Author Topic: 1973 Yamaha Stock Carb Tuning  (Read 11118 times)

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Offline machina

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1973 Yamaha Stock Carb Tuning
« on: July 22, 2014, 02:45:18 AM »
Hello All, looking for ideas and insight on my setup.



I have a 1973 TX500 with a 1977 single piece head on it, with a fresh valve job and decking.  I have the rounded exhaust ports instead of the oval once that came stock with a 73.  I have some 17" reverse cone shorty mufflers attached to the headers.  The main baffle is punched out, and the cone is packed with fiber glass.  So it is a pretty free flowing exhaust. 



For right now, I have a set of Pod filters attached to each carb.  I know this is not ideal for laminar flow, I am working on a design for a single air filter using this K&N filter and some 2" Tubing. ( K&N RU-2850).  I have all the stock Jetting in. #125 Main, #50 Slow Jet, #50 Pilot Jet

Those of you with sharp eyes will notice that I have modified a set of header pipes I have to accept a Wideband OČ Sensors.  I will be using my Innovate LM-2 reader to try and get this tuned in as good as I can get it.




Now I will describe where I am at now.  I have run a leak down on the motor and I am showing around 95% leak down on both sides (100psi on the in guage, 95psi showing up on the backside guage after the restriction oriface).  Compression is around 150psi on both cylinders after warm up.  My timing is as close as I can get it with my timing light.  The spark looks good and strong.

For right now, I am trying to tune up the pilot system on the bike for idle.  After I warm up the bike, I proceed to calibrate the sensor using the LM2 (you are supposed to do this every time you unplug the sensor from the LM2 unit).  I then connect the sensor up to my #1 cylinder to getting a AFR reading.  With the pilot screw turned out 1 turn, I am showing a 10.0/1 AFR.  This is the part that is puzzling me, I have Pod filters and an open exhaust on the bike.  This would lead me to believe I should have an AFR higher than 14.0/1.0, or a lean condition.

Typing this out, I have been looking at my FSM and I have come up with something I want to try out.  I am looking at the slow air and main air jet sizes.  They should be #60 for the main, and #110 for the slow.  I am guessing that I might have them backwards.  Which would restrict the air on the slow and give too much air on the main.  I will look into this tomorrow after work and update this post.

If you have any questions, suggestions or improvements on what I am doing let me know please.  If I can get this to work right, I might be able to help a lot of people with similar setups to mine.

1973 TX500
Dual Disc Front Conversion
'77 One Piece Cylinder Head Conversion
Yamaha Aftermarket Period Correct Oil Cooler

Offline machina

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Re: 1973 Yamaha Stock Carb Tuning
« Reply #1 on: July 22, 2014, 02:48:49 AM »
Posting a few more pics of the bike for those interested.







1973 TX500
Dual Disc Front Conversion
'77 One Piece Cylinder Head Conversion
Yamaha Aftermarket Period Correct Oil Cooler

Offline dean

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Re: 1973 Yamaha Stock Carb Tuning
« Reply #2 on: July 22, 2014, 01:24:23 PM »
For right now, I have a set of Pod filters attached to each carb.  I know this is not ideal for laminar flow 

You should not be tuning the  "for right now"   set up. You should start the tuning process once you have a proper intake and exhaust on the bike, or you will be chasing your tail  :D
However "for right now" : Make sure the inner shoulder of those pods do not obstruct or block the atmospheric slide ports or the air jet ports on the intake side of the carb. It is explained here: http://www.xs500forum.com/index.php?topic=952.msg5152#msg5152

 
The main baffle is punched out, and the cone is packed with fiber glass.  So it is a pretty free flowing exhaust. 
Those of you with sharp eyes will notice that I have modified a set of header pipes I have to accept a Wideband OČ Sensors. 

It may just be the pics, however your exhaust mod with the O2 bung adapters appear to be a much smaller inside dia that is actually restrictive (?)  The stock Keihin carbs don't like short, open exhaust by the way.


 

Offline machina

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Re: 1973 Yamaha Stock Carb Tuning
« Reply #3 on: July 22, 2014, 02:57:26 PM »
For right now, I have a set of Pod filters attached to each carb.  I know this is not ideal for laminar flow 

You should not be tuning the  "for right now"   set up. You should start the tuning process once you have a proper intake and exhaust on the bike, or you will be chasing your tail  :D
However "for right now" : Make sure the inner shoulder of those pods do not obstruct or block the atmospheric slide ports or the air jet ports on the intake side of the carb. It is explained here: http://www.xs500forum.com/index.php?topic=952.msg5152#msg5152

 
The main baffle is punched out, and the cone is packed with fiber glass.  So it is a pretty free flowing exhaust. 
Those of you with sharp eyes will notice that I have modified a set of header pipes I have to accept a Wideband OČ Sensors. 

It may just be the pics, however your exhaust mod with the O2 bung adapters appear to be a much smaller inside dia that is actually restrictive (?)  The stock Keihin carbs don't like short, open exhaust by the way.

For right now will have to do for the rest of this year.  For what I have planned using some aluminum tubing and that K&n Filter would take me a little bit of time.  As for the exhaust header.  All the TX500 exhausts, if you cut the chrome off of them have an inner exhaust tube within the chrome outer tube.  It flairs out towards the exit of the header to meet up with the muffler.  At least the 3 sets of exhaust headers I have here (oval and round) have that setup.  Their shouldn't be any more restriction in the setup than a stock header.

I am sure that the filters are not blocking the air port for the air jets, because it acts the same and shows the same AFR numbers with or without pods.  I will be pulling the carbs tonight, as I suspect I might have the Air Jets backwards.  Other than that, I am not sure why it shows such a rich AFR on stock jets and settings with pods and an open exhaust.
1973 TX500
Dual Disc Front Conversion
'77 One Piece Cylinder Head Conversion
Yamaha Aftermarket Period Correct Oil Cooler

Offline steve

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Re: 1973 Yamaha Stock Carb Tuning
« Reply #4 on: July 22, 2014, 04:34:18 PM »
I'm really looking forward to seeing your results!  When I was tuning my bike, I often wished I had a way to get actual air/fuel ratio readings, like you have there.  All I had to go by was the "feel" of the bike when riding it, and some plug chops (which are OK for rough impressions of engine condition, but not always clear and distinctive enough for fine tuning). 

Offline machina

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Re: 1973 Yamaha Stock Carb Tuning
« Reply #5 on: July 22, 2014, 04:42:47 PM »
Yeah, I am hoping all goes well.  For the idle, I can test in my garage with a fan going.  For the slow and main circuits, I need to ride the bike.  I need to run it at the correct throttle spot too.  I am not sure where each circuit opens up.  I think it is idle(not touching the grip) to 1/8 is the pilot/idle circuit.  Then it is 1/8 to 1/2 for the slow.  Then 1/2 to WOT for the main.  If anyone else has info on this please let me know.  When I test on the road I will have to mark my grip so I know where to keep it so I can data log for the proper circuit on the Carbs.
1973 TX500
Dual Disc Front Conversion
'77 One Piece Cylinder Head Conversion
Yamaha Aftermarket Period Correct Oil Cooler

Offline dean

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Re: 1973 Yamaha Stock Carb Tuning
« Reply #6 on: July 22, 2014, 05:24:33 PM »


Idle fuel is controlled by the pilot circuit.  To lean it , try turning in the mixture screw 1/4 turn ( 3/4 out from seated) .  Make sure you are measuring a/f with a warm engine. Make sure the rubber plug to the pilot/slow jet passage is in good condition and in place or the pilot circuit will get fuel directly from the bowl instead of the main jet to the pilot circuit and it will be rich.  (The fuel to the pilot/idle circuit is supplied via a passage from the main jet. )   Also check your float height- if the fuel level is very high, you will be rich at idle.   

Just off idle, the slides will not lift yet- the fuel mixture is still supplied through the slow/ pilot circuit to the  bypass port. The slow/ pilot jets are in series and always the same size.  (It seems silly and redundant, however this keeps fuel flowing when the throttle is closed under hard braking:  "bandaid" ) 

1/4 ish throttle the slide lifts and the needle controls fuel flow form the main .  Higher the slide, the more air through the body and a balance of fuel through the needle jet. This continues through mid range and up to wot) 

At wot, the needle is fully lifted and the main jet flows open.

Make sure when you do you street runs at various throttle positions, you are putting a decent load on the engine

(A bit rich at idle is not as worrisome as being  lean under load at higher rpms)


Offline machina

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Re: 1973 Yamaha Stock Carb Tuning
« Reply #7 on: July 23, 2014, 12:46:43 AM »
So, as I suspected, I had my pilot and main air jets backwards.  Once I swapped them, the bike fired right up and ran great.  I took it for a spin to warm it up, taking care not to go to hard into the mid range.  Since I am most likely still lean there with the #125 main jet.  After I warmed it up, I pulled into my garage and it actually idled beautifully.

Again #125 main, #50 slow, and #50 pilot.  #60 air jet on the main circuit, and #110 air jet on the slow circuit.  Pod filters bolted directly on the carbs and my packed and debaffled 17" reverse cones.

I had it at 1 1/4 turns out (stock) on the pilot screw to start with.  I then warmed up my AFR Sensor and calibrated it in Air.  Connected it to my OČ bung on the exhaust and fired it up.  I started getting a reading of around 11.2/1 AFR.  I then turned that carbs pilot screw down to 1/2 turn out to get a reading of about 12.2/1. 

So turning it in leaned it out a bit, not allowing as much fuel in.  Now, if I wanted to get it around 12.5-13/1 AFR my thinking is that i would have to open up the #110 slow air jet to get more air into it, so I could then use the pilot screw to adjust the AFR closer to the 13.0/1 with a pilot screw turn out of around 3/4 - 1 1/4.

What do you guys think?
1973 TX500
Dual Disc Front Conversion
'77 One Piece Cylinder Head Conversion
Yamaha Aftermarket Period Correct Oil Cooler

Offline berniebee

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Re: 1973 Yamaha Stock Carb Tuning
« Reply #8 on: July 23, 2014, 12:43:36 PM »
Can't help with the carbs, but just wanted to say that I like that seat.

Offline dean

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Re: 1973 Yamaha Stock Carb Tuning
« Reply #9 on: July 23, 2014, 01:18:50 PM »
Like I said before, having the a/f slightly rich at idle is not as big a concern as a possible lean condition everywhere else. Because of that short, open exhaust system , you will never get a perfect a/f throughout the entire throttle range with the stock CV carbs and will most likely have flat spots anyway. To err on the side of mildly rich will protect your engine as you go on with tuning.

(Did you check your float height ? What is your float height set at?  Is the rubber plug for the pilot/slow jet port in place?  )

If you look at the slow jet, it has a built in emulsion tube that combines the air from the slow air jet with the fuel. The air is basically there for the purpose of atomizing fuel before it is introduced into the circuit. It is not intended to be increased or decreased in size to affect a/f mixture.  It is the job of the mixture screw to adjust a/f ( it does not change the mixture, it increases or decreases the volume of air/fuel "mixture" supplied from the pilot circuit to the pilot and bypass ports. I would be wise to move on to the next range as a needle jet or main change will affect the idle again anyway.



 

Offline machina

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Re: 1973 Yamaha Stock Carb Tuning
« Reply #10 on: July 23, 2014, 03:50:20 PM »
Can't help with the carbs, but just wanted to say that I like that seat.
Thanks, it is just an eBay special.  Got the foam and the pleather, just had to take apart a ratty seat and use the pan.  It works pretty well.  If I were to make one myself or have a professional do one for me.  I would round the edges where your legs would sit on it, because it does dig in a little there.

Like I said before, having the a/f slightly rich at idle is not as big a concern as a possible lean condition everywhere else. Because of that short, open exhaust system , you will never get a perfect a/f throughout the entire throttle range with the stock CV carbs and will most likely have flat spots anyway. To err on the side of mildly rich will protect your engine as you go on with tuning.

(Did you check your float height ? What is your float height set at?  Is the rubber plug for the pilot/slow jet port in place?  )

If you look at the slow jet, it has a built in emulsion tube that combines the air from the slow air jet with the fuel. The air is basically there for the purpose of atomizing fuel before it is introduced into the circuit. It is not intended to be increased or decreased in size to affect a/f mixture.  It is the job of the mixture screw to adjust a/f ( it does not change the mixture, it increases or decreases the volume of air/fuel "mixture" supplied from the pilot circuit to the pilot and bypass ports. I would be wise to move on to the next range as a needle jet or main change will affect the idle again anyway.

Yeah the floats are set, can't remember the number since I set them last year using a float gauge, I think it is 22mm.  The plugs are there, although there is a bit of a gap (1/16-3/32) before it touches the clip that mounts to the main jet.  I might try and modify it to make sure that the plug is not falling out a bit and letting in a little gas.

If and when I start making adjustments to my main, it will richen up my idle even more.  That would tell me I would want to open up the slow air jet a bit, since I can't go past 0 turns on the pilot.  And I would rather have some play up and down on the screw then none one way and all the other way.  I am shooting for 12.5-13 for the AFR, I know a little rich is better than lean, but just a little rich.  Otherwise you start fouling plugs like I did when I had my air jets backwards.  What do most drag racers run?  12.0?  They don't care about mileage or fouling plugs since they probably replace them frequently anyways.

If I did open up the air jets I will be stepping it up in .05 mm increments.  I have a set of micro bits that range from .25 mm to 1.00 mm in .05 mm increments.  So I should be able to go slowly with the tune.  I do have to run and grab another set of plugs, as the ones I was using the other night were from when I was fouling out plugs bad.  I just cleaned them up.

When I start stepping up the Main jet, should I be messing with spacing the needle up or just tuning with the main jet?
1973 TX500
Dual Disc Front Conversion
'77 One Piece Cylinder Head Conversion
Yamaha Aftermarket Period Correct Oil Cooler

Offline dean

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Re: 1973 Yamaha Stock Carb Tuning
« Reply #11 on: July 23, 2014, 05:39:43 PM »
I recommend you move on to obtaining your baseline a/f ratios at the various throttle positions (under load ) before you proceed with drilling or changing anything. 



 

Offline steve

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Re: 1973 Yamaha Stock Carb Tuning
« Reply #12 on: July 26, 2014, 02:14:23 AM »
Yep, Dean's right.  The way to get useful information out of that A/F meter is to begin by taking readings with the current (stock) jets and settings.   I'd like to see baseline A/F readings for idle, quarter throttle, half throttle, three-quarters throttle, and full throttle. 

Here are the stock jets and settings:

JETS

Main jet #120
Main air jet #60
Slow jet #50
Pilot jet #50
Slow air jet #110
Starter jet #45

SETTINGS:

Pilot screw one turn out, = or - a quarter turn.

Float level: 22mm

« Last Edit: July 26, 2014, 02:28:36 AM by steve »

Offline machina

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Re: 1973 Yamaha Stock Carb Tuning
« Reply #13 on: July 30, 2014, 01:05:32 AM »
Settings

#125 - Main
#60   - Main Air Jet
#50   - Pilot Jet
#50   - Slow Jet
#110 - Slow Air Jet

Float Level at 22mm
Pilot Screw Turned out 1/2 Turn ( already ran the AFR meter with it idling to come up with that)

Pod filters mounted directly to the carbs
Stock Headers mounted to 17" reverse cone megaphone mufflers packed with fiberglass.

So I went for a nice 20 minute ride with my meter attached and secured to the bike.  All of the cabling tucked away so it wouldn't catch on anything.  I then used some painters tape to mark throttle positions in 1/4 turn increments.  I started recording the ride, but didn't record correctly so I don't have a graph to show.  I did pay attention on all the throttle positions though.  Here is what I got, as close as I can remember.

All numbers are in Air/Fuel (# on the screen is the parts of Air to 1.0 part of fuel)

Idle - 11.9 - This number stays fairly consistent at 11.9, but it will jump a bit from 11.7 -12.2
1/4 - 10.8 - Guessing the main is starting to leak a little fuel into this part of the circuit, hence the richer mixture while riding
1/2 - 14.1 - This is where the main starts taking over most of the fuel delivery.  This is where it starts to stumble a bit (because it is running lean)
3/4 - 16.5 - Obviously starting to go really lean because the fuel is being delivered by a too small main jet.
WOT - 18.1-19.0 - I ran this for about 30 seconds, didn't want to run it to much longer than that and risk damage at the higher RPMs.

So, looking at this, the main obviously needs to get bigger.  I will start with going from 1.25mm to 1.35mm.  This should richen up the slow circuit also, so I will have to run it again to see how rich it gets, and decide if I want to open up the slow air jet a bit (.60mm to .65mm).

Let me know what you guys think.
1973 TX500
Dual Disc Front Conversion
'77 One Piece Cylinder Head Conversion
Yamaha Aftermarket Period Correct Oil Cooler

Offline dean

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Re: 1973 Yamaha Stock Carb Tuning
« Reply #14 on: July 30, 2014, 01:08:34 PM »
Napkin math says 135 on the main at WOT may be ok.  Do ONE adjustment at a time, then measure the result and confirm before moving on to something else. (  It is clear that you really, really, really  want to drill out that slow air jet, however I recommend you tweak the slow circuit last as the fuel for the slow/pilot jets come from the main. Again, a slightly rich idle will not hurt anything )


Also NOTE, as I said before:  with that open exhaust and those pods, you will have flat spots (stumbles) with the stock CV carbs -even with good a/f ratios at specific key points. Transitions may not be smooth. Don't expect them to be unless you want to a install proper exhaust and intake system.  :)