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CB400F failure

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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:

--- Quote from: K2-K6 on October 31, 2024, 03:16:11 PM ---Doesn't it get too rich at altitude if set correctly at sea level  ?

--- End quote ---
Yes

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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:

--- Quote from: Binman180 on October 31, 2024, 12:36:15 PM ---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.

--- End quote ---

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

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