HELP!! 1980 Yamaha XS650 special. Surges, loses acceleration

[N8WAVES]

Fist off I'm new to bikes but i have done a lot of work and learning with this bike.
Its a 1980 Yamaha XS650 Special.

My bike has currently been un-rideable for about 6months now.
I will describe my symptoms as best as i can along with what i have already done to try and fix it.

My bike the best way i can describe it is it wont hold a constant acceleration while the bike is in gear.
If the bike is up on the stand there is no issue as i cycle through the gears and accelerate the engine. Once i get on it and start to ride if i hold down on the throttle and try and keep the bike cruising at (lets say 3000 rpms) The bike will "surge", "buck", decelerate and accelerate with out the manual movement of me moving the throttle at all. It now also has a hard time staying on at lights as well along with this problem.
This problem was a fast onset there was nothing leading up to this. The same day i noticed this problem my headlight went out as well.
I took apart the carbs thinking it was a carb problem they were perfect and clean. I looked to the fuel line and even put a new in line fuel filter in and still no avail. I traced the throttle cable and found no disturbances. I checked the plugs and wires and also no issue.
From here i was thinking electrical so i turned it into a local shop. The mechanic there kept it for 2 months and fixed the head light but could not fix the main problem. He checked my carbs as well and adjusted the floats and could not identify the problem. He said from here he might look at the electronic ignition (your thoughts??). I was thinking what about the Throttle position sensor(TPS) but does this bike have one?? I have no clue!!!
I'm so lost as to what to do and where to go. The bike has such a good engine it starts right up with no hesitation. I want to fix this and have spent countless hours working and asking for help and have only found walls.
Can you please help? ANYONE!!! please be very descriptive because my knowledge is slim.
Any help would be much appreciated!!!
Also the mechanic looked at the clutch and said it was a bit worn but nothing unusal or needed to be fixed on a 1980.

 

[milldinaire]

Check your regulator rectifier. In the book it will tell you what you need to know about that. Also check your rotor too. They go bad and drain your battery. But it sounds like. Your reg rec. I had the same issue. It is the gray box with the white wires going to it I believe. I'm not at home and gong off the top of my head.

 

[milldinaire]

Reg rectifier check that.

 

[milldinaire]

Our bikes have to maintain a certain voltage to stay running and it sounds like to me you are not getting that voltage under load. I can tell by your headlight situation.

 

[retiredgentleman]

Possibilities:

Fuel starvation.............blockage at the petcock(s), remove from tank and check it. If it is the vacuum operated type, it may not be opening fully...............I don't trust them and replaced mine with manual petcocks. Is the vacuum line to the petcock in good condition?
If the vent in the fuel tank filler cap is blocked, its possible to pull a partial vacuum inside the tank, which will starve the carbs of fuel.

Electrical........the original 4 position fuse panels (glass fuses)are worn out and may produce high resistance intermittantly..........they can heat up..........they can cause voltage drops.........replacing with automotive blade fuses is a must on these bikes. The kill switch and the ignition switch can be corroded which again will cause voltage drops to the ignition coil. All electrical connectors are suspect for causing voltage drops , until taken apart and cleaned up bright and shiny.

Make sure you have a good battery..............12.5 volts with the bike off. 13.5 volts at idle and 14.5 volts at 3000 RPM. Correct voltage is fundamental to good operating of these engines.

 

[xjwmx]

Once i get on it and start to ride if i hold down on the throttle and try and keep the bike cruising at (lets say 3000 rpms) The bike will "surge", "buck", decelerate and accelerate

If it only does that when you're trying to maintain a steady speed and doesn't' do it when you're accelerating, I'd say the problem is none of the above. Something about riding style can cause it to happen. The bike I had for MSF class did it, and the instructor told me to find the "sweet spot" or some such. The bike switching from trying to pull vs trying to coast, I think. My xs will do it occasionally, I don't know why. Let someone else ride it without telling them you have a problem with it, and see if they experience the same thing. Among other things to check are valve adjustment, freedom of carb piston movement, and drive chain tension.

 

[retiredgentleman]

Here is another area to think about:
http://www.xs650.com/forum/showthread.php?t=4718

Weak TCI magnet???

 

[milldinaire]

You need only search two places. check your rotor resistance and check your reg rectifier. one is bad. like the gentleman said, make sure your battery is 12.5v off. fire it up and test voltage at battery at idle if it is 13.5 your rotor is okay. if less that might be your problem. if you throttle up to i believe 3000 You should get 14.5v if less you should check your rotor and regulator rec. also watch your headlight if it goes up and down in brightness with throttle it could be your reg rec. I had the same problems once only it was the windings in my rotor went bad. no charge at all. If you find it is your rotor. There is a guy who will rewind your rotor for cheaper than it is to buy one off ebay. also it performes awsome at near idle even. good luck I hope you find your issue.

 

[xjwmx]

Unfortunately that's bullshit. And - if the headlight increases and decreases in brightness with throttle, it's actually a sign that your charging is working

 

[milldinaire]

No it just means you have a shitty charging system and need to have it looked at.

 

[milldinaire]

I have never heard of the riding style excuse, it is a good one though. Maybe you need to have your instructor help you with that. As far as this guys bike goes it came on suddenly. Not from riding style.

 

[xjwmx]

No it just means you have a shitty charging system and need to have it looked at.

You do seem know you have to rev it to 3000 to get over 14v. You know it's less than that until then. Any guess about how that might make the light behave?

 

[milldinaire]

I hear what you are saying but a noticeable dimming of the head light is not caused by a 1 to 2 volt drop. My headlight does not do what is being described.

 

[xjwmx]

I can look at my meter and see my voltage go from 12.5 to 14.5. I will see my headlight and every other light be brighter at 14.5 Two volts is plenty to be visible in the lights. If you see this it means your charging is operating normally.

 

[milldinaire]

Ok you have established your charging system works for you. Now back to this persons bike. The loss of your headlight along with the stumbling can be attribute to your battery voltage dropping below 12 volts. Check your battery voltage with a voltmeter. Of below twelve volts check your charging system. That is it. If it is higher, talk to xjwmx. He'll help you with your riding style. Nice to meet you xjwmx. It is lunch time woohoo!

 

[xjwmx]

Sorry to bother you with electronics and motorcycles and instructors and the like

 

[650Skull]

N8WAVES, Welcome to the site....................... It is hard enough to know who to believe when asking questions on a site, especially if you are a new member.

Look in the Tecnical section and look up Electrical and go through the threads, also make sure you look in the Maintenance and General Troubleshooting, and look up
"Bike won't start? Running rough?" and
"Buying guide - Getting to know your bike"

Riding style may cause a symptom, but unfortunately changing the riding style won't fix the problem.

+1 RG

80G Special come out in 2 models.
One is the "Special 2" with spoke wheels and the 78/79 Special Taillight with a chrome grab-rail and tin side panels, seat hinges from the side. ................Not sure if this model had points ...............If it has it would have the separate Reg/Rec, If so junk them and replace with a aftermarket one or build one yourself, Pamcopete has a write up on how to do this.

80SG Mags with large taillight at the back of the seat with an alloy grab rail and plastic side panels, Seat clips and locks in. This model has the Yamaha (Hitachi), electronic ignition. Check to find out if this model has the combined Solid state Reg/rec

Learn to continuity test and use a volt meter then learn to check out your wiring and electrical parts..................workshop manuals, hard copies and online. ........Again check out the tech menu, look under Other[/COLOR]

Learn as much as you can and then some one here will be able to help you out also helps you to get to know who to listen to

Go to More smiles and you are allowed to use

 

[xjwmx]

Riding style may cause a symptom, but unfortunately changing the riding style won't fix the problem.

If riding style is the problem, fixing that will fix the problem

 

[vagabond]

N8WAVES, have you managed to identify and fix the problem?

I recently took at 1979 Yamaha XS650 Special II out on the road for its first long distance test after restoration and, after about 175 miles, it started doing exactly what you describe happened to yours. At 50mph on the highway, it started surging/bucking, acceleration topped out at 50 even if I opened up the throttle all the way, it quickly overheated. I gave it a 20 min rest, thinking I had simply driven it too hard (max speed 65mph...so that shouldn't be the case), then got it back out on the road. Within 5 min it was doing the same thing and becoming dangerous to ride in crazy CA traffic, but it made it home. The next morning, it was doing the same thing cold, except it was nearly impossible to release the clutch into first and dying at idle. The left carb was giving me more problems than the right. Adjusting the air/fuel mixture richer helped a tiny bit but not really. By later that day, the bike was no longer roadworthy. Having just bought the bike, I was pretty devastated.

Talking to a friend of mine who restored the bike and knows it well, he immediately suggested making sure the pilot jet is clear. The pilot jet as I understand is responsible for 0-25% throttle. So cleaned out the carbs thoroughly with carb cleaner, including the float valve assembly, the tube where the float valve seats, main jet, needle jet, throttle valve (avoid carb cleaner on the diaphragm...which I learned afterwards), basically every hole where fuel passes through the carb. Do each carb separately and don't swap parts between them, especially the float valve and diaphragm/needle/throttle valve assembly. I recommend not pushing anything through the jets and holes, as the metal is sensitive and you could make the hole wider and enrich the fuel mixture. When the pilots are clean, if you hold them up to the light you should get a round light coming from the other side. These need to be really clean inside, so shoot plenty of carb cleaner through them. When reseating them, screw them in lightly, only until they seat comfortably. Any more could damage them.

That being my first time taking a carburetor apart, I swapped parts shamelessly and missed the subtleties that make carbs work. After 3 days, I noticed that the BS 38 floats were uneven--one was at least 4-5mm above the other, so I took forum member xsjohn's advice and measured 27mm from the carb body, on the very inside of the ridge on which the gasket seats, making sure the top of each float, without pressing down on the float valve, sat exactly at 27mm. BS 34 floats will have different specifications, not sure what they are but you can find it on the forum. If one of these is too low, apparently this can flood the bowl with fuel enough to prevent the cylinder from firing. When you put the diaphragm back on, make sure the underlip along the edge of it seats perfectly into the groove on top of the carb body on which the diaphragm cover seats.

Test to make sure the pressure in each diaphragm is sufficient and uniform to assure there are no holes or what not by removing the air filter (if the carbs are on the bike), pushing the piston all the way up and covering the oblong hole just above the air outtake. A functioning piston/diaphragm should take at least 30 seconds to fall completely. Mine took about 18 seconds, which proved sufficient.

Also be sure to sync the butterfly valves. A quick on-the-fly way of doing it that's 95% accurate is to take strips of business card, put them under each butterfly valve, and pull them out slowly, making sure they both offer equal resistance. If not, adjust them with the screw between them.

If you haven't yet checked the spark plugs, take them out in the wire that connects to the outer end, ground the firing end against the engine, and press the starter. You should get a strong blue spark. Examine the ends of the plug. A dark greasy soot means it's been running too rich; dry black soot too lean; cappuccino brown just about right.

I also drained the fuel, took off the tank, cleaned out the inside with carb cleaner, then let it dry for a while to make sure the B12 was dry.

I then disassembled the petcock and tap valve assembly, cleared it of sediment (there was quite a bit in there from the tank), put it all back together, and checked the fuel lines through the fuel filter and entire line all the way to the carb to make sure they are both delivering fuel.

Before staring the bike up, make sure the battery's charged. Just in case, you need a slow charge charger 2AMPs or less for at least 3-4 hours for a partial charge, 6-8 for a full charge. This gave me issues with the starter until it was charged.

I put the carbs back on, make sure there was enough oil in the bike, adjusted the air/fuel mixture to 2 1/4 turns to start out with, and after 3 days, with the heavens opening to singing angels, the bike not only started but idled smoothly and revved smoothly past 2000-3000RPM again, but when I took it out on the road and adjusted the air/fuel mixture on one side to take care of the popping and hesitation, it was working perfectly. My first huge success in motorcycle maintenance.

Hope this helps!

 

[weekendrider]

vagabond,
Welcome to the site. Nice write up. With your attitude this isn't the only success you will have fixing these.