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Post by maicohunter on May 25, 2008 23:47:11 GMT -5
Help !! My 1979 sr500 is driving me crazy. I am a pretty good wrench when it comes to these things but even my friend whos a Factory Harley Mech seems stumped by my bike. It starts easily hot or cold.The timing,valves and carb are set and everything is by the book. It runs great,pulls through all the gears without any hesitation. The problem: cruising speed i either pull in the clutch or slip her into neutral and close the throttle the engine continues to increasingly rev all the way up to 5000 rpm if i let it.If i drop the clutch slightley putting drag on the motor the idle will drop back into the correct range.My first thought is a air leak between the carb and the head but i have sprayed wd40 around it and it has no effect. Would a crank seal cause this to happen? The head gasket seems fine and i have performed a compression test with consistant 130# readings any help would and i would be greatful.
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Post by stevec5000 on May 26, 2008 0:09:17 GMT -5
The engine is just an air pump, more air makes it go faster but it can't go faster by itself for no reason unless it's getting extra air from someplace or maybe the air is staying the same but the timing is changing. Either you have a leak in the input or the throttle is sticking open or timing is off. Shouldn't be too hard to find the problem by elimination, make sure the throttle cables aren't sticking, take the air filter off and make sure the throttle is closing and if it still goes faster with the throttle closed there has to be an air leak whether WD40 has any effect or not. Have you checked the timing? If the throttle is completely closed the engine has to slow down but slightly open and timing change could make the speed change to speed it up.
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Post by solo2racr on May 26, 2008 0:15:48 GMT -5
. Would a crank seal cause this to happen? 2 stoke...Yes. Air leak. 4 Stroke....No. But it would probably leak oil out. I have run into this before (when I was working at a bike shop 25ish years ago) with other bikes. I just can't remember for sure what the problem was specifically. All I can think of right now is that it is running lean. I just can't remember why or what to do about it.   |
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Post by maicohunter on May 26, 2008 1:03:57 GMT -5
The throttle is fully closed ,cables are new and free. Timing checks out right on. and the intake is not leaking. When i first start it up it idles normally,once i ride it and its warm thats when it goes crazy.If i just let it do its thing it will slowley rev to infinity.
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Post by stevec5000 on May 26, 2008 1:16:14 GMT -5
An engine will normally speed up when it gets hot if it's too lean but not usually that much, couple hundred RPMs maybe, adjustable by idle speed screw. Still, it needs air to run so cut off the air at the intake and it has to slow down and stop. I would suspect a leak in the input that opens up when hot. If the throttle is really closed and it still runs too fast it's getting air from someplace else bypassing the throttle.
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Post by maicohunter on May 26, 2008 1:21:39 GMT -5
Thats where i started looking but havn't had any luck finding it. Keep in mind as its reving up [throttle closed] if i just let the clutch out putting the least amount of drag on the motor i can get it to drop back to between 1100-1300 rpms. Then if i give the throttle another blip it slowley takes off again,climbing and climbing in rpms. I am sure the throttle is closed,no binding cables and the plug is not lean nor rich. its perfect..
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Post by stevec5000 on May 26, 2008 7:43:20 GMT -5
Still it has to have more air to run faster so it must be getting in somewhere. Think of it this way, if there was an obstruction and the intake was blocked the engine would die since it can't run with a vacuum, you have just the opposite problem.
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Post by solo2racr on May 26, 2008 11:00:19 GMT -5
once i ride it and its warm thats when it goes crazy. There's your answer. Look for cracks in the intake side of the head leading into the intake port. When the engine heats up and expands, the crack gets wide enough to suck in air. You won't find it with spraying WD-40 because it will evaporate off the hot engine before it can get though the crack and effect the engine RPM. |
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Post by maicohunter on May 26, 2008 12:57:41 GMT -5
That makes sence except for when its racing and i let out the clutch a little to drag the idle back down it will stay down until i open the trottle just a bit and then it starts reving again. Im running a 1 3/4 pipe with a suppertrapp "White Bros" its jetted at 30 pilot and 320 main. This morning it seems a bit rich.
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Post by solo2racr on May 26, 2008 13:49:08 GMT -5
That makes sence except for when its racing and i let out the clutch a little to drag the idle back down it will stay down until i open the trottle just a bit and then it starts reving again. Im running a 1 3/4 pipe with a suppertrapp "White Bros" its jetted at 30 pilot and 320 main. This morning it seems a bit rich. Maybe not enough vacuum at idle to draw air through a crack??? 30 pilot seems right. should have a 2.0 or 2.5 slide with that. I just changed mine from a 3.5 to a 2.5 and having to drop the pilot size from a 50 to a 25. People don't think about slides to much but on these big singles, they have a huge impact. 320 seems rich. I'm at a 260 with alot more done to the engine. BTW.......what kind of carb? ? |
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Post by maicohunter on May 26, 2008 21:34:40 GMT -5
My carb has a 3.5 slide. Stock airbox.The only thing thats not stock is the 1 3/4" pipe with a supertrapp.
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nsman
New Member
Posts: 25
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Post by nsman on Jun 6, 2008 17:43:38 GMT -5
Was this bike working well for an exteded period and then this problem appeared or was there a change made to something?
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Post by jimnmad on Jul 8, 2008 0:55:49 GMT -5
I'm having exactly the same problem. same motorcycle too. Did you ever resolve? What was the solution?
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Post by maicohunter on Jul 8, 2008 23:51:49 GMT -5
It seems to be better,not as crazy as it was.I never came to any conclusions as to why it happens.I fiddled with the mixture screw a bit and it seems to be very sensative.
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Post by jimnmad on Jul 9, 2008 22:39:37 GMT -5
MIne was acting exactly as you described. See my post on the other fast idle thread. Jim
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