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Discussion Starter · #1 · (Edited)
Hey there!

I purchased a 1978 CB400T II a couple of months ago and it's my first motorcycle. I thought it was in really great shape, looks clean, and only had 8k miles on it. It fired up right away from dead cold, idled great, didn't smell, and ran well enough when I tested it, as well as a bunch of other stuff, so I took it home! All of the parts were original (so I was told, anyway). I did see that even the tires were the original tires (!) so I believed the fella I got it from. The crazy part is, those tires were still supple and didn't have any visible signs of dry rot, inside or out! (I still replaced them because I would like to keep alive)

I didn't get above 35-40 MPH while testing it, however, and after I purchased it, I tried to take it for a short ride on the highway. I could get it up to 55 MPH if I rested just so on the throttle in 5th gear. When I pull back the throttle above halfway in any of the gears, I lose power. Once I get the engine to around 5k RPM in any gear as well, I also lose power. I'm a frequent flyer on reddit and got a ton of help over there, but I really like this community and wanted to join in, as well as get some advice! I've basically copied here what I've written there, with a lot of extra information. (For those curious, here's my thread there.)

Here is what I've done so far to find out what the issue is:

  1. Checked the screen before the petcock -- no block, gas flows smoothly (and I added an inline filter) as well
  2. Checked the fuel line for any breaks or cracks, looked good (replaced it when I added the filter)
  3. Checked for vacuum leaks. I sprayed some carb cleaner around every part around the carburetors that I could see/think of. There wasn't any change in how the bike idled.
  4. Checked the air filter. It was still slightly oiled and very clean. Even then, I replaced it with an OEM one.
  5. The brakes are fine and they don't rub.
  6. Checked the air stems on the tires and they don't appear to be slipping.
  7. Took the carbs off about 10 times and cleaned the everloving bejesus out of them. It started off with a good gentle scrubbing because they were already pretty clean. Then I scrubbed them again. Then I soaked them for half of a day and scrubbed them again. Then I cleaned each of them using an ultrasonic cleaner for about ~2 hrs each, twice. (I really hope that I didn't screw up the aluminum. I know it's brittle and porous.) In my naive and very amateur opinion, I'm almost sure it isn't the carbs. Replaced the o-rings, gaskets and float needles. Checked float height and made sure jets were right size. I have 78/105 for primary/secondary, but I do have a 112 as well, and tried that, but the result was exactly the same, so I switched it back to the 105. (** Based on threads I've found in here, however, I don't believe these are the stock jets for my year... Also, this thread here states that the 78 CB400T II should have the VB21As, not Bs. I got got! That makes sense why the jet sizes would be wrongish then.)
  8. Reset the cam chain tension.
  9. Did the valve clearance adjustment.
  10. Carbs synced (not bench synced, but actually synced while running)
  11. I also checked the ignition timing and that was fine!

My exhaust looks and sounds fine, though I'll admit that I haven't done thorough look over it.

For information on the carbs, I have VB21B carbs. I tried opening choke while the bike was running and it just died, so it's not a lack of fuel.

There is build up on the spark plugs (Denso X24ES-U), however, so something is causing the fuel to not be used up completely. The spark plugs are gapped to speck and when I checked for spark, it was a strong, bright blue flash. I ordered some B8ES and B7ES plugs just in case as well (it's in the 30s in the PNW where I am, and was told that hotter plugs may help)

Just in case, I took a second look over the valve tappet clearances and made 100% sure that they were correct.

After that I moved on to checking resistances using this thread here.

These are the values I got:

Place MeasuredResult (Ω)Spec (Ω)
Spark Plug Coil0.4.35 to .55
Spark Plug Wires7.99K7.2K to 8.8K
Spark Plug CapsLeft: 4.72K; Right: 5.37K5K
Stator Advance Pickup Sensor1.004K at male end of plug; 127.8 going TO the stator135
CDI Power Source83.485
Primary Pickup Coil204.7207

That thread above states:

The stator ohm readings listed can be off no more than 1 ohm, more than that will have a negative effect on the timing advance
...and some of these are off, so I'm not sure how to interpret it. Is it strictly off by more than 1 ohm? If so, then do I need to rewire everything?! Or do I just need to clean contacts?

I checked the timing again. It was slightly off at idle, so I'm a little unsure.

Here are some images:

Here's the timing at idle and the advance

For the timing advance, I was somewhere between 5000 and 6000 RPM. The manual says it should be in between the marks at 5350 RPM. I think I was above that with it between the marks, like ~5500 RPM.

And I also checked the compression with the throttle opened, here are some images of the gauge for each cylinder:

Here's the gauge with the compression reading

Values are:

Left side: ~163 psi; Right side: ~150 psi

Book says: 185 ± 15 psi. So both sides are low :( I've checked these twice after getting the engine hot and with two different compression tester kits and they both read the same. Oil on the threads of the tools, both spark plugs pulled, and throttle pulled open.

The manual also says this:

If the compression pressure is low, check the following items:
  • Leaky valves
  • Improper valve tappet clearance
  • Blown cylinder head gasket
  • Worn piston/cylinder

I can scratch out the valve tappet clearance because I've checked that twice now. The other three I have not checked and am unsure how off of the top of my head and don't know what order I should check them. (I'm a noob!) I've been reading a lot on here, but I'm a little bit overwhelmed by it. The first thing I'm going to do is a leak-down test. I have found this thread for a procedure as well as this guide which looks pretty good. I've yet to do that. Gotta rent an air compressor. I don't have many tools because I'm a poor student.

I've posted this gigantic thread in here because I've figured it's probably an engine issue.

What do you guys recommend I do next?

And also thank you so much for taking the time to read all of this! As soon as I get some more time, I'll head over the introductions area and share some pictures and a little bit about myself. :)
 

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So, in short, it starts ok, runs ok cold and hot but has no pwer after say 5 to 6 k revs.

The ignition advances ok.

When cold and hot??

When hot, with bike on centre stand and idleing in neutral, if you increase revs all is ok to about 6 k revs when misfiring/surging takes place?? More throttle opening just causes more misfiring and engine refuses to rev over 5 to 6 k revs ??

Connect your strobe timing light, with a clip over ht lead semsor ?

Does the stobe flash all ok up to the point where is misfires, then just goes crazy, stops flashing properly, with no flash when misfing??

Road test, red neon flashing plug caps are available and cost only a few $$, easy to fit and give a visual indication ( flash ) that HT is passing thru them. With these fitted for testing, on a safe staright bit of road ( up hill as well ) on a dark evening, do they stop flashing when the misfire happens ?? I assume yes they do.

Verdict, internal shorting out of the genny cdi charging /source coils, and/or failing cdi, and/or failing ht coil, and/or ig kill shorting/flashing/AC leak to earth. Pretty much the whole ignition system really.

Do you have any AC volts data from the wires from the genny, there is a list on here somewhere, see my and others threads on this matter. Not at home and memory of 400 genny wires a bit poor.

green is earth, light blue, dark blue, brown, white and one other, the colour of which escapes me at present, it may be pink. As a rule, AC output will rise with revs.

If AC volts drops from about 55 v AC at idle to about 25 volts AC at about 5k revs on the source/charging coil wires , the misfiring starts to happen. Seen this many times now.

Recheck all genny /cdi/coil connections and try with cdi black/white wire disconnected. This wire is earthed/grounded to kill ignition. If this disconnection cures the problem, then there is a fault either in the main loom, ign switch or handlebar kill switch, the latter being most likely.

You may have a HT coil/cdi fault also, not to mention fueling, but rule out ht first.

If you have a scope, or access to one, you are half way there.

PS does the bike vibrate a lot ?
 

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Ignition timing looks fine. Resistance values are fine. It sounds like it may be starving for fuel under load. Are you sure you have the primary and secondary jets in their correct locations? The secondary jet (105) should be below the needle. The primary (78) should be between the secondary jet and the mixture screw.
 

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I'm going to suggest that you replace the stator. Any manual transmission stator from 1978 thru 1986 CB/CM 400/450 will fit and work.
 

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Discussion Starter · #6 ·
@daydreamer

PS does the bike vibrate a lot ?
I'm not too sure. It doesn't seem to vibrate any more than I would think it would.

Recheck all genny /cdi/coil connections and try with cdi black/white wire disconnected.
Disconnecting this wire made no difference. The bike would just stay running if I disconnected that and turned it on. I cut off the fuel and let it die out, then plugged it back in. Same deal as before.

@dtsmjr8dan

I'm 100% positive that the jets are in the correct positions. I've double, triple, and quadruple checked them and they're good. I've gone over the carbs 10 times probably because most other threads I've read about issues like this, the problem was with the carburetors.

So I purchased a new CDI unit from cdimagician on eBay, and the bike actually runs worse. I swapped out my plugs with D7EAs and it runs a *little* bit better with those, but still runs crappy. Bogs even more now and back fires a lot.

@longdistancerider

Since I replaced the CDI and the bike runs worse now, I'm going to try and find a stator. I'm having a really hard time, though, as I can't seem to find one that doesn't cost $1,000,000.

I'm thinking about just selling the bike. This feels like more trouble than it's worth. :(
 

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The reason for replacing the stator is that the advance pulsar resistance is way out of spec. The Primary pickup is reading low also.
Where are you located?
 

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Discussion Starter · #8 · (Edited)
I'm in Portland, OR.

So those being even just a few Ohms off can make that much of a difference?

I can't afford to have the stator rewound, and unless I can find a working used one for ~$80-100 or so, I'm not going to be able to afford that either. I'd be willing to try to rewind it myself.

Thanks so much for your help, by the way.
 

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i might have one within spec in the backyard, but wont be able to check til later in the week.
to be honest, i also have reservations about getting rid of it in case mine craps out.
however, longdistancerider upgraded his with one from another bike, so i may investigate that...
will pm you if i decide to part with it.
 

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Discussion Starter · #10 ·
That's amazing, thanks so much for the offer. I'll keep searching. If you do decide to part ways, just let me know how much you'd like for it.
 

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The part that needs repair isn't one you would be able to do, there's only one place I'm aware of doing it, Rex's Speed Shop in the UK
I have a cannibalized stator here, missing some of the harness and the primary pickup, have to check if the windings are in spec.
 

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Discussion Starter · #12 · (Edited)
Ah... so I'm guessing that, based on the information I found at Rex's Speed Shop - Sales 01580 880768, the advance pickup sensor and maybe the pickup coil are wound in a manner which requires a CNC machine. Is that correct?

Just to be clear - the "Stator Advance Pickup Sensor" is the same thing as the "advancer pulser" right? Also, the "primary pickup coil" is the same as the "fixed pulser"? Looking at the FSM, that's how I'm interpreting it.

Edit: Also, with the resistance decreasing in the coils, does that mean that they're loosening up? That loosening up would make it so that there'd be a few mm or cm of slack, which would slightly reduce the number of loops in the coil, which in turn slightly reduces the resistance?
 

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I don't know how they are actually doing but a CNC machine sounds right.
Yeah, my terms don't quite match Honda but they are the same as you read
Unsure why the resistance is dropping off on these, possibly it's a shorting out process that eliminates some of the windings.
 

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Discussion Starter · #14 · (Edited)
Here's hoping I can find one! It looks like it would be pretty expensive for me to have my stator repaired by the fellas over in GB.

Standard hourly rate: £35/hr

Trigger coils - prices start from £75.00 for repairs (these would be the pulsers)

So it'd be at minimum £150 for just the pulsers, and then £35/hr for labor and probably more than that.

So let's just say, somehow, 3 hours of labor, so £105 + £150 = £255 = $322.36 USD

Uff da...
 

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Cycle Heap off the 205 or Mike's in Salem are the 2 viable ones right now. You can call Cycle Salvage in Redding CA, Wayne might still have one that I checked out to be good
 

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So the only question I kinda have for you.
you mentioned the plugs do not look good ?
are you sure it is not a carb issue ? these carbs are a pig....
what are the plugs saying ? hot cold ?
just wanted to make sure since it was not clear by the post.
 

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Discussion Starter · #17 ·
So, I feel bad necro-ing my own post from ~3 years ago, but I guess I have an update, and I still have the same problem with my bike. Unfortunately, I've been quite busy these past 3 years and only recently got the resources/time to start digging into it again. I've been running it once a month, changing the oil, and keeping it clean and well-stored.

As I said, the state of things is the same. The motorcycle runs the same as it did when I made this post. Here's what I've done since then:

  1. I sent my stator off to Rex's Speed Shop (Home - Rex's Speed Shop). This is what they had to say:

    I have tested your stator, the high speed tests fine as does your lighting and fixed pickup coil.
    Your low speed is in range but if you have had problems starting or cutting out when hot i would suggest that this is the main problem.

    Your values for the pickup sensor are different to the ones that we have on record would it be possible to email a copy to us.
    I am unsure weather the advance pickup sensor has failed due to honda supplying a large gap in the Ω reading. They state 150Ω - 260Ω yours reads 211Ω
    This is mid range and tests on others that we have had in are around this value.
    ...

    I think the best coarse of action would be to rewind the low speed as it is already suspect.
    While this is being done i will have a look into a replacement pickup. We have not been able to repair them yet but i think we can get good replacements.
    I agreed to having them rewind the low-speed coil and replace the pickup. I didn't take a pic before I put it back on the bike, but it looked good and I trusted their work.

    Unfortunately, this didn't fix the issue.

  2. Ordered a set of used carbs (VB21A) from Ebay. I rebuilt/cleaned them, and swapped them out and the bike ran slightly better, but it didn't still fix the issue!

Soooo, now I've just decided to order a slew of parts and clean up a bunch of things. I've ordered a rebuild kit from 4into1.com (Engine Rebuild Kit - Honda CB/CM400 - 1978-1981) and another carb rebuild kit with the cutoff valves. I also ordered a set of piston rings. I'm working on getting the engine off of the motor. I ordered a new ignition coil for the helluvit, even though the plugs had good spark. But I figured if all of this other work still doesn't fix it, I might as well try one more thing.

So I'll probably take some good pictures and update this thread with what I see and what I've done. I should probably finally make an intro thread too, hah.
 

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Discussion Starter · #18 ·
what are the plugs saying ? hot cold ?
just wanted to make sure since it was not clear by the post.
I didn't answer this - sorry. The left, or port, carb has some soot, looks like it's running rich, while the right carb is definitely scorched/white and is running lean. I've made sure the carbs were synced over and over. But since it's been a minute, I'm just gonna go ahead and rebuild/clean them again. I ordered new boots and was planning on sealing the "boots" on the air box as well.
 
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