OzVMX Forum
Marque Remarks => Yamaha => Topic started by: Ozadvntr on March 14, 2016, 11:55:04 am
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Hi, just making sure I buy the correct Mikuni Carbie for a 1979 YZ250F - 38mm or 36mm?
thanks,
Mick
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from memory only will check the manual after but 36mm
get 2 manuals 1 for the YZ250F
and 1 for the carby SUDCO mikuni tuning manual 5th edition, GOD wrote it, its all I have to say about that!
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36mm
Basically any carby from yz250\400\465-E\F\G\H will do but you will need to rejet to suit.
Number #3 slide
50 pilot
Q-0 needle jet
F-16 needle (I think)
390-410 main
Obviously location heat altitude will all be determining factors.
I can double check the jetting but they are the starting point.
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Bloody hell Perko how do you remember this detail after all those Beers ;)
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Ant comments on the air correction bleed jet guys?
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Bloody hell Perko how do you remember this detail after all those Beers
I don't.
Just guessing
Ant comments on the air correction bleed jet guys?
? ? ? ? ? ? ? ?
Do you mean the air mixture screw? Or the needle jet mixture? I will double check that one too but I thought it was Q-0 & needle F-16 but as I said "Off memory"
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Number #3 slide
50 pilot
Q-0 needle jet
6F16-4 (4 is clip position)
390-410 main
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i am talking about the small jet which is at the bottom of the inlet side of the carby it bleeds air to the main and corrects mixture at high rpm. from memory it should be approx 1mm dia id, some people just take it out on bigger bores.
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Still not sure what you mean but there is an air bleed jet on the 32mm. The larger ones dont have the jet just the hole where it would go.
Once you drill something out it's a bit of a pain to go back. Have to make a very small bush.
YZ125 No 4 jet
http://www.boats.net/parts/search/Yamaha/Motorcycle/1978/YZ125E/CARBURETOR%20YZ125E%20-%20F/parts.html (http://www.boats.net/parts/search/Yamaha/Motorcycle/1978/YZ125E/CARBURETOR%20YZ125E%20-%20F/parts.html)
YZ250 no air jet
http://www.boats.net/parts/search/Yamaha/Motorcycle/1979/YZ250F/CARBURETOR/parts.html (http://www.boats.net/parts/search/Yamaha/Motorcycle/1979/YZ250F/CARBURETOR/parts.html)
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yep, thats the one i meant...
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I just checked an old test and everything I read suggests a 38mm Carbie.
I also understand that 36mm will give an increase in torque. Is this correct?
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Definitely 38mm according to my info and samples.
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36 mm carby on the 77 yz 250 d , 38 mm on the E and F , 400 had 38 mm from the D model on
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thanks, Just ordered a new 38mm. Just over $200 delivered to me with all the correct jetting in a separate kit.
thanks for the advice.
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38mm
Stand corrected
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i am talking about the small jet which is at the bottom of the inlet side of the carby it bleeds air to the main and corrects mixture at high rpm
So does it lean the mixture out or richen it?
I got one in the bellmouth of my 38mm for my IT400D and that was lean everywhere. Spoke to the Mikunioz bloke and he said leave it in there and jet around it.
Which is an excellent business strategy when you need 9 needle jets and a bucket of mains to stop the thing from cooking.
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i am talking about the small jet which is at the bottom of the inlet side of the carby it bleeds air to the main and corrects mixture at high rpm
So does it lean the mixture out or richen it?
I got one in the bellmouth of my 38mm for my IT400D and that was lean everywhere. Spoke to the Mikunioz bloke and he said leave it in there and jet around it.
Which is an excellent business strategy when you need 9 needle jets and a bucket of mains to stop the thing from cooking.
The bigger it is the leaner and the smaller the richer, but mainly has more of an effect at the top end. Not normally something you would change if it's the original carb.
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no, it was installed in the 'miles to buggery too lean' new carb.
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Perhaps applicable to this air corrector issue..in my Weber-carbs-on-Mazda-rotary tuning days we would use known diameter safety wire appropriately restrained in the air jets as a calibrated restriction for a quick, cheap checks.
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no, it was installed in the 'miles to buggery too lean' new carb.
Taking the air jet out will only make it leaner. Sounds like you need to find someone with a box full of jets to borrow till you get it correct. If the original carb was lean everywhere as well, perhapes you have a big air leak that needs fixing.
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www.iwt.com.au/mikunicarb.htm
Go here , sounds like you guys need to understand how to properly diagnose and set a carb.
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www.iwt.com.au/mikunicarb.htm
Go here , sounds like you guys need to understand how to properly diagnose and set a carb.
There is no information on that link about the main air blend jet so it is quite irrelevant to fred99999au's question.
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Mine was more a rhetorical question about the function of the air bleed.
Genuine carb didnt have it replacement carb did and it was lean everywhere.
It runs Ok now and is as reliable as a woodburning stove. I was wondering what the function of the air bleed was.
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Leave the jet in there and jet around it.
The information is very relevant to the understanding of the operation of a carb. and setting of the same.
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Mine was more a rhetorical question about the function of the air bleed.
Genuine carb didnt have it replacement carb did and it was lean everywhere.
It runs Ok now and is as reliable as a woodburning stove. I was wondering what the function of the air bleed was.
The function of the air bleed? The pasage from the bell mouth through to the top of the needle jet allows air to be mixed with the fuel to aid in atomisation of the fuel as it is drawn into the air streem entering the engine. At low engine speeds there is only a small amount of air passing through this passage or jet if fitted. At high engine speeds there is a much greater flow of air which causes a pressure difference across the air jet, this in turn means more fuel is drawn out of the needle jet with less air added. If we make the air jet smaller this pressure difference is increased at high RPM making it richer but because the flow of air through the jet at low RPM is only small the smaller jet has virtualy no effect on mixtures at the lower speeds.
The size of this jet and other subtle differences in passage sizes are the main reason that fitting a new carb with stock size jets doesn't always work.
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www.mikuni.com/pdf/vmmanual.pdf
Attached is the Mikuni super tuning cat. which is very helpful
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i am talking about the small jet which is at the bottom of the inlet side of the carby it bleeds air to the main and corrects mixture at high rpm
So does it lean the mixture out or richen it?
I got one in the bellmouth of my 38mm for my IT400D and that was lean everywhere. Spoke to the Mikunioz bloke and he said leave it in there and jet around it.
Which is an excellent business strategy when you need 9 needle jets and a bucket of mains to stop the thing from cooking.
Without the air corrector the mixture would get richer with rpm the air corrector(as the name suggests) keeps that in balance. Early mid 90's 250 road bikes put solenoids on the air corrector to pass emissions tests. With modern electronics we can "rejet' without touching the float bowl.
http://www.flo-commander.com/