CB400F failure

[Binman180]

Hi All,

Took my CB400F for a trip along the Pyrenees, Bilbao to Barcelona but unfortunately fully complete the return trip.

The route I took meant the bike covered approx 750mi with a mixture of 30mph and 80mph roads - some riding was enthusiastic and some riding was cruising. The fuel I used was whatever standard e10 fuel as "super" wasn't always available.

At some point, the bike began overheating, smoking, had an unstable idle and would not accelerate past 60mph prompting me to cease riding for the remainder of the trip through fear of damage.

The bike was rebuilt some years ago using a mix of OEM/pattern parts (in particular the rings were aftermarket) and has covered approx 12000 miles.

Bike is back home and currently (slowly) in the rebuild process.

On dissassembly, there appears to be some minor damage to the pistons, rings, bores and head. (see the pictures attached). It appears to be localised to cylinders 2 and 3, cylinders 1 and 4 are visually ok and have good compression.

1. Any ideas on what could have happened or some ideas on what might have caused this situation? it would be pointless to rebuild and ignore this as it could happen again.

2. Are any of these components savlageable? The cylinders are actually smooth, can't feel anything by fingernail but there are a couple of dents. Probably due to the piston rings breaking up. Perhaps a rebore to 0.50 and use the DSS aftermarket piston/ring kit again?

As for the head, it does not look like theres damage (on initial look over) to the valves, valve seats or sealing faces - could this damage be removed with a rotary tool and tidied up? or is a replacement head necessary?

3. other misc information

- Engine oil used halfords petrol/diesel oils with a higher zinc content changed at 1200mi intervals.
- So far, it does not appear that theres been oil starvation. It looks like the head has been recieving plenty of oil but further inspection of the pump etc still needs to be done.
- Carbs have genuine jets and parts, no obvious signs of blockages etc.
- Genuine points ignition which appears to be adjusted correctly (via strobe light) and is functioning.

[Alexrayz]

Hey man! That is a sad view, sorry about that!
My first guess would be overheating from an improper mixture?
Did you adjust the air/fuel mixture? At altitude there is less air available so the mixture becomes richer. Without adjustment it can get the balance off kilter and mess with the performance.
What kind of smoke did ya have? And was it struggling to idle or was it hanging revs?

Cheers and good luck for the rebuild.

[Johnwebley]

Sorry to hear you have issues

It looks like pre ignition, detonation, or pinking


Maybe the ignition was over advanced?

Or wrong grade plugs,

Or running to lean,

Best wishes

Sent from my SM-A546E using Tapatalk

[Nurse Julie]

Broken rings, pre ignition, running lean, very similar damage to the damage I had on my 400

[McCabe-Thiele (Ted)]

Not sure if it would have prevented the damage  in your cases (probably not as it changes the burn temperature) ,   I've always been a fan of using some UCL like Redex.
It was very popular at filling stations in the 1960's when I worked on an Esso  Petrol Forecourt (no self service back then) even though all petrol was still leaded.

The forecourt had a hand held dispenser, one shot per gallon (2p) or for a tank full of fuel we had a large static dispenser that would deliver a Long Shot for 10p.

[K2-K6]

A thread about this type of problem here

http://www.sohc.co.uk/index.php/topic,14609.0.html

 

[K2-K6]

It would be interest to hear which spark plugs were in use too.

Can you post binman?

[Bryanj]

Timing slipped or "over enthusiastic" riding from cold, possibly ring groove to ring clearance too large you will need at least 1 piston but what does 3 look like? And before any decision on that a light hone on the cylinder(s) in question plus very carefull inspection and measurement, by the time you have 2 piston kits plus rings it can be as much a 4 piston kits, oversize is same cost as standard so only extra is a rebore to be "as new"
At what altitude did the problem start as possibly weak from thin air

[Binman180]

Hi all,

to answer some additional questions

1. My own suspitions similar to others is lean condition and detonation which deteriorated the pistons and ultimately the piston rings. since the top ring is gone on cylinders 2 & 3.
Cylinder 3 has damage to the same type and extent as cylinder 2. Cylinders 1 and 4 seem ok and no visible damage but do look like they have been hot. All pistons will be replaced and all cylinders will be inspected and reworked.

2. Thank you for the link K2-K6. I have read that post before and I think even replied to it. I'd like to think I learned from the information provided and It did influence my riding style but perhaps I didn't adjust enough. I try to keep the rev range a bit higher and not roll about on the throttle too much accelerating in high gears.

3. The colour of the smoke was blue, which I guess tracks.

4. The spark plugs used were NGK D8EA, the plug caps were the NGK ones DSS sells.

5. Timing was configured with the help of this video https://www.youtube.com/watch?v=3933ouJ1wrI and I used a timing light. (this isn't to say that I didn't make a mistake of some sort, of course.)

6. I didn't adjust the mixture dependant on the altitude I was at. I can't say I was being that observant about the altitude. a quick google suggests the highest I got was probably about 900m elevation.

[K2-K6]

Doesn't it get too rich at altitude if set correctly at sea level  ?

[McCabe-Thiele (Ted)]

How many miles were you riding in a typical day without stopping could this be a factor on an air cooled engine?

[Johnwebley]

Doesn't it get too rich at altitude if set correctly at sea level  ?

Yes

Sent from my SM-A546E using Tapatalk

[K2-K6]

Mixture shift, for altitude reason, shouldn't have played a part in this then, logically.

[K2-K6]

It is linked to one "set" of the ignition system though.

Points, condenser, coil (resistance) etc.

Carbs possible, but less likely as there's not a specific link between 2 & 3 that's not on other combination ie 1 & 2 for example.

[400 Cafe Racer]

Hi All,

Took my CB400F for a trip along the Pyrenees, Bilbao to Barcelona but unfortunately fully complete the return trip.

The route I took meant the bike covered approx 750mi with a mixture of 30mph and 80mph roads - some riding was enthusiastic and some riding was cruising. The fuel I used was whatever standard e10 fuel as "super" wasn't always available.

At some point, the bike began overheating, smoking, had an unstable idle and would not accelerate past 60mph prompting me to cease riding for the remainder of the trip through fear of damage.

The bike was rebuilt some years ago using a mix of OEM/pattern parts (in particular the rings were aftermarket) and has covered approx 12000 miles.

Bike is back home and currently (slowly) in the rebuild process.

On dissassembly, there appears to be some minor damage to the pistons, rings, bores and head. (see the pictures attached). It appears to be localised to cylinders 2 and 3, cylinders 1 and 4 are visually ok and have good compression.

1. Any ideas on what could have happened or some ideas on what might have caused this situation? it would be pointless to rebuild and ignore this as it could happen again.

2. Are any of these components savlageable? The cylinders are actually smooth, can't feel anything by fingernail but there are a couple of dents. Probably due to the piston rings breaking up. Perhaps a rebore to 0.50 and use the DSS aftermarket piston/ring kit again?

As for the head, it does not look like theres damage (on initial look over) to the valves, valve seats or sealing faces - could this damage be removed with a rotary tool and tidied up? or is a replacement head necessary?

3. other misc information

- Engine oil used halfords petrol/diesel oils with a higher zinc content changed at 1200mi intervals.
- So far, it does not appear that theres been oil starvation. It looks like the head has been recieving plenty of oil but further inspection of the pump etc still needs to be done.
- Carbs have genuine jets and parts, no obvious signs of blockages etc.
- Genuine points ignition which appears to be adjusted correctly (via strobe light) and is functioning.

Hi Binman 180,

Observations

My opinion FWIW
1.It looks as though the ring breakage through the groove, getting onto the top of the piston has caused all the peppering and marking to the top of the piston, head and cylinder wall.
2. The valve colour from combustion looks to be on the Weak mixture side, adding to piston crown temperature.
3. There are signs of burnt oil laquer around the piston upper half.

See below an opinion I gave to a member regarding concerns over oil temperature. My gut feeling is that the centre 2 cylinders will run hotter simply because they are in the middle of a heat mass.

"Hi,

Having fitted both an oil pressure guage and  temp guage to my CB400F some months ago, I can confirm that in normal riding you will easily be running at 95c + with 40ish psi @ 3000 and if you give it the beans for a few miles you will be at 120c with 15psi @ 3000. I suspect the reason a low 4.5psi oil pressure switch used was to prevent worry of riders seeing the light blinking at idle on a hot engine.

It is likely that cylinders 2&3 are running even hotter.
**Piston temperature in the area of the ring grooves and piston crown will be well in excess of the 120c Oil temp.

IMHO If your seals and gaskets are newish, there is a definite case for synthetically improved oils** which wern't available in the day to handle the higher temp protection better, and I will be running some tests next spring.

I think one of the major reasons for piston ring and groove failures is the deposits and gumming caused by burnt 10w 40 oil which cannot handle the excessive heat conditions at the top of the piston."

Regards

Dennis