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Post by thevillageidiot on Nov 4, 2005 18:51:09 GMT -5
I just rebuilt a '78 SR500 from the ground up (was sitting nicely for 15 years). It runs, it might jump, but i will ride for ever if i could only figure out why it wants to idle a 4000 rpms instead of the 12-1300 it should be. any advise would be a great help.
Thanks
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Post by G Man on Nov 4, 2005 20:42:34 GMT -5
If you could give us a little bit of background on how the bike is set up. What carb, jetting, engine mods etc... Have you checked for an air leak at the intake flange? Is the idle adjustment correct for the type of carb you have? Any throttle/cable binding or sticking?
Talk to you soon,
G Man
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Post by thevillageidiot on Nov 5, 2005 2:59:18 GMT -5
36 MK, a white bro. pipe with a supertrap muffler and 12 baffels/spacers. rest of it is stock....havent found any bindings in the cable and no air leaks. and i dont know of a specific idle adjustment thing.... I am new to this kind of bike, and this is the first i have rebuilt from the ground up.
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Post by StewRoss on Nov 5, 2005 6:39:21 GMT -5
Hi mate...
Some other little considerations:
slide in the wrong way around
Needle not under retainer plate, insted situated on top.
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Post by StewRoss on Nov 5, 2005 6:53:55 GMT -5
...aargh, hit the wrong key again...cont'd...
idle screw too far in...fairly basic ...screw in the side of the carby, screw in for more ...out for less...pushes on a ramp on the side of the slide...nothing to it...
Mixture screw set incorrectly...this one is situated on the lower part of the carby body in the side....above the float bowl...engine stopped...screw it all the way in...CAREFULLY; don't don't screw it home hard...very lightly...count the number or turns as you go...write it down just in case...
Once it is gently fully in...not screwed in hard!!!...back it out again about 1.5 turns as a start point...start the bike up and wind it in or out slowly to get the fastest idle point...then use the main idle screw to get about 1,000rpm or so...hopefully that will all work and it will run nicely...
still worth checking the needle plate to make sure the needle is in correctly...plate should sit over the circlip and hold it down in place...the whole thing will be held by the return spring...
really if the slide was in backwards one you got it going it'd probably run near flat out...the slide cutout goes to the intake side (bellmouth) of the carby
Hope this assists ...
SR
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Post by thevillageidiot on Nov 5, 2005 16:03:53 GMT -5
will give it a shot thank you. I was told one screw was fuel rich, the other was the air rich. didnt think one was an idle screw. Will give all that a shot this weekend and get back to you.
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Post by thevillageidiot on Nov 7, 2005 0:22:11 GMT -5
I played with the screws on the carb for a little while and finaly managed to find the perfect combination. now all I will have to do is find out any adjustments I will have to make when I head up to the mountains.
Thanks for the help guys wont be long before I know I'll need it again.
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Post by StewRoss on Nov 7, 2005 4:08:42 GMT -5
The pilot jet system is there to sort out the running at low or near closed slide. Adjusting the screw in or out adjusts the amount of air, supplied to the pilot jet to mix with the fuel.
If you look at the carby inside you'll see a little outlet downstream of the slide (..to one side of it) this is where the fuel is ported from the pilot system into the engine with the slide down. The air comes in to the jet from one of the air bleed holes in the periphery of the carby's inlet.
The pilot system also operates at very low throttle settings to supplement the low suction on the main jet outlet, about 0 to 1/8 throttle. Getting this right helps with nice smooth take-offs etc.
The idle screw simply pushes on a ramp machined into the side of the brass (..or alloy) slide and forces the slide up more or allows it to go down more. That increases the amount of air coming in and therefore the suction over the main jet outlet...(...where the needle goes into).
Unless you're changing altitude dramatically don't concern yourself much about jetting changes. You can probably sort it out within the range of adjustment provided.
Main jet changes can usually be done within the range of movement of the circlip on the needle as well. So long as it was fairly right in the first place. There are lots of variables with the adjustment of these carbs if you start playing.
One good place to start with the main jet is a plug chop. That entails taking the bike out along a secluded and fairly straight road. Holding it wide open for a short while as you ride along and then killing the engine quickly. Coast to a stop and then remove the plug to see the colour of the tip. It should be a nice light tan colour. Too white, try raising the needle a notch and check again, too dark the opposite. If it is always too light (lean) or dark (rich) possibly go up or down a size in main jet and start the needle from the centre notch again. Engine's that use oil can also mess up the plug readings as well....
Hmm, hope I haven't confused you too much...
SR
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