Motorcylce Experts Please!

[keithpip]

Don't have anywhere else to turn so I thought I would ask here. I'm restoring a Honda 1976 CB500T. Prior to any restoration I noticed that the engine had a bad skip. Testing confirmed that the right cylinder wasn't firing consistently. Since I planned on completely redoing the ignition I felt confident that the problem would eventually be solved. I suspected that the spark plug wire was bad. Both wires looked original to the bike and integral with the coils, so they were easily 34 years old. I replaced the points, condenser, new Dyna coils, new spark plug wires, new battery, new plugs, set the timing etc. You guessed it, same problem! Mysteriously the "dead" cylinder switches back and forth from the right side to the left. I'm at wits end trying to find the short. I've gone back over the new coils making sure everything is grounded the way it should. Also I don't see any problems with the wiring harness or connectors. I'm to the point of pulling out my Model 29 and putting a round thru the crankcase. Has anyone run across a similar situation? Finding electrical shorts is not my strong suit. Any help would be greatly appreciated.

Keith

 

[m657]

Just going on a fading memory here....IIRC some of those Honda twins from that era fired both cylinders at once, as the crank was not offset, so it actually acted as a single power impulse.

They ran a little rougher and eventually the factory changed the crank to 180 offset power impulse. I'm sure some of the 305 Super Hawks were like this as I had a couple and it was a small issue of the day depending on which version you had.

Not for sure about this model, but might help in your ongoing evaluation.

 

[Scrapper]

Have you made sure you do not have a tight valve? (check valve tappet clearance @ Top Dead Center). If you do not have a spec just use .003" feeler gauge. If that's ok and timing of the valves and points are good then check to make sure the carburetors (especially the right one) isn't clogged and course the tank must be clean also.

 

[JWiley]

Does it have a distributor? (gawd, it's been soooo long.) If so, is it installed correctly, i.e. not 180 out, all internals good?

From your description it sounds as though something in the internals is flaky.

 

[Beauetienne]

Have you made sure you do not have a tight valve? (check valve tappet clearance @ Top Dead Center). If you do not have a spec just use .003" feeler gauge. If that's ok and timing of the valves and points are good then check to make sure the carburetors (especially the right one) isn't clogged and course the tank must be clean also.

I agree. I would also check the valve clearances.

 

[scooter123]

First thing I would suggest you do is run a compression test on the engine. That will allow you to eliminate poor exhaust valve seal or bad rings as a factor.

Next I would check the electrical power side of the equation by checking the voltage at the battery with the bike running. If your not seeing at least 11.5 volts, start checking for a bad ground somewhere, poor alternator output, or a bad voltage regulator. My hunch is that your problem may be a weak spark and low voltage will cause this. One way of confirming a weak spark issue is to see if the problem is worse at near full throttle because as the charge pressure increases a stronger spark is required for reliable ignition. A weak spark will also become more of a problem as RPM's increase due to coil saturation.

Many years ago I had a 1974 CB450 and it had a single throw crankshaft and fired both spark plugs at once. My current bike is a 1983 V45 Sabre and it's uses a similar scheme with the front bank firing the plugs as a unit and the rear bank firing as a unit. So, with paired cylinders, it is standard practice for Honda to fire both plugs at once. An easy way to see if you have a single throw crankshaft is by putting one cylinder at TDC and then checking the piston position on both cylinder using a probe in the spark plug holes.

If the cylinders are paired, it requires a lot more power from the electrical system so it's important to make sure it's up to the task. Honda also uses a VERY basic and somewhat cheezy voltage regulator on these bikes so it's not unusual to have one get weak.

Finally, you'll also want to check the color of a newly installed set of spark plugs. What you want to see is a light tan color after a short moderate power ride. If they are black with soot, your carburation is rich, if it's dead white, your running lean. Both issues require correction although running rich isn't as harmful as running lean. Chamber temps when running lean can get high enough to melt a piston on a sustained ride, so if your running lean, get that corrected ASAP. Unfortunately, you'll probably have to fix that miss before you can get an accurate plug reading because a missing engine will almost always read rich due to the raw gas being run through the cylinder.

PS, when I restored my Sabre in 1999 I stripped the wiring harness from the frame and sandblasted every single connector. Then, when I put the bike back together I used silicone dielectric grease on every one of those connectors. While it took a bit of time to do this, it's saved me countless hours of trying to track down an electical gremlin. In a worst case scenerio, you may have to do the same thing because all it takes is one hidden corroded connector to have you contemplating dynamite.

 

[GKC]

I'm no expert, but I do hang out some on a forum where there are lots of them...try VTX Owners Association. It's a forum for Honda VTX riders, but there some of the guys have owned/ridden/repaired everything on two wheels. You have to join to post, but you can read and search as a guest.

 

[Bruce51]

I had a similar problem on a CB 350 with points. It would only miss with the point cover on going down the road. Somehow it would short out on the cover. I put a piece of black electrical tape around the inside of the cover and the problem went away.

Bruce

 

[310Pilot]

I can see many possibilities, especially since it is an older bike with unknown history. They include: 1) worn rings/pistons/valves; 2) carburetor problems, which could range from being gummed up inside (highly probable, in any case) to an improper re-jet at some time in the past; 3) carburetors undoubtedly need to be synchronized; 4) worn camshaft or cam gears/chains; 5) the little scoot just wants to annoy you so it will get more attention; 6) all of the foregoing, and many more. At a minimum, it would be wise to run a compression check (a leakdown check, if possible), check the timing, check the cam timing, check valve clearances, pull and rebuild the carburetors, then synch the carburetors. Good luck getting it running well.

 

[millbilly]

Pull out your model 29 and put a couple through the block.....be sure and video it so we can see it !!! THEN go get a Harley Davidson !!!
Just my 2 cents........Thats what I would do.....

 

[G.T. Smith]

I think all of these responses are good. I didn't read anything about the balance tube between the exhaust pipes if is so equipped. If it has one make sure it isn't smashed together or leaking, or eliminated altogether. And synchronize the carbs for sure.

 

[Marshwheeling]

You have received some excellent mechanical advice, but the best suggestion echoed your original instinct to plug the thing with your Model 29. The mainstream motorcycling press does not pan a new motorcycle very often, but the CB500T stands out as receiving one of the two worst reviews I have ever read. (In case you are wondering, the other was the 1973 Yamaha TX750.) The press hated the 500 almost as much as it loved the CB450 it derived from. Vibration, carburetion, and the hideous brown seat consigned the 500 to the motorcycle hall of shame.

This is the rare case where the Harley really might be an improvement.

 

[2krkrider]

I know for a fact that if you use a M29 on the left hand side of the CB500T engine with the carbs off, a white smoke/vapor will come out of the intake. What ever problem you have now with the bike will never bug you again.

As fun as that was, it was unsatifying as then all I had was scrap metal. Selling a bike by the pound is not rewarding.

All of the advice about what to fix is good. My bet is a compression test will show low compression. Digging in the engine will show an issue with the valves.

 

[bkp3gen]

I would either take the carbs off and clean them or go get a can of GOOD carb cleaner (BG44 or similar) and pour into the tank. It sounds like you've done all the correct electrical trouble shooting, the next least costly thing to do is clean the carbs. If you go the cleaner in the tank route, if and when the bike starts running smoothly, you can fill the tank and dilute the rest. I've done this several times with old bikes I've helped people with and about 50% of the time it works. The technique I've used is to pour it in, ride or run the bike for a while (enough to get the cleaner distributed) and then let it rest for several hours or overnight. Do the same thing the next day and it should start running smoother. If not then it's probably not the carbs. In any event, you aren't out much and since it's that old, they needed cleaning anyway,

 

[zercool]

If you're that far into a rebuild, you probably either already have or don't need a shop manual... but I think I have a copy of the Haynes or Clymers manual for the mid-70s CBs kicking around if it would help. If you can use it, drop me a PM, consider it karma.

 

[cp1969]

Clean/rebuild the carburetors. It takes very little in the way of dirt, gum, varnish, etc., to cause problems. You may be looking at replacing the diaphragms as well.