Topic: CB500 dies after warm up

[hughes1962]

Got a non-runner '71 CB500. Rebuilt the carbs. Rewired the spark leads. When cold it starts up and idles nicely at about 1500 rpm. At 3 minutes the revs drop by about half. At about 4 minutes it starts coughing. Soon after that it dies. Won't start again until it sits for a good while. Any ideas? Thanks.

[Deltarider]

Is fuel supply to the carbs OK? When it dies after 3 minutes is here still fuel in the float chambers?

[hughes1962]

Thanks. Yes, fuel supply is good, flowing freely. Still has fuel in float bowls when it quits.

[Deltarider]

I suppose you know how to operate the choke. You could unscrew points cover and watch for excessive arcing between points when he engine starts dying.

[TwoTired]

Got a non-runner '71 CB500. Rebuilt the carbs. Rewired the spark leads. When cold it starts up and idles nicely at about 1500 rpm. At 3 minutes the revs drop by about half. At about 4 minutes it starts coughing. Soon after that it dies. Won't start again until it sits for a good while. Any ideas? Thanks.

Air filter clean as new?

Choke on = lever up. Use when cold
Choke off = Lever down.  Set here when warm.

It is normal for the idle setting to change after warm up.  Set the idle speed AFTER engine warm up.   Hold the throttle open manually during warm up.


Check for head pipe temps.  Cold(er) ones indicate a cylinder that isn't firing.

[hughes1962]

New air filter and I'm operating the choke as it's supposed to be. #2 header is SLIGHTLY cooler than the others, but you'd need a accurate thermometer to measure the difference.

[hughes1962]

OK, with points cover off and engine running (briefly) I can see arcing in both sets of points. The flash looks (to me) about the size of half a BB (unscientific, I know). How much of an arc is too much? What does this indicate? Thanks.

[josuepdx]

What's your air/fuel mixture screw set at?

[Medyo Bastos]

does it have any oil? sounds like overheating to me...

[bwaller]

You likely have a vent blocked somewhere.

[commando1954]

Check the colour of the spark plugs. It sounds like it is running too rich. Check the float level and needle positions.

[TwoTired]

When you rebuilt the carbs did you clean the main emulsion tubes?

Have you checked the jet sizes, marked and actual?
What slide needle position was selected?
Did you verify the float height?

Describe the spark plug tip deposits.


Don't worry much about spark at the points.  Do worry about gap as measured at high point of point cam and then make the opening transition at the F mark.

Tank vent clear?  It is in the cap.  Quick test is; run it with the cap open.  Are all the tubes air and gas connected to the carb open and clear?  Around here, wasps like to block the open ends with nests.

[hughes1962]

Thanks for all the tips - it's great to be able to tap into such a huge amount of experience - I'm gonna try them all tomorrow and I'll let you now what happens.

[abreeze]

Hi Have similar problem,run my 73 cb500 4 for about 30 mins, then dies, smoke coming top number one cylinder around exhaust and rocker cover. Sounds like it wants to seize , cylinder really hot. Cool down for a short while and away it goes, then same again. Recent new rings but oil light on all the time, wiring new, sender new

[TwoTired]

Hi Have similar problem,run my 73 cb500 4 for about 30 mins, then dies, smoke coming top number one cylinder around exhaust and rocker cover. Sounds like it wants to seize , cylinder really hot. Cool down for a short while and away it goes, then same again. Recent new rings but oil light on all the time, wiring new, sender new

The sender turns on the oil light below 5-7 lbs pressure.  If you run it long enough without oil pressure, you'll be shopping for a replacement engine.  You new rings won't get any splash lube at such low pressure.

If there is any doubt about low oil pressure, it is sure nice to have a gauge on there, if only temporarily as a test instrument.

Do check the oil level. And, perhaps the oil pickup screen above the bottom pan.  And certainly the oil pump.  If the tappet area on either end of the cylinder bank is not splashing oil under the inspection covers, the oil pump or the flow restrictor orifices to the cam should fall suspect.

No way of knowing if your bike has the same issue as o.p.  Probably should start a separate thread, to avoid confusion.

[jmelche2]

Got a non-runner '71 CB500. Rebuilt the carbs. Rewired the spark leads. When cold it starts up and idles nicely at about 1500 rpm. At 3 minutes the revs drop by about half. At about 4 minutes it starts coughing. Soon after that it dies. Won't start again until it sits for a good while. Any ideas? Thanks.

You should update your profile to show your location.  You might find a local member willing to come and give you a hand. 

[abreeze]

Hi
Thanks, updated profile.
Think I will burn the Honda and finish the Matchless G5 off. Good old british stuff. So crude and simple amazes me how it would ever run. Japanese advanced technology killed off the British Bikes, but I still remember from the 60's you could get any Brit bike running from the side of the road.

[TwoTired]

...but I still remember from the 60's you could get any Brit bike running from the side of the road.

Which was a good thing considering that it was required far more often.  Practice, practice, practice!

I once had a 62 BSA twin, btw.

[abreeze]

Good point, perhaps it should be patience, patience, patience
Weathers crap here so maybe I am in a bad mood

[hughes1962]

Happy to report a solution to the "dies after warm up" situation but not happy to report that evidently I'm a dumb***.

SHORT STORY: It was a faulty petcock.

LONG STORY (for whoever's interested) When I got the bike I had taken the petcock apart and cleaned it so I thought it was OK. Turns out that in the "On" position gas was not "flowing freely" and in the "Off" position gas was trickling out so I was burning the fuel in the bowls but the gas wasn't coming in fast enough to sustain combustion so it would start gasping and die.

After it sat for a while the gas would trickle until the bowls were full  and the process would happen again. When I opened a float bowl drain gas would come out and when I checked the fuel lines they had gas in them so locating the problem was a complete mystery to me.

This morning I was tinkering with it, hoping some flash of insight would come to me. I changed the mixture settings and it wouldn't start - I thought "So that's not it not it...", before it would at least idle for 3 minutes.

Put the screws back to the original position - still wouldn't start. Checked the sparks - they were nice and hot. Sprayed starter fluid in the airbox and it fired up but immediately died. That's when I thought "Whoever suggested fuel starvation could have been right..."

So I rigged up a remote fuel supply that would run gas straight into the fuel tubes between the 2 carb banks. It took a minute to fire because the bowls were bone dry but when it fired up it idled nicely.

At 3 minutes it was still running so I set up a couple of small electric fans to blow on the heads and barrels to replicate a slow cruising speed. As the Brits say, "RESULT!" It sat there idling for 15 minutes with nary a cough or sputter.

Among the lessons learned: (1) Check the simple stuff first and thoroughly. (2) Just because you cleaned something doesn't mean it's working correctly. (3) It takes roughly 3.5 minutes to burn the fuel out 4 CB500 float bowls.

Thanks for all the helpful advice - you guys are great.