Topic: 1973 CB500 Runs and then dies after 10 min of riding, I searched

[KCRSXTypeS]

Hey guys, my first post here.  I got my 500 from my grandpa, it was his first motorcycle back in the 70s and he gave it to me for free after not riding it for 25 years.  Now I finally got it back up and running, rode it around the block and the bike dies after 10 min.  Had the carbs professionally cleaned and rebuilt and bench synched (then we synched the carbs ourselves though we MIGHT have screwed something up).  It may be overheating but I don't know how to tell.  I searched and other people said that it might be a blocked vent hole in the gas cap but I tried running it with the tank open and still have the problem.  Im starting to think it might be the petcock.  I just bought the petcock brand new but no matter what setting the petcock is on (on, off, reserve) it still feeds fuel.  We also had a sticking float bowl that we just tapped with a wrench and it stopped spewing fuel all over the transmission.  The spark plugs look new, the condenser and points look new (installed new 25 years ago).  Anything else anyone can think to check?  The bike is great, pulls hard (has a hesitation below 2.5K that I need to figure out), and I would really hate to get rid of it just because of something simple.  

[TwoTired]

Air filter clean as new?
Valves adjusted too tight?

[KCRSXTypeS]

Air filter clean as new?
Valves adjusted too tight?

Unsure, it is still the airfilter from the 80s lol.  I mean it is clean, nothing blocking it.

[KCRSXTypeS]

We havent checked the valves yet.  A little scared to get into that and screw it up even more.  The repair manual we have SUCKS @$$!  Its the old Clymer's manual.

[TwoTired]

Take the filter out for a test to see if it is the problem cause.  Honda says those paper filters should be changed every 6 months.  I think the paper fibers collapse over time and humdity, etc.  This make them more restrictive it behaves like adding choke.  I usually switch mine to the Foam Uni Nu-4055.  Its reusable and they last for 15 years-ish.

The carb overflow drains are supposed to have rubber tubes to route the over flow under the bike rather than on it.

Yes, clymers have limited usefulness regardless of age.
You can download the Honda shop manual from a link on this forum.

[KCRSXTypeS]

Take the filter out for a test to see if it is the problem cause.  Honda says those paper filters should be changed every 6 months.  I think the paper fibers collapse over time and humdity, etc.  This make them more restrictive it behaves like adding choke.  I usually switch mine to the Foam Uni Nu-4055.  Its reusable and they last for 15 years-ish.

The carb overflow drains are supposed to have rubber tubes to route the over flow under the bike rather than on it.

Yes, clymers have limited usefulness regardless of age.
You can download the Honda shop manual from a link on this forum.

They do have the tubes on them now...at first they didn't though .  I may try the airfilter but why would it run fine for a few min then die because of the airfilter.  I would think it would run crappy all the time.

FYI, I don't even need choke to start the bike.  I just blip the start button and she fires right up!

[technojock]

I can think of a couple things to check. 

Sometimes the handlebar kill switch will corrode up and have a poor connection.  This would get worse after it heats up.  While you're at it clean all the connections and pack them with dielectric grease.  A weak connection will get worse as it heats up. 

You could also have a bad coil or spark plug cap.  Check all the plug caps for resistance and run the bike off a drip tank if you can to see if one of the coils gets hotter than the other one.

Also pop off the fuel lines, install a couple of short tubes and run them into a quart glass jar.  Turn on the gas and it should be able to fill the jar 3/4 up with a smooth fuel flow.  Any hesitation is bad news.

I second checking the air filter.  The could very easily be a mouse nest in there.

Tony

[TwoTired]

I may try the airfilter but why would it run fine for a few min then die because of the airfilter.  I would think it would run crappy all the time.

FYI, I don't even need choke to start the bike.  I just blip the start button and she fires right up!

Carburettors don't compensate for temperature, or altitude differences.  A cold engine metal doesn't atomize the fuel well and richer mixtures are required to start it up.  That is why the choke is there.

That you can start it cold says it is already rich, possibly due to a restrictive air filter.  As the engine warms, the fuel atomizes much better, making all available hydrocarbons pair with the oxygen.  However, too much fuel won't light off if there isn't enough oxygen.  So, it can stop running with higher normal temps as the air filter can cause too much fuel draw from the fuel jets in the carburetor.  There is a narrow range of air/fuel mixture ratios that will burn and if the ratio goes outside that range, no combustion will occur.

Even if you have new spark plugs, an over rich condition should place black fluffy deposits on the electrodes.  The black deposits are conductive, and can short the plug so that the spark no longer needs to jump the gap.  This will make ignition difficult, or not occur at all.  Examine the center electrode insulator.  If the insulator is coated with conductive carbon, the purpose of the insulator is defeated.

Pull your new plugs and look for these deposits.  Remove your air filter and run the bike, to see if the condition improves.  If so, get a new air filter.  Be aware, that having no air filter can cause a too-lean condition, and can make throttle response worse.

The carbs are jetted for the air restriction of a new, clean air filter.

Cheers,

[KCRSXTypeS]

Carburettors don't compensate for temperature, or altitude differences.  A cold engine metal doesn't atomize the fuel well and richer mixtures are required to start it up.  That is why the choke is there.

That you can start it cold says it is already rich, possibly due to a restrictive air filter.  As the engine warms, the fuel atomizes much better, making all available hydrocarbons pair with the oxygen.  However, too much fuel won't light off if there isn't enough oxygen.  So, it can stop running with higher normal temps as the air filter can cause too much fuel draw from the fuel jets in the carburetor.  There is a narrow range of air/fuel mixture ratios that will burn and if the ratio goes outside that range, no combustion will occur.

Even if you have new spark plugs, an over rich condition should place black fluffy deposits on the electrodes.  The black deposits are conductive, and can short the plug so that the spark no longer needs to jump the gap.  This will make ignition difficult, or not occur at all.  Examine the center electrode insulator.  If the insulator is coated with conductive carbon, the purpose of the insulator is defeated.

Pull your new plugs and look for these deposits.  Remove your air filter and run the bike, to see if the condition improves.  If so, get a new air filter.  Be aware, that having no air filter can cause a too-lean condition, and can make throttle response worse.

The carbs are jetted for the air restriction of a new, clean air filter.

Cheers,

Well, its not the air filter.  I ran it without the airfilter and same problem only this time it happened more quickly.  I am quickly coming to the conclusion that its a bad petcock.  I pulled the petcock bowl off before i tried the airfilter thing and the fuel was clean and the screen was clean.  While the bowl was off the petcock was just continuously dripping fuel as if it was leaking.  My thought is that I have a bad petcock.  No matter which setting the petcock is on it leaks fuel out of the nipples.  I think that in reality it is stuck in a "leaky" state and that it isn't providing enough fuel to keep the float bowls full.  The bike dies after it starves for fuel then when the float bowls are full again it is fine.  When I pulled the petcock bowl and emptied it I didn't wait for it to refill completely before riding again which is why I think it died more quickly this time.  Also, once it started dieing I opened the choke a little bit and it ran A LOT better for a few seconds and then died anyways which still leads me to believe that it is a fuel problem.  If it is indeed a fuel rich problem then how do I go about leaning out the mix?

also, I had thought it could be a bad ignition coil as well but I felt them and they were barely warm to the touch.  It could still be the culprit but I don't know.  Where can I source new coils from?  I think I would like to replace them anyways.

edit: BTW...this website is a FREAKIN GODSEND!  I have been searching high and low for some answers to some of my problems and forums are always the best place to find help from a common collective knowledge.  You guys are great and you can expect that I will be on here A LOT!  Anyone on here from the Kansas City area?

[KCRSXTypeS]

bump!

[TwoTired]

The fuel selector is a fairly simple device.  Leaks can occur when the 4 hole gasket hardens.  But, the drip you have is probably the two fiber washers failing (or missing) on the two screws that hold the valve to the tank body.

As for fuel flow, test it!
 Time how fast it fills a container and note the volume.
These bikes, at worst, get about 30 MPG at 60 MPH.   At that speed it takes 30 min to travel 30 miles, using 1 gallon of fuel.  15 minutes would use 1/2 gallon, 7.5 minutes would use one quart.  3.6 minutes would use 1 pint.  1.8 minutes would need 1 cup of fuel.
So, time how fast the fuel comes out of the tap to fill one cup.  If it takes less than 2 minutes, the fuel tap isn't the problem.


I still think you should replace the old filter.  And, I still think you should look at spark plug deposits to determine lean or rich combustion conditions. 
Also, when it quits, check the head pipe temps.  Do all cylinders fail in unison? Or, do some stop before others?  Ones that feel colder can help point to fuel issues at the carbs.

Still haven't checked the valve clearances, I suppose.   

[KCRSXTypeS]

The fuel selector is a fairly simple device.  Leaks can occur when the 4 hole gasket hardens.  But, the drip you have is probably the two fiber washers failing (or missing) on the two screws that hold the valve to the tank body.

As for fuel flow, test it!
 Time how fast it fills a container and note the volume.
These bikes, at worst, get about 30 MPG at 60 MPH.   At that speed it takes 30 min to travel 30 miles, using 1 gallon of fuel.  15 minutes would use 1/2 gallon, 7.5 minutes would use one quart.  3.6 minutes would use 1 pint.  1.8 minutes would need 1 cup of fuel.
So, time how fast the fuel comes out of the tap to fill one cup.  If it takes less than 2 minutes, the fuel tap isn't the problem.


I still think you should replace the old filter.  And, I still think you should look at spark plug deposits to determine lean or rich combustion conditions. 
Also, when it quits, check the head pipe temps.  Do all cylinders fail in unison? Or, do some stop before others?  Ones that feel colder can help point to fuel issues at the carbs.

Still haven't checked the valve clearances, I suppose.   

lol, how am I supposed to check the head pipe temps?  With my hand? haha

[TwoTired]

lol, how am I supposed to check the head pipe temps?  With my hand? haha

Yup. That's how I do it.  The "flash touch" method.  You can pass your hand through open flame if you do it quickly.  Your hand will still feel warmer after the fact.

Start with a very very short touch and release.  Then repeat until you get a sense of the time you can hold on during the flash and how hot your fingers feel.  You fingers are mostly off the pipe.  Always assume the next pipe is way hotter and you can't hold it on nearly as long, until you discover the flash touch frequency, and sense of heat it imparts.

If it is glowing red, don't touch it at all.

I've heard others spit on the pipes and watch for equal sizzle.  But, these are mostly panzies.

[Alan F.]

 

+1

Or you could use an infrared thermometer to get spot on readings at the second it quits... for comparison later.

[KCRSXTypeS]

+1

Or you could use an infrared thermometer to get spot on readings at the second it quits... for comparison later.

If it turns out to be a rich or lean condition, how do I change the air/fuel mix?  Carburetors are fairly new to me (although i understand the basic workings and parts).  I am used to just plugging in my laptop to my vehicle to change the air/fuel mix or change timing.

[scunny]

I spit on my finger then touch the pipes, hopefully that makes me half a pansy.
I doubt it's a rich or lean issue, petcocks got my vote

[fishhead]

Touch the pipes with a clean shop rag in your hand and you wont burn your self. This might be the way panzies do it, but at least you'll be a panzy with out blistered hands. 

    I had a KZ 900 in the shop that the owner put brand new fuel lines on and it would run fantastic in the shop and for about 3 miles on the road. More than 15-ish minutes of riding, it would start to run like crap.
 It turns out that the new fuel line he had installed (bought new at an auto parts store) had a dead bug in it and would keep the carbs from getting gas on more than 15-ish minutes of riding.

[HondaMan]

This sounds like bad condensors.

These gadgets, if not used, will dry out inside (they were paper and wax inside 25 years ago) and develop low-resistance shorts inside between the plates. They will run until the can of the condensor heats up from the short, then it shorts enough to leak the current past the (open) points. Then the sparks stop.

Typical of cars with points, too, that have condensors older than 5 years old.

Honda gets $28 apiece for these: get some aftermarket ones for about $3, they will be fine.

[KCRSXTypeS]

This sounds like bad condensors.

These gadgets, if not used, will dry out inside (they were paper and wax inside 25 years ago) and develop low-resistance shorts inside between the plates. They will run until the can of the condensor heats up from the short, then it shorts enough to leak the current past the (open) points. Then the sparks stop.

Typical of cars with points, too, that have condensors older than 5 years old.

Honda gets $28 apiece for these: get some aftermarket ones for about $3, they will be fine.

Hmmm...this sounds like a plan.  I will go ahead and check the spark plugs (probably should just replace them since they are at least 25 y/o) and adjust the fuel accordingly (although I have no black smoke for rich and no blue smoke for lean), will get a new petcock, will replace the condenser, and will check the valve lashing.  I'll do all of that in order so that if I get the problem solved we will know which solved it.  My vote is still for the petcock too though but the condensers sound feasible too.

[TwoTired]

Though certainly possible.  The probability of BOTH condensers going bad simultaneously would seem small.

For carb tuning, you can still attach your laptop to the bike.  Tap into the grey wire and enable the comm by selecting the bar control switch L three times and the R twice.  This will open a dialog in Intercal and give you access to adjustment parameters.  You will also need a Kanji interpreter/translator.

Refer to the following chart to address specific tuning aspects:

[KCRSXTypeS]

Though certainly possible.  The probability of BOTH condensers going bad simultaneously would seem small.

For carb tuning, you can still attach your laptop to the bike.  Tap into the grey wire and enable the comm by selecting the bar control switch L three times and the R twice.  This will open a dialog in Intercal and give you access to adjustment parameters.  You will also need a Kanji interpreter/translator.

Refer to the following chart to address specific tuning aspects:

Ok...that just slipped past me like a vasoline covered ninja...

[Spanner 1]

+ 1 with TT on low fuel delivery to carbs, 10 mins. is the critical run time to where the float bowls can't fill fast enough to catch-up with the demand.( in this case )...my bike goes a full 5 mins. with the gas OFF !! Bunged-up petcock, full of rust......time to clean the tank or have the prob. re-occur again and again...IMO.
Also, rust in petcock rubber gasket will cause it to drip into fuel line even when OFF....mine sure did !

[Achmed]

Yeah it could be a restriction inside the tank. Check or replace the fuel lines, too. The picture TwoTired posted lets you know which parts of the carb are metering fuel at various throttle positions. You need to know what's inside the carbs in order to read the chart.

[Alan F.]

rust in petcock rubber gasket will cause it to drip into fuel line even when OFF....mine sure did !

This is the rubber washer with 4 holes, right? 
I believe we have a winner for the petcock malfunction.

Though certainly possible.  The probability of BOTH condensers going bad simultaneously would seem small.

It would, but condensors just don't age well, it's one of the reasons that particular wheel was reinvented.

[TwoTired]

For carb tuning, you can still attach your laptop to the bike.  Tap into the grey wire and enable the comm by selecting the bar control switch L three times and the R twice.  This will open a dialog in Intercal and give you access to adjustment parameters.  You will also need a Kanji interpreter/translator.

Ok...that just slipped past me like a vasoline covered ninja...

This was a geek joke for the computer nerds who use laptops on their vehicles...   
But, don't worry, the vasoline will wash right off.

The chart is no joke, but you'll have to crack open the shop manual (and the carbs) to change the fuel metering circuits.  This bike doesn't need no stinkin' computer, just a mech that's smarter than the machine.

Cheers,