'78 CB400T II Losing Power Around ~5500-6000 RPM (Max speed 50 MPH); Low compression
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  1. #1
    Junior Member zaemz's Avatar
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    Jan 2017

    Question '78 CB400T II Losing Power Around ~5500-6000 RPM (Max speed 50 MPH); Low compression

    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 Measured Result (Ω) Spec (Ω)
    Spark Plug Coil 0.4 .35 to .55
    Spark Plug Wires 7.99K 7.2K to 8.8K
    Spark Plug Caps Left: 4.72K; Right: 5.37K 5K
    Stator Advance Pickup Sensor 1.004K at male end of plug; 127.8 going TO the stator 135
    CDI Power Source 83.4 85
    Primary Pickup Coil 204.7 207

    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.
    Last edited by zaemz; 01-10-2017 at 03:21 AM.

  2. #2
    Senior Member drydreamer's Avatar
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    Aug 2015
    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 ?
    Last edited by drydreamer; 01-10-2017 at 04:51 AM.

  3. #3
    Senior Member dtsmjr8dan's Avatar
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    Oct 2011
    Windsor, Ontario, Canada
    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.
    1979 CM400T (rider),
    1981 CM400T (in surgery awaiting transplant)
    1982 CM450 (parts bike)
    1982 CB450 (parts bike)
    Past rides: Vincents, Triumph, BSA's, Suzuki's, Twingle, Corgy, others.
    Remember, today is the tomorrow we worried about yesterday.
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  5. #4
    Senior Member doode's Avatar
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    Mar 2014
    Great work trouble shooting so far!
    I second drydreamer with stator or cdi issues.

  6. #5
    Super Moderator longdistancerider's Avatar
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    Oct 2011
    Vancouver Washington (text 408-239-9580)
    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.
    Jim O'Brien
    1979 CM400T "road bike" modified for travelling
    1978 CB400T1 restored
    1972 CL350 survivor restoration

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