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1978 Honda CX500
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Discussion Starter #1
Hello! This is my first post on this forum so forgive me if I do anything wrong. I am also extremely new to motorcycles, so my knowledge may be limited so forgive me for that as well. I do have some experience with engines from my engineering degrees, but nothing very hands on. I am a huge fan of the forum and it's helped me immensely so far with the project. Sorry for long explanation but figure its best to be detailed and explain everything I've done to eliminate tests I've already performed.

Recently I have decided to work on a 1978 Honda CX500 standard that my grandfather acquired in that same year. The last registration was from 1981 and the bike only has 1500 miles on it and seemed in decent shape. I've started doing some work on the bike (repainting parts and cleaning, getting a new battery, beginning to modify rear frame and have spent a good bit of money on new parts). Obviously I should not have done this before making sure the bike was running well but here we are. At this point I've spent 10s of hours on researching as well as money on parts so I would like to continue the project. My grandfather also assured me that the bike should be able to run and in good condition before starting.

I decided it was time to get the bike running so I got a new battery and hooked it up. Took me a while to get power to all the parts before realizing there was a broken fuse. Switched that and all lights came on and seems all blinkers and such worked. Tried to hit the push start and if I remember correctly the bike turned over for about one second then stopped and buzzed by solenoid. Figured it was solenoid by checking with multimeter, tried to take apart and clean with no luck so got a new one. Under $15 so nbd. Also figured it was issue because I wired directly from starter to battery and the starter turned (I did take it off of the engine to test). I was told this was normal, but my battery does drop from about 12.7 to 7-8 volts when I push the start button and have the multimeter connected directly to the battery. Also checked wires from switch to solenoid and didn't get any Open Loops with multimeter test. Anyway, not really important for post as I now believe all electrical problems seemed solved for now. Only explained to show how the bike turned over then stopped.

I decided maybe the problem was with the engine itself as when I had the starter on the engine and wired directly it would make noise at the starter but not turn over the bike. Figured it was seized so I grabbed a socket wrench and tested the front manual motor crank under the radiator. It would move a little then stop when turning clockwise (from front of bike). I opened the side engine inspection bolt and looked in and could see the parts moving while turning the front, but eventually they would stop and I didn't want to force it harder than I already was. The bike was stored in WV in the USA, so it is rather humid, so it makes sense moisture could have gone up the exhaust and carbs and caused things to seize after not being ran for like 38 years. Figured it maybe was piston rings so I added a 50/50 mixture of Marvels Mystery Oil and diesel (about 1.25 oz of fluid to each spark plug hole) and let it sit for 4-5 days. Tried using manual crank again with no luck. Also tried putting it in highest gear I could and rolling back and forth but the back rear wheel skids after about a quarter turn. I can roll bike fine in neutral as well as in gear with the clutch pulled in. I am not sure if this is related but I have been having trouble with the shifter and it seems like parts don't always want to engage and ill press down or up and nothing will happen/click. Took me about 5-10 minutes of wiggling it around with my foot to get it back into neutral. Figured maybe some things are just loose (or I'm really really bad at lifting my foot up and down, like I said I'm completely new to bikes).

So I guess my main question is how can I unseize the engine (if that's really my problem).... are there other things I should be testing? Possible it's locked at the drive shaft/final drive? CAM chain? I'm definitely out of my league when it comes to this stuff so just wanted to make sure I am not overlooking something simple/an easy fix. I'd really prefer to not have to take the whole engine out and take it apart as I'd have no idea what I am doing. If I have to I will, but only as a last resort as I'd get in there and not have any idea what to be looking for. What I also am confused about is how I got the bike to turn over for a second when I first put in the new battery then it stopped. In my mind, if the pistons were seized and attached to the cylinder walls then the engine wouldn't have turned over at all before stopping. Maybe only one piston is stuck and it was able to move a little before stopping? I'd like to learn more about it, but realistically may be better to just get a whole new engine. Right now I am just trying to get the bike to turn over (then I'll tackle the carbs and such). Thanks for any help and taking all the time to read my long message.
 

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I don't think your engine is seized. If it initially cranked for 1 second, that would be at least a few revolutions.
It sounds like a weak battery, to me. Did you fully charge it before the attempted start? Put a charger on it (2A, if you have that setting) over night before another attempt.
When you tried turning it manually, did you take out the spark plugs? You might just be working against compression.
 

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1978 Honda CX500
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Discussion Starter #3
I don't think your engine is seized. If it initially cranked for 1 second, that would be at least a few revolutions.
It sounds like a weak battery, to me. Did you fully charge it before the attempted start? Put a charger on it (2A, if you have that setting) over night before another attempt.
When you tried turning it manually, did you take out the spark plugs? You might just be working against compression.

Thanks for the reply! Have pulled all spark plugs out and charged battery on all attempts. I have two AGM batteries and have charged and tried both. Both reading over 12.5 volts before starting attempts. Haven't tried it with the new solenoid, but figured if i cant turn engine over manually there's no point. It is possible it did not turn over initially and the noise I heard was coming from something else, but to my unexperienced self it sounded like an engine turning over for a half second. Not sure what else it would be. Again, it was very brief and I wasn't expecting it to turn over then stop so suddenly after fixing the fuse issue so hard to remember that well after almost two weeks. In my mind if I am unable to turn the engine with crank/wheel in gear then its not an electrical issue.
 

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1978 Honda CX500
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Discussion Starter #5 (Edited by Moderator)
can you turn the motor backwards a bit?
I assume this is what you mean:

Using the 17mm bolt on the front I can turn it counterclockwise (from the front) yes and it will rotate things that I see through the timing inspection cover on the side of the engine. I read that if I turn too much it will unthread the bolt and that its supposed to really only turn clockwise, so I only gave it a revolution or two in this direction. So if I understand you correctly then yes I believe I am able to rotate the motor backwards some using the manual crank at the front of the bike.

I should also say that the bike was running when it was put away. It was also left in a garage away from the elements but with no cover and was not meant to sit for that long. Gasoline was left in the fuel tank and I imagine the carbs are really gunked up as well because of this (and who knows what else)
 

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The torque value on the primary gear bolt is the second highest on the whole bike, so unless you're working against a truly seized engine, you're unlikely to undo it. You say you "only gave it a revolution or two" counter-clockwise. If that's the case, you should be able to rotate it forward the same amount. Again, plugs out, clutch disengaged or transmission in neutral.
 

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The torque value on the primary gear bolt is the second highest on the whole bike, so unless you're working against a truly seized engine, you're unlikely to undo it. You say you "only gave it a revolution or two" counter-clockwise. If that's the case, you should be able to rotate it forward the same amount. Again, plugs out, clutch disengaged or transmission in neutral.
Maybe also take the valve covers off to see if the camshafts move while you manually crank it. Just checking to see if everything is still mechanically connected.
 

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Maybe also take the valve covers off to see if the camshafts move while you manually crank it. Just checking to see if everything is still mechanically connected.
No need to remove the valve covers, just see if the fan turns with the turning of the crankshaft.
 

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Discussion Starter #9
The torque value on the primary gear bolt is the second highest on the whole bike, so unless you're working against a truly seized engine, you're unlikely to undo it. You say you "only gave it a revolution or two" counter-clockwise. If that's the case, you should be able to rotate it forward the same amount. Again, plugs out, clutch disengaged or transmission in neutral.
What was happening to me is when i would turn the front bolt while the bike was in neutral I could get it to turn about a quarter turn/half turn clockwise then it would become tight and not move anymore. That is with respect to the position of the bolt before I had touched it. I could turn the bolt counter-clockwise that same amount back to that initial position then more past that. I would stop turning counter-clockwise after I went about a revolution past the initial position because I assumed all I was doing was loosening the bolt. If I then went and turned clockwise again, I could spin up to the initial point and another half/quarter turn past that before it would get tight again. It seems it is stopping in the same spot because when I looked through the timing inspection bolt hole there was a very small mark on the flywheel that ended up at the same spot. Doing all this with the plugs out and the bike in neutral. Would that not mean its seized? Sorry for my ignorance like I said I'm new!
 

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Discussion Starter #10
No need to remove the valve covers, just see if the fan turns with the turning of the crankshaft.
You mean the fan behind the radiator ya? Fan is mechanically connected and should rotate with the crankshaft if I am able to rotate it I assume?
 

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Maybe also take the valve covers off to see if the camshafts move while you manually crank it. Just checking to see if everything is still mechanically connected.
So if I mechanically crank it with the valve covers off I should see the intake/exhaust valves open and close meaning everything up until then would be connected ya?
 

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When you turn it counterclockwise, do you see the fan and/or flywheel (inside the inspection port) turning? If neither turns, you are in fact loosening the primary gear bolt. If only the flywheel turns, you have a broken cam chain.
 

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Provided the history you’ve given us about the bike is accurate, I would guess your first thought is the problem.
 

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Discussion Starter #16
When you turn it counterclockwise, do you see the fan and/or flywheel (inside the inspection port) turning? If neither turns, you are in fact loosening the primary gear bolt. If only the flywheel turns, you have a broken cam chain.
I will test that this week. Very informative reply thank you
 

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1978 CX500, 1982 GL500 Silverwing Interstate, 1980 CX500 Custom
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You mentioned that you are new to motorcycles, but... If you've managed to take the front cover off, attach a 17 mm wrench and socket, and can see the crank rotating through the timing cover - then you've done great !!

You mentioned that this is a 1978 model, with low miles. These all had a recall due to the "Cam Chain" tensioner. Dealers resolved this as part of a recall. When they completed it, they punched 3 marks next to the Engine Serial Number (found on the left hand side of the engine):

205682


The fact that you can rotate the crank a small amount would seem to indicate it is not truly "seized", if it is in neutral. It's not the "Gearbox" - if you can select 1st or second gear easily, which also rules out the "Clutch"..

A suggestion was made above to remove the cylinder head valve covers, this might be really helpful. If you do that on both sides, it exposes the "Inlet valves" (at the back of the engine, near the battery & air filter), and the "Exhaust valves" (to the front, close to the muffler down pipes). The valves are part of the 4 stroke cycle (suck - squeeze - bang - push) and are opened and closed as part of that, allowing fuel/air in and exhaust out.

They are controlled by the "Rocker arms", which are moved by the "Push-rods" - these are close the frame. They are moved by the "Camshaft" inside the bowels of the engine.

What you could be happening is as a result of a "Mechanical failure" of the Push-rods, a "dropped" or stuck valve hitting a "Piston". Worse still - it could be part of the internal moving parts connected between the "Crankshaft" and the "Camshaft" that moves the "Push-rods"., i.e. a damaged "Cam Chain" due to the tensioner as mentioned above.

If all appears to be OK, try removing the "Spark plugs", and use a small long wooden dowel. Rotate the crank again (slowly) and ensure the dowels move up and down. If they don't you might think they are seized - they could be. But it's also possible that the "Connecting rod" ("Conrod") between the crankshaft and the piston has broken.

If you end up having to do a rebuild, you will hear a lot on the CX 500 forums, click on the link to learn about the "Triple Bypass" (for the the "Cam chain" and tensioner/guide blades, the "Stator" and the "Camshaft" mechanical seal)..

There's a lot of help available in this forum, and everybody here will give you great advice. Good luck with your diagnosis and fixing it !!
 

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The bike was stored in WV in the USA, so it is rather humid, so it makes sense moisture could have gone up the exhaust and carbs and caused things to seize after not being ran for like 38 years.
Where is the bike now, Quin? I imagine it's nowhere near me, but there might be an experienced member nearby who can help you evaluate.
 

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So if I mechanically crank it with the valve covers off I should see the intake/exhaust valves open and close meaning everything up until then would be connected ya?
As others have said, watching the fan for motion should be enough since it is also driven by the cam chain. I would have jsut been looking for some rotation of the camshaft. From your descriptions you may not be able to turn it enough to see valves open and closing.
 
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