Question NC700X won't turn over...?

[lootzyan]

Decompression devices can be found on manual start (kick, hand crank, etc) engines...

Thank you. I have a lack of knowledge about engines with manual start. (Except for my lawn mower)

 

[TNHoosier]

Thank you. I have a lack of knowledge about engines with manual start. (Except for my lawn mower)

Many of the "easy start" mowers with a pull cord have an extra cam lobe that briefly opens the exhaust valve just enough to lower compression so it is easier to start. This comes at the expense of power once running.

 

[bigbird]

I owned a 1973 Yamaha RT360 single cylinder 2 stroke enduro motorcycle. It had a manual operation compression release to aid ease of starting. It used a cable from a handlebar mounted control to open a tiny valve in the cylinder head. It worked well and was appreciated, as the force to operate the kickstarter and the occasional kickback was greatly reduced.

 

[showkey]

The NC compression test protocol and specifications have come up several times in prior posts:

Cut and pasted from the prior post:
“Repeated the compression test on the NC without opening the throttle. ( all other variables were the same)
Reading was close to 130 psi
Repeated compression tests with open throttle and it was again 250 psi.”

I am away from a shop manual but the compression spec with the throttle open is 250PSI.

 

[spark82]

If compression pressure is higher than expected, my thoughts would be pointing to some kind of extraneous material like fogging oil being introduced into the cylinder, but a dead giveaway to that would be excessive smoking on startup.
A blown head gasket could allow coolant into the combustion chamber, causing that nasty sludge produced from engine oil and coolant mixing.
Is your coolant overflow level low?
Is your oil milky looking?
Are your spark plugs looking OK?
Any excessive anti seize used on your spark plug threads?

No sign of oil and coolant mixing. Spark plugs look OK.
By the way, maybe I put it the wrong way, it seems that my compression is higher (as between compression cycles there seems to be no difficulty to turn the engine), as we couldn't measure it the usual way. Without plugs the engine turns "freely" (with minimal constant resistance, from friction I suppose)

 

[spark82]

Decompression devices can be found on manual start (kick, hand crank, etc) engines. Never seen one on an electric start engine but it does not mean they do not exist.

@spark82: When you say the torque was "similar" with the plugs out, how similar? Within 5%, 10%, ?

We "tested" it with one of those adjustable torque drivers, we adjusted it until it started ratcheting while turning the main shaft. Without plugs the two bikes were inside 1-2 clicks of setting. Not an exact way, only made for comparison. With plugs in, mine needed significantly higher setting. We started thinking, came up with that decompressor thought (I'm working on an old fire pump, hand starter, that one has it, seemed a viable idea at the time, I thought maybe NewConcept NC is different than other bikes, I know it's not used on electric starters, or at least very rarely). Back to thinking and poking I guess (my mechanic would disassemble the engine if I wanted but until all the other options are out, I'd like to keep it together.)

 

[Joppi]

Decompression devices can be found on manual start (kick, hand crank, etc) engines. Never seen one on an electric start engine but it does not mean they do not exist.

@spark82: When you say the torque was "similar" with the plugs out, how similar? Within 5%, 10%, ?

I saw it on a Honda quad bike that had it

 

[TNHoosier]

In my 2021 service manual the "standard" compression is 261 psi (1800 kPa) @ 470 rpm with throttle fully opened.

The only suggested cause for high compression is carbon deposits in the combustion chamber or on piston head.

If your compression was too high, I would expect you would have detonation or 'pinging' especially under load.

 

[bigbird]

So, then possibly some kind of buildup on the piston crowns or cylinder head or valve faces causing reduced cylinder volume leading to a higher compression ratio?
Was the engine knocking (pre-ignition) once warmed up and running under heavy load?

 

[lootzyan]

I admit that I did not pay attention to this note of yours:
"The bike can be started by pushing and seems to be working fine, idling and revving well. So there's no indication to take it apart ..."
I am confused about the meaning of this. If it were a mechanical failure, such as increased friction of bearings or pistons, or an incorrect valve timing, such failures would very quickly disrupt the operation of the engine. High temperature, irregular engine running, etc. But, according to your observations, this does not occur. So if the problem isn't the engine then what? Check engine starter again?
Excessive carbon deposition on the piston surface and combustion chamber may only be due to excessive oil burning when the engine is in a high wear state. This would be easily detected if the oil level is checked regularly. And the engine compression would be noticeably lower, not higher.

 

[spark82]

Hi everyone, good news! Problem not solved yet, but at least found: it seems there was a material or heat treatment fault in my camshaft, there was a small crack in one of the lobes (correct term?) which couldn't be seen at first (I needed a magnifying glass to see it and it could be felt by a fingertip), but it started to wear out the rocker roller (not sure on the correct term on this either), and it must've hit it more that it was supposed to and the roller started to "stick". That was causing the hard start, as it tried to push the rocker and the valve, it acted as a brake on the camshaft (while also starting to wear out the camshaft lobe itself). The camshaft and the rocker is toast anyway, we grinded it a little with Dremel to get rid of the fault line and voila! starts as new. While the engine was running, it didn't cause any symptom, no sound, no vibration or anything, and the engine easily overcame of the resistance it seems, but it was a bit too much for the starter.
So I'll need a new camshaft and a rocker (and maybe I'll replace the chain and the tensioner, as it seems this whole thing caused the chain to wear out sooner, as there seems a little low reserve left in the tensioner)
Thanks for all the input for everyone, the thread can be closed.

 

[TNHoosier]

Hi everyone, good news! Problem not solved yet, but at least found: it seems there was a material or heat treatment fault in my camshaft, there was a small crack in one of the lobes (correct term?) which couldn't be seen at first (I needed a magnifying glass to see it and it could be felt by a fingertip), but it started to wear out the rocker roller (not sure on the correct term on this either), and it must've hit it more that it was supposed to and the roller started to "stick". That was causing the hard start, as it tried to push the rocker and the valve, it acted as a brake on the camshaft (while also starting to wear out the camshaft lobe itself). The camshaft and the rocker is toast anyway, we grinded it a little with Dremel to get rid of the fault line and voila! starts as new. While the engine was running, it didn't cause any symptom, no sound, no vibration or anything, and the engine easily overcame of the resistance it seems, but it was a bit too much for the starter.
So I'll need a new camshaft and a rocker (and maybe I'll replace the chain and the tensioner, as it seems this whole thing caused the chain to wear out sooner, as there seems a little low reserve left in the tensioner)
Thanks for all the input for everyone, the thread can be closed.

Congrats on finding the problem. That was certainly an uncommon cause and effect.

 

[670cc]

Congratulations on finding a fault. This is a disturbing find, as it is not charactistic of modern, especially low mileage, engines to fail in this way. It is certainainly not what I would expct to see on a modern Japanese built Honda.

 

[lootzyan]

For me, I do not know if "congratulations" is the right word in such a situation. Rather, "I'm sorry that happened to you." The repair cost can be significant. Sometimes it may turn out that the entire used cylinder head, e.g. from eBay, may cost less.

 

[TNHoosier]

For me, I do not know if "congratulations" is the right word in such a situation. Rather, "I'm sorry that happened to you." The repair cost can be significant. Sometimes it may turn out that the entire used cylinder head, e.g. from eBay, may cost less.

That is true. It appears the rocker arms are sold as a set for $311 and the cam shaft is $165. Ouch. That does not include gaskets, labor, etc.

I would be looking for used parts if they can be found.

 

[lootzyan]

...there was a material or heat treatment fault in my camshaft, there was a small crack in one of the lobes (correct term?) which couldn't be seen at first (I needed a magnifying glass to see it and it could be felt by a fingertip), but it started to wear out the rocker roller ...

I think the reason is the opposite. As the valve rocker arm roller has stopped rotating, it has damaged the cam surface.

 

[spark82]

That is true. It appears the rocker arms are sold as a set for $311 and the cam shaft is $165. Ouch. That does not include gaskets, labor, etc.

I would be looking for used parts if they can be found.

There's a shop in Germany that sells parts from "totaled" bikes. I think they have engine parts, I'll check that out.

 

[spark82]

I think the reason is the opposite. As the valve rocker arm roller has stopped rotating, it has damaged the cam surface.

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Yeah, it's likely, that would be my first guess as well, but I thought the camshaft had been the cause because of the crack-like nature of the fault. A stuck roller would wear out the lobe, not crack it (one would think). Anyway, I suspect it must've been a material fault, not a machining one, I owned Hondas before, and a lot of my friends did and do, haven't heard about a similar kind of fault ever. I guess it's a one in a million thing, maybe I should buy a lottery ticket. If not for the worn lobe, it would be enough to replace the rocker. But seeing the surface hardening being damaged, I'll opt for replacing the whole thing (with the chain and the tensioner).
This season I'll use my vintage 250ccm single-cylinder two stroke bike - it has no camshaft and it's a kickstarter. I have another project going on, I bought a big basket of parts for a 45-year-old firetruck and the water pump engine inside it, my wife would surely divorce me if I'd buy all these additional parts on this years budget.

 

[lootzyan]

Yeah, it's likely, that would be my first guess as well, but I thought the camshaft had been the cause because of the crack-like nature of the fault. A stuck roller would wear out the lobe, not crack it (one would think). Anyway, I suspect it must've been a material fault, not a machining one, I owned Hondas before, and a lot of my friends did and do, haven't heard about a similar kind of fault ever. I guess it's a one in a million thing, maybe I should buy a lottery ticket. If not for the worn lobe, it would be enough to replace the rocker. But seeing the surface hardening being damaged, I'll opt for replacing the whole thing (with the chain and the tensioner).
This season I'll use my vintage 250ccm single-cylinder two stroke bike - it has no camshaft and it's a kickstarter. I have another project going on, I bought a big basket of parts for a 45-year-old firetruck and the water pump engine inside it, my wife would surely divorce me if I'd buy all these additional parts on this years budget.

For us here to advise the next move is like playing chess with eyes closed.
That's why I previously advised: "If you can notice unusual resistance to manually turning the engine with the spark plugs removed, it is time to consult an experienced mechanic." Even for purely economic reasons.
Good luck.

 

[mercurydog47]

Hello everyone, I have a strange problem. The bike in question is a '13 NC700XA. In the autumn I had starting problems, engine moved a little but the starter couldn't produce enough speed to start the bike. I thought maybe a bad battery, so bought a new one (old battery was a little weak). The season came to an end, but after installing the new battery a few times I still felt like starting the bike is a little difficult as if like the starter motor turns the engine a little slower than usual. Now today a friend of mine came to check out my bike as he wants to buy something like that, and I failed to start the bike (oh the shame). The new battery was on trickle charger, so one would think it's OK, but it acted as a dead battery. All it did was to turn the engine a little, but I felt it couldn't get through the compression (so it didn' really make a full turn.), the display blank while I pushed the starter button, it came back to life after, headlight looks OK, fuel pump sound normal (not with as a dead battery) I tried a new car battery also and the starter couldn't turn over the engine with that either. Moves a little but stops and after I release the starter switch I hear the engine moving back as if compression pushes it back. I attempted several times but I only to managed to get a few turns, but not near enough speed to start the engine. (The battery starts a car.) After that I tried to touch the jump cables directly to the starter motor cables to eliminate the starter relay from the equation, the result was the same as before. Now I removed the starter motor to check if there was something at fault there, but couldn't find anything obvious, I made the basic measurements, bearings, brushes look OK.
All in all I feel as if the starter motor is weak or the engine is hard to turn, I cannot really say it differently. I'll put the starter motor back tomorrow. Any ideas, what might be the problem? Thanks in advance for any input.