Super Rat with reed jetting

[tahoethumper]

Hello, I recently finished overhauling a super rat engine with a reed and I found that in order to get the engine to idle decently, the air screw (low idle adjust) had to be turned in to where it was only 1/8 out. The engine kicks off great, but has it's issues at idle because of it's leanness. I pulled the jets off of the carb (24mm mikuni) and found a pilot size of 45, and wrong, "round" type main jet (#97?) in place of the correct "hex" type main jet(No. 220). Could anyone please help point me in the right direction for jetting a super rat with a reed induction? I do have stock super rat specs, but not with a reed and I'm sure the jetting would be different than stock. Any help is much appreciated!

(The pipe I'm using is from Torque Engineering, I'm in Seattle so not high in elevation, and I haven't got my k&n yet, but will be small 2"x3")

[racerclam]

Well you will probably want to start with about a 300 main and go up a couple sizes on the pilot, with air screw out 1/8 indicates too lean of pilot. With reeds the signal at the carb is much less therefore jetting needs to go up to compensate for that situation,

Rich

[Zyx]

Someone needs to explain to me what reed induction has to do with jetting. I have never noticed a difference. For any given stoichiometric mix that works, reed induction just lets more of it stay inside the crank during certain parts of the rotation. More mix per cycle, but not more or less gas and/or air versus piston port.

[bchappy]

Let me take a stab at it and Rich can jump in and correct me. I just know that every engine I have set up with a reed needed larger jets by a size or two or it would seize up. My theory is that with a piston port when the skirt starts opening the intake there is already a sizable vacuum so I imagine it jerks air through the carb quickly. With a reed it starts pulling through the carb as soon as the piston starts up so the flow (signal) is more even and does not pull as much fuel.

[hodakamax]

In a well ported reed valved engine you have added more intake through the bottom holes in the piston and added more transfer ports in the cylinder and piston. We are pumping more air through the engine and thus needing more fuel. I'm thinking as I write and could have to edit this soon! Lower holes are mostly intake and uppers are transfers. If you are pumping more air you have increased the velocity through the carb and probably changed the flow patterns. Also the reed stops most exiting pulses that occur in piston port engines out the carb. All food for thought but it does seem you do need to increase jet size.

Still thinkin' on this one!

OK, I'm back with a theory. ( It's hard to remember all this from 45 years ago!) Let's say we've increased the intake and transfers by 25%. We are using more air and increasing the velocity through the carb by the same amount. (at full throttle). More velocity should pull more fuel through the jet up to a point. You cannot keep making more vacuum because it is limited by atmospheric pressure. Looks like we need a bigger jet like Bill said!

Max

[racerclam]

Needing more jet is not because more fuel is necessary but that the jet restriction of fuel flow mut be decreased to compensate for the engines ability to create a signal to the carb due to the restrictive reed valve , not to be richer than it was as a piston but just to correct the mixture again. Thats why the piston needs holes in it or in the case of a modern case reed engine having direct crankcase access with the piston not involved, more time is necessary to get the air in , 360 degrees of availability . When I add a case port to an engine a little less jet becomes necessary since the intake port is now connected to the crankcase creating a stronger signal to the carb . And technically a piston port engine has ability to produce more power on top end than a reed engine (BUT) almost everything must be lost in the rest of the power band because of reversion , loss back through the carb. How ever thats why V force reeds work so well , they have much more reed tip area makeing them lots more sensitive to open and easier to close since they dont have to open as far either . More volume less reversion more power every where. Im doing a V force conversion right now for anothe rcustomer with a combat wombat for street use . They are great every where.

Rich

[hodakamax]

Rich, It sounds like you're on top of it in a modern way! A lot has changed since my 2-stroke engine building 45 years ago. Like I said, it was just a theory but I think that it is probably true that air velocity through the carb is not directly proportional to the fuel flow. Lots of factors to consider and it's always fun to discuss 2-stroke technology. Keep up the good work!

Maxie

ps--unless it's top secret I'd like to know more about your work. Tell us more if you can!

[racerclam]

I guess the beat way to know more about my work is by going to my web site richstaylordporting.com there is alot on there . Thanks for your interest.

Rich

[Bullfrog]

One of the most significant reasons for reeded engines requiring richer jetting than piston port engines (assuming same carb, same pipe, same compression, same porting/port timing) is that the reed valve eliminates backward travel of air fuel charge through the carb.

In a standard piston port engine, air travels through the carb toward the engine as the piston moves up - air rushes in to fill the partial vacuum in the crankcase below the piston and it picks up fuel as it goes through the carb. However, when the piston starts back down again, some of the air/fuel mix in the crankcase is forced up through the transfer ports and some of the air/fuel mix is forced backwards through the carb. This pass through the carb in the "wrong" direction picks up additional fuel. Then when the piston starts up again, and air starts to flow in the "right" direction, the air picks up fuel again (3rd pass through the carb). To be sure, only a small portion of the incoming air will have picked up fuel three times, but it is most certainly enough to have an effect. Since all this reversing and picking up of extra fuel is part of normal operation, normal carb tuning provides that the engine will run correctly with all that unseen stuff going on. (Have you ever seen a "mist" or "fog" at the air inlet end of the carb when you have the air cleaner off? There you go. Direct evidence of the process described above.)

A reed valve stops that reverse flow of air. Without the reverse flow, the carb must be tuned to provide all the fuel the engine needs with only ONE pass of the air through the carb. This alone requires richer jetting - before we even get to discussions of flow obstruction, "signal" or other things.

Ed

[racerclam]

Yep what Ed said , Back when I was doing jet ski work and tuning them while running the machine in a stationary tank it was kinda cool watching the fuel cloud over the carb at Wide open throttle , but both piston port engines as well as reed engines had the cloud and neither one could be seen at low speed , how ever the reed engines cloud was about 1/2 of the piston port engine , reeds are not 100% efffective , rotary valve is another story with a mechanical positive intake closure. Even 4 stroke engines experience this reversion problem especially with large overlap cams , In drag racing tracks will often demand carb coverage to the rear if they are exposed to keep fuel vapor off the windshield .

Rich

[hodakamax]

Hey Rich, I went to your site and being an old "porter" from the distant past, was very impressed with what you are doing and your portfolio, (so to speak)! Thanks for the info!

Max

ps---Good discussion gang! I think we are all on the same page.

[bchappy]

I am an old porter from the past also but I never had the necessary tools to do the transfer ports like Rich does. He did a job a couple years ago for my Super Combat and I was amazed. Beautiful work.

[hodakamax]

Come on Rich, show Bill and Max the tool(s) that you do those little transfers with. I too am amazed!

Max

[tahoethumper]

Thanks for all of the great insight guys- very cool stuff. I ended up going up in jet sizes and was able to achieve a much better idle, and with the air/fuel screw two turns out (much better than 1/8 out, hehe).

https://www.youtube.com/watch?v=q--Fn09ervY

[hodakamax]

Checked the video, sounds great!

Max

[Zyx]

In a well ported reed valved engine you have added more intake through the bottom holes in the piston and added more transfer ports in the cylinder and piston. We are pumping more air through the engine and thus needing more fuel. I'm thinking as I write and could have to edit this soon! Lower holes are mostly intake and uppers are transfers. If you are pumping more air you have increased the velocity through the carb and probably changed the flow patterns. Also the reed stops most exiting pulses that occur in piston port engines out the carb. All food for thought but it does seem you do need to increase jet size.

Still thinkin' on this one!

OK, I'm back with a theory. ( It's hard to remember all this from 45 years ago!) Let's say we've increased the intake and transfers by 25%. We are using more air and increasing the velocity through the carb by the same amount. (at full throttle). More velocity should pull more fuel through the jet up to a point. You cannot keep making more vacuum because it is limited by atmospheric pressure. Looks like we need a bigger jet like Bill said!

Max

Since you are suggesting theory, allow me to disagree a bit. Reed valving provides a more or less one way gate for intake charge. I know it isn't perfect, but compared to piston port, it is largely one way. Porting the piston does not provide more air, nor does hogging the transfers. Nor does any of this increase charge velocity. It is the other way around. Reeds and piston porting increase duration, but otherwise reduce velocity. Hogging the transfers actually increases crank volume, thus further reducing velocities as well as dropping crankcase pressure. Air, as well as air/fuel charge, is a fluid albeit a compressible one. Fluid dynamics demands that a given volume of fluid passing through a smaller pipe increases velocity, passing through a larger one reduces velocity, all other things equal. So far, all of the changes attendant to reed valving and porting reduce velocities and internal transfer pressures. What is increased is duration and volumetric control.

As far as more air, first it isn't air but air/fuel charge. Nor can the piston pump more charge than its crank volume and under piston displacement provide. In order to increase charge beyond static displacement you have to have high mass due to velocity, which we have effectively reduced. So I dont believe reeding provides a cramdown effect on charge, or at least less so than piston porting. What you might gain in displacement efficiency due to the one way nature of reeds, you give away to some extent by reducing velocity. Actually I believe you can better maximize velocities, crank pressures and peak power with a piston ported motor. BUT, you get a much broader power band with reeds so you can go a little more radical on porting knowing you don't have the peaky power band you would otherwise have with a raised exhaust port or enlarged intake timing.

The original question of mine was WHY a reed engine needs more fuel, and the only suggestion that comes close in my mind is the reduction of reversion in the Venturi. Beyond that, every benefit of reeding has to do with torque and a broader band provided by the fact that charge reversion is reduced by reeds even though the intake port is essentially uncovered through more of the stroke than it otherwise would be.

[racerclam]

Ok Here is the picture you wanted , since I had the camera out I shot som shop pics too

Rich

[hodakamax]

Hmmm---AZ, I bet (again), If you would measure the air velocity after the carb on a stock engine at a given RPM (full throttle) and do the same after installing a reed valve and porting cylinder and piston you would find a substantial increase in velocity of the air (or fuel/air). There is also a horsepower
increase because we have increased the pumping efficiency of the engine. More fuel+air=HP. The reed has stopped the mixture from going backwards (MINUS velocity) as much therefore increasing the total velocity through the carb.

Maxie

[hodakamax]

Wow Rich! Just got the pictures and am running out the door after fencing with AZ -- I'll check them out and comment later!

Thanks!

Max

[Zyx]

Max,

That's not how intake pulses work. Reed blocks generally interfere with induction versus an ordinary manifold intake. It takes energy to deflect the reed, which also creates turbulence, creates intake pulse delay, and decreases volocity. Piston port intake charges will reach an ideal velocity depending on port timing, which affects power band by using velocity to create the effect of mass to counter reversing pressures over the piston. Raising an exhaust port raises the point at which power is maximized because it takes more velocity of the intake charge at higher rpm to overcome port overlap, and albeit with a diminishing return, gets the needed velocity at higher engine speeds.

The timing events going on in a two stroke are complex. Altering those timing events alters how much power is made, and when. Reeds assist in making power because instead of, for example, 150 degrees of intake timing, with charge speeds starting slowly, gaining incredible speed, and then slowing down again, we have essentially 180 degrees of timing opportunity. The displacement of the machine is fixed by the bore and stroke. Crank volume is fixed by its design. Transfer pressure is a result of those constants, with rpm as a variable. Done right, a piston port two stroke can make tremendous power, but the more hp it makes, the less torque and the higher up the power band largely because the needed charge velocity used to counter port overlap only occurs at those higher rpms.

Reeds change the dynamic, both by delaying or interfering with intake pulse, and by again interfering with the pulse when it tries to revert. The boost you get from holding the charge in the system, between reeds and crank, occurs at lower speeds and over a much broader range which allows the engine to make more torque than it otherwise would, but rarely helps it make more horsepower than it could for a given porting combo.

I began studying internal combustion engine theory in 1955 by reading my fathers SAE Journal magazines. Later, by working with porting two strokes and V-8's, and by designing expansion chambers using mathematical formulae. There is zero way to place a reed block, which at static pressure constitutes a 100% blockage of flow, into an intake system and then expect it to create increased flow rates. Not possible. It does create increased intake duration, which is how it puts more fuel, hence more output, in the engine, but does this mostly at moderate to high midrange speeds. Once the engine is screaming, the reed system just gets in the way.

Not to mention reed flutter, supersonic shock problems at the reed tip at high speeds, massive turbulence, and other things going on in there. We use reeds because in the balance, we get more grunt out of our bikes, with a far more user friendly power band. If we wanted no more than peak power, we get that without the reeds, but also without much usable power from idle up to the point where it all starts to kick in. In a road racer, this is not such a big deal because it isn't a stop and go environment. Get on the pipe, stay on the pipe. To some extent also the same in MX, except if all you have is light switch power band it is a bitch to ride, so we either back off the radical porting, or use reeds to supplement the midrange while still having a good punch on top.

We are all aware of the seat of the pants benefits of reed systems, but those benefits do not come as a result of increased intake velocities.

[hodakamax]

Simplifying it even more, If you're pumping more air through a hole in a given time you've increased the velocity. What else could be really happening?

Maxie

[bchappy]

I can kinda follow this sparing and I am not smart enough to argue with you guys, but I know if you install a reed you will usually need to increase the jetting.

[hodakamax]

Thanks Bill, you are old and wise--No doubt about it! Observations are always good evidence. (and seized engines).

Maxie

C'mon gang, I need some help! AZ can type way faster than Max!

[Zyx]

Simplifying it even more, If you're pumping more air through a hole in a given time you've increased the velocity. What else could be really happening?

Maxie

Retained intake charge, less reversion. Got nothing to do with the speed the charge achieves at the peak of the intake pulse. Speed would be a bonus, but you can't put a spring loaded gate in the way and expect faster speed through the gate. Even when the reeds are as open as they get, they are not open that far, and at that point the opening is something like 50 or 69 percent of the otherwise available manifold if the reed assembly were not there. Reed cages are made big because they are restrictive by nature, so you have to overcompensate and make them large.

Put a reed cage on a piston ported cylinder but leave out the reed petals. See how it runs.

[Zyx]

And I think we hijacked this thread.