My thoughts on tunning today 5-16-12

This site may earn a commission from merchant affiliate
links, including eBay, Amazon, and others.
C

ChuenouXiao

Senior Member
Messages
1,560
Reaction score
1
Location
California
I notice that when tuning my OBR..... that I may be a bit lean on the low speed so I rich it about an hour. When I did that, the engine is about to die which is normal so I lean the idle screw to compensate the low speed needle from killing the engine. The result is a more responsive low end power :D So what I'm trying to say is that did I just discover a way to better tune my engine properly? The idle screw is there to help you hold the idle when you tune correct?
 
Last edited by a moderator:
If it has a wt 990, Install a 3/4" velocity stack, replace the stock pressure spring that controls the fuel needle with Walbro # 98-344. buy a bunch of carb butterfly plates, screws, red lok tite, a set of needle files. File a notch -start small- on the throttle plate so the notch straddles the 3 holes you see in the carbs bore. If you eventually make the notch too large, you simply will not be able to slow the idle down. this is harmless. you gotta experiment with the size of the notches and find the carbs- engines "happy place". The hotter engine ya have, the more this will help it really rock--- these parts are cheap to by also.
 
alfred e numan said:
If it has a wt 990, Install a 3/4" velocity stack, replace the stock pressure spring that controls the fuel needle with Walbro # 98-344. buy a bunch of carb butterfly plates, screws, red lok tite, a set of needle files. File a notch -start small- on the throttle plate so the notch straddles the 3 holes you see in the carbs bore. If you eventually make the notch too large, you simply will not be able to slow the idle down. this is harmless. you gotta experiment with the size of the notches and find the carbs- engines "happy place". The hotter engine ya have, the more this will help it really rock--- these parts are cheap to by also.
Click to expand...
I heard that velocity stack are prone to air leak?
 
Do not use a velocity stack with a built in choke. They all will allow dirt-dust into the engine. A regular -plain velocity stack will not leak if installed properly.
 
alfred e numan said:
Do not use a velocity stack with a built in choke. They all will allow dirt-dust into the engine. A regular -plain velocity stack will not leak if installed properly.
Click to expand...
So what you explained above is fine tuning the needle through modding of the area near the needle huh? Interesting... do you do this to your RC? :D Sorry I am nooby so I don't really follow that well...
 
also, making this notch -- the carb "sees" this notch as a tiny velocity stack - the notch increases the velocity of air over the 3 holes in the carbs bore.- it will make the engine have more fuel- this is a rather simple explanition, these 3 holes in the bore operate in 2 fashons- and 3 stages. depending on the engines rpm. Experimenting- trial and error is part of the fun of any hobbie, these mods will not cause your engine to go lean- just the oppsite- it will cause the engine to go rich- you just have to readjust the 2 fuel screws- I would reccomend buying a 990 to experiment with- and save the carb that came with the obr- at the stock settings.- pardon the spelling.
 
The pressure spring under the primer bulb has 2 functions. it controlls when the carb gets more fuel, and also controls- tells the carb when to stop feeding fuel. its called -in order, pop off pressure-- reseat pressure. here is a example from testing-- The stock pressure spring on a WT990 pops off @ 32 psi. it reseats @ 28 psi. the pressure spring i mentioned above- Walbro part # 98-344, pops off @ 26 psi. Re seats @ 17 psi. Do not attempt to change the pressure spring by cutting- or stretching it. Its best to buy a factory spring designed -built for the carb. You are not modding the area around the needle. you are simply changing this little spring to a spring with lighter settings- tension.
 
alfred e numan said:
The pressure spring under the primer bulb has 2 functions. it controlls when the carb gets more fuel, and also controls- tells the carb when to stop feeding fuel. its called -in order, pop off pressure-- reseat pressure. here is a example from testing-- The stock pressure spring on a WT990 pops off @ 32 psi. it reseats @ 28 psi. the pressure spring i mentioned above- Walbro part # 98-344, pops off @ 26 psi. Re seats @ 17 psi. Do not attempt to change the pressure spring by cutting- or stretching it. Its best to buy a factory spring designed -built for the carb. You are not modding the area around the needle. you are simply changing this little spring to a spring with lighter settings- tension.
Click to expand...
I see.... tension huh... very interesting!
 
Alfred u think Chue can complete carb. Modding and spring pressure swapping with out incident.for seasoned engine and dirbike tech this comes easy .Chue leave your 990 stock and tune the best u can.because once u start swapping carb. Springs and notching the carb.u don't always get the result u set out for.run stock and let the mad scientist work for guys like Alfred..lol...that's a complimate Alfred.talk to u guys later...Ildy...
 
Ildy666 said:
Alfred u think Chue can complete carb. Modding and spring pressure swapping with out incident.for seasoned engine and dirbike tech this comes easy .Chue leave your 990 stock and tune the best u can.because once u start swapping carb. Springs and notching the carb.u don't always get the result u set out for.run stock and let the mad scientist work for guys like Alfred..lol...that's a complimate Alfred.talk to u guys later...Ildy...
Click to expand...
Yea, Its sorta like The music by Men at work- The tracks- Its a mistake- and the track- Jeckel and Mr. hyde- lol! - ya gotta love Colin Hayes voice- cool eyes too-- lol
 
Back
Top