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1978 Cb400TII Not Reaching Top Speed

1836 Views 11 Replies 3 Participants Last post by  longdistancerider
Hey guys,

I have a 1978 Cb400TII that I've been working on trying to get it tuned up after it was given to me by a family member. The bike has 24K miles on it and has not been sitting so any long period of time, it was used pretty regularly before I had it. The engine runs well by my standards, this is my first bike so I'm not too experienced in that regard. It will start right up and doesn't really get bogged down through the gears/RPMs. The issue I seem to be having with it is that the speed tops out at about 55-60mph, and from what I've read online this bike should get up to 80-90mph. I did a compression test and it was about 150psi in each cylinder. I also did a leak down test at TDC on each cylinder and found less than 20% leakage. I've seen that the sprockets could be changed out but my front has 16 teeth which I think is stock. There is what I think to be a small leak in the head gasket as there is a small amount of oil that is around that area I just haven't cleaned it off yet to see f its leaking out or not. I'm not really sure where to go from here so hopefully, someone can advise on what to do to bring the speed up.


Other things I've done to this bike since getting it.
changed the oil (Delvac 15w40) and the filter,
changed the brake fluid and adjusted the rear brake
adjusted the clutch
Cam chain adjustment,
cleaned and lubed the chain,
cleaned and reoiled the air filter
cleaned/adjusted and Synced the carbs
replaced the battery (AGM battery)
adjusted the slack in the chain
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These will hit close to if not the 100 mph mark stock when everything is correct.
Delvac oil? is this oil JASO MA rated? I know Rotella is. I used motorcycle specific oil, Castrol 4T, so I won't ever have clutch slippage until it's worn out.
Correct sprockets for your year/model are 16/37.
I would check the carb sync, probably fine, and set the mixtures.
Check the timing advance, it should be progressing about 1* per 100 rpm until @4500-5000 rpm. There is a sticky thread in the Electrical section on Diagnosing the Ignition system. It sounds like you may have a stator or CDI problem
The oil is Mobil Delvac 1300 super I thought it was just an alternative to Rotella or others like that. But after looking at JASO MA rated oils It looks like it's not on the list. should I just drain the oil and put in the proper kind? like Castrol 4T. yes, thats the same number of teeth in the gears I have so I guess that's stock. I just did the carb sync last weekend and put the mixtures screws 2 turns out. that seems to sound the best. I'm not sure I fully understand what you mean here "progressing about 1* per 100 rpm until @4500-5000 rpm" but it sounds like ill have to do some more reading on the Ignition system, I'll definitely read that sticky thread.
thank you Jim for pointing me in the direction of the Electrical diagnosis. I ran all the tests you suggested in that post here are the results:

Coil Test:
Primary Side (Yellow and Green wires)- 1.1ohms
Secondary Side (spark plug wires w/o plug end caps)- 8.30K ohms
Plug End Caps: 4.71K ohms for both left and right

Stator Tests:
Pink and Green wires- 135 Ohms
White and Blue wires- 86.8 Ohms
Brown and Light blue wires- 209 ohms


Also, thank you for your explanation I'll have to get a timing light and see if those markings will lineup.
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As you can see some of your readings are a bit high, this will affect the performance. Coil won't produce as much voltage as it could due to high Primary resistance and the high Blue/White reading from the stator. Advance is going to be slow, might not get full advance until 6,000 or higher.
Do you think this alone could be causing the bike to not reach its full potential? Is there a way I could confirm this? would I be able to see the timing not aligned with a timing light? Also, is there anything I could do about this short of buying a new stator? They are kinda expensive so I just wanna make sure that the issue before I go out and buy one.

Again thank you for all your help!
so I realized that I didn't account for the resistance in the multimeter leads, I've taken the resistance measurements again with better results.

Coil Test:
Primary Side (Yellow and Green wires)- 0.6ohms
Secondary Side (spark plug wires w/o plug end caps)- 8.30K ohms
Plug End Caps: 4.71K ohms for both left and right

Stator Tests:
Pink and Green wires- 134.5 Ohms
White and Blue wires- 85.4 Ohms
Brown and Light blue wires- 207 ohms

So it seems I'm in the clear based on the new measurements so from your sticky post about the ignition it seems like my issue might be the CDI box? I've also done some reading about the GM ignition coil mod do you think doing this would be advantageous?
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Ok, so I checked the timing with a timing light. and it looks like there is definitely something up with the timing advance. at idle using, both right and left spark plug leads the indicator is in line with the "F" mark. Although when I rev up the motor to around 5K rpm there is no advancing the indicator stays at the "F" mark. Although I will say the reading on the left spark plug lead gave me a bouncing reading as in some of the time the reading wouldn't be exactly on the "F". so I'm not totally sure if that means anything.
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