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CB400T

Elwyn

Well-known Member
Joined
Nov 25, 2021
Total Posts
68
Total likes
0
Location
Warrington, UK
Hello

1978 CB400T Dream

It's never been a great starter

We've had the carbs stripped down and rebuilt by a proper mechanic
It still didn't start well

I recently had a rush of blood to my head and did a bit of home fiddling as well as sending things away to real people

Summed up below:
New battery, fully charged
Starter motor brushes and O ring replaced
New ignition coil, HT leads, spark plug caps (David Silver)
New spark plugs
New starter solenoid (David Silver)
New rectifier / regulator (David Silver)
New CDI (Rex Speedshop)
Stator taken out, sent away to re rewound, returned, refitted with rotor also put back in place
3 fuses replaced
Fresh petrol / gas in the tank
Key in
Turn to ignition
Neutral selected
Press the starter
Starter motor springs into life
Engine still won't start

I said a few choice words

Any advice from anyone out there?

I also had to change the short fuel hose from the fuel tap
Will that have resulted in a loss of pressure?
Might be part of the problem?
If so, how to I overcome it?

Thanks in advance

Elwyn
 
A few questions for you.
Compression? Crank the engine until the gauge stops rising with the throttle held wide open and choke off(knob pushed in).
New plugs? NGK D8EA?
Stator rewind? was the primary pickup and advancer pulsar repaired as well?
Your definition of "doesn't start well".

Just to clear things up the electrical system; battery, starter, solenoid, voltage regulator, etc., will have no affect on starting other than to have the engine cranking fast enough. The ignition system is powered directly from the stator via the Blue and White wires. The engine will actually run w/o a battery if it's push started.
Your fuel line should be 5.5mm, 5.3 minimum. Before starting turn the petcock on and wait for @1 minute to allow the float bowls to fill.
Honda's recommendation for cold start is Choke fully on, pulled up, push starter button. Do Not operate the throttle while starting. After engine starts immediately decrease the choke to get a stabile idle of 1200-1500, after 1/2 minute push the choke in fully to open them and operate the throttle as needed to keep an idle speed. You can download the 1978 CB400 Owners Manual from the library, pages 28 & 29.
 
A few questions for you.
Compression? Crank the engine until the gauge stops rising with the throttle held wide open and choke off(knob pushed in).
Answer : sorry, I don't have a compression tester

New plugs? NGK D8EA?
Answer : NGK

Stator rewind? was the primary pickup and advancer pulsar repaired as well?
Answer : This is the description from the specialist:
  • Ignition source coil rewind, and
  • Internal pick up repair


Your definition of "doesn't start well".
Answer : Using the starter button, the starter motor turns
Eagerly
Choke out
No throttle fiddling
Starter turns, but the engine doesn't fire
Just the starter turning
It used to be just the same, with occasional bursts of fire / spark that would occasionally "catch" and the bike would start
It was always very hit and miss
It's now 100% miss


Just to clear things up the electrical system; battery, starter, solenoid, voltage regulator, etc., will have no affect on starting other than to have the engine cranking fast enough. The ignition system is powered directly from the stator via the Blue and White wires. The engine will actually run w/o a battery if it's push started.
Answer : understood. The replacement parts were an act of desperation. Having had the carbs stripped down and rebuilt (by a local, professional motorcycle mechanic), but still suffering the "won't start" issues described above, I just thought "balls to it" and placed orders for the easy bolt on / bolt off electrical components in the misguided hope that the scattergun effect would solve the problem by chance. It didn't. It was very much out of desperation and frustration.

Your fuel line should be 5.5mm, 5.3 minimum.
Answer : that's what it is, thanks

Before starting turn the petcock on and wait for @1 minute to allow the float bowls to fill.
Answer : I'll try that, thanks
It's fresh fuel too

Honda's recommendation for cold start is Choke fully on, pulled up, push starter button. Do Not operate the throttle while starting. After engine starts immediately decrease the choke to get a stabile idle of 1200-1500, after 1/2 minute push the choke in fully to open them and operate the throttle as needed to keep an idle speed. You can download the 1978 CB400 Owners Manual from the library, pages 28 & 29.
Answer : thanks. That's what we've been doing, but not getting to the part where the engine starts.

Also
I can't find the manuals on this website.
I'm obviously not looking in the right place

Many, many thanks

This is greatly appreciated

Regards

Elwyn
 
Did you test the petcock for flow when you had the fuel line off? Can also check the carb bowls for fuel by cracking open the drain screw(s).
 
Hello
Fuel flows well from the petcock
It was replaced about 2 years ago
I've checked both spark plugs
Both are sparking
Both are wet (damp, not dripping) and smell of petrol / gas
I've not checked the carb bowls via the drain screws
Not actually sure which ones are the drain screws!
I'm usually under an old Mini
I like bikes, but their workings are a bit of a mystery to me
Thanks

Elwyn
 
Hello
Fuel flows well from the petcock
It was replaced about 2 years ago
I've checked both spark plugs
Both are sparking
Both are wet (damp, not dripping) and smell of petrol / gas
I've not checked the carb bowls via the drain screws
Not actually sure which ones are the drain screws!
I'm usually under an old Mini
I like bikes, but their workings are a bit of a mystery to me
Thanks

Elwyn
Drain screws are at the bottom of the carb bowls. Thin flexible tubing is attached to direct drainage to the rear of the engine and floor.
 
Thanks
The photos help
I thought the drain screws might have been the brass things right at the bottom (only because they're at the bottom
Red rectangle below
Clearly not
I'll go and loosen the correct drain screws now
Just so I know, what are these brass screws (red rectangle)?
I found a parts website that simply describes them as screw-sets, but with no other description / clue

7DujMxFDMw8AAAAASUVORK5CYII=
 
Drain screws cracked open a little
Fuel dripped to the ground via the fitted overflow pipes
That's a good thing, I assume?
Now tightened back up
 
I suspected a wiring fault / something I'd done wrong
From a similarly named forum, I took a look at the kill switch to CDi connection, thinking I'd done something wrong
I've just taken the tank back off
The bullet connector connects to a corresponding bullet connector on a wire that disappears into the main wiring loom, that I assume connects to the kill switch
Still can't get the bike to start though
With or without that kill switch (I assume) connection
I tried it with that kill switch bullet connector undone in case the kill switch is faulty
Fully charged battery, starter motor doing its thing, plugs are sparking, fuel in the carbs and cylinders, but it won't fire up at all
 
I suspected a wiring fault / something I'd done wrong
From a similarly named forum, I took a look at the kill switch to CDi connection, thinking I'd done something wrong
I've just taken the tank back off
The bullet connector connects to a corresponding bullet connector on a wire that disappears into the main wiring loom, that I assume connects to the kill switch
Still can't get the bike to start though
With or without that kill switch (I assume) connection
I tried it with that kill switch bullet connector undone in case the kill switch is faulty
Fully charged battery, starter motor doing its thing, plugs are sparking, fuel in the carbs and cylinders, but it won't fire up at all

What color is the wire with the bullet connector you mention? The ignition kill wire should be black with white stripe
 
Yes, black with a white stripe
Spark plugs are sparking
Would they spark if the kill switch was doing its thing?
 
Further desperation kicking in
I've ordered a compression tester from the kind Mr Bezos
Cylinder compression was queries earlier and it's one of the checks in the service manual for diagnosing and fixing a non starter
I don't know what the symptoms of low / out of balance pressure would be
Might that be the cause here?
 
Yes, black with a white stripe
Spark plugs are sparking
Would they spark if the kill switch was doing its thing?

If the kill switch was working properly, the plugs would not spark when the kill switch (or the ignition switch) was off, and the plugs should spark when both switches are on
 
Thanks again
Ruling things out but not getting close to the root cause
I'm not sure of images can be posted into messages, but I'll try
In the gathering of wires and cables with the fuse box and the rectifier / regulator, I have a green cable / wire that isn't connected to anything


Red arrow
Any idea what it might be?
There's no other corresponding loose / unconnected wire / cable

spare wire!.JPG
 
Last edited by a moderator:
The Green wire in the picture would be used with a 3 terminal flasher unit, you have the 2 terminal US type installed.
Spark plugs that are fuel fouled/soaked need to be replaced. Might go ahead and pick up a box of 10 so you have them handy. Once fouled they're pretty much junk.
Sounds like you had Rex's do the stator from the wording of the repairs. Good people. His CDI's are good as well. Using the shotgun approach isn't the most cost effective but it does cover all the bases and I've done the same before.
Have the valves been adjusted?
What carb numbers do you have, stamp at the left side of the body right above the float bowl seam.
 
Thanks
I'll ignore the green wire
The valves were lapped in (that might be a UK phrase?) in 2019 as part of the carb work / engine gasket replacement work we had carried out by our local bike mechanic
I'll check the carb numbers shortly
 
I would adjust the valves, this will be a regular maintenance procedure. Lapping the valves is not the best idea with these engines. The valve face only has a .0002" hard face and lapping can go thru that easily.
2 hours charged for cleaning the carbs tells me they did a cursory cleaning not a in depth type which takes more hours to do correctly. There's a sticky on Rebuilding VB Carbs in the Fuel section. I hope they didn't change any of the brass parts, jets/needles.
 
From memory, they spent more hours than they billed us for. The overall 8 hours was probably not quite as the 2 and 6 they put on the invoice
No brass parts (jets / needles) were changed
The main part of the job was to strip things down to get to various perished gaskets
That's what they charged the majority of the labour for
Working on the carbs was something that I asked them to do while the opportunity presented itself
 
VB31's, same carb as our VB21's. Largest venturi used on these engine, 32mm. VB22/32 are 30mm and have an accelerator pump.
So you should have the Black adjustable floats which is great. The idle/slow jet is pressed in so removal for cleaning can be problematic but doable. To give an idea of the time needed to go thru and clean these carbs member Randall just finished his, 3 days later. I spend 2-3 days going thru them, lots of ultrasonic time.
As a wrench the billing time is almost always less than actual time spent.
 
Brass screws...I recall only 2 on each carburetor, 1 being a drain screw (horizontally oriented) and the other being the "pilot mixture" screw (vertically oriented)...
..Have you verified that the vertically-oriented pilot-mixture screws of each carburetor are unscrewed 1.5 turns out from closed ?

Reminder of the 3 ingredients to ignition:
1) gas
2) spark
3) air

"Initially, pilot mixture screws should be screwed clockwise all the way in until they gently bottom-out or "seat" in their holes and they then should be unscrewed a specific number of turns or fraction thereof dependent upon the carburetor they are installed so to enable an engine's starting. They have little effect at engine speeds faster than idle speed"

..the Clymer published service manual states an initial unscrewing of 1.5 turns for the 1978-79 CB400T

Mouse nest in the air filter housing box ?
If not, is your engine's air filter good and clean ?
If yes, are the rubber duct boots between the air filter box and rear-side of carbs snug in the box and clamped well onto the carbs ?
If yes, are those rubber duct boots intact and not split apart nor torn anywhere along their lengths ?

The shop manuals "Library" here at this forum will lead you to the Honda Factory service manual for your cycle and within it you will find a more lengthy checklist of "troubleshooting" items if you haven't found it already.

​GOOD LUCK !
 
Checked the pilot mixture screws
Just by looking at them, it was impossible to tell so I followed the guide above
They're both definitely now 1.5 turns back from gently bottomed out / seated

No mice, nest, debris etc in the air filter box
It's clean and clear
The air filter is almost new
Rubber duct boots between air filter box and carbs were replaced in 2019 and remain in great, as new condition, tightly / firmly secured to the carbs

It still won't start
 
Has the compression tester arrived yet?
Going back thru all the posts it seems that you have consistent spark every time you've checked which is great. With fresh plugs the cylinders will be getting fire. Only 2 things left in the equation, compression and fuel. I'm leaning towards fuel at the moment. These engines will run as low as 135 psi but be difficult to start.
 
Hello Mr Distance
Mr Bezos promised to deliver the compression tool on Saturday, but he must have been busy playing with his space rocket.
It’s due to arrive tomorrow.
I also need to check the screws on the underside of the carbs. The ones with the needle and spring. Fully seated then wound back 1.5 turns, I believe.
A simple check / adjustment.
Lots of other advice received
Check valve clearances, timing chain tension, cam timing (not at all sure about that one), balance chain tension, carb float height, choke flaps (again, a bit stumped).
Plenty to go at
 
Timing chain tension is checked/set with the engine running at idle speed.
Mixture screws can be 1.5-2 turns out
Balancer chain adjustment is a maintenance item, not related to the problem.
I've yet to see cam timing off on one of these engines unless someone actually messed up during assembly (not Honda).
Checking that the choke plates are actually closing when the knob is pulled is a good idea, means pulling the air box boots loose to see.
 
The compression tool thing has arrived
I've read and watched youtube videos on what to do
All clear

However, everything I've read or watched refers to compression testing a warmed up engine
Mine won't start and I live in the north west of England where nothing is warm

Are there any known cold engine compression test figures?
There's no point in testing the engine cold and comparing those results with target compression readings for a warmed up engine

And would the "up to 10% discrepancy between cylinders" apply to a cold engine?
Or not

Thanks in advance
 
A warm/hot engine is preferred but you have no choice in the matter. Run it cold, throttle held wide open and cranked until the gauge stops rising. 10% variance is fine. Report your numbers and lets see what you've got.
 
Cylinder pressures taken cold
Dry
LH : 80 psi
RH : 58 - 60 psi


Left hand cylinder pressure Dec 21.JPGRight hand cylinder pressure Dec 21.JPG

I then squirted a little oil into each cylinder through the spark plug holes and re-tested the compression
Again, with the engine cold
Results:

LH : 95 psi
RH : 90 psi

Left hand with oil squirt cylinder pressure Dec 21.JPGRight hand with oil squirt cylinder pressure Dec 21.JPG


Within 10%
Good

Piston rings need replacing?
Both sides?
Makes sense as a matter of maintenance / presented opportunity

Check and adjust valve clearances as a sensible maintenance task while the oily bits are exposed?

Is that correct?
 
Check and adjust valve clearances as a sensible maintenance task while the oily bits are exposed?

Is that correct?

They're only exposed if you've removed the "valve cover" from the top of the engine. So, if you've done that and it remains removed then yes, you ought to check the clearances between the valve stem "tappets" and their rocker arms with a properly-thick "feeler gauge" when the piston for that cylinder is at Top Dead Center (TDC) and that cylinder's valves are "loose".

This procedure also requires the engine's left side-cover to be removed for access to the alternator's flywheel with its punch-stamped timing marks, one of which is used to set TDC for each cylinder using the center bolt of the alternator/flywheel to rotate the flywheel counter-clockwise to align the mark. Spark plugs removed, of course, during this procedure...SEE the FSM for this "valve clearances" check in which you'll either verify they are properly gapped OR need a correction from being too loose (gap is too large) or too tight (gap is too small)
 
^^^With one caveat, of course - the valves are to be adjusted at TDC of the compression stroke on each cylinder. Since the SOHC 400 is a 360° crankshaft (meaning the pistons go up and down together), both pistons will be at TDC at the same time but one will be on exhaust stroke while the other is on compression stroke. To determine which stroke, watch the intake valves open and close during crankshaft rotation and then watch for TDC immediately afterward for adjusting that cylinder.
 
Those are low. If your testing with a hose on the tester (I prefer the cheap short rubber nipple directly in the spark plug hole) a not robust battery for the starter and the other plug in (remove both), no air filters needed and, as previously said, Full throttle, no choke. These things could easily be robbing our readings by even 50lbs. If around 130 should still run, gutless, but run. My 78 CB400Tii after 20 year nap in a leaky garage popped 175/180 with 16K on the clock, was maintained (I know the PO). You did not mention mileage did you? Even if your rings are shot it should start maybe smoke some. Don't lose heart this college of knowledge you're in is worth it.
 
It's done 60k miles
It was a rubber hose tester
It's been well maintained according to the paperwork

Side cover coming off
TBC to be located

Rocker / valve cover already off

Oh what fun
 
Oh what fun

So true, but that's the ownership of a vintage Honda. 60,000 miles may well account for the level of wear on the internal parts, and it must have had some level of decent maintenance along the way or it would not likely have made it that far. Sounds like you're in for an engine rebuild.
 
I'm applying my flimsy knowledge from fettling my old Mini
It's not directly transferable
Valve / rocker cover off
8 long, holding down bolts slackened then removed in the right order
Set aside and marked up to go back in the same locations

Looked in the Haynes manual about what to do with the cam / timing chain
Absolutely rubbish
Tried youtube
Nothing

Any guides on here so I can work my way down to the pistons to replace piston rings and whatever else i find on my way?
 
I'm just a hack compared to these professors here but they help you get it going and you'll know where you stand for future work.
 
Valve / rocker cover off

Any guides on here so I can work my way down to the pistons to replace piston rings and whatever else i find on my way?

I STRONGLY RECOMMEND you access the Factory Shop/Service Manual (FSM) in the "MANUALS LIBRARY" here at this forum

Haynes ARE rubbish and while the Clymers are better than Haynes, they are frowned upon - NOTHING BETTER THAN the FSMs



And if you paid someone to rebuild your carbs because you don't have the skill(s) or tools to have done so yourself or you considered it but (due to their complexity) you chose to hire that out, I'm a little concerned that you're now dismantling the top end to conduct a piston ring replacement job yourself...
 
Any guides on here so I can work my way down to the pistons to replace piston rings and whatever else i find on my way?

Jim (LDR) is the resident expert on these engines, I've never touched one myself. I'd be skeptical of any YouTube video on pretty much anything about our vintage Hondas, so many out there think they know how to properly work on these bikes but they really do not. Did you not download the FSM from our library?
 
i have the FSM
reading it now
Haynes is worse than rubbish

Since 2019, when I had the carbs done by a local mechanic, I spent most of all 3 or 4 UK Covid lockdowns taking a Mini apart, buying specialist tools, replacing, refurbishing, reassembling, taking the engine out, doing a top end rebuild, replacing front and rear sub-frames, preplacing the entire braking system, replacing suspension, steering rack, steering column etc etc

It's been a learning experience

Every time I do something for the first time, it's daunting and I seek advice from those who've been here before me
As I work through each process, I get the jobs done

These days, that is

Not pre 2019
 
An update
Removed and cleaned up:

  • Rocker / valve cover
  • Camshaft holders
  • Cylinder bolts
  • Camshaft and sprocket (timing chain held in place)
  • Cylinder head
  • only the parts of the cam chain tensioner needed to remove what I need
  • 4 inlet valves, springs, oil stem seals, seating washers, collets etc
  • (exhaust valves left in place as they don't seem to be available to replace them)
  • side cover (for access to rotor / flywheel)
  • carb insulators disconnected (4 bolts, one of which refused to budge)
  • (found no O rings between the carb insulators and the cylinder head, so I've ordered 2)
  • cylinder
  • (found no O rings between the bottom of the cylinders and the gasket, so I've ordered 2 of those too)
  • Pistons, piston pins and c clips
  • Con rods left in place
  • Exhaust pipes (L&R, front)

Everything has been cleaned up
No scores, pitting, marks etc
Everything that should be smooth is smooth

New parts fitted (so far)
  • new pistons, piston rings (the root of the low compression, I hope), piston pins and c clips (I hate those c clips)

New parts to fit as part of the reassembly of the above:
  • all the gaskets
  • the o rings that were missing
  • one new carb insulator (damaged getting it off)
  • 4 new carb insulator bolts
  • 8 new inlet valves and oil stem seals
  • 2 exhaust pot gaskets (crushable copper washers, by the look of them)

The factory service manual is proving to be invaluable

So far so good

Will it work after all of this?

No idea

Enjoying myself
Hoping for the best

Also checked the choke cable operation while I had access to the carbs
All working fine

More (dull) updates to follow
I have to work, so progress is slow
Work gets in the way

Regards

Elwyn
 
With 60K on it, a rebore and oversize would be expected, I'd think due to ovality and taper. I'm a novice in this territory.
 
With 60K on it, a rebore and oversize would be expected, I'd think due to ovality and taper. I'm a novice in this territory.
Surprisingly these cylinders tend to hold shape for longer than expected. I did mine at 50K and reused the original pistons since they were in spec and the bores were also in spec.
 
Still waiting for hard-to-get O rings, but the other parts have arrived
I've checked the cylinders with a digital measuring caliper
Perfectly round
Correct sizes
No ovality
No scratches, scores etc
As soon as the postman makes his way through the rain, re-assembly can begin
 
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