Topic: CB550 dies after 10 minutes +/- 5 minutes

[ktm520]

OK, searched on this one and found some threads.

I first diagnosed a clogged vent and replaced cap as I had one from another build and also went ahead and pulled the carbs just to check them out again, found 2 needle clips crushed replaced those, resynched, floats done by tube method and all right around 22...new coils as well.

Bike runs as good as they get, starts with choke briefly, no hiccups, backfires... air screws all between 1.5 and 1.75 with stock jetting,exhaust.

After 4 or 5 miles (much better than is was before) and most consistently after a good stretch of road, bike will die soon after deceleration or soon after. Hard to detect if it going lean before hand (sign of low fuel in bowls). After a a minutes or so bike will start back up, but problem comes back even a bit sooner.

I still think I am dealing with something that would effect all cylinders, so loss of fuel flow is still on the table. I am going to remove petcock (again) and assure no restriction. A few questions:

1) Is there a flow test of any kind for fuel delivery out of petcock e.g. volume/minute or equivalent?

2) Thinking of rigging a secure tube I use for float adjustment and reproducing to see if fuel is in the bowl, sound reasonable?

3) It is the duel lined petcock, any specific problems related to this, worth considering conversion with splitter or is there no advantage?

4) Barring findings of above, raise float level slightly?

5) Any thoughts on the electrical side?

6) All ideas welcomed, bike runs too good to not get this one figured out.

Cheers!

[Maraakate]

Have you tried to readjust the idle screw itself?  The idle will "move" a bit after it warms up.  When it stalls, pull the choke up a bit and try to start it.  May just need some fine tuning.

[seanbarney41]

Bring along an extra spark plug...when it dies hop off quick, pull some caps, and check for spark

[robvangulik]

Isn't just the fuel cap ventilation clogged? No air supply=tank vacuüm= no fuel to carbs.

[ktm520]

Bring along an extra spark plug...when it dies hop off quick, pull some caps, and check for spark

That's a good idea along with the float adjust hose. Should narrow it down.

Thanks

[Jerry Rxman Griffin aka MuthaF'er]

Isn't just the fuel cap ventilation clogged? No air supply=tank vacuüm= no fuel to carbs.

+1 if it acts as if you are running out of gas like when you need to switch to reserve

Loosen the cap when this starts occurring to allow it to vent

[seanbarney41]

Guys, he said in first post that he already replaced the fuel tank cap...not that fuel flow might still be a problem

[Jimcg]

But the vent could be clogged in that cap too if it was laying around a long time since used before. Easy thing to try to open up the cap a bit next time just to see if it makes a difference.

/Jim

[RAFster122s]

With a bit of care you could do a long clear tube on one of the carb drain screws and then with the end of the line tethered so it won't get knocked out or end drop below the bowl you have a quick way to see if your carb has fuel in the bowl.
This could reduce the number of things you are chasing.
Have you rebuilt the petcock, in the case of the 74 with rebuild able petcock. If the petcock won't flow enough fuel then you are going to have running issues.
Chinese petcocks on later models can have more restrictive flow, but haven't heard this being a common issue. Most spend the money on oem petcocks because of build quality.

[ktm520]

With a bit of care you could do a long clear tube on one of the carb drain screws and then with the end of the line tethered so it won't get knocked out or end drop below the bowl you have a quick way to see if your carb has fuel in the bowl.
This could reduce the number of things you are chasing.
Have you rebuilt the petcock, in the case of the 74 with rebuild able petcock. If the petcock won't flow enough fuel then you are going to have running issues.
Chinese petcocks on later models can have more restrictive flow, but haven't heard this being a common issue. Most spend the money on oem petcocks because of build quality.

Yes agreed on the aftermarket for the double petcock and it is difficult to find OEM, that is why I am considering going to a single and splitting the line.

I will post back as I will not have time to diagnose until Friday. First plan is to run with tube from bottom of bowl and carry spare plug to rule out the obvious. Bench fuel flow looked good with new cap, but perhaps there is a dynamic effect. The new cap greatly improved issue, so petcock is on the list, but again flow looks good on bench.

Has to be something upstream whether fuel or electric.

[RAFster122s]

You do run points, not an electronic ignition? Asking to rule out an overheating electronic ignition component...

[ktm520]

You do run points, not an electronic ignition? Asking to rule out an overheating electronic ignition component...

Running points

[Rosinante]

You are on the right track and getting some good suggestions, I see.  Engines need only three things to run properly:  Adequate compression, which can be ruled out unless there is mechanical damage or LOTS of wear.  Proper air/fuel mixture.  Spark, properly timed.  Spark can fairly easily be ruled out.  I have seen engines fail to produce a spark, and yet produce that spark with a spark plug located outside the combustion chamber, BTW.  I would connect a spark plug and lay it something grounded, and see the spark.  But no spark was happening when the wire is connected to a spark plug installed in the engine.  Something about a spark under compression being a different animal than a spark at atmospheric pressure.  Because of this, when I test for spark, I use an inductive timing light so that the current is tested with the spark plug in situ.


At any rate, my guess and most contributers here believe your problem is fuel.  The clear hose idea is a good one. 

[Kevnz]

Agree with comment about spark under compression. Had a car fail to restart after gassing up, tested usual suspects and had plenty of spark. In desperation, as nothing else seemed likely, I replaced ignition coil and car burst into life. Coil was 12 years old at that time. Was driving along 20 minutes later at 75mph, thinking car had never gone so well and realized I was still in 3rd (of 4) gear.

[rotortiller]

1) Is there a flow test of any kind for fuel delivery out of petcock e.g. volume/minute or equivalent?

Running WOT the bike would drain a 16  liter tank in just under two hours at 160kph. That would give you a crude LPH consumption so call it 8 LPH for the sake of a grossly over stated argument. 133cc or more per minute coming out the petcock should satisfy most metric riders.I expect the normal flow rate to be much higher. LOL

[Rosinante]

This may not have much value regarding fuel flow estimates, but it is a data point:  I recently drained the old gas from the tank of my 750 K8 and I timed it.  The fuel hose had a small inline fuel filter, which likely reduced flow volume.  Still, a gallon drained in about 10 minutes.  That would be six gallons per hour.  At a fuel consumption rate of 35 mpg, that would be 210 miles.  I rarely ride this bike faster than 210 mph.