Topic: 1971 CB500 Running Issues

[Montgomery]

Hi There-

Let me preface this by introducing myself and my bike. I am a broke out of my mind student at San Jose State University, and found this bike for $300 a few months back, currently registered and running, on Craigslist. I've been riding dirtbikes since I was a child, but have never ridden a streetbike until this venture. I realize that you get what you pay for, and am not looking for any miracles. With that in mind. . .

The bike has tons of cosmetic issues and leaks a fair amount of oil, but usually runs decently. It was a daily rider until about 2 years ago. As of late, however, it has been giving me a decent amount of issues. Yesterday I rode the bike back from a friends house 10 miles away. It was probably 85-90 degrees outside. The bike never idles consistently, but thats something I can live with from a $300 motorcycle.

About 9 miles in it starts losing power under acceleration, so I pull off the road and it splutters to a stop. Check the fuel level, switch to reserve even though it doesn't need it, and give it a few cranks with the starter until it dies (probably 5, 3 seconds cranks, which is how long the battery normally lasts) and proceed to go at it with the kick starter. Kick and kick and kick for forever without getting it to start, although it did try a few times. after 15 minutes or so it finally it springs to life, so I let it idle, put my jacket and helmet back on, and take off. Bike runs fine for another 3/4 of a mile, and then dies as I'm pulling out at a green light. I then repeat the same process as before, but only get 1 or 2 attempts with the starter. After about 15 minutes, the bike starts, runs poorly, but enough to go two blocks, and dies again. No starter at all. At this point I'm in the shade, so I push it home.

After getting home I go to double check the oil level, and the cap is so hot it burns the crap out of my fingers as soon as I touch it. After it cools down I checked again, and it was still at the full line.

Where should I start tinkering? With a dirtbike background I'm not used to bikes as complex as this.

Is that amount of heat normal? If no, is it running lean?
Could the battery be so old and shot that the ignition is just too weak?
Time to clean the carbs?

Bike still has the stock exhaust (one rusted through) and the stock airbox has been replaced with 4 (Ok, 3. One is missing) individual cone shaped air cleaners.


If anybody has some thoughts on where to start for a broke student, I'd love to hear them. Again, not expecting miracles, just some budget advice.

[NCSUEngineer]

sounds like a charging issue.  Same #$%* happened to me when my field coil was dead.

I would start with a full battery and it would eventually die - after a little while it would start back up. It was like the battery was recharging itself very briefly.

Anyways, test your field coil - there is a white and green wire coming out of a cloth sheath under your stator cover (you dont have to remove the stator cover, just pull on the wires and they pop out of their connections) test the resistance between them to make sure its around 5 Ohms.

[1timduke]

If you're missing one of the pod filters, your carbs, therefore your cylinders, won't ever be in sync, they will fight each other, robbing power that should go to the wheels.

Pull your plugs and see the condition then tell us what they are.

Where is the oil coming from?

Get some exhaust patch and plug that pipe (especially if it's in the header pipe)

If you don't have 'em, you'll need a multimeter and some metric tools, not too many, just a few.   

Welcome to the forum.

-Tim

-Tim

[Montgomery]

I have the pod filter for cylinder #1, all I'm missing is the clamp, so I'll get that and mount it asap.

Pulled the plugs today, looks like I've got a range from too lean to too rich. That should make it fun to balance them out. Picture attached.

Washed the bike so I could more easily see where the oil is originating. There is some leaking out from the sending unit for the low oil light, as well as from the head gasket to the #1 cylinder. It really only leaks when it is running.

The pipe for cylinder 3 is. . . Probably a write off. The weld at the joint between the muffler and the pipe is totally rusted through, they're not even connected. I'm not sure if tape will help any.

I also went out and replaced the battery. The old one was 4 years old, so I figured it was due. Bike starts within 1 crank on the electric starter now, and after only few a kicks manually.

[fmctm1sw]

couple of those look oil fouled.  Valve stems seals maybe?

[TwoTired]

Has the fuel tank been lined?  Is the cap vent plugged?  If air can't get into the tank the fuel won't exit.

[Montgomery]

fmctm1sw: Yeah, one plug definitely is oil fouled. No bueno.

TwoTired: Don't think the tank has been lined. Just the typical mostly clean bare metal interior. How can you tell if the cap is plugged? I can see the small hole protruding from the cork, I guess I could go put my face on it and try to blow through.

Just got back from riding the bike through downtown San Jose, after adjusting the mixture per the directions found here: http://forums.sohc4.net/index.php?topic=5410.0. Bike seems to be running better, although a few times towards the end of the ride the power dropped off sharply and significantly, making acceleration painfully slow, even at a high RPM. It righted itself after a block or 2 each time. Checked the petcock screen when I got back, it was clear.

The plot thickens.

[TwoTired]

You need to figure out why all the plugs don't show the same style deposits.

Start with the standard tune up procedure required every 3mos/3000 miles.

Then make sure the carb couplers are sealing, vacuum sync the carbs.

If the individual pods are olied, make sure there is equal oil in each one.  Differences in restriction at that point will drive everyone crazy tring to compensate carbs to unequal inlet restriction.

Do check that each choke butterfly stays open and equal across the bank.

What do you know of carb history, rebuilt or otherwise tampered with by previous owners?

If the tank vent is the problem, opening the cap will rectify the issue while open.

I'm a bit reluctant to have you bring the bike over for diagnosis and fix issues, given you can spend no money on the bike.  Some things can't be expected to last forever and simply need replacement when necessary.

[Montgomery]

Did as much of the tuneup as I could given the resources. Tried to adjust the tappets, but couldn't get my feeler gauge crammed down in there; will have to get a different one off of ebay. I am also going to see if we have a timing light at work today (I work at an aicraft maintenance operation).

I have a friend who have owned a CB550 in the past, I'll see if he has a vacuum sync setup.

All the butterfly valves stay open: no problem there.

I don't know much of the history of the carbs, other than they are the originals to the bike. Everything on the bike is stock, other than the dents and black primer on the tank.