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

[HondaMan]

This bike doesn't need no stinkin' computer, just a mech that's smarter than the machine.

Based on some recent SOHC4 bikes I've just fixed in my backyard for local riders who went first to local "expert shops", I think that breed has gone the way of moral politicians, TT....
 

[KCRSXTypeS]

+ 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 !

Tank has been fully cleaned and prepped with POR-15 and the petcock is brand new.  I took the float bowl off but haven't taken off the valve yet.

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.

The fuel lines are brand new and the carbs have been fully cleaned and rebuilt.

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.

This was the exact thought that we had also I just haven't had a chance to check the bike yet since its at my parents.

This bike doesn't need no stinkin' computer, just a mech that's smarter than the machine.

Based on some recent SOHC4 bikes I've just fixed in my backyard for local riders who went first to local "expert shops", I think that breed has gone the way of moral politicians, TT....
 

There is an expert shop very near me though   The guy does nothing but rebuilds old bikes and he was the one I had clean and rebuild the carbs.  If I could get the bike to run long enough to get it to him (about 5 mi.) then I will just ride it to him and have him finish the bike and trouble shoot any problems.

[Achmed]

Ride fast and you should be able to make it, if that's what you want.

[KCRSXTypeS]

Ride fast and you should be able to make it, if that's what you want.

Well I dont really mind having it shut down for 10 min except its not licensed yet so if a cop shows up no dice...

[CBGhia]

While running, after about 5 minutes pop the top on your gas tank and see if you get an air gushing sound like when you open a fresh can of coffee.    You could have a clogged vent on you tank and that will create suction and cause the problem you describe. 

You may have POR 15'd over the vent as well (I have seen this happen on classic cars).

[KCRSXTypeS]

While running, after about 5 minutes pop the top on your gas tank and see if you get an air gushing sound like when you open a fresh can of coffee.    You could have a clogged vent on you tank and that will create suction and cause the problem you describe. 

You may have POR 15'd over the vent as well (I have seen this happen on classic cars).

I didn't mention that...I did try that, didn't work.  I actually shot compressed air into the vent and heard a POP! and saw some dust but that was about it.  I have ridden with the cap open and it doesnt help.  Also, I didn't have the cap closed when i POR-15'd the tank...I just duct taped over the gas fill hole

[Alan F.]

Just a logic check....you didn't por15 with the petcock on the tank, right?  How much restriction is the por-15 causing at the petcock mount area?  What year tank is this? (petcock left or right?  Thread on or two screws?)  If it's a later tank with the "in the tank filter thingie" did you happen to remove it before coating the tank? maybe it's clogged?

[KCRSXTypeS]

Just a logic check....you didn't por15 with the petcock on the tank, right?  How much restriction is the por-15 causing at the petcock mount area?  What year tank is this? (petcock left or right?  Thread on or two screws?)  If it's a later tank with the "in the tank filter thingie" did you happen to remove it before coating the tank? maybe it's clogged?

Petcock off when POR-15'd.  The tank is 1973 (Left) and the petcock is the 2 screws...hope that helps.

[Spanner 1]

Time to do the flow test as TT mentioned...let the gas flow for 2 or 3 minutes, I think.
Or did you do that already ?

[KCRSXTypeS]

Time to do the flow test as TT mentioned...let the gas flow for 2 or 3 minutes, I think.
Or did you do that already ?

Havent done it yet...been pretty busy.  So, lets get the sequence right.

Pull the fuel lines off the petcock and just let it drain into a measuring cup...1 cup in under 2 min. then it isnt the petcock?

[Spanner 1]

Sounds about right,but it should be obvious if flow starts-out good and then diminishes to a trickle over a couple of minutes or so...

[KCRSXTypeS]

Sounds about right,but it should be obvious if flow starts-out good and then diminishes to a trickle over a couple of minutes or so...

What could cause it to start out fine then taper off?

[KCRSXTypeS]

Also, the exhaust on the bike is a 2-1, I don't know what the stock exhaust is supposed to be but could this cause anything?

[Spanner 1]

Foreign matter in the petcock...bits of POR swept in ? Happened lots of other posters !!

[KCRSXTypeS]

Foreign matter in the petcock...bits of POR swept in ? Happened lots of other posters !!

Just an update, going to check petcock fuel flow, correct the timing, replace the condensors, and adjust the valves, and put in some SeaFoam tomorrow.  Will let you know if anything fixes it.

[KCRSXTypeS]

Turns out it was the petcock.  We tried testing the fuel flow and it would only drip on every setting.  In 2 minutes we only got a teaspoon of fuel.  So, we took the petcock apart and there was snot in it...looked like silicone sealant.   Cleaned it, put it back together, tested fuel, and we have full fuel flow.  So now, now more dieing!

[Simpson]

It always seems so obvous when you know the answer afterwards. 

I really enjoy reading these threads.

Congrats! Now take it for a long ride after you get it legal for the road.

[wardenerd]

Have a 77 550 been running ok except at idle.  Runs good sometimes but when cut off starts back on 2 cylinders.  It will not start as easily as it did.  I checked the petcock flow and it ran full on for 15 secondss.  seemed fine.  I checked the plugs   2 were wet one was normal and one looked like it did out of the box.  Ideas?

[Gordon]

  Ideas?

Start a new thread.  You're not going to get as much help by piggybacking on somebody else's.   

[TwoTired]

  Ideas?

Start a new thread.  You're not going to get as much help by piggybacking on somebody else's.   

+1

And do a complete tuneup, which should be done every 3000 miles anyway.  New spark plugs, note deposits on the old ones, new air filter, etc.

You know, mods can make them run poorly.   And, if you don't tell us about mods, we think it is all stock and give advice accordingly.

[eideteker]

Sorry for the thread revival, but I've got an extremely similar problem with the exact same bike ('73 CB500).  I've already: replaced the petcock (with a brand new one), tried riding with the gas tank open (previous owner POR'ed the tank), checked/reset valve lash, replaced plugs, and checked head pipe temps (yep they're all hot!  ).  I have not yet: replaced condensers/points (the bike sat for a couple years after receiving an ignition tuneup, so they are as new), or checked ignition timing (need a light, really should get one). 

Overall it runs really, really well - especially after resetting the valve lash, there's a lot less upper-end noise now - but it dies out of nowhere.  I've narrowed it down to being parked on the side stand for any period of time with a warm engine...if I park it on the center stand it'll run for miles, but leave it on the side stand (even with the petcock OFF) and it will go for about 5-10 minutes and die.  I usually stand there with the bike upright for about 3 minutes (long enough to collect my thoughts and burn a smoke) and it will fire up and get me home.  What gives?  I want the bike to be reliable, but I'm already getting a little annoyed at how I keep throwing money at the same problem.