Intermittent Misfire Problem

[danc27]

Hi everyone,

I have a '77 CB750 Supersport that's recently been having intermittent running problems. I've just replaced the coils (dynatek 3ohm) because the last pair developed cracks in the insulator around the central metal post (whether that was me leaving the ignition on without the engine running whilst testing lights etc or a manufacture fault I don't know). Anyway, brand new dyna coils 2 weeks ago, dyna s ignition about a year old. I am running K&N filters and a motad exhaust, however everything is rejetted (pilots, main, needle) to compensate for this, and the filters are attached via some custom 'velocity stacks' that are replicas of the original from the airbox, and it has run well with this setup.

The problem seems to happen only when the bike has warmed up/gotten hot and it has gradually gotten worse to the point where it is unrideable after 2 minutes of riding now. It started about a week ago where it sounded like 1 or 2 cylinders stopped firing very briefly, and then cleared up, same thing happened the next day but for a little longer, checked what I could briefly but didn't find anything. Next day it happened for a lot longer (letting up for short bursts and then happening again) and I had to ride like that for the last 5-10mins of my journey. Fiddled with things and took the journey home easy. Next day I only got 5 minutes down the road and it happened, letting up for very short bursts, but I had to just turn back and nurse it home. I can start it fine, leave it idling for some time (10+ mins) with no apparent problem while it warms up and I look for the problem, as soon as I take it out and put some stress on the engine for 2 minutes I get the misfire. If i then leave it idling while I look for the problem, it will go back to normal, which makes it incredibly hard to find.

It seems like it only happens to 1 or 2 cylinders, so I would assume it was ignition or carb related. I have done a plug check a couple times and one plug has been wet (a different one each time though). I have taken the carbs off and gone through them to no avail.

I'm kinda stumped as to what it is. I'm beginning to wonder whether it's the dyna ignition and maybe it started packing up when the coils died, and perhaps from the same cause. I'm not sure how I would test this though, especially when it disappears when left idling.

If anyone has any ideas it would be much appreciated, I'm at a loss. Thanks.

[Bryanj]

Check your battery and charging output as if the voltage is low the electronic ignition don't like it

[danc27]

The charging system appears fine, over 13 volts at idle, around 14 at 2,500+. I'll check again though, and check the battery with the ignition off too.

Does anyone know how to check if the dyna s is functioning properly?

[danc27]

Ok, It APPEARS to be fixed. Because it's an intermittent problem I'm not going to relax just yet. I checked all the connections in the ignition system, a couple of bullet connectors going to the coils seemed loose, so I squashed them a bit to tighten the connection and just generally fiddled with all the wires, took it out for a 20 minute spin and all seemed well. I won't hold my breath until I've given it a couple more runs at least though.

[K2-K6]

Hope you've got it with that run around the connectors, It's always odd if it goes away without real reason and maybe leave you stuck somewhere.

As for checking the electronic ignition....the easiest way is to put a points system back on if you have one to give a straight comparison......or run an oscilliscope on it to check switching.

Suppose a strobe-timing light would also give you a quick look at it as well.

Fingers crossed you've sorted it though....nice weather today to test it.

[bluemouse1006]

Hi
I have had the same problem over the last few day s
I had fitted an electronic ignition last year and used bullett connections
One had be come loose and the bike started on two cylinders then when warmed up
If it kept running the other two came on song
I soldered all the coil wires and touch wood it's been fine
Hope yours is ok
Amos

[Lobo]

Unsure whether the same, but my K2 had to be nursed home recently and would not 'pull' with any appreciable throttle.  This issue only manifested itself after a long run (ie hot), and most notably the first outing on the new DSS electronic ignition - which to be honest, looked awfully 'cheap' and 'unrobust'.

Because I'm a simple fella, and able to cope with points etc on the roadside (but not circuit boards) that was enough to have me reinstate the good old original Honda points system. No probs since, other than £100 poorer.

[Waggles]

Had similar on my F1 after its rebuild. I don't have electronic ignition but it turned out to be loose connectors in the points wires where they join the loom near the rear brake switch, the wires are a bit tight and can pull out easily. Worth checking if your electronic system still uses these wires.

[danc27]

It actually turned out to a carburetion issue... float bowl overflow tubes (after taking the carbs off and stripping the internals a bunch of times!)

I had heard something about this before, someone had said that their carbs had been sucking rain water up through the overflow tubes and into the float bowl, the problem is cured by cutting the ends of the tubes at an angle to stop droplets forming over the hole... But, mine were already cut like that, plus I had checked on at least two occasions whether there was any water in the float bowls, and there wasn't, so I had decided that it wasn't the problem.

That was until, after a stroke of genius, well really just desperation and 'try anything and everything' logic, for some reason I pulled one of the overflow tubes off and blocked the overflow outlet with my finger, and hey presto that cylinder started to cut out after a few seconds. Seems as if blocking this creates a vacuum that either cuts off the fuel to that bowl, or the exact opposite causing it to flood.

The problem didn't always crop up when it was raining which led me to believe it wasn't caused by water, in hindsight it was either wet road conditions or the day after rain etc. I guess there was water residue/droplets in the tubes that only created a problem when there was significant enough draw/vacuum on the float bowl i.e. high revs etc. Which would explain why I could leave it idling for 15+mins no problem, but after riding it for a bit it would start dying on me. Also if it drew the droplet all the way into the float bowl the problem would clear up I guess, which would explain some of the intermittency.

Bit of searching, I came up with this: http://forums.sohc4.net/index.php?topic=107640.5
Looks like they are the only vent from the float bowls to atmospheric pressure. Running them without the tubes at the mo with no problems, but a clean and reroute should fix it. They're the PD carbs if anyone is having similar issues.