Topic: cb500 misfire

[Kevnz]

Hi Folks,
I am having a problem with my CB500 K1. It starts from cold on the button and idles smoothly and reliably. It will rev to the redline in the first 4 gears, but stumbles just off idle up to about 3k revs and then runs sweet. I have
1 adjusted the camchain
2 Adjusted the points\ timing
Adjusted the tappets
Cleaned the slow running jets in the carbs and adjusted the float levels
 The ignition advance unit is free
Nothing has made any improvement. Using the choke doesn't help.
This is a standard machine with standard exhaust system, and until recently was running fine.
I'm just about at my wit's end, I 've had the carbs off 3 times and found nothing. Plugs look good. Has reasonably new coils and points\condensors.
Any ideas?
Thanks in advance,
Kevin

[bryanj]

Replace plugs first and then condensers

[Deltarider]

It will rev to the redline in the first 4 gears...

No CB Four will get to redline in top gear, except on a dyno stand

... adjusted the float levels... and until recently was running fine...

What made you think the floats needed adjustment? Could it be you have enriched too much?

... but stumbles just off idle up to about 3k revs and then runs sweet...

Are you sure the slow jets are all in place? I once found one fallen down in the bowl  . Furthermore it would be interesting to know if you have the US carb setting or the European. For the sake of experimenting I have changed my carb setting to what we find described in an American booklet, edited in 1977. Here's the difference: Europe has needle in middle (3^rd) position, US 4^th, the latter is somewhat richer, Europe had the airscrew turned out 1 turn ± ¹/₈, US had 2 turns out which is leaner. Since I have my carbs set to US spec, I have a little bit of the symptom you describe. In general I do not recommend changing the needle position unless PO has messed with them. I hope you have not cleaned the slow jets with material harder than brass and accidentely reamed them. What airfilter do you run? Provided ignition* is OK, I suspect a too rich mixture. Your and until recently was running fine gives a clue where to look first: the things you have done recently.
* CB500/550s should regularly be inspected for possible arcing between plugcaps/plugwires and the head. Easy test, best seen in the dark.
For condensers watch: //www.youtube.com/watch?v=t3cuvGVwnjI  With engine running, see if there's excessive arcing at the breakerpoints. A tiny bit of sparking is normal, excessive sparking is not good. In this vid left condensor (1+4) is bad, right condensor (2+3) is good. Test is best performed with engine fully warmed up.

[Kevnz]

Hi Everyone,
Thanks for the input, much appreciated.
The bike was running fine but started running ratty without my doing anything to it. Just doesn't get used a lot, but has reasonably fresh fuel. Have discounted plugs as I figured they would be more likely to cause trouble at higher revs than down low. Needle height settings haven't changed, and I adjusted the float levels as a process of elimination; set at 22mm as per manual. The worst was about 25mm, but 3 of them were within 1-2mm. Made no difference. Ditto condensers, figure they would likely cause misfiring at higher revs. There is no abnormal arcing. Feels like a carburation issue as it occurs just as the throttle is opened. Is worse under load, though. Plugs not sooty. Used to pull smoothly in top from 2000rpm, so quite frustrating.

[Erny]

Couldn't it be advancer issue? Too much advance in low revs? Full advance shall be from 3000-4000, so this could explain, no?

[dave500]

check the plug caps resistance,has it got pods on it?

[Kevnz]

Full marks to Dave500. I checked the ( standard) resistor plug caps and no3 is high resistance. The other 3 read about 4.9K ohm, this one shows a dis ( infinite resistance), but it varies, starting a read at about 10- 20k, but then counting up to infinity. May be voltage dependent because it would run fine once the revs were up. Haven't fitted a replacement yet, but jury-rigged a standard cap onto the coil lead and the bike ran as it used to. ( Bike's been laid up over the Southern Hemisphere winter. I'm old and soft) Must say I really didn't think these would be the cause as the symptoms pointed to carburation, but I was running out of options. And it's a worry if you can't sort out a fault with a carbs-and-points bike! Thanks for all your help, may be back if a new cap doesn't fix it , but at least I'm more confident now.
Have a great Xmas everyone,
Kevin

[dave500]

replace all four man,that way its a proper job done and not another miss fire just around the corner!

[Kevnz]

Further update, the plug cap did not cure the fault. It ran ok for the test ride and next day was back to its bad old ways. I think I've finally fixed it now though ( ever hopeful) . Cylinder 2 was the errant culprit, if you pulled the plug lead the idle never changed, the pipe was slower to heat than the others and it would pop and spit until revs were up. The float needle would occasionally bind,allowing a low float level. Presumably vibration at higher revs let it open up. Cleaned all the float jets with fine sandpaper and all now seems well. 2 faults in the same cylinder, designed to confuse a bear of little brain. Then the tail light went out. Not a simple bulb failure, oh no. Broken wire in the headlight dimmer switch. What a pain to fix that in situ as didn't want to pull the cable out of the handlebar. Much patience required.