Author Topic: 1973 Yamaha Stock Carb Tuning  (Read 6198 times)

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Offline guylr

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Re: 1973 Yamaha Stock Carb Tuning
« Reply #15 on: July 30, 2014, 02:49:10 PM »
Your idle seems overly rich considering how far in the pilot mixture screw is. The standard setting for that screw is 1 to 1-1/4 turns so you are likely getting extra fuel from somewhere. Do check the rubber plugs that should be in the pilot jet tube are held in firmly by the retaining spring clip. You may need to tweak the tab a little.

To address your mid-range stumble, try shimming the both slide needles 1mm with two 0.5mm washers as used on Mikuni carbs (Yamaha p/n 3F7-14952-00-00 or 90201-037F2). I'd also go up to a 140-145 main jet to start. As lean as you are at wide open it may take more though.

Guy
« Last Edit: July 30, 2014, 02:57:49 PM by guylr »
Now Retired Yamaha guy. Cut me and I bleed "Cinquasia Red"

Offline machina

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Re: 1973 Yamaha Stock Carb Tuning
« Reply #16 on: July 30, 2014, 05:24:31 PM »
Your idle seems overly rich considering how far in the pilot mixture screw is. The standard setting for that screw is 1 to 1-1/4 turns so you are likely getting extra fuel from somewhere. Do check the rubber plugs that should be in the pilot jet tube are held in firmly by the retaining spring clip. You may need to tweak the tab a little.

To address your mid-range stumble, try shimming the both slide needles 1mm with two 0.5mm washers as used on Mikuni carbs (Yamaha p/n 3F7-14952-00-00 or 90201-037F2). I'd also go up to a 140-145 main jet to start. As lean as you are at wide open it may take more though.

Guy

Yeah, there is a decent gap between the bottom of the clip and the rubber plug.  I will bend the tab down some when I take them off tonight, as I suspect that is letting in a little extra gas.

I will probably start out with a 135 main and see where that puts me.

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.  :) 

 

And you really, really, really don't want me too. :)

Just trying to get it to run as good as I can with the stock carbs.  I don't really feel like spending a lot on new carbs for it.  If messing with the air jet doesn't prove fruitful, I can always swap in a stock set again.  I will be trying as much as I can not to mess with them, but if just messing with the fuel jets doesn't do it for me I will adjust the air jets.
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 #17 on: July 30, 2014, 09:39:51 PM »
Wow, this is interesting!  You're well on your way to getting this sorted out.

I think guylr is right: you want to figure out why the A/F mixture at idle is (almost) within the proper range, even though your pilot screw is not set anywhere near normal specs. 

BTW, is everybody in agreement that we are aiming for between 12:1 and 14:1 A/F ratio throughout the rpm range?  Or should the range be narrower than that?




Offline steve

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Re: 1973 Yamaha Stock Carb Tuning
« Reply #18 on: July 31, 2014, 12:04:26 AM »
Machina: please post a photo of the inside of the rubber mounting flange on your air filters.  The shape of the inside of the flange can have a huge effect on tuning.   Yes, I know you said it runs the same with or without the filters, but sometimes one problem can mask another.

Offline steve

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Re: 1973 Yamaha Stock Carb Tuning
« Reply #19 on: September 09, 2014, 04:15:50 PM »
Any updates?   

Offline TXRAT

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Re: 1973 Yamaha Stock Carb Tuning
« Reply #20 on: September 09, 2014, 06:18:53 PM »
rich fuel mix is 12:1 , 14:1 is "perfect" so im aiming at 12:1.

Offline prican8069

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Re: 1973 Yamaha Stock Carb Tuning
« Reply #21 on: February 03, 2015, 05:38:34 PM »
Any update on this topic?

Offline machina

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Re: 1973 Yamaha Stock Carb Tuning
« Reply #22 on: March 08, 2015, 07:58:52 PM »
I have been working for the last couple of months, and since this project was taken up right before winter, I did shelf it.  For the time I did get to ride it, it had run as good as it had ever ran.  I will be making more attempts to tune it in once the weather gets warm enough were I will enjoy being out in the garage.  But since I am in Minnesota, it will take at least another month.
1973 TX500
Dual Disc Front Conversion
'77 One Piece Cylinder Head Conversion
Yamaha Aftermarket Period Correct Oil Cooler

Offline dailydose1

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Re: 1973 Yamaha Stock Carb Tuning
« Reply #23 on: November 29, 2017, 08:15:14 PM »
SUPER OLD POST I KNOW! but anyone know what he is doing for his oil filter setup there? looks like going from engine case in bot to oil cooler then back in right hand screw port on engine?