Mystery stumble between 1100 and 1700 rpms solved... Kind of.

happychappy

XS650 Member
Messages
15
Reaction score
0
Points
1
Location
round rock tx
Hello, sorry to make my introduction like this. I have been reading this forum since I bought an xs650 from CL a while back. Since I've owned it, it has never run well. It was stored for a while before I bought it. The po said it was running well when he put it up. It was running rough when I went to see it. It ran with a stumble between 1100 and 1700 rpms.

I should also say that this bike is not in riding condition at the moment. I have been slowly working on parts of the bike that need attention as frustrations rise and fall with different aspects of the project. I'll go over what I'm working with so you have a frame of reference (the bike below).

From what I read the stumble I was/am tracking was a pilot jet adjustment that is typical after an intake configuration change. Having gone from the stock air box to the uni I thought this was an obvious culprit and put a lot of time in that. I have taken the carbs off at least 10 times in the last 3 months and tried everything form a 34 to a 50 pilot jet. Yesterday was a breakthrough day. I attached the charger and set it to "start"50 amps and fired it up. To my surprise it ran with no stumble, the best it has ever run. Literally after disconnecting the negative terminal from battery and revving the engine there was the stumble again, connect it again and the stumble is gone. Needless to say I thought this was a good test for the future if I ever have this kind of low rpm stumble since this seems to be a pretty consistent issue with these bikes. I thought I would share the testing procedure and ask you to check the validity of the test. Maybe it will help someone in the future more quickly diagnose a possible issue.

Procedure
1 Turn bike off
2 Connect Charger terminals to battery
3 Set charger to "12 volts"and the "Start"
4 Turn on bike and search for stumble.
5 If there is no stumble remove the negative terminal from the charger and see if your stumble is back. If so you can safely assume the timing chain tension, the valve gap, carb timing, air mixture screws, jets, intake boots and seals are fine and that your issue is somewhere between your rotor/stator (Charging) and the spark plug (Ignition).

Today I am going to try to clean up the charging system make sure its pushing enough juice at idle and test the coils with my multimeter. Having heard what has happen so far any advice you have that could get to me a faster resolution would be gratefully appreciated.


The bike:
1980 XS650 Special bought stock
What I have done
Removed the old wiring harness except for the rotor
Replaced with a harness from TCBros
Removed stock airbox
Added uni pods
Disassembled and cleaned the carbs (looked pretty good for having been stored)
Re jetted pilot to a 47. (seems to be happy there)
Clutch push rod oil seal replacement (leaked oil)
Swapped out the clutch push rod for a single rod (Mikes XS set up)
Timed the cabs with homemade manometer
Replaced battery with one from TC bros.
Replaced the air mixture screws (Old ones were pretty bad and looks like the previous owner had to do a lot of adjusting)
Adjusted the cam chain tension (Was a little loose)
Verified the gap clearances on the intake and exhaust vales.

I sincerely thank you for all the helpful info I have found here.

Thanks
HaPPYCHAPPY
Austin TX
 
Hi happychappy and welcome,
looks like you got the procedures aced.
Some pointers you may not be aware of:-
XS650 parts swap between model years like LEGO.
Check that your 1980 still has the original carbs.
The quick check is those carbs should have thin steel diaphragm covers.
If they have thick cast aluminum covers they're off an earlier model.
The bike will run fine but the earlier carbs have real skinny slow running needles
that can break their ends off in the carb bodies and eff up your slow running.
 
Welcome to the forum, happychappy. There's several XS resources just south of you, members DogBunny, Goldtooth garage, Limey Bikes, the_nutt, Maxx650, others.

The Limey: http://www.limeybikes.com/

It sounds like you may have a weak trigger magnet. When the charging system is working, the magnetism of the rotor can overshadow a weak trigger magnet. When you jumper the battery, the charging system probably turns off power to the rotor, allowing the trigger magnet's magnetism to be detected.

One of DogBunny's TCI threads:

http://www.xs650.com/forum/showthread.php?t=30582

Try using the forum's 'google custom search', upper left of the page, and type "rare earth magnet". Read through some of the threads, and see if they explain it better...
 
Thanks for the info. I pulled the coil off and tested it. It tested good and it was verified by a local shop owner when I took it to be looked at. During our conversation and story telling I told him about the battery test and he suggested it could the stator and that he also thought the coil was good. When I tested from primary to primary I got 2.8 ohms, he got 3.6. after some BSing I got home and pulled the stator off and found a couple of things wrong. I found that one of the brushes was not screwed in all the way and flush with the housing because it was crooked and the black wire from the pickup coil had been pinched inside the sleeve behind the chain. When I opened it up to inspect it I found it had been almost completely cut.


After repairing these two things I put the stator back on completely confident I had resolved the issue. Kicked it over and over and over and over and nothing.... Hooked the dang battery up and boom runs great. After this failure I researched the magnet idea from TwoManyXS1Bs. When I figured out where it was on the rotor I found that mt magnet had begun to disintegrate. Luckily I had a refrigerator magnet that was exactly the same diameter as the magnet that was there. After gluing it on with a 2 part epoxy and waiting for about an hour it fired right up and runs great without shore power...haha

Thank you fredintoon for helping me ID the carbs they are the correct BS34 with the shinny tops, and a HUGE Thank You to TwoManyXS1Bs for pointing me in the pickup coil magnet direction. I have some pics and some videos. The pics are obvious so are the videos. I think I ma going to fiddle with the pilot jet some because I'm running a big 50 and I'm pretty sure I don't need it. It was part of the carb troubleshooting process. the timing fluctuates when you rev the motor but I have read that its normal when not at idle.

https://www.amazon.com/clouddrive/s...oGORXHR1jeMf5pU?ref_=cd_share_link_copy_flash

https://www.amazon.com/clouddrive/s...eipRAWRM1sqyjYU?ref_=cd_share_link_copy_flash

https://www.amazon.com/clouddrive/s...QunPIgHyeLQK2gM?ref_=cd_share_link_copy_flash

https://www.amazon.com/clouddrive/s...960DZ1ZAgF8siGY?ref_=cd_share_link_copy_flash


Video of it running.

Before

https://www.amazon.com/clouddrive/s...YBjT4Am5CIuSNTA?ref_=cd_share_link_copy_flash


After.

https://www.amazon.com/clouddrive/s...BlCew10yuiKp0hg?ref_=cd_share_link_copy_flash
 
Back
Top