[Weston Jezek]

Hey Y'all,

I just bought a 1973 Honda CB450. It's been running fine until a couple days ago.

What happens is I'll be cruising along in 3rd or 4th gear at <30mph and suddenly lose about half my power. When I slow to a stop and put in the clutch the engine dies, but when I release the clutch the engine will sputter back to life and eventually die again as I come to a rest.

I put in a new battery a few days ago, but it already seems dead.

Any thoughts on troubleshooting this issue?

Thank you!

 

[Dodsfall]

Charge the battery at .75 to 2 amps until it tests full. Load test the battery and check the voltage. It should read somewhere around 12.8 volts or higher.

Install the battery, making sure the connection is clean and tight. Test the voltage at the battery terminals with the motorcycle running at about 3000 RPMs. You should see something in the neighborhood of 14-15 volts on your meter. If not, your charging system has a fault.

 

[Weston Jezek]

What equipment do I need to buy to do this?

 

[Dodsfall]

A battery tender, a multimeter, and a motorcycle battery tester.

 

[Weston Jezek]

Dodsfall: Thank you very much for your help! Please excuse my inexperience here, I'm trying to save $$...What is the difference between a battery load tester and a Multimeter? Can the multimeter serve both purposes?

 

[Dodsfall]

The multimeter will test for voltage and continuity. A load test will test for battery strength. It is possible to have a battery that shows full voltage, but is actually faulty when a load is put on it.

Edit: Many auto parts stores will load test a battery for free.

 

[joewasaki73]

since the battery is new, you can get by without a battery load tester (the seller will usually load test it for free); but a multimeter is a must have. i prefer the ones that have a beeping continuity tester.

 

[FlipFlop]

Owning this bike it is a good idea to have real tools to work with.

A good Multimeter, a metric set of sockets both deep and regular with extensions, metric wrenches, Sparkplug gap tool, chain cleaner, and chain lube, and the manual for that bike.

The factory manuals usually have a trouble shooting check list in there, always IMO follow that first then start thinking outside the box. The process Dodsfall gave you is almost verbatim for how to diagnose and trouble shoot a battery/charge issue on almost any motor vehicle or motorcycle.

https://drive.google.com/file/d/0B_ZumJE59-T4YjUwNjllMjgtZDNlYi00ZTcxLTgyNDEtZmIxOTlmM2ZlNTZl/view?ddrp=1&hl=en# <---downloadable PDF manual

 

[cmonSTART]

Completely agree! That bike is now 45 years old, so even in good shape chances are you're going to need some tools every now and then. A multimeter is one of those tools that chances are you'll use often once you own one.

Great bike - I owned a couple of those!

 

[Weston Jezek]

wow, thank you for the support everybody.

I'm going to follow these steps and follow up after I know whether or not the charging system has a fault.

Can I parallel path with any other recommendations on what the power loss may be? More specifically, It seems like I am losing power on one cylinder before the other dies.

 

[cmonSTART]

Always a good idea to go through the points and tune them up - they can be the source of a lot of issues. Also check that the coils, wires, and plug caps are all in good working order and not old and cracked. You can test the primary and secondary windings in the coil with your new multimeter - the manual should have the procedure. You can also spray a bit of water on the plug wires and caps - if the bike begins to miss they're bad.

If the battery is dead though that's likely a big issue.

 

[WintrSol]

Hey Y'all,

I just bought a 1973 Honda CB450. It's been running fine until a couple days ago.

What happens is I'll be cruising along in 3rd or 4th gear at <30mph and suddenly lose about half my power. When I slow to a stop and put in the clutch the engine dies, but when I release the clutch the engine will sputter back to life and eventually die again as I come to a rest.

I put in a new battery a few days ago, but it already seems dead.

Any thoughts on troubleshooting this issue?

Thank you!

'lose about half my power' - engine power, or lights going dim?

Without a Voltmeter, unless your lights are dimming, you can't tell if the battery is discharged. It could be a blocked air vent in the gas cap, causing the fuel to stop flowing. If the battery load tests OK, but it is an electrical problem, it could just be the fuse; it is a Japanese standard, which is a slightly different size than what you normally find in the US. A US fuse will be loose in the holder, creating a high resistance which blocks the power as it heats. I replaced my fuse holder with one that holds the small blade type, so that never happens again.

 

[slumlord]

It also can be other weird things like corroded connectors. Since it is a Honda , check how tight and good the main connections,[heavy wires] going from the multiple pin connectors are. In other words, from the battery to the switch to the fuse panel and then on to each individual fuse. While you have them apart, look for green corrosion. I had one pin from the fuse panel feeding from the ignition in horrible shape on a 1974 CB360. That bike would cut in and out as power came and went. Once I opened that white connector, the problem was obvious

Just today, I replaced the contact area on the ignition switch on the CB750. A night and day difference on performance. Emglo makes aftermarket contact blocks for Honda switches because they are no longer available from the manufacturer. You keep the old lock cylinder and key and change out the working parts of the switch for about ten bucks.This bike has been acting up for a year, and I was looking in the wrong place.

There is a sticky thread on this site about voltage drop. That thread has helped me to find and repair these kinds of wiring problems.

 

[FlipFlop]

If the battery is dead though that's likely a big issue.

Yeppers bike batteries don't like the big discharge and likely hurt its life in the end so put a new battery on your shopping list once you get the issue sorted.

 

[cmonSTART]

There exists something called "threshold voltage." That is the minimum voltage an ignition system requires to operate and produce a spark. It's different from bike to bike depending on the specifics of the vehicle - I can tell you that my Volkswagen's threshold voltage is 8.6 volts though. When a bike's charging system doesn't work, the battery discharges to keep things operational until it reaches the threshold voltage, where it can no longer produce a spark. If one coil, wire, or plug is "weaker" than the other it will generally show up here first as the voltage drains down.

So, while you may be able to ride for short periods with your charging system inoperative it will eventually put you on the side of the road.

 

[Weston Jezek]

'lose about half my power' - engine power, or lights going dim?

Without a Voltmeter, unless your lights are dimming, you can't tell if the battery is discharged. It could be a blocked air vent in the gas cap, causing the fuel to stop flowing. If the battery load tests OK, but it is an electrical problem, it could just be the fuse; it is a Japanese standard, which is a slightly different size than what you normally find in the US. A US fuse will be loose in the holder, creating a high resistance which blocks the power as it heats. I replaced my fuse holder with one that holds the small blade type, so that never happens again.

engine power!

 

[WintrSol]

engine power!

So, not electrical, unless the switches between the battery and coils are in need of cleaning; those switches are the key switch (not likely, but possible) and the run/stop switch.
More likely from the symptoms described is lack of fuel. Fuel line routing from the fuel valve to the carbs, clogging filter screen/bowl, or blocked cap vent are most likely here. Next time it starts to slow down, pop the fuel cap; if it picks back up soon, you have your answer. BTW, there is a proper way to route the fuel lines, too, and you could be starving one side if it's not good. A sticking float could also be involved.

 

[Weston Jezek]

So, not electrical, unless the switches between the battery and coils are in need of cleaning; those switches are the key switch (not likely, but possible) and the run/stop switch.
More likely from the symptoms described is lack of fuel. Fuel line routing from the fuel valve to the carbs, clogging filter screen/bowl, or blocked cap vent are most likely here. Next time it starts to slow down, pop the fuel cap; if it picks back up soon, you have your answer. BTW, there is a proper way to route the fuel lines, too, and you could be starving one side if it's not good. A sticking float could also be involved.

Could this also be killing the battery? Because the lights have been getting dimmer despite new battery.

 

[WintrSol]

Could this also be killing the battery? Because the lights have been getting dimmer despite new battery.

When you responded 'engine power', I thought you meant the lights were not dimming. Different answer. If you still have the stock selenium rectifier, which is the orange square with many fins, that could be the problem, as selenium isn't great to begin, and fails with time. Next is the stator connections at the back of the transmission, which can be corroded; in the older models, part of it was switched on with the headlamp, but you may not have a headlamp off switch (forgot which model this changed). If you have the headlamp off/low/high switch, the wiring for the stator goes through the headlamp bucket, where the switch is easily bypassed, if you install a modern rectifier/regulator. Next comes the regulator, good for its day, but there are many better now.

The heavy battery negative cable connects to the frame and engine at the back of the transmission, and this area probably needs a good cleaning; mine dropped the battery cranking speed to little more than 1/2 normal, until I cleaned that contact, and the frame parts under it.

 

[Weston Jezek]

Hey Guys - Quick update.

So I took out the petcock and the filter was super dirty. I de-rusted the tank and filled up with new gas over the weekend. Just took the bike for a spin and realized that the engine power loss issue is only happening at higher RPMs / Higher Speed. When I cruise the neighborhood at <30 I'm good. But when I go over 40 the engine just gets bogged down and sputters in and out of full power and dies as I come to a start. Maybe I need to clean the carbs? If the petcock was filthy, likely the carbs are, right? But It's definitely happening more at higher speeds or when I get on the throttle.

Also, I slapped the battery on the tender for a good 18 hours over the weekend and it only read maybe 9.5 volts at the most (12V Lead Acid) and averaged at around 8.5. The new automatic tender told me it was fully charged though... Just load tested it at 3000rpm and it read between 6V and 9V ish...maybe I'm not using the multimeter correctly or something? Added a pic for reference.

Seriously appreciate your help, thanks for helping me learn.