CB250RS Rebuild Progress

[Laverda Dave]

Hmm, no MOT today for the bike. I begun to fill the tank and to my dismay the fuel tap has developed a really bad leak as a result of not be used for 10 years.
Unfortunately nobody makes a rebuild kit for this particular tap and I've been forced to order a new one from CSMNL at an eye watering €119!!!! S/H ones on ebay are going for £30 and are untested. I hope they deliver quickly, spring appears to have arrived  .

[Laverda Dave]

It's finished  .
I fitted a new gearbox driveside seal last week and put the engine back together earlier this week. Everything now back on and adjusted.  I took it out today for a 50mile shakedown and all is good. Only weird problem is the neutral light staying on in second gear, not sure why, it was OK a couple of weeks ago? Otherwise all good. Thanks for the words of encouragement when the seal blew and it was an engine out job .
Thanks to Piki for fantastic decals and to Menno for being a wizard with the spray gun. I'll keep the revs to 5k for the first 300 miles and then slowly build them up to 6.5k until the 500mile service.

[Nurse Julie]

That is lovely, such a stunning colour. There are going to be lots of Honda SOHC owners taking it easy for the next few weeks, what with all this running in of newly built engines !!!.

[K2-K6]

I always felt they were a good looking bike with nice proportion,  set off well with the twin pipes. Fabulous job you've done in bringing this one back to life.

It looks great. Hope it all runs smoothly now and you've got the bugs out of it.

[Spitfire]

Great job, it looks really nice, I always liked the looks of those bikes.

Cheers

Dennis

[haynes66]

well done mate, youve done a cracking job there. enjoy the ride.

[Piki]

Congratulations mate, very good job!!!  Fantastic bike!!

[K2-K6]

Generally looking around today and found the following

https://www.ebay.co.uk/p/Honda-Cb250rs-Cb250-RS-Xl250s-1979-1981-Engine-Crank-Rod/27007292260

Could be of interest.

[Laverda Dave]

Thanks Nigel. I've just returned from a few days in Devon to find the new main bearings have arrived from Simply Bearings. Just waiting for the con rod kit to arrive and will then take the crank to Bedford for a rebuild. I'll keep an eye on the link you sent in case the worst happens!
Dave

[K2-K6]

In the interests of trying to avoid a repeat, it's always horrible to see someone's work in building their engine result in damage like this,  thought I'd offer a constructive view of what appears to have gone wrong.

The question I posed about the plug,  I couldn't see from the photo what condition the electrode was is.  What I was looking for may need a magnifying glass to inspect the round plug electrode for signs of melting as this can induce pre-ignition conditions that would lead to failure.

As I understand it,  a holed piston results from early or pre-ignition. Meaning,  that the whole mixture has been spontaneously ignited before the maximum ignition advance point, in effect trying to make the motor go backwards while it's being spun forward by mechanical connection to road speed. This is normally always catastrophic. Heat builds veeeeery quickly and passes above melt point of the components in seconds if consistently applied.

The plug electrode is just one source of this,  if too hot it will glow and and can ignite the mixture as compression comes on during the phase leading up to the engine's set timing.

As Bryanj pointed out,  there may be a question mark over ignition accuracy which has the same potential if too much advance is applied. This should be easy to check for by making sure any timing marks are accurate and strobing it to verify.

Basically,  you either need compromised ignition or a very hot component that will glow hot enough to ignite mixture to get to this status. Another source can be the exhaust valves.
From the photos it looks as though the exhaust cam lobe has scoring / wear visible,  this would indicate that the engine has lost valve clearance on the exhaust valves. If they're kept even a very small amount off their seats they will be unable to lose heat and can get hot enough to give you pre-ignition.
Inlets are cooled by incoming cooler charge so don't generally do this.

If you look at the head photo there appears to be a site of combustion between the exhaust valves and the edge of the head. It would probably take less than 30 secs running to hole the piston in these conditions.
Running conditions were also against you as I believe it runs a accelerator pumped carb? So leaner constant running and richer during throttle twisting.
Now it should have no problem running at that mixture,  but if you've already got a mechanical compromise it'll give you no safety leeway regarding cooling the combustion chamber from throttle application.
It just looks like it lost exhaust valve clearance ( assuming the plug electrode is not melted)
It'll probably be ok if after rebuild if you can identify what happened and take steps to avoid the same conditions.

[Laverda Dave]

You have made a lot of good points Nigel.
I have checked the electrode on the plug with a magnifying glass and it shows no sign of burning.
The mark on the head are fine particles of the piston.
It could be the ignition module as you and Bryan say, the only   answer I can provide on that is its the original ignition module before I rebuilt the bike.
Going back to your previous post I think may provide a possible clue, the piston material as it was a pattern part? Could this be the cause if the material used wasn't up to Honda spec?
My biggest question though is where did all the oil dissappear to over the 200 miles when I checked it (when it was just under maximum). It hadn't used any oil in the first 200 miles and it was leaking any or burning any (I followed the o/h for a good while to check if it was burning any and to check if it was running rich (it wasn't). When I stripped the bike after the blow up there was about an egg cup of oil in the sump but a lot of oil type mist in the air filter chamber, I imagine this was as a result of blow by following the piston damage?
The good news is the engineer has begun the crankshaft rebuild today. I have the new mains and conrod. He's going to see how much he needs to remove from the cylinder bore and let me know what size piston I need. When he does I shall ask the piston supplier if they can advise of the materials used and if they are the same as a honda piston. If I can find a genuine piston and rings I'll use those but I think they are even rarer then a sandcast CB750!
Thanks for you help 

[HWG27]

You have made a lot of good points Nigel.
I have checked the electrode on the plug with a magnifying glass and it shows no sign of burning.
The mark on the head are fine particles of the piston.
It could be the ignition module as you and Bryan say, the only   answer I can provide on that is its the original ignition module before I rebuilt the bike.
Going back to your previous post I think may provide a possible clue, the piston material as it was a pattern part? Could this be the cause if the material used wasn't up to Honda spec?
My biggest question though is where did all the oil dissappear to over the 200 miles when I checked it (when it was just under maximum). It hadn't used any oil in the first 200 miles and it was leaking any or burning any (I followed the o/h for a good while to check if it was burning any and to check if it was running rich (it wasn't). When I stripped the bike after the blow up there was about an egg cup of oil in the sump but a lot of oil type mist in the air filter chamber, I imagine this was as a result of blow by following the piston damage?
The good news is the engineer has begun the crankshaft rebuild today. I have the new mains and conrod. He's going to see how much he needs to remove from the cylinder bore and let me know what size piston I need. When he does I shall ask the piston supplier if they can advise of the materials used and if they are the same as a honda piston. If I can find a genuine piston and rings I'll use those but I think they are even rarer then a sandcast CB750!
Thanks for you help 

I've only just caught up with this and seen the photos in the other thread. What a shame that is. I've only ever done that with 2 strokes and that was both times a simple high compression/low octane incompatibility. It's  a very unusual thing to happen with a bike like that.  I have a spare CDI lying around somewhere that I know to be good, and you'd be quite welcome to try it. These thing do though tend to either work or not. I would not have thought it possible that the CDI would suddenly advance the ignition for instance. I'd be looking at the mixture tbh. Pattern piston should be fine imo. There's plenty of wiseco xl250 stuff about that you could use, although you pay a premium.

[Bryanj]

The 400 wasn't the cdi unit that went wrong but the flywheel generator and they just went bad for no apparent reason

[AshimotoK0]

On the 400N and the the CX 500 the ignition advance is done by pulser coils whose output modifies the 'normal' trigger waveform. I was told it was based on a Maserati patent. On those bikes the CDi Unit, pickup coils and high voltage output on the alternator (which charges the capacitors) go bad. I don't have the shop manual for the CB250RS to compare but it wouldn't surprise me if the RS doesn't employ a similar design. What people do on those bikes if the alternator HV coil goes bad is to fit a 12v to around 200v inverter which then powers the CDi. The only disadvantage being that you need a charged battery to start the bike.

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Chapter from CB400 Honda manual here:-

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[K2-K6]

That's interesting Ash, I wondered how they worked to advance the timing.

I was struggling to get my head around that schematic on the right as it looked like the timing was being retarded to me. Then realised I had a vision of clockwise rotating crank as my reference,  on which you'd have to move to the left to advance 

The timeline,  indicated with the arrow,  is going to the right and starting from TDC is effectively extending to earlier in the crank cycle.

Would it be true then that losing that high voltage pulse would stop it advancing?  As that's what it looks like to me.