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Post by mcjongs on Sept 30, 2024 4:53:51 GMT -5
Hi guys!
I have an 78 SR500 with some odd problems. It starts to go rough in the mid RPM for then to go clean all up to max rpm.
The bike is relatively new to me and I have seen that the previous owner had changed out the wiring harnesses with some homemade wires and very bad connector choices in the headlight.
After i received the bike I have:
• changed battery
• new reg/rectifier, (14,4 volt steady independent of RPM)
• new stator (used but checked to be OK)
• Rewired in accordance with a wiring diagram, including change of ignition switch to a OEM type earthing out the CDI/Coil when off. (The former did not that)
• gone through the VM36 carb with a jetting that I know works for these bikes including XT's that I have several of.
The only thing I have not done is to change the CDI unit. Another thing that puzzles me is that the wiring harness does not have connectors. only cable shoes on the wires going into the different connectors. I have to a certain extent changed to insulated cable shoes but not for the CDI unit.
I have no experience with possibly faulty CDI units but is it (if faulty) likely to cause this hesitation in the mid rpm register that I have?
I struggle to just buy a new CDI unit and then find out that it is something else.
Coil could be a source for fault but it pulls strong on high rpms/loads so I doubt that.
Jon
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Post by andy on Sept 30, 2024 15:45:05 GMT -5
Jon, having had a CDI go down on me and talking to others I can tell you that they either work or don’t, there’s no detioration period. I was so lucky when mine gave up, I was close to home and near a mate’s house 🙏.
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Post by mcjongs on Oct 1, 2024 3:13:17 GMT -5
Ok, Thanks for answear
Maybe I have to buy a Powerdynamo system. Very good experience with those.
Regards Jon
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Post by redgalvin on Oct 1, 2024 7:24:30 GMT -5
Jon, having had a CDI go down on me and talking to others I can tell you that they either work or don’t, there’s no detioration period. I was so lucky when mine gave up, I was close to home and near a mate’s house 🙏. When my '78 CDI went out the spark wouldn't advance in the 100+*F. heat, but I limped home 1500 miles. When the temp. dropped into the 80's I could run at highway speeds. |
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Post by redgalvin on Oct 1, 2024 7:30:23 GMT -5
Oh yeah, The sealant on the back of the CDI unit was shrunken, cracked, separated from the nearly rusted through case. POS only lasted 40 years and 80.000 miles! Never buy a Yamaha again!
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Post by andy on Oct 1, 2024 12:06:44 GMT -5
Jon, having had a CDI go down on me and talking to others I can tell you that they either work or don’t, there’s no detioration period. I was so lucky when mine gave up, I was close to home and near a mate’s house 🙏. When my '78 CDI went out the spark wouldn't advance in the 100+*F. heat, but I limped home 1500 miles. When the temp. dropped into the 80's I could run at highway speeds. 1500 miles ? I didn't get 15 yards  ,it was just like turning the ignition off |
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Post by stevep on Oct 2, 2024 13:42:37 GMT -5
Couple of things come to mind to check and cross off the list: Correct gapping of the Pulse coil air gap. (I never stop banging on about that. But it's an overlooked item that, if out of spec, will cause trouble.)
The inlet manifold: I never got more than about 3 years out of one, with various issues like rubber separating from the metal, or the vac spigot detaching.
Lastly, has it got a standard silencer but a non-standard downpipe? A big bore (single skin) downpipe with standard silencer needs a torque cone to replicate the back pressure you'd get from the standard downpipe. Hesitation and misfires would fit the bill here!
All of those would make a difference if they were currently 'out of whack'.
And of course it could just be a range of things just not quite in spec.
Spark plug gap?
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Post by stevep on Oct 3, 2024 10:19:17 GMT -5
On re-reading your original post, the coil is a cheap item to replace and I remember when I fitted new coil and wow, the difference it made. I know they are supposed to work or not work, but I suspect you can get progressive failure and my situation was it failed to rev past about 4k then fitted a new coil and bingo!
Worth a shot I reckon, especially if the one on the bike is the original item. It's long past renewing!
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,it was just like turning the ignition off
