Topic: Frustrating CB500K3 carb issues

[Deltarider]

There are ways around ethanol. There are alternative, socalled designer fuels like BP Ultimate and Shell V-Power, which don't contain ethanol, at least not where I live. Last year I have travelled the US Southwest, but can't remember they had more than 10% ethanol containing fuels. There may have been some rare E85s, which is a total different category ofcourse. Ever since the US started messing with gasolines, people have found ways around it. I remember that long time ago, some bikers that preferred to run at least a bit of lead, mixed 1 part of AVgas to 40 parts of regular gas. As far as additives, I'm no expert, but there may be some that soften eventual negative side effects of alcools. For the last months I have run a 50% E10 and 50% V-Power mix and my bike likes it.

[Ozzybud]

You can get Ethanol free gas in all 50 states.
It's all I use in my bikes, classic cars and lawn equipment.

https://www.pure-gas.org/

[newday777]

From what I understand the states have higher levels of ethanol in fuel than we do.

10% in the states. Some in the government were pushing it to 15 but got railed by the manufacturers.

More that the alcohol problem, the chemicals that are added that do damage to the carbs and environment, groundwater.

[Deltarider]

A little anecdote. My first parttime job - weekends and vacation period only, as at 15 I was still in highschool - was attendant at a gasstation. Innumerous times my hands have been exposed to gasoline when I was pumping too fast. Automatically the flow was cut off, not preventing a gulp of gas over the hand that held the pistol. It cooled my hand a bit. In those years you didn't worry about these things.
I still remember the day the first self service station opened. It was a mile from our gasstation. For the Dutch: it was when the Coen tunnel was opened in 1966. The company was Mobil. From then on more and more selfservice stations conquered the market. I cannot surpress the suspicion that the big oil companies were well aware of the health risk of the work attendants like me did and expected claims sooner or later and so decided to spread the risk over all of the population. The drivers of the trucks that delivered the gas had always been obliged to wear gloves in all those years before self servicestations were introduced...

[Teddyhoeg1982]

So an update, although not very positive.

As mentioned I got a set of period correct PD42B carbs with all original Keyster brass. I cleaned, mounted them and started the bike today. Runs exactly as before…

Idle is generally ok but a little unstable. Will sometimes rise to 2000 rpm or seemingly drown itself slowly. To keep idle at 1000 rpm+, I need to give a little more throttle adjustment screw than usual, which makes idle hang and drop too slow. With less throttle adjustment idle does not hang but idle will drop to an unstable 700-800 rpm and cause the engine chain to make unpleasant rattling noises.

I will next try to sync the carbs and check if there are any problems with my ignition setup. It sometimes shoot out of the exhaust or drops a spark I think.

So generally it starts fine from cold with choke. When warm, it runs ok with light throttle, but will bog and hesitate when more than 50% throttle is applied.

[scottly]

Will sometimes rise to 2000 rpm or seemingly drown itself slowly. To keep idle at 1000 rpm+, I need to give a little more throttle adjustment screw than usual, which makes idle hang and drop too slow. With less throttle adjustment idle does not hang but idle will drop to an unstable 700-800 rpm and cause the engine chain to make unpleasant rattling noises.

When it's idling at 2000 RPM, try putting it in gear and letting the clutch out a bit: if the idle speed drops and stays low until you rev the motor, then stays high again, you should check the ignition advance mechanism for weak springs.

[Teddyhoeg1982]

Will sometimes rise to 2000 rpm or seemingly drown itself slowly. To keep idle at 1000 rpm+, I need to give a little more throttle adjustment screw than usual, which makes idle hang and drop too slow. With less throttle adjustment idle does not hang but idle will drop to an unstable 700-800 rpm and cause the engine chain to make unpleasant rattling noises.

When it's idling at 2000 RPM, try putting it in gear and letting the clutch out a bit: if the idle speed drops and stays low until you rev the motor, then stays high again, you should check the ignition advance mechanism for weak springs.

I think it may act like this. Any guidance on how to check if they advancer works as intended, also any knowledge of there are aftermarket advancers of good quality? OEM does not seem available.

[scottly]

Do you have a timing strobe? When idling, the timing should be at the F mark, and when RPMs increase to 2500 RPM+, the timing should advance to the full advance marks ", then return to the F mark when the engine slows down. If the springs are too weak, the advance won't return to the F mark, and the extra advance makes the engine idle speed faster, until forced to slow down by slipping the clutch while in gear. Stay away from aftermarket advancers! Most of the time, the original springs can be shortened by cutting a coil off.

[Teddyhoeg1982]

Do you have a timing strobe? When idling, the timing should be at the F mark, and when RPMs increase to 2500 RPM+, the timing should advance to the full advance marks ", then return to the F mark when the engine slows down. If the springs are too weak, the advance won't return to the F mark, and the extra advance makes the engine idle speed faster, until forced to slow down by slipping the clutch while in gear. Stay away from aftermarket advancers! Most of the time, the original springs can be shortened by cutting a coil off.

I will try to get a strobe light and check this

[Ozzybud]

So an update, although not very positive.

As mentioned I got a set of period correct PD42B carbs with all original Keyster brass. I cleaned, mounted them and started the bike today. Runs exactly as before…

Idle is generally ok but a little unstable. Will sometimes rise to 2000 rpm or seemingly drown itself slowly. To keep idle at 1000 rpm+, I need to give a little more throttle adjustment screw than usual, which makes idle hang and drop too slow. With less throttle adjustment idle does not hang but idle will drop to an unstable 700-800 rpm and cause the engine chain to make unpleasant rattling noises.

I will next try to sync the carbs and check if there are any problems with my ignition setup. It sometimes shoot out of the exhaust or drops a spark I think.

So generally it starts fine from cold with choke. When warm, it runs ok with light throttle, but will bog and hesitate when more than 50% throttle is applied.

Please get your bike charging and ensure you have good voltage to the coils static and dynamic.
This is clearly a ignition problem not a carberator problem.

[HondaMan]

Am I hearing the primary chain clattering loudly in the video? If so, that can indicate wide variance between carbs' slide openings, causing erratic torque on the crankshaft and resulting in chain rattle, even against the upper engine case if worn enough. This is settled with the vacuum gages and synching process. For example, if 2 carbs are more open than 2 others, and are opposites in the firing order (like carbs 2&3 open slightly more than 1&4, or 3&1 more than 2&4, etc.) then 2 of the cylinders' sparkplugs should show darker than the other 2, helping to figure out where the trouble is starting.

[Teddyhoeg1982]

Am I hearing the primary chain clattering loudly in the video? If so, that can indicate wide variance between carbs' slide openings, causing erratic torque on the crankshaft and resulting in chain rattle, even against the upper engine case if worn enough. This is settled with the vacuum gages and synching process. For example, if 2 carbs are more open than 2 others, and are opposites in the firing order (like carbs 2&3 open slightly more than 1&4, or 3&1 more than 2&4, etc.) then 2 of the cylinders' sparkplugs should show darker than the other 2, helping to figure out where the trouble is starting.

Yes just replaced the carbs and will need to sync. I will acquire a vacuum gauge and sync.
At 1000+ rpm it does not rattle, but it obviously loses a spark here and there making rpm drop and rattling to appear.
So all in all I will now move to charging and ignition and try to go through everything.

[HondaMan]

Yes just replaced the carbs and will need to sync. I will acquire a vacuum gauge and sync.
At 1000+ rpm it does not rattle, but it obviously loses a spark here and there making rpm drop and rattling to appear.
So all in all I will now move to charging and ignition and try to go through everything.

One thing that may help to 'ease in' to the source of the problem is: back off a little bit on the spark advance timing. I have found that when chasing carb troubles (lean burning issues, like in the 750K8 especially, and the CB550F3) it helps to smooth out the engine somewhat by applying less spark advance until the carbs are well sorted. Then the timing can be pushed back to normal with less troublesome testing. I usually set the idle timing to be in between the "T" and "F" marks, by just dialing the points plate CCW a little bit. This is especially effective on the 550 because of the very long intake runner between the carbs and the engine (and this is MUCH worse with the Murray setup, to the point of becoming 'touchy', because of its even larger intake plenum volume).

Since you have described that above mid-throttle the engine falters, this points to a lean main circuit situation, if the spark timing is normal or even slightly retarded. If the spark timing is advanced more than normal, this carb-mixing situation can become un-repairable until around 7500 RPM when the ram-speed of the incoming air mass overwhelms the spitback pressure in the long intake tract: this is how the Murray carbs work, from what I have seen. They are very tenuous below 5000 RPM, too.

One thing I haven't seen mentioned is: the intake manifold O-rings where the manifolds bolt to the head. These wore out somewhere around 1985 when they got hard: they are the same size as the O-rings in the little valve caps on the engine. These will leak on every intake stroke if not sealed, making the engine run lean and hesitantly above 3500 RPM or so. I have seen on the 500 and 550 both when they would not accelerate because these O-rings were simply too hard to seal.

[Deltarider]

Aim at an idle of a true 1200 rpm and no less. Verify your onboard tacho is accurate and not like the K3 I once worked on, which indicated 1400 at a true 1000 rpm.
All owners of CB500/550 should be aware that the plugcaps can arc to the head, due to their location. I've seen it many times. So easy to check. Have it idle in the dark and look for sparks between the plugcap and/or its lead to the head. It was known as the #1 dent in these bikes reputation of a legendary reliability. Plugcaps should routinely be replaced every 20.000km imo.