Topic: First real CB500 problem

[Priscbs]

For some background, this '72CB500 k1 is the SWMBO project. I have ultrasoniced the carbs, eletrolysized the tank, freed up what was a frozen engine that is part of a bike that was last registered in '82, adjusted the valves, installed new battery, aircleaner, points, condensers, plugs, and trimmed the plug wires. The plugs have a nice hot spark.

So, today I got the ignition system all in line and pulled it out to fire it up. It spit and sputtered for a while. Then it pretty much came to life.

But, it will fire on the first kick and sounds pretty good. It runs for about 10 seconds, will rev or idle, then it bogs down and dies. Turn it off, wait a minute and same thing all over again.

Got any ideas?

A matter of seconds didn't seem long enough for the gas tank vent, especially at idle, but I popped the cap anyways and that ain't it.

[edbikerii]

The later 77 and 78 CB550 carbs have a fast-idle cam that causes the engine to rev slightly higher when the choke is applied.  The earlier bikes do not, so the rider is responsible for keeping the revs slightly higher during warm-up.  Does this sound like it could be the problem?

[mattcb350f]

Will it idle longer before quiting then it does when it's reved up. If so, sounds like fuel starvation.
Try putting on the choke just before it dies, if it comes back to life then it's lean or running out of fuel.
 Matt.

[Priscbs]

I am going to check fuel supply tomorrow.  I forgot to mention that I cleaned the petcock assembly and replaced the screen and it o-ring.  It seems to die quick and hard when it does.  Does 10 or less seconds at idle seem long enough to drain the bowls?  Not trying to question anyones knowledge, just asking more questions in the pursuit of knowledge.

[mark]

My 550 can go a couple miles with the petcock off, then sputters. Please don't ask how I know that. Some folks around here can sync their carbs without an extra tank - just the fuel in the bowls. We're talking minutes here, not seconds.

Dittos on perhaps needing choke and/or throttle.


Happy trails.

[Priscbs]

It seems to do it regardless of choke position, and while opening the throttle

[TwoTired]

Are you using the stock airbox/filter?

Are you concerned about operation before it is thoroughly warmed up?  Or, is the symptom you are annoyed with while the engine is cold?

Did you set the float height to 22mm?

What Idle air bleed screw setting did you use?

Are your carb rubber couplers in good shape and well sealed?

What are the numbers on your spark plugs?

[mattcb350f]

I wonder if the fuel isn't flowing into the carbs fast enough. How long do you leave the fuel on before starting it?
Try leaving the fuel on for a while before trying to run it. This will make sure that each carb is completely full of fuel. Then, it should run for a few minutes. This is of course if the floats are set correctly 

It's hard to imagine that the problem is anything but fuel starvation. you say it's got good spark, and even if it had poor compression it shouldn't do that.

However, to rule out an ignition problem completly, check the spark imediately right after it dies. It's a long shot, but ignition on occation does do some weird things 
 Matt.

[Priscbs]

I haven't gotten it fully warmed up as it only runs for a few seconds. 

I am running the airbox with an uni replacement. 

I did set the floats with my calipers. 

I messed with the idle screws when I was trying to get it to start. I got a sweet spot somewhere between 1 and 2 turns out, but will have to go bottom one out and count.  (very sensitive, those screws)

The carb rubbers all look amazingly good for a 35 yo bike

Plugs are stock D7EA

Will check the fuel supply and spark after idle today.

[lostinthe202]

Did you check for spark on all four?  Could it be you're only running on 1/4 or 2/3 without realizing it?  Did you check over all the connections in your ignition system (coil ground wire, signal wires, etc.) to make sure everything is tight and clean? Timing and points gap? 

I'm just tossing out suggestions.

Good luck!

[TwoTired]

I haven't gotten it fully warmed up as it only runs for a few seconds. 

Carb adjustments are made when the engine is at normal operating temperature.  When cold, fuel atomizes poorly and this detracts from engine efficiency.  The choke compensates for poor fuel atomization, and the right hand on the throttle compensates for poor idle efficiency.  You should need 1500 to 2000 RPM and partial choke to keep it going during warm up.  My CB500 has a thumbscrew under the twist grip to add friction and hold the throttle setting.

I am running the airbox with an uni replacement. 

That's what I run on most of my bikes, too.  I like the set up.

I did set the floats with my calipers. 

I asked about this because a float level set too low can hinder throttle transition and make the bike lean overall.  This could make the warm up period, troublesome.

I messed with the idle screws when I was trying to get it to start. I got a sweet spot somewhere between 1 and 2 turns out, but will have to go bottom one out and count.  (very sensitive, those screws)

The throttle transition from idle needs a rich mixture to compensate for vacuum loss when the throttle is opened.  If you open the idle air screws out too far, I guarantee that you will have throttle transition issues.  Both when cold AND when at normal temps.  Do NOT adjust these for peak RPM at idle as that create a too lean condition for throttle response.
Are those air screws original, or after-market replacements?  The tips should have hollow ends as this reduces their sensitivity.   Set them to one turn out, and fine tune them later when you can test for throttle transition from idle while riding the bike.

The carb rubbers all look amazingly good for a 35 yo bike

The question is, are they sealed at both ends?  Air leaks here cause lean conditions.


Cheers,

[Priscbs]

An update on my continuing problem.

I have a nice hot spark on ALL 4.
Am getting plenty of fuel (not fuel starvation).
All the timing and gaps are correctly set.
Float levels are set.
Idle screws are at1 turn out.
I adjusted the valve clearances.
Have checked for intake leaks with my turbo torch (best method IMHO).
The exhaust is not blocked and free flowing.

All along, it seem mechanical to me.  It is just the way it dies, so firm and abruptly.  giving it throttle doesn't help.  Yesterday, I ran it for maybe 20-30 seconds.  The 1 and 2 cylinder side exhaust (4-2) starts smoking the longer it runs. It started to bog down like it does just before it dies. I gave it some throttle trying to force it to keep running. This lasted for maybe another 5 seconds then it came to a very abrupt halt like always. However, this time I immediately tried to kick start it. I couldn't get it to budge. It usually kicks over just fine, but for about 15, seconds it was all locked up.  Then it started turning over again.  These bikes have sat for the last 25 years.  Could there be crap in the jugs causing them to sieze when it warms up?

[754]

Look for a pinched or obstructed fuel line..
 or a partially plugged brass tube in the petcock.

You can put another piece of fuell hose on petcock and test flow into bottle or cann.. best done outside..

[TwoTired]

If the engine is indeed seizing, check your oil pressure/circulation.  Or, at least see if oil is getting up to the cam/rocker area.

You might also squirt some oil into the spark plug holes to lube up the piston/cylinder interface and see if it runs longer.

In the ideal world, I'd look into the cylinder bores with a borescope.  But, it's starting to sound like you have rings rusted onto the piston lands.

[Priscbs]

Thanks Twotired, some good advice I will try out.  I bought a cylinder hone, but hope I can get away with not using it.

Side note;  again, there is NO fuel starvation problem.

[Priscbs]

Oh, and I think I have good oil pressure because when I had the stator cover off, that stupid plug would shoot out at even the slightest touch of the starter button.  However, I am not really sure how else to check that oil is getting everywhere it should in sufficient supply.

[754]

Plug is sealed with an O-ring, nothing else holding it.. kicking it should pop it out..

[bryanj]

if it locks and frees like that i give it 90% its the cam siezing and that means new/different head cam cover and cam and the reason is blocked restictor jets in the cylinder bas dowels

[number13]

Yep, bet you any money it is oil starvation.
It sounds like the exact same problem I
had on a old Triumph when I accidentally
reversed the oil bag lines. 

[Priscbs]

okay.  So, again, how do I check/ fix this oil problem?

[bryanj]

Remove all the tappet cover and peer inside with a torch, everything should be oily but i bet it looks dry, blue and burnt.
You could try starting it with the covers off and everything outside should aquire a coat of messy black stuff.
If I am right get a COMPLETE new top end set and a used head/cam/camcover from any 500 or 550 on ebay,probably, then with standard hand tool plus a torque wrench, a manual, patience, care and this forum strip it down to base gasket and re-build.

Oh and remove the sump pan to clean the oil pump screen but that has an "O" ring gasket that you can use again

[Priscbs]

Good new so far, I pulled the valve cover today and everything looks nice and oily with no heated looking steal.  What's next

[bryanj]

With the cover off and plugs out spin it on the starter and make sure that oil bubbles up both ends, if so keep going down, I'm happy to be proved wrong about the cam, just hope you havent got big problems lower down

[goon 1492]

Yeah that is good news but maybe you should pull the oil pan and check the sump too to make sure your getting enough oil.

[Priscbs]

even with slow kicking, I have oil bubbling out both sides.