'80 XS650 falls on its face in 4th/5th gear

Hey guys, Im at my wits end with this bike a little background on myself since Im new....had an 81 xs650 in high school that I tore down to the frame and rebuilt, and rode for 4-5 years before moving on to sports bikes...currently ride a Hayabusa.

A friend of mine has this '80 650 that has been sitting for awhile....purchased new by her dad, and only has 12000 miles on it. It hadnt run in awhile, so we rebuilt the carbs and put a new battery in it and everything seemed great. Sat for awhile again, and started having issues. It needed a new chain/sprockets anyways, so I said Id do it all and get the bike back to her after. Replaced the chain/sprockets, throttle cable, cleaned up the carbs, and adjusted the clutch cable. Now the bike feels sluggish down low (I think, Its been awhile since I rode one and Im used to the power of the Busa), and once you get up into 4th/5th it wont get above 3k RPMs and tops out at 50mph.

Ive since pulled the carbs back apart, inspected and cleaned everything, made sure the slides were clean and no holes in the diaphragms, adjusted the floats to spec per the carb guide on here, made sure they were adjusted (sync'd), replaced the spark plugs, HT leads, and coil, air filters, etc. Ive got the mixture screws out 3 turns from bottom on both sides......

I know this is long, but please bear with me....one cylinder appears to be running warmer than the other (you can physical feel a temperature difference in the exhaust gas)...that spark plug looks normal. The left side the plug almost looks wet when you pull it. That being said, the bike idles great, and once you get moving it revs fine until you get into 4th, where it seems to fall on its face. I dont know what else to try and check; it was running fine 8 months ago so it has to be something I caused. The only things I havent tried yet is adjusting the valves, or checking the cam chain...there is no tapping from the valves, and the cam chain isnt making any noise either (I remember from my 81 that it is very loud when its out of adjustment). Any thoughts on why it wont go over 50mph? Or why it tops out at 3k RPM in 4th/5th? It just stops pulling....

Replace the stock fuse panel with automotive blade type fuse holders. The original fuse panel is worn out and can cause electrical problems.

Low voltage will cause poor engine running. Your new battery may not be receiving a full charge from the charging system. Measure the battery voltage at 3000 rpm..................if all is well you need to see 14 volts.

Welcome to the forum, Bluestreak06. Three other things you could check:

Put a timing light on it and confirm getting proper advance.
Check for fuel flow restriction.
Check for exhaust restriction.
(We had one in the shop that drove the mechanic nuts. Finally found some plugs in the exhaust, from those tree-boring wasps)

retiredgentleman said:

Replace the stock fuse panel with automotive blade type fuse holders. The original fuse panel is worn out and can cause electrical problems.

Low voltage will cause poor engine running. Your new battery may not be receiving a full charge from the charging system. Measure the battery voltage at 3000 rpm..................if all is well you need to see 14 volts.

Click to expand...

I will look into that. The battery did go dead the last time we were out riding (brand new battery), but havent had it happen again since Ive had it. I know its holding a charge, and it jumps up when you start (i think it was around 13.2 at idle), and up to 13.4/.5 when reved up. I will do a more indepth test later on tonight to make sure.

TwoManyXS1Bs said:

Welcome to the forum, Bluestreak06. Three other things you could check:

Put a timing light on it and confirm getting proper advance.
Check for fuel flow restriction.
Check for exhaust restriction.
(We had one in the shop that drove the mechanic nuts. Finally found some plugs in the exhaust, from those tree-boring wasps)

Click to expand...

Petcock filter is clean, flow through the petcock is consistent, and its deff. getting to the bowls. Ill have to pick up a timing light before I can check on that. As for the exhaust, that will be something to look into. Thanks for the ideas guys, just want to get to the bottom of it and get the bike back to her

Just went out and checked on the battery voltage. I havent had it on the battery tender for about two weeks now, and Ive just been running around town on it. with the bike off its showing 12.5, and showing about the same on idle. When I pull the bike up to 3k RPM's, it only gets up to about 12.8V max. Starting to sound like this could be an eletrical issue.......

Bluestreak06 said:

Just went out and checked on the battery voltage. I havent had it on the battery tender for about two weeks now, and Ive just been running around town on it. with the bike off its showing 12.5, and showing about the same on idle. When I pull the bike up to 3k RPM's, it only gets up to about 12.8V max. Starting to sound like this could be an eletrical issue.......

Click to expand...

Remove both brushes and measure. Replace if they are 3/8" or less.

While brushes are out, measure rotor resistance from one slip ring to the other slip ring. Should be around 5 to 5.5 ohms.

The regulator may have failed.

retiredgentleman said:

Remove both brushes and measure. Replace if they are 3/8" or less.

While brushes are out, measure rotor resistance from one slip ring to the other slip ring. Should be around 5 to 5.5 ohms.

The regulator may have failed.

Click to expand...

Outside brush was 9.6mm, inner brush was 9.72mm. Rotor resistance was a little tougher to get (hard to get consistent reading), but saw 5.1 ohms for a short period of time. Other then that it was bouncing between there and about 3 ohms. Im going to order some brushes and get those replaced since thats a lot cheaper then the rotor.

Will update once I get them in....but still looking for any other suggestions on why one cylinder seems to be running cooler then the other side.....

The rotor has to be at5 to 5.5 ohm's. No less, no more. These stock exhaust system's are double walled pipe, and are known for the inner pipe collapsing closed. That could explain your different exhaust temp's, even as your testing them. Easiest way to find out is to remove the system, disconnect the muffler's and roll a quarter sized ball through them. While the exhaust is off, a very short run around the block should let the engine breath and have more power. The regulator's are a known problem, along with the rotor's.

Up in the XS650 TECH section you will find a "Charging System Guide" It explains how to trouble shoot your charging system. Just follow the steps.
All you need is a multimeter.
Leo

Update now that Im back from the rig.....got the new brushes in with no change. Decided to check the rotor again and couldnt get it to give me a reading over 1 ohm. Needless, ordered a new rotor for it. Going to pull the exhaust as well this week while its down and make sure there are no obstructions.....

Another update......got the new rotor installed, and the battery at least charges now. Can only get it to go up to 13.5v at 3000 RPM. Going to cut the fuse panel out and wire in blade style fuses and see what happens. Just want this to be over haha

You may want to replace the 2 tail light bulbs with an LED tail light. Those two #1157 bulbs draw a lot of current compared to LEDs.

Ill look into it. I cut out the old fuses and replaced them with blade ones. Fire the bike up and was able to get the voltage to climb to 13.8v at 3000 RPM, but it took it 30 seconds to get there. Ill take it for a spin tomorrow and see if this has helped the issue of no power in 4th/5th gear....if not, Im really stumped after everything Ive changed.

It could be that your battery is not up to full charge and thats why your voltage only rise's up to 13.5 volt's. Even with a fully charged battery 14.2-14.5 is close to the normal area, but anything above 13 volts is usually good. Eventually your battery will get a full charge if you run it long enough. I, myself, use a battery tender for all my machine's. No question about a fully charged battery then.

BTW, the stock charging system only put's out a maximum of 16 amp's, and alot of that is used to run the bike, charging system, light's, ect, so R.G's suggestion is very helpfull as it give's a couple more available amp's for battery charging.

It was new a few months ago, and has been on the battery tender quite a bit over the last month. Battery seems to be holding charge(12.5-12.7V by itself). I guess to test it I can pull the battery from my Hayabusa and drop it in the bike since I know for sure it is good.