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Discussion Starter · #1 ·
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
 

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Hi there!

Indeed, your bike should be reaching for 80-90mph or even the ton. I know because I've reached it on the same bike.

The maintenance you did is great, as you've addressed the air flow and the all important brakes.

However, I think you should dig deeper on the fuel and sparks. You can remove the carbs and check if the Pistons are moving freely. They are vacuum actuated so if they stick for whatever reason you won't get enough fuel to make her run hard on the top end.

Change the spark plugs, as they get worn. Once you do that, run with her for a while. Then give her a whack in 1st gear and cut the motor on the top rpm (with your hand on the clutch). Check the spark plugs. They should be tan, not white nor smokey black. The electrode shows the top end of the rpms, while the base shows the bottom end. If white, you are lean. Black, you are rich in the mix.

Also check the petcock. Remove the gas from the tank and change the petcock for a new one, while checking for rust. This is very important, and often overlooked. The petcock should provide enough flow to keep the bowls full when on wide open throttle. Do not use any fuel line filter, as they never have enough flow, and the petcock screen should suffice.

Feel free to chime in if you have any problems.
 

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Discussion Starter · #4 ·
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.
 

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You could be getting high rpm/load clutch slippage. I'd change the oil out. Because these engine maintain an oil bath in the head for the cam/rockers I'd also tip the bike as far to one side as possible before draining to empty those somewhat.
What's meant about the 1 degree per 100 rpm is the actual ignition timing advance. Total advance per factory spec is 43 degrees BTDC, +/- 2, at 4500 to 5350 rpm. Full advance on the rotor is the 2 lines to the left of the T and F marks, 43 is in the middle. So with a timing light running at idle the F mark will line up with the cast arrow pointer in the upper case edge. As you raise the rpm you should see the timing moving towards the 2 lines. Some timing light have a sort of accurate degree wheel that allows you to change the position of the light, you can set it for 15 degrees at idle and the light will flash at the T mark instead of the F.
 

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Discussion Starter · #6 ·
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.
 

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Discussion Starter · #8 ·
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!
 

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Discussion Starter · #9 ·
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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You'll need to check the timing advance before replacing the CDI. It may be ok and the issue elsewhere.
The GM coil mod will give you easier starts, smoother idle and it just feels smoother on accel. This is all because the GM coil is putting 30-50K volts out vs. the OEM coil which new was only @15K.
 

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Discussion Starter · #11 · (Edited)
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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OK, you have zero timing advance. Time for a new CDI. Choices are CDI Magician on eBay or Rex's Speed Shop in the UK
I don't see a current listing on eBay for CDI Magician
Here's the info on Rex's Honda CB400 CM400 CB450 CM450 CDi Unit - Rex's Speed Shop Nice thing about this one is it's plug-n-play, no cutting and soldering wires
 
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