Reed valve

skitchfish

Member
Oct 27, 2010
222
0
16
Michigan
Yeah you guessed it! actually it was the color of molten gray aluminum ( like a bad weld job color) LOL There is no picture in the NGK ad that looked like this one. LOL

It's the beauty or the ugly option of having two mixture screws on a walbro carb. At first I was very careful and even went to using a old treadmill someone had discarded to tune under load in my garage. This was fine for a little bit and eliminated all 4 stroking until the treadmill decided it was tired of doing that many Rpm. I had no 4 stroking and a decent color plug.

Never being satisfied to leave well enough alone, I attached rubber hoses for screw drivers to my mixture screws and went for a ride. As you can see the result of what happens when you hold throttle wide open and turn the high speed screw with a rubber hose ( I do not advise tuning this way to anyone) I have never heard a china girl scream like that right before it decided that was enough.
 

Venice Motor Bikes

Custom Builder / Dealer/Los Angeles
Mar 20, 2008
6,631
439
83
Los Angeles, CA.
Whenever you add a reed valve you need to jet the carb real BIG & then carefully test smaller jets until the 'sputtery' feeling goes away, (& then stop)!!!

Jetting any smaller will feel like there's more power but it's actually getting close the 'too lean' territory that burns pistons! :(
 
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maniac57

Old, Fat, and still faster than you
Oct 8, 2011
4,484
16
0
memphis Tn
The line between just right and melted parts is VERY FINE.
I ALWAYS jet a little in the rich side for engine life.
A little bit more oil and fuel keeps things under control and really doesn't hurt power all that much.
Unless you're running a hardcore rebuild every race motor, pushing the jetting is simply asking for trouble.
My engine pulls clean when accelerating, and blubbers a bit when at constant speed.
Simply the nature of a piston port two stroke, and it's not something I care enough about to run a reed valve, which is about the only way to really stop four stroking everywhere (when done right anyway)
 
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Dec 12, 2015
43
6
8
Illinois
I can't say anything bad about the stock carb, it idles perfectly and throttle response, I couldn't ask for more from this chins motor. Wondering if I lengthened the runner from the reed valve to the carb, would that give me the low end torque I'm looking for?
 

exokinetic

New Member
Mar 18, 2016
108
3
0
Lake Forest, CA
I can't say anything bad about the stock carb, it idles perfectly and throttle response, I couldn't ask for more from this chins motor. Wondering if I lengthened the runner from the reed valve to the carb, would that give me the low end torque I'm looking for?

Without a well tuned exhaust pipe (expansion chamber) to take advantage of the increased volumetric efficiency at that low-rpm, then likely no, it wouldn't.

The further you move the carburetor away from the engine, the longer it takes the signal from the engine to reach the carburetor.

In other words, the only way the carburetor knows how much fuel to deliver, is the "vacuum" signal it receives from the engine.

When you accelerate (or decelerate) or change rhe engine load (uphill/downhill etc.) that signal changes as the engine now requires a different mixture.

The longer the intake tract is BEFORE the carburetor, the longer TIME it will take this signal to get to the carburetor.

You will experience this as "delayed" or generally slow throttle response.

Not something anybody wants.

In order to achieve the effect of a "tuned" intake system, OEM manufacturers use as short as possible path from the intake port to the carburetor, and then add an "air-box" onto the other side of the carburetor.

The size of this air box is tuned to the frequency required to boost power at the desired RPM (usually low-RPM in an effort to correct the dreaded off-idle bog)

You thus gain all the advantages of the tuned intake system, without any of the throttle response drawbacks of a long intake runner.
 
Dec 12, 2015
43
6
8
Illinois
Thank you for the detailed mechanics involved. I have been hearing forever about a longer intake = more torque. It only makes sence that would mean throttle lag. I have sold Volkswagen that has about 2 feet to go from the carburetor to the heads, but its torquey, because it makes no horsepower.
 

exokinetic

New Member
Mar 18, 2016
108
3
0
Lake Forest, CA
its torquey, because it makes no horsepower.
Nail on head.

Making good torque down low AND big HP up high is the holy grail we are all aiming for... and it always end up more complicated than pure theory would suggest ;)

I do however recommend looking into air boxes.

If I remember the research correctly, you make the air-box the exact volume of the engine displacement...or was it crankcase volume...

I think it's engine displacement, because the concept work exactly the same for 2 strokes and 4 strokes, even diesels.

And once you have that, fine running is done to the volume, the size the hole that goes to the carb, and the inlet hole, to dial in exactly where it is going to boost power, so you end up with smooth strong power delivery from idle, all the way up.


I do EXTENSIVE work developing these engines for race use, development of an air-box for the engines is pretty high on my current list of things to create ;)
 

exokinetic

New Member
Mar 18, 2016
108
3
0
Lake Forest, CA
And just for more interesting info...

In an effort to attack this exact problem, many incredibly innovative manufacturers (Mercedes, BMW, Porsche, Jaguar, and probably many of the American manufacturers as well) use what is know as "variable length intake runners".

There are lot of ingenious ways to implement this theory mechanically, but it basically goes like this:

At a minimum there are 2 intake runners for each cylinder (one long, one short, tuned for their respective power band) and solenoid valve, or vacuum rake-off or some other similar device to actuate a valve that directs the intake path to either the short, or the long intake runner.

The system is then designed so that at a specific RPM the actuating mechanism switches from the long intake runner (for low RPM's) to the short intake runner (for High RPM's).

This system has become highly developed, Porsche has a system that is "constantly variable" using a novel shape cam lobe in the intake tract, they can vary the effective intake length to ANY length, between two extremes. In this case the intake length varies constantly with the engine conditions, so no matter what rpm/load the intake length is optimized.

Very cool stuff, gotta love them Germans.
 
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Dec 12, 2015
43
6
8
Illinois
I did drill some extra holes in the air cleaner cover, because it only drew air from the bottom, also replaced the air filter with a thicker piece of foam, not too closed cell, free flowing in other words. My concern now is if it raining/wet, the back tire throws water on
It which could easily be sucked in and easily trash the intake side of the piston, not to mention the cylinder.
 

exokinetic

New Member
Mar 18, 2016
108
3
0
Lake Forest, CA
Yea I hear yah on the water.

And I have seen tests that show there aren't really any filters that work great a preventing water transmission, without hurting performance.

Seems like it is just an unfortunate fact of life... another benefit of the air-box style set-up, and hiding the inlet somewhere out of the air-stream + fenders to keep water from spraying all over the bike.
 

sbest

Member
Nov 3, 2015
343
2
18
Nova Scotia
Yea I hear yah on the water.

And I have seen tests that show there aren't really any filters that work great a preventing water transmission, without hurting performance.

Seems like it is just an unfortunate fact of life... another benefit of the air-box style set-up, and hiding the inlet somewhere out of the air-stream + fenders to keep water from spraying all over the bike.
Outerwears silicon impregnated screen will keep water out of your filter.
http://www.outerwears.com/
I'm not using it on the MB yet but they work ok on the KTM.

These motors are so easy to experiment with.
Last winter, before I bought my new camera, I tried various "airbox" shapes:

The long "trumpet" at top was a torque monster but no speed.


This was the best one.


Some JB Weld and a waterbottle on a jar lid, too easy.
Test them on a hill with a GPS as a speedo.


Cutting the bottle at an angle broadened the power spread and is easily angled to miss the mud spray off the tire.


Cuz we can't be afraid of a little rain can we?

I had a problem with the stock filter letting dust through, damaging the cylinder.
This finer foam is working fine. You are right about getting the carb as close to the piston for signal too.
I'll probably mount the carb closer yet later. Extending the bottle "airbox" gave worse performance.
I need more shortness!

Steve
 
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exokinetic

New Member
Mar 18, 2016
108
3
0
Lake Forest, CA
Outerwears silicon impregnated screen will keep water out of your filter.
http://www.outerwears.com/
I'm not using it on the MB yet but they work ok on the KTM.

These motors are so easy to experiment with.
Last winter, before I bought my new camera, I tried various "airbox" shapes:

The long "trumpet" at top was a torque monster but no speed.


This was the best one.


Some JB Weld and a waterbottle on a jar lid, too easy.
Test them on a hill with a GPS as a speedo.


Cutting the bottle at an angle broadened the power spread and is easily angled to miss the mud spray off the tire.


Cuz we can't be afraid of a little rain can we?

I had a problem with the stock filter letting dust through, damaging the cylinder.
This finer foam is working fine. You are right about getting the carb as close to the piston for signal too.
I'll probably mount the carb closer yet later. Extending the bottle "airbox" gave worse performance.
I need more shortness!

Steve

So awesome, too east not to try.

I will be trying this for sure.
 

Rasor235

New Member
Jan 19, 2021
5
0
1
28
Here's pics of experiment gone bad
This looks like you bought the wrong piston for your bike. There’s some that have the long rod and some the short. You have to get a high or low hole piston and it looks like it jammed into the spark plug and broke a hole. That’s what it looks like from the pictures