Misfire and running on 2 cylinders when hot?

[Hudson.]

I recently got my 78' 550 road worthy. Recent work includes a total carb rebuild (floats are all set, everything clean and clear), pod filters (I know!), larger main jets, valve clearences, cam chain tension, vacuum sync.

Bike easily starts without choke (too rich?), but idles very nicely at 1000rpm. Theres some slight spluttering at part throttle which im sure is down to fine tuning of the jets and needle positions.

However, after a 30min ride last night, most of which it was running great, i opened the bike up in 3rd gear when joining the motorway, revs up to approx 8k and the bike spluttered. I backed off the throttle and the bike misfired 8 miles home on what seemed like 2 cylinders. Id been at WOT a few times before hand with no issues and it pulled great.

Sure enough when i got home I took out the plugs to find plugs in 2 and 3 a lot more coked up than 1 and 4, although not wet, just black in comparison to the biscuit colour of 1 and 4.

Im still learning all this, but am I right in thinking this is an ignition issue? Where should I start looking?

Dont have much time to work on the bike at the moment and really need it for next weekend, im tempted to buy new coilpack and points but itd be nice to diagnose the issue instead of just throwing money at it?

[hairygit]

Ignition fault would be my first thought, so check the condition and security of the points, the condensors and all the wiring leading from them. Some Honda fours, 550's I think were prone to the yellow and blue wires working loose where the snap connectors join in the rubber shield near the back brake pedal.

[Bryanj]

If its a 78K with PD carb and starts without choke then the carbs just aint set right

[Johnwebley]

check the resistor plug caps,

 they normally have 5k resistance,but over time,when hot ,they fail,

I had one go 10 days ago,new cap,smooth as silk again ( I hope,well it run sweetly now)

does anyone understand electricity and resistance?

 with electronic ignition,I get a FAT BIG spark,so I run 33thou plug gap,how does 5K resistor plug caps affect them,would the bigger gap
cause the cap to fail sooner ??

[mike the bike]

I does sound like an ignition problem.   There's two separate, independent ignition systems, one firing 1 and 4 and the other firing 2 and 3.
Try to determine which by feeling the temperatures of the header pipes.
What ignition set up have you got;  is it the original points or have you gone electronic?

[matthewmosse]

Ignition related I am sure from the pair of cylinders going like that - assuming the filters have had a similar life ( possible for outside pair to get more weather , maybe washing out any oil etc, depends on the pod filter type. I have had issues in the past with condensors malfunctioning when hot ( belive it was that, I described the symptoms to a collage mechanics tutor who recommended chainging the condensors and this seemed to resolve it.

[mike the bike]

I agree.  I rode home 70 miles on my 400four going like a 200two.   I was just a dodgy capacitor.

[Hudson.]

Thanks for the input guys!

Ignition fault would be my first thought, so check the condition and security of the points, the condensors and all the wiring leading from them. Some Honda fours, 550's I think were prone to the yellow and blue wires working loose where the snap connectors join in the rubber shield near the back brake pedal.

Points look to be in nice shape, no pitting or grime etc. All the connections are secure and appear in good nick too, i checked at the connections you mentioned up behind the brake/frame and theyre solid and clean.

If its a 78K with PD carb and starts without choke then the carbs just aint set right

Thats what i thought! I have the screws set 2.5 turns out, too much? Mains are 110, stock idle jet, stock needle position, new clean pods and an open exhaust.

check the resistor plug caps,

 they normally have 5k resistance,but over time,when hot ,they fail,

I had one go 10 days ago,new cap,smooth as silk again ( I hope,well it run sweetly now)

does anyone understand electricity and resistance?

 with electronic ignition,I get a FAT BIG spark,so I run 33thou plug gap,how does 5K resistor plug caps affect them,would the bigger gap
cause the cap to fail sooner ??

Checked the caps and they're all 5k ohm which appears to be right.

I does sound like an ignition problem.   There's two separate, independent ignition systems, one firing 1 and 4 and the other firing 2 and 3.
Try to determine which by feeling the temperatures of the header pipes.
What ignition set up have you got;  is it the original points or have you gone electronic?

Im pretty sure after the other evenings ride it was 2 and 3 that were down, as when i pulled the plugs they were really sooted up in comparison to 1 and 4. This was making me think ignition/coil/points etc too.

It's the stock points system, but could possibly be made up of after market non-genuine bits.

Ignition related I am sure from the pair of cylinders going like that - assuming the filters have had a similar life ( possible for outside pair to get more weather , maybe washing out any oil etc, depends on the pod filter type. I have had issues in the past with condensors malfunctioning when hot ( belive it was that, I described the symptoms to a collage mechanics tutor who recommended chainging the condensors and this seemed to resolve it.

The filters are brand new UNI pods which i know a fair amount of people use on these bikes without issues, the condensors definitely dont look new on this bike so could potentially be an issue.

I agree.  I rode home 70 miles on my 400four going like a 200two.   I was just a dodgy capacitor.

70 miles is impressive! haha


Well, this morning i checked the plug caps and snipped the leads back a couple of mm's even though they looked good. I went for a 1 hour ride with a spare plug in my pocket, so if i started running on two i could check to find where the loss of spark was.

I suppose its a good thing that the bike ran faultless the whole time?! I tried a few WOT pulls aswell and it was fine. Although the clutch cable snapped and i had to ride back through the gears without stopping, good fun!

I rode it to its MOT on wednesday night and it was pouring with rain, a friend of mine said he had a few issues in the wet with cylinders 2 + 3 going down due to being soaked from the front wheel with no mudguard (i dont have a front mudguard either). This sounds possible, but is kind of weird that when i rode it the next day in the dry it was fine up until it got hot?


Heres the ignition



Thanks again!

Rich

[mike the bike]

Make sure you seal where the HT leads exit the coils to keep the rain out.  The best stuff to use is "STICKS LIKE SH#T".   It's more permanent that silicon sealant.

[Spitfire]

Yes that's where the rain gets in, in the olden days I used to seal where the HT leads go into the coils with Araldite, nowadays there are lots of other stuff available

Cheers

Dennis

[MCTID]

There are holes in the sky, where the rain gets in.....but they are ever so small, that's why rain is thin.

Spike Milligan RIP.

[mike the bike]

I loved Spike Milligan's sense of humour, and Ivor Cutler

[matthewmosse]

I also like Milligan humour, I first discovered his scribblings from his war memoirs 'Adolf Hitler, my part in his downfall' a work of genius. Sadly his work isn't repeated that often on tv.
RIP Spike.

[Chris400F]

And me. The guy was mad, but brilliant.

[MCTID]

Well if you Gents put me on the spot....I would have Spike as Prime Minister anytime! I have always said that....not got anything particular against the present lot but somehow Spike would have probably made a damn good job of a very complicated business......as would most blokes who had lived through WWII (Im biased obviously as well because he was a Gunner.........who are (IMHO) just about the finest breed of Men on Gods earth!

As you say - RIP.