OzVMX Forum
Marque Remarks => Yamaha => Topic started by: BA #63 on June 01, 2015, 02:25:30 pm
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any ideas why my sc 500 is starving for fuel out of the corners put new 38mm carb on back to standard spec
feels like the fuel is off then comes back when bike is upright and twist of the throttle
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Float hieght?
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carb off again :( :( thanks
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check fuel tank filter to tap????
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put new tap on was still the same
cheers
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Check the breather in the cap.
cheers.
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And check the new tap has big enough holes in it, ie can it flow enough fuel? Alvin had a similar problem with a new tap on his SC 500.
Foss
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put the yz 360 tap on still the same will do a bit of f%c*&_#@ around bit I think maybe floats as I did not check as the carb came pre set from the states
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Check the needle and seat size; it could be in effect the main jet.
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make sure your'e running the SC carbies needle,& emulsifying jet,(hint really fugging rich bigtime),Alvins' was a bitch to tune/ride with the new mikuni,till all the original "middle" bits went back into it,sweet as , :P
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Just a late thought--hasn't popped the crank seal out behind the mag?, stator plate would probably stop that happening though. Not a piece broken off a reed petal?. Also, as has been said, must be kept plenty rich on the needle/emulsion tube [mid range] well, & the main jet--ok everywhere--big piston. Used to race one pretty much out of the crate [bosses one] & left it rich midrange [stock std jetting], another sc500 on the day i'm thinking of [grass TT] ended up seizing--think at the end of a long straight as he rolled it back. Anyway, just my thoughts & maybe not relevant in this case. Cheers. [one more--not base gasket partially sucked in].
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also, blocked breather [breathers] into the bowl--bit of mud. Had a yz80H did that & wouldn't take full song for long before giving running out of fuel symptoms. Bit of mud had got right up into the breather high up on the side--which leads to the bowl. Thought maybe the vacuum caused by full song was stopping the float from dropping--whatever it fixed it--kid happy again, had me head scratching for a bit.
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thanks for all the info will take it out and play I still have another one to start on next I suppose more head aches
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just one more trip down memory lane--can't remember to turn the compressor off at night [till it starts up in the early hours grrrr] but can remember taking the balancer chain off the SC back in the day--only for one meeting & put it back on as too much vibration.
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Hi Brent,
I have seen your Carby set up, disregarding the intake manifold leak and the carb is not an original SC500 carb, have you done the basic's.
The SC500 is a reed valve motor as your aware, these motors require a larger pilot jets than a piston port motor and or a four stroke.
The carb you have purchased either with the bike or after the fact, is it jetted close to O.M.E spec ?
The std SC 500 carb uses a 80 pilot jet, a O8 needle jet and a 6F16 jet needle. you say the carb you brought was jetted to suit, if brought locally its likely its got this set up.
Pilot jet 30 (VM22/210), Needle 6DP1, Needle Jet Q-2 (166 Series), Main Jet 330 (4/042), Air Jet 0.5, Slide Cutaway 2.5 (VM38/24), Needle valve 3.3
In the above info, if this what your carb has in it, you have issues!
The pilot is way to small and the needle jet combo is to large.
All these jets effect the lower throttle open position running.
Provided the electric's are good, strong spark etc and the bike starts first or second kick in its current state, a question you can answer is, does the bike maintain steady speed at low throttle in a lower gear. any running issues will be highlighted by applying this basis running test.
If the bike runs poorly in the lower throttle percentage ranges the bikes not jetted correctly or has an mechanical or electrical issue.
If the bike push's you though corners when the throttles closed, its lean, either a leaking seal/manifold etc or a small pilot jet.
You say the bike starving for fuel, often its the opposite, don't get court chasing a issue. I watch bikes around the traps. often its not what it first appears to be.
I had massive issues with my 500 for quite a while after I first got it running and learnt a ship load about carby set up because of it, the clearance between the needle jet and jet needle is measurable, this clearance is critical to the 1/8 throttle position to my SC500 and my other bikes.
I have a value that I stay within for all my bikes, once the value is known I can run any needle jet & jet needle combo there after, provided the 1/8 throttle opening value does not change greatly.
I will not discuss the tunes I run in my bikes in this forum, But ask Wayne and his sons what I have done to their bikes ( 26 & 13 ) and the results on correct jetting, or Robert about his/Fred's TS250 Reed valve mod or Mark Steel ( 174 ) about the reed valve mod and jetting on his CR250M are just a few.
Even chat to me or bring bike me the bike one weekend! you have my number.
Regards Gibbsy
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I am running a 1984 YZ 490 carb on my SC 500.............however, I have changed all the internals back to the '74 specs, as shown below. I still had starvation problems until I fitted a larger fuel tap........also, as I only race dirt track, I use a 460 main jet. I hope this helps......... :)
(http://i396.photobucket.com/albums/pp45/triumph51/SC%20500%20Carb%20Settings_zpsbgymmwnh.jpg) (http://s396.photobucket.com/user/triumph51/media/SC%20500%20Carb%20Settings_zpsbgymmwnh.jpg.html)
(http://i396.photobucket.com/albums/pp45/triumph51/006_zpss6bvfqxv.jpg) (http://s396.photobucket.com/user/triumph51/media/006_zpss6bvfqxv.jpg.html)
(http://i396.photobucket.com/albums/pp45/triumph51/009_zpsd2zr6qyk.jpg) (http://s396.photobucket.com/user/triumph51/media/009_zpsd2zr6qyk.jpg.html)
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Thanks I will be going out next week end to play with it a bit I tried two taps already let you know the out come
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change your needle jet to a larger one like a P or Q, it could also be blocked. your mains can only flow as much as the Needle jet will deliver in.
Also fit the largest fuel bowl needle and seat you have it should be in the #3.5 stamp zone and have 4 or more holes