CB400/4 ENGINE STRIP, ASSESSMENT AND REBUILD BY JULIE - NOW COMPLETE

[Nurse Julie]

That looks good if it’s not top secret what has been done to get to that condition

No secret. Hot wash to remove all grease etc.  Acid dipped to remove all old paint / oil. Another hot wash. Soda blast and hand finish to remove any old corrosion that may be left in the nooks and crannies. Painted with engine enamel, air cured for 24 hours then final cure in the oven. The good outcome is definitely due to the prep.

Julie, was final painting farmed out or DIY?

Trigger did it and he has many engines on the paint list at the moment but, I did squeeze mine in.

[Rozabikes Tim]

That looks good if it’s not top secret what has been done to get to that condition

No secret. Hot wash to remove all grease etc.  Acid dipped to remove all old paint / oil. Another hot wash. Soda blast and hand finish to remove any old corrosion that may be left in the nooks and crannies. Painted with engine enamel, air cured for 24 hours then final cure in the oven. The good outcome is definitely due to the prep.

Julie, was final painting farmed out or DIY?

Trigger and he has many engines on the paint list at the moment but, I did squeeze mine in.

Mmm favouritism then.... Rattle can job or proper set up?

[Nurse Julie]

That looks good if it’s not top secret what has been done to get to that condition

No secret. Hot wash to remove all grease etc.  Acid dipped to remove all old paint / oil. Another hot wash. Soda blast and hand finish to remove any old corrosion that may be left in the nooks and crannies. Painted with engine enamel, air cured for 24 hours then final cure in the oven. The good outcome is definitely due to the prep.

Julie, was final painting farmed out or DIY?

Trigger and he has many engines on the paint list at the moment but, I did squeeze mine in.

Mmm favouritism then.... Rattle can job or proper set up?

Favouritism, ha, your having a laugh Tim. Customers come first and as I'm told regularly, I'm not a customer    Rattle can enamel. Trig heats the cases and the paint  before applying, it gives a much better adherence and finish.

[K2-K6]

Online K2-K6
SOHC Master
Join Date: Jun 2010
Location: Wimbledon London
Posts: 1480
View Profile  Personal Message (Online)

400 Four detonation
« on: Today at 09:05:04 AM »
QuoteModifyRemove
Alright,  I know I " go on about it" but been looking at why I feel detonation can easily occur on this engine given the right / wrong conditions.

As others pointed out you normally associate lean mixture with something like this,  which is not obviously the case if jetted properly,  you'd have thought?
A characteristic of slide carbs is that you can create it with how you ride,  even if setup is correct.


Above should be a dyno trace that Jensen has kindly shared of his 400 Four running in 4th gear with throttle rolled open in that one gear to build the power curve you see in top traces.

The really interesting bit is the lower trace,  left scale is mixture with rpm along the bottom. It's obvious from that peak leading up to 4000rpm that it's running toward an over lean condition. Complete burn is considered to be 14.7 parts air to 1 part fuel. Most engines won't get that close. During acceleration you'd expect closer to 12 to 1 to make it move,  with cruising state in between the two.
It's not a wrong setting on here,  just characteristic of running from low rpm to high in one gear which is needed for the dyno plot.

What it does is the vacuum the engine draws at low rpm with throttle wide open cannot match the flow the carb venturi is making,  resulting in loss of vac on the main jet,  and so less fuel than ideal is pulled from float chambers ( it's this bit that PD carbs fill in with accelerator pump,  clever a?)  you can't jet your way out of it as the carb and main jet is almost fully open. It's really a failure of slide carbs used in this way and what would be termed "lugging " the motor in high gear.

In addition,  the ignition advance reaches max around 3000rpm and quite early for full load application ( more suited to cruising on light throttle to give good economy)  so in that lean peak area you've also got probably too much advance for a short period.

Put those conditions together,  perhaps riding along into a headwind,  fith or sixth gear,  trying to keep up speed at lowered rpm and you've got classic detonation. Riders with experience would recognise it's not pulling properly,  but if the circumstances are held like that without the engine accelerating through that, it will most likely detonate. It'll only take a few minutes like that to start the destruction of the pistons and rings.

You can't jet it differently,  it's just how you ride it. They need to be revved if you want them to go.
You've either got to back down the throttle or drop gears and spin it to higher revs.
Modify message
[ Guests cannot view attachments ]

[K2-K6]

The above post is copied from another thread that I've started with a relevant heading to make it easy to find in future.

http://www.sohc.co.uk/index.php/topic,14609.msg118849/topicseen.html#new

Thought I'd better explain as Julie requested that I post it over here otherwise it may look a bit like random ranting by me.

[Nurse Julie]

That's really interesting Nigel. So, going back to the mess on my pistons / rings on my engine strip thread, it would make sense to me how my engine got in such a state. If a PO had not ridden it as the bike likes to be ridden for optimum air / fuel mixture ie, high revs and the engine 'pulling' a majority of the time then it makes total sense how the damage had occurred. I have said many times that the CB400/4 likes to be ridden like a 2 stroke, high revs and minimal lugging and I can see how riding it for long periods in the wrong gear / too lower revs can start a chain of events leading to damage.
A lot of riders think the CB400/4, being a smallish engines bike, are easy to ride but it's totally the opposite in fact, they are bloody hard work if ridden properly, definitely not a bike for the cruiser or the lazy rider.

[Nurse Julie]

All the above makes total sense to me of how, after detonation, the pistons/ rings where damaged to such a great extent on 3 of the pistons/ rings. But, that still does not explain why 1 piston and rings were totally unaffected. I'm now thinking that this was the end result of a chain of events. I think, as Mr Davo and others have commented,  the bike having had been left standing for at least 8 years prior to me owning it, that we know of, rust had started forming between the rings and the bores and someone had unseized it either, forcibly or just by trying to start the engine, the rings sustained damage and the process of destruction had begun. I think the unaffected piston was due to the stroke the piston was on when the bike was laid up.

[K2-K6]

It may be linked to storage and stuck rings,  you'd not want to rule out something obvious if you were to recommisiion an old engine. It's possible it could be a reverse of the logic though. If you had the rings stuck on one piston,  then the resulting loss of compression would make that faulty one less likely to detonate. Float height on one carb or leaking float valve would make it run rich,  possibly on one pot. Even something as simple as having one plug with it's gap facing a different way in the chamber will have some influence.

It's something that can easily be done that will completely wreck pistons in even the most pristine condition. I stood in on a dyno run for quite a radical tuned car engine in which they were running it under load to make sure they didn't get it detonating in use. Completing igniting swings ( running at varied static advance to make sure they had the most advance they could use while defining what's too far so they could get ideal setting) it was running close to flat out when it started to exhibit that. It sounds like something from general description to be minor when described as "pinking" but when standing that close to it,  it really does sound like you're listening to the beginning of an explosion! I stood outside the doorway as I imagined being hit by bits of hot engine it sounded so bad. It was only a second or two as they obviously wanted to avoid any damage,  but nevertheless gave an indication of the violence that it can exert if left unchecked.

[Nurse Julie]

I re assembled the crankshaft today. Even though initially I had thought there would be signs of damage and wear on the shells due to the break up of the pistons / rings and the debris going around the engine, all the engine shells, to my amazement show no sign of damage, or excessive wear and measure well within the service limit. Therefore I am using all the original shells throughout the re build.

I am so scared of getting anything out of sequence, or position, or put back in the wrong place, I have to keep everything in order, or just 'so', hence the conrods on a handy screwdriver, in order !!!

I blew all the oilways out with the airline to make certain they were thoroughly clean, even though I had given the crank a thorough clean previously. I also cleaned, again, all the bearings, both front and back to make sure there was no contamination when I fitted the shells to the con rods. I then covered the shells with bearing guard, torqued up the conrod bolts and today I am using pink engineers paint as the visual reference to myself that I have torqued the bolts.

[ Guests cannot view attachments ]
[ Guests cannot view attachments ]
[ Guests cannot view attachments ]
[ Guests cannot view attachments ]

[Nurse Julie]

I then set about fitting the top and bottom crank case shells, which I had numbered and bagged up on strip down as top and bottom. Once again, I gave them a thorough clean, again. I fitted the 2 primary chain guards, with the indent facing backwards towards the gearbox end. I also fitted the reduction gear and the photo shows the stopper bolt and the lock washer that holds the gear shaft in place.
[ Guests cannot view attachments ]
[ Guests cannot view attachments ]
[ Guests cannot view attachments ]

All in, not a bad bit of work today. I'm very slow but want to get it right (obviously!!!) but also want to learn as much as I can throughout the process. I also have to keep thinking what is position No1, for example, as at the moment the top case, being upside down is, in effect, back to front but I'm gradually getting the knack. It's lovely working with everything nice and clean.

[mike the bike]

Your photos are spot on. 

[K2-K6]

Very nice.

I can see a problem though,  that mug has no tea in it,  may have to attend Guy Martin training course on the benefits of tea during engine building 

[Bryanj]

Its also way too small, you need a proper Lancashire pint pot!

[Laverda Dave]

Looking good Julie.
Word of warning for you about UPS delivering your CMS parts, you have to home when they deliver as CMS insist on your specific signature. They will not deliver to a neighbour even if you ask them to. After three failed attempts at delivery you have to collect them from the UPS depot.
Don't ask me how I know this  !

[Rozabikes Tim]

Very nice.

I can see a problem though,  that mug has no tea in it,  may have to attend Guy Martin training course on the benefits of tea during engine building 

Its not tea its Linconshire potatoe Vodka☺