My Superdream CB250N engine woes!

[UrbanBadger]

Hello folks,

I'm looking for a few words from the wise! First of all I'm new to the wonders of the motorbike but do know a bit about engines so I'm not completely green.
https://i173.photobucket.com/albums/w73/conneltreble/IMG_0121.jpg

I've done as much research as I can but am at my wits end as I don't feel any closer to getting it fixed.

A few months ago I bought a Honda CB250N. When I first rode it all was good but I went back two days later and there was no spark, after swapping the coil and CDI onto another working bike it was narrowed down to be a faulty generator. I swapped this over from a spares bike I have and it promptly died on the same day! Only ridden twice so far!

I bought a rewound generator (ignition coils only) from www.bbbikeshop.co.uk and also bought a new battery.

After fitting these to the bike it ran but had the following problems which it may have had before but having only ridden it twice I'm not sure! Rode it two work 5 days like this.
- Sometimes it would struggle when up at about 3000rpm (timing advance revs?)
- Won't elec start when hot, it will bump start and sometimes kickstarts.
- Black plugs
- Popping airbox occasionally
- Exhaust backfiring occasionally
- Gunson colour tune plug showed it running rich at high rpm.

Carbs were taken off, stripped and cleaned although to be honest they were surprisingly clean.
- One had a black sooty deposit inside it.
- One idle air needle had an o-ring missing.
- Two float bolt fastener hole threads were stripped
(All of the above were rectified, both idle air screws fully wound in and back out two turns.)

The carbs were refitted, new plugs and caps fitted and the engine fired up and revved up great. After a few minutes it started to pop a bit on deceleration, the hot start problem was still there but a Gunson colour tune plug showed the correct fuel to air ratio.

Next I did a compression test, 110 psi on the left cylinder and 120psi on the right. It should be 185 +/-14 psi but considering its 30 odd years old thats not too surprising.

Next I checked the timing using a xenon lamp and it was sparking at the the 'F' mark but didn't seem to advance when it wouldn't rev up.
When it did rev up the timing seemed to advance up once but the light was flickering a bit (not in the way its meant to!) when I tried again the timing light wasn't always flashing, sometimes not at all (I checked for a good connection). I thought the gun might have be faulty checked it on my car an it was fine throughout the rev range.

My thoughts were the CDI isn't working properly despite testing it on another bike or the pickup on the generator isn't working.

Today I got a replacement CDI from a breaker (he said if it didn't work he would change it but only once!) I tried it but it didn't make a difference so will go and change it next week. I also fitted a new pattern coil I had.
I then took the pick up off another generator and swapped it with the one on the bike, now it seems harder to start!

The battery is on charge so I will be back down tomorrow, any advice anyone can offer would be much appreciated!

[smegballs]

Has the air filter been cleaned replaced??

From what I've read popping on the overrun usually is air leaking into the exhaust and igniting hot exhaust gases. Especially if there is extra unburnt fuel from running rich.

I'd check zorst and airfilter.

Nice tidy SD by the way, how much was it?

Mine was 200 quid but nowhere near as tidy.

Cheers, Rich

[UrbanBadger]

Cheers, it was £450 I saw another for a bit less but it was much shabbier although at least it ran

I had the air filter out and it has been cleaned and oiled by the previous owner.

Funny you should say that about the exhaust as it is leaking a bit at the head and collector box. I might try taking it off and refitting it, surely that would cause the other problems though would it?

[smegballs]

Leaky zorst header on a four stroke is nowhere near as dangerous for the engine as on a two stroke. Still its not how things are meant to be so it should be rectified.

Its entirely possible that an old bike such as this has many faults with it. All these faults can produce varying symptoms that overlap or obscure each other making it hard to trace down what really is wrong.

So for starters I would try and stop you zorst leaking as it obviously is not meant to leak!

You may then find that this say eliminates the backfiring which may then make the other problems easiers to hunt down. By correcting each thing wrong as you find it you are more likely to suss out the nasty little bugs in old engines like these.

Cheers,

Rich

[UrbanBadger]

Thats it up and running up to the red line again!

Put a new CDI on it and rewound the pick up coil (pulser coil?) that sends the signal to advance the timing at thats it going again. Well chuffed! Feel like I've learnt something

[MadDogMcQ]

Sorry to trench up an old thread, but I was wondering where you got the pick up coil rewound? Same place you got the stator??