Missfire at low revs

[florence]

Not sure what the problem is, runs very badly, popping and misfiring and chuffing back through the carbs at low revs, drops to to 3 or 2 cylinders momentarily, but when I open up and keep above about 4000rpm it runs like a dream.  I thought it was the points because when these aftermarket points get old the timing goes all over the place.  However, installed a good used genuine set and reset ignition timing and still has exactly the same problem.

Does anyone have any ideas?

S

[Colonial-Clive (yindi)]

Bad earth on coil maybe, Look at your bullet connectors make sure there not all corroded up.

[florence]

good thought, will try that.

[hairygit]

Also check where the wires from the points join the main loom, just behind the rear brake light switch. They sometimes got knocked /pulled at that junction, and make sure both parts of the bullet connectors are clean (bright and shiny) and tight.

[petermigreen]

If it's only happening at lower revs sounds like carburation issue on the idle circuit.
If it was and electrical connection, wouldn't that be an issue at any revs?
Just another opinion.

[Rob62]

It could be ignition timing, maybe a bit retarded and at higher revs the auto advance just helps it enough? ... I’ve only ever had a similar problem on a 2t yamaha which is definitely too rich on the idle circuit, so you may be right to suspect the carbs.

[Rob62]

Actually on thinking about it i have had a similar issue, running ok then sudden missfiring and chuffing at low revs and then springing into life at higher revs. It was on a v twin 2 stroke NSR 250 with an expensive and rare electronic ignition module and i feared the worst... But all it needed was a new pair of spark plugs and it immediately ran like a dream. A lesson in the importance of checking the cheap easy solutions first...

[Bryanj]

Plug capor lead

[florence]

OK, so checked timing again and definitely fine.  I will check the plugs and caps and leads, they are a bit of a mess, and covered in all sorts of 25 year old road dirt.... I also just realised that I cannot remember when I last took out the spark plugs.  Until recently it has just been amazingly reliable.  I will continue to investigate along these lines and report back..  Thanks for suggestions.

[florence]

OK, so looking at the coils, they are the original ones and they always go wrong in the rain so maybe should replace anyway as part of process of elimination.  Does anyone have a view about best place to get replacement coils?

[Trigger]

Have you tried snipping 5mm off each of the HT leads ? I had the same problem one, and found a small bit of corrosion on one of the lead ends. Always worth a try 

[florence]

Unfortunately there isn't enough left, I've done that in the past, now they only just reach 

Took off the carbs today just to make sure.  Found lots of crud in number one and thought that must have been the fault, but, I put it back together and now it runs slightly better but still cutting out on one of the cylinders every now and then.

[Johnwebley]

OK, so looking at the coils, they are the original ones and they always go wrong in the rain so maybe should replace anyway as part of process of elimination.  Does anyone have a view about best place to get replacement coils?

  I got a pair of compatables from Boyer electronics,
I had several from DSS,but they kept failing,and only 6months warrenty

  I assume its the 703, with 4ohm resistance

[petermigreen]

I used these at recommendation of Green1. No problems.

https://www.ebay.co.uk/itm/Honda-CB500-K0-K3-Four-71-74-Ignition-Coil-1-/192366027453?hash=item2cc9e8a2bd

Peter

[Green1]

I run those on my 550 and 750 cheap as chips and work in the rain