 stroke port timing modifications.

The question you ask about the front loading requires a post with a lot of mathematics to explain in detail so I won't go that route. When cylinders are custom ported, the end result is going to be an increase of air flow or a piston and/or ring that catches the edges of the port that was "over ported" effectively trashing the bore (ask me how I know this). More air requires more fuel to match which is why jetting changes are needed. Here's a direct example.

If you have a completely stock 50cc engine and slap on a PG pipe, you'll notice that as soon as you open the throttle it'll bog then die. Put back the stock pipe and everything will be fine. This is because the PG pipe is offering a lot less resistance towards fighting the intake/exhaust overlap period so more of the intake charge that is supposed to remain in the cylinder is going straight out into the PG pipe. Whatever intake charge that remains in the cylinder isn't enough to combust so the engine stops running all together. The PG pipe has too much internal volume and there isn't enough air flow to pressurize the pipe to stop all that extra overlap charge from leaving the cylinder using the stock jetting. The correction that's needed is a huge jet increase (that realistically doesn't exist for a stock carb).

[Arnadanoob]

.............
There is another contributor to the popping sounds you hear in a exhaust system in a car called reversion. That's a condition that starts with the exhaust system being pressurized with more exhaust gas velocity and volume (from pushing on the throttle) and the driver suddenly lifts off the throttle during decel abruptly reducing the exhaust gas velocity and volume causing a pressure differential and turbulence in the pipes and silencer, if it has one................

All exhaust systems have reversion or better yet reversion pulses. *, Johnson and Evinrude V-4 outbards run a modified exhaust so that each bank sees a reversion pulse from the other bank, making the exhaust think there are 3 cylinders in each bank. This is because 2 stroke exhaust tuning works best with 3 cylinder on each pipe.

True, in engines with more than 1 cylinder, the effect can be fully utilized. On a moped engine, it is very difficult since we're dealing with only 1 cylinder. Scavenging is part of the process that uses the exhaust gas pulses as a pump to pull more intake charge into the cylinder. It's like pulling on a length of rope from 1 end and watching the other end come to you. On moped engines we used tricks like using a custom pipe made of heat-retaining material (thick metal) then wrapped it so more of the heat remained inside of the J and silencer sections. Hot air = faster air = more air velocity. The faster you make the exhaust gases, the corresponding effect is better scavenging. The main problem we ran into when we used to build drag bikes is the carb was massive using huge jets (over 175 on the main) on a stroker. Extra fuel = rich exhaust gases + very hot pipe = need to watch EGT otherwise BOOOOM!

[jeremydde]

Your absolutely correct regarding the PG pipe paired with the stock carb. I tried it out before I installed the OKO 28 and I couldn't get the thing to accel at all. It would idle, then die out immediately, no matter what jet I tried.

[Lunytune]

Arnadanoob saith

The question you ask about the front loading requires a post with a lot of mathematics to explain in detail so I won't go that route

That's what I thought. Basically, I was very amatuerishly saying that when you install the overbore, then upjet or go larger carb you exceed the potential of the exhaust. That's when it's time for change, and you are absolutely right, lots of math that flew over the cuckoo clock. (they don't call me Luny for nuthin').

So then... I installed the 50mm overbore on my 2001 AF16E and upjetted like a good boy. Plug is nice chocoholic, and lots of power, empty airbox. I installed new OEM filter and box cover, and it bogs. What is the economical solution to run with filter protection? Is it the filter or the cover which is restricting air? Can I cut out the extra square as we've been talking in the other thread? Possibly drill holes in the cover? Or is the filter the problem?

[Arnadanoob]

The airbox is designed to allow a certain amount of air at max capacity, at some point the demands on the airbox will be so severe that you'd have to start drilling holes or something of that nature to increase it's breathing ability.

The foam material isn't much of a restriction however oiling it as the stock manual suggests will impose a greater restriction than without oil. The end effect is the engine will run richer as a result of being starved air. UNI makes pod filters with inside diameters that vary. Measure the outside diameter of the carb mouth/body with a caliper and match it to the internal diameter with what you find here. http://www.unifilter.com/online%20catal ... ersal.html

[Wheelman-111]

Greetings:

Do you know if the 1" will squeeze over the 27mm stock carb bellmouth?

[Arnadanoob]

I'm sure it will. that's only about 1.5 mm difference I think.