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Discussion Starter · #1 ·
I have the workshop manual but it seems to be based around the 185, 200 and 250 model. Is the adjustment the same for all three models, TDC no1 cylinder, release lower dome nut then re-tighten, just dont seem right to me.
I see that the 400 model has to be running while adjusting??
Did they ever do a workshop manual just for the 250?
 

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Check the mechanism on CMSNL.com.

I don't believe any of them has/had a "rachet" mechanism...

Cam cover off, flywheel set to T, slacken the middle of cylinder nut at the rear, mechanism should auto adjust. Watch the clevis pinned rear blade go down a bit.
Nip the nut up. All done.

Some pictures of the cam area might be good.

When done, how much cam sprocket timing marks "lean back", and how much lift of cam chain from top of sprocket .

You have done valve working clearances ?

Balance chain next.....

Don't shear those special cam cover bolts !

I use a slightly similar method insuring all slack/play is taken from the front run of the chain, then auto adjust, but that's just me.

Enjoy....
 

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Check CMSNL for balance chain mech as well.

Quadrant spring loaded to auto adjust, auto adjust anticlockwise.

If slot left after autoadjust, happy days.....if no slot left, get a clutch cover gasket.....note dot position on end of shaft, look thru window upwards and to front ....it can just be seen, but only just.

Don't rotate crank during autoadjust.

Where is the dot using clock face....?
 

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To adjust any Honda, and I mean any year with a tensioner system short of an auto-adjust; meaning, the crank position in the book is saying and I paraphrase, 'set crank 15° past TDC so [wink-wink] the valve spring does not move the crank backwards as you now have two hands to work in back of the cylinder.'

Where, in my book of, 'oh look, I can set the crank at any position as long as I have the front chain rung taught, and the back is loose in the rear, I hold the crank at [pick a degree] and now with the other hand, I retighten both 8's. With this style tensioner.... I break both upper and lower first, then tap both bolts to make the spring do its thing.

Never set a cam chain with engine running. The theory of 'that in motion' was when the crank was on the down stroke, and now set the chain before it BDC and slows down. I didn't think you could time that, silly!

Signed,
U B Playing Wit Day Big Boys
 

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Discussion Starter · #8 ·
Seems i have to make a major step back on my bike. Been looking at pictures on CSMNL and there are some real big differences on the CM250 depending what country they were sold in.
I got info on my bike from David Silver Honda spares in the UK and my bike was made in 1981. If you look on CSMNL you will see that the crank cases are a horizontal split where as other CM's are vertical split cases.
Also it has long bolts through the head, barrels into the top half of the crank cases where as other have studs.
Also it is chain drive.
My bike was registered in 1983 so as far as the paperwork goes it's a 1983 bike which means my bike sat around for two years in the showroom.
My manuals for the later models and that has been throwing me a bit, does anyone have an electronic (PDF) version of the manual. I don't mind shelling out some shekels for it.

I appreciate that parts may be used across all models but i can see some differences as well.
Back to the camchain tensioner, i have been releasing the lower nut when in fact i think it's the top nut i think, both nuts are dome nuts

Any help on the correct manual would be a great help.
 

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Hi Hawkeye,

So yours has a vertically split c/case ?

Clarify the exact model either from DSS or CMSNL, engine number will help as they are listed on CMSNL and DSS will be able to tell you.

Vertical split engines are a different animal to the horizontal split ones.

You have had the info for the horizontal split shell bearing crank, the vertical split ones are pressed up crank and not that common in the UK.

However in the nineties from memory the cb250 and I think ( but not sure ) the cb400 had vertical split c/cases.

That was 30 years ago.....
 

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Hi Hawkeye, decide/deduce/look up exactly your model which may turn out to be a "grey import" into the UK.

Notice slightly different mechanism to the cm250t for the British market, a different animal .

I would be only slackening, that's slackening, not remove, the upper bolt, to allow auto adjust to take place.

When that's happened, nip up tight. It's a 6mm bolt so don't be a Gorilla.

If the system is suffering from "sticktion", a gentle tap or excercise will wake it up.

Whilst you are readjusting the 4 valve working clearances, check for timing dot creep back once cam chain is adjusted, and cam chain lift from the top of the cam sprocket.

I assume ( which can be a dangerous thing to do from time to time, solid crank or pressed up crank etc. ) the cam "bushes" are in good order ?

No balance shaft to worry about.
 

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Discussion Starter · #12 ·
Thanks for the reply guys.
I used to work at a Honda/Yamaha/Suzuki dealership in the mid 80's and went on courses for them as well so know about tightening bolts. But i never saw a CM model of any type.
I will do as you say about the cam chain tensioner, and give it a tap.
British bikes had vertical crank case's and are a real PITA to assemble compared with horizontal cases.
I have visited another forum and download a full Honda workshop manual for the CM400 and this is more like my bike but a bigger capacity.
For my bike CM250T 1981
What are the tappets set to, my (not correct) manual state 0.002 inlet and exhaust, 0.002 feels real small for me. The CM400 is 0.008, i know being a bigger bike clearance may also be bigger.

When i asked about the cylinder two head bolts that will not come out there it is not corrosion holding them in as the end of the bolt will be inside the crankcase, but i have no idea what is stopping them from just screwing out, the other six are free as birds.
I can't find a workshop manual of any kind for my bike, and i have looked. I will have to upload the CM400 manual as it is really good and a genuine Honda one.
 

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So confusion does not take over, state which type cm250 you are referring to.

The superdream like solid crank, 6 valve engine, horizontally split, call it the 6 valve engine

Or the non superdream like pressed up crank 4 valve engine vertically split, call that the 4 valve engine.

As you will remember on some hondas, 2 thou was a common ( but not the only ) valve working clearance.

I have no 4 valve cm engine info, even the cb250, the 1990's version, which was a vertical split 4 valve engine. I still may have that sales brochure but will take some finding.

I do have info for the superdream like cm model 6 valve engine.

Removing cylinder head "studs", the long "wasted" cylinder head studs like in a c90 engine that screw into the crankcases, can be problematic. Heating the alloy casing and/or cooling the stud, or bolt for that matter, helps, and sometimes is your only option.

Sonic screwdrivers have not been developed yet, or have they......a tomorrow's world April the first maybe or are they a reality these days....

As a side note, Yam XS250, remember them ? They did have a cam chain "ratcheting" adjustment system, but the engine was far from good, always sounded as if the engine was on the verge of siezing up !

Anyway, which type engine you having head bolts/studs yriuble with, and, which type engine you want cam chain and valve clearance info for ?

Probably better to start new threads perhaps ?
 

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Discussion Starter · #15 ·
The cam chain adjust has two mounting points, one in the barrel and one in the cylinder head, which one is the adjuster.
I have also seen mentioned that the engine should be a tick over, is his correct?
 

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Have another look on CMSNL at the cam chain adjusting device for your exact bike.

It has an upper bolt that holds the units top to the cylinder head and is the fixed part of the devise.

The spring acts to pull the sliding member downwards, thus making the actual curved adjusting blade more curved taking up the chains "slackness".

The sliding member is "clamped", "locked" by the square headed "bolt" ( which looks similar to a UK 70's /80's ford car battery clamp bolt, the flat terminal battery lead terminals ) which passes thru the cam chain tunnel, thru the tunnel wall and clamped by the nut in the centre of the cylinders cam tunnel.

The nut half way down and in the centre of the cylinder blocks rear, is the auto adjusting mechanism lock nut.

Slacken this nut, clamping effect of sliding part is lost, spring action auto adjusts adjusting blade downwards.

Once this has happened, nip up nut tight.

The adjusting blade is clevis pinned to the sliding member at the top and visible in camshaft area, centre rear.

The bottom of the adjusting blade, the curved one, is held in place via a balance chain slipper bracket and bolt.

I would not auto adjust with engine running, nor would I press downwards on the clevis pin to allow "extra " adjustment.

If you have had the cylinder head off, the front blade's upper "pegs" fit in two slots either side of the cam tunnel in the cylinder block.

The bottom of this blade fits in a trough involved with the balance chain slipper A, but can be fitted incorrectly. If you look down into the cam chain tunnel, you will see the trough. A mirror and torch may be needed.

If you need pictures of these parts and locations, just ask.

Actual component pictures can be seen on CMSNL.

So, cam chain adjustment, engine cold, cam cover off, flywheel cover off, set flywheel to T, slacken the cylinder block central rear adjusting nut, watch auto adjust happen, nip and tighten nut, job done.

How are cam sprocket timing marks, level, or any "lean back" ?

Also, cam chain lift from top of cam sprocket, how much daylight seen between chain and teeth when cam chain lifted ?

I like to ensure the cam chain front run is taught during auto adjust.

When auto adjust had taken place and nut is locked, move flywheel back and forth one degree, not much at all, watch for chain link movement to indicate wear in each link. This is akin to cam chain lift.

It can show wear in the whole drive train. Try to not let the cam sprocket move at all for more accurate wear assessment.

All of this procedure with cam and flywheel covers removed, takes say 10 seconds.

Check adjust valve clearances, my book says clearances for this type of cm 250 could be 0.12mm inlet, 0.16mm exhaust, or, 0.10mm inlet, 0.14mm exhaust cold.

Engine no. Cb250te-2100001 and subsequent
Frame no. Mc05-2400001 and subsequent

Check on CMSNL using your frame and engine numbers, they do appear near the model classification.

I don't have internet other than mobile phone, so pictures will be phone pictures.

However, I can scan and save to Sd card, transport card and reader to an internet pc some distance away and send from there....a time consuming process.

Pictures of the cam area if this bike please.

Is this the bike with head bolt problems ?

Balance chain adjustment as per superdream, have you found the DOT meantioned and after auto adjust, how much adjusting slot remaining ?

The dot position relative to clock face and remaining adjusting slot is quite important.
 

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The cam chain adjust has two mounting points, one in the barrel and one in the cylinder head, which one is the adjuster.
I have also seen mentioned that the engine should be a tick over, is his correct?
As I have recommended, trick is to loosen both, tap both upper and lower heads, or you can snap the rear wheel in gear backwards, then load forward and hold. Tighten the top first, the bottom last.

Signed,
NOLTT (no one likes Turtle's Twirling)
 

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Discussion Starter · #19 ·
I have done all you said with the cam chain tensioner but will do again keep flywheel in place and chain slack at the rear of chain.

On a different note, late last night in bed thinking on this. Honda seem to have made these bikes in a bit of a mix match for the few years they were in existence. Been looking on CMSNL web site i have noticed that they are all looking very similar.
On my bike i have found I have the CM400 forks, speedo rev counter cluster, i know someone could have fitted these at a later date. But looking at pictures on CSMNL my bike lean more toward a 400.
I will be looking and frame no's and engine no's later today.

Can I ask has anyone ever actually done a top end strip down. I am still confused as to why two of the bolts are solid and will not undo. The other six are as free as birds. The bolts definitely are not exposed to outside where corrosion could occur.
Actually breaking the bolts would be ok as long as they broke in the middle, if the break just above the crank case that would be a whole lot of trouble.


Daydreamer
You quote "Check adjust valve clearances, my book says clearances for this type of cm 250 could be 0.12mm inlet, 0.16mm exhaust, or, 0.10mm inlet, 0.14mm exhaust cold."

That is real big for a Honda regarding valve clearance. The manual i have for a CM250C is 0.05mm inlet and 0.05 exhaust. Can i ask what book/manual you have?
 

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So some more confusion.

Your cm250 engine is 6 valve horizontally split engine ?

That's 3 valves per cylinder, one exhaust and two inlets.

The inlets are controlled via a "forked" rocker arm.

Is this the case with your engine, 6 valves, horizontally split crank cases ?

Post a picture of the cam shaft area.

Over...
 
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