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Discussion Starter · #1 ·
Hello everyone. This is my first time on this forum, and I'm looking for some expertise.

I recently purchased a 1978 Honda cb400t Hawk II. The engine would turn over but it wouldn't start. Well come to find out that the engine is pretty messed up. So I'm looking to swap the engine.

I know that the frame is compatible with other cb400 models. I found an engine from an '82 cb450sc nighthawk. So I just want to make sure that this engine will fit on the bike.

Thanks a bunch
 

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Any SOHC 400/450 will fit. Nighthawk is a 6 speed. Carbs generally need to be cleaned, on any old bike that has been sitting, before it will start.
 

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Do you know if the 82 nighthawk is currently running or did you just find a used engine, a whole running motorcycle would be handy.
 

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Discussion Starter · #4 ·
Do you know if the 82 nighthawk is currently running or did you just find a used engine, a whole running motorcycle would be handy.
Hey, thank you guys for the replies. Sorry I haven't been super active. But I just wanted to update this thread with my progress. As an update to the old engine, turns out the timing chain snapped while the engine was running, it hit the tensioner and it basically exploded and sent metal chunks into the crank case, which then proceeded to destroy the balancer chain. I feel like an idiot for not figuring this out before I bought it, but live and learn. Though somewhat in my defense, the timing chain happened to snap with the the cam shaft at TDC, so the exhaust valves were shut, making the Kickstarter feel like there was decent compression (boy was that wrong).

But the 'new' used engine is in running condition, a shop was parting out an '82 nighthawk that the owner couldn't get a title for. The motor is surprisingly clean for its age. Goes through all the gears, snake camera pics of the inside of the cylinders were very clean. This parts bike was running when the shop bought it, but since they parted it out, they weren't able to show it running. While a whole parts bike would have been handy, that wasn't available in my area unfortunately. But I picked up the engine and have it on my frame now. Pretty much everything from the 400 transferred over to the 450.

The only difference (besides sparkplugs) I have found is that the neutral safety switch off of the 400 had a single wire coming from it, and the one on the 450 has two wires. The second wire on the 450 switch is for the over-drive indicator light, which the 400 doesn't have so I just left that wire disconnected.

I was this close to getting it started up when the starter motor stopped working. Not sure if this is the solenoid dying or the motor burning out. But when I went to start it, the starter motor made a terrible noise and then wouldn't work after that. So I've ordered a new starter motor and hopefully that fixes that issue.

Two issues I haven't gotten to yet is the starter button not working, and the dash indicator lights not turning on. The backlights for the tach and spedo work, but the neutral, oil pressure, high beam, and turn signal indicator lights on the dash don't light up. I haven't begun to chase down these leads yet though so we'll see where that takes me.

I will try to keep updating this thread as I continue to work on the bike. If you guys have any tips/suggestions that I might be overlooking, feel free to let me know. Thanks!
 

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Did the guys that sold you the engine do a compression test on the motor before you bought it?
"making the Kickstarter feel like there was decent compression" <- and this does not equate to a compression test, not even close, you test the cylinder compression with a pressure gauge that sticks into the spark plug hole, then you spin the motor over to get a reading. You take that reading and compare it to what the service manual says compression should be.

Test your electrics right from the start, begin with the alternator which actually produces the electric power and work down stream. If the alternator ain't working right nothing down line of that will work right.
 

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Discussion Starter · #6 ·
Did the guys that sold you the engine do a compression test on the motor before you bought it?
"making the Kickstarter feel like there was decent compression" <- and this does not equate to a compression test, not even close, you test the cylinder compression with a pressure gauge that sticks into the spark plug hole, then you spin the motor over to get a reading. You take that reading and compare it to what the service manual says compression should be.

Test your electrics right from the start, begin with the alternator which actually produces the electric power and work down stream. If the alternator ain't working right nothing down line of that will work right.
Yes they did a compression test on the new engine. The new engine doesn't even have a Kickstart. That was just my backwater method of testing that old engine and clearly that didn't work out for me, haha. And the alternator is working, getting power to the regulator rectifier and the battery. I have spark, so the coil and igniter are good. I think I have to replace the starter solenoid though. Thanks for the tip!
 

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I really like numbers when it is about compression tests being done. Number for each cylinder.
You need to look at the wiring diagram to know if spark indicates a working alternator. Again it's all about numbers when you are talking about an alternator output being good. What is the voltage outputs on all of the leads independently and what is the static resistance of the coils in ohms. Both will be specified in a shop service manual.

Solenoid can be meter tested for integrity before you go replacing it needlessly.
 

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Do a voltage test at the battery terminals,
disconnect the starter motor connection and do a meter test there when the starter voltage is applied, if they are not the same numbers you have resistance somewhere.
 
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