New to Honda Z50... Engine dead. Help!

Q

QRSamuel

Member
Skip to the second big paragraph if you don't need a backstory on my bike ;)

Hi everyone. I recently (2 days ago) got my hands on a 1974 Honda Z50J. The plan was to get it certified and road legal in NZ, however it lost all compression after I tried to tune the carb yesterday - It would start fine while not using the choke however it refused to start at all while using it, so I figured I'd iron out the issue, then... kaput. Shouldn't have touched it :cry:. Anyway, the engine it has installed is from an ST50 Dax and it has a manual clutch. After removing the head, it appears to have the wrong piston. At TDC it appears to stop a few mm before reaching the top of the cylinder, with a heap of carbon buildup in the area it doesn't extend to. Compression was just under 3kg/cm2 after it stopped running however I'm not sure the shorter piston would cause it to be that low... as it was running and driving when I purchased it. Also after disassembling I found the left hand thread for the intake manifold it stripped... Annoying but I just chucked a lot of silicone at it however no change - I was thinking It could be a vacuum leak due to that stripped thread prior to compression testing it. I found a few other loose fasteners on/in the engine too and it feels like something has come loose and is scraping on the flywheel. There is a lot of resistance when turning the flywheel even with the head removed, and if I look through one of the holes in the flywheel I think I can see what looks like scored metal towards the rear of the interior. I did mess with the cam chain tensioner, as there was also a noise coming from the engine prior to it's death - perhaps whatever it caught in the flywheel, however I loosened it again to ensure that it wasn't an overly tight cam chain causing the resistance, and it wasn't. There were also very fine metal particles in the oil when I emptied the engine and my last oil change was the day I purchased it so that's not good. A cause of the metal particles could be the upper cam chain sprocket cover, as I detached it to inspect the cam chain, and after removing it a second time I noticed it had been scraping against the sprocket - how this happened I do not know. Mismatched parts? I used 0.7L 10w-30 SAE as specified on the right side engine cover too, so I'm decently sure nothing I did caused this.

Anyway, my big question is, will an eBay CDI conversion kit for the Z50 work if installed in a CRF50 engine? I ask this as having to potentially purchase a bunch of parts for my current engine (new piston, and maybe cam chain rebuild kit) may exceed the cost of just buying a CRF50 engine from a wrecking bike near me. I was also planning on doing the CDI conversion at some point anyway so I don't mind buying the kit. While doing research I saw people saying due to the updated stator design you have to swap out the wiring loom and coil on the Z50 for the CRF50 one or it won't work, or connect up right - and add the CRF50 CDI, no duh. To avoid doing this I was wondering if I could install that kit and keep the old wiring loom. I'm aware that Honda changed the taper on the crankshaft so that the old style flywheel no longer fits, and from pictures I've seen online it looks like the kit might fit, but my main concern is compatability between the new style flywheel and the ebay kit. Is anyone able to shed some light on this, or show me some other paths I can take? Any input is appreciated. Cheers, Samuel.
 
The Bike :cool::
20250219_154136.jpg


Carbon buildup cylinder edges with (short?) piston:
20250221_134629.jpg


Scored metal on interior of flywheel:
20250221_134725.jpg


Scraping on upper cam chain sprocket and cover:
20250221_134821.jpg
 

Attachments

  • 20250219_154136.jpg
    20250219_154136.jpg
    3.1 MB · Views: 42
Interesting, what head is on it? That cam gear is certainly not stock. It also looks like someone installed a 12v CDI piston on there and the piston is 3mm below the deck. They run like crap when they are setup that way. I'd guess someone put a 12v chinese top end on there.
 
Hi @fatcaaat - Here is a photo of the writing on the head. I have no idea what it means as I am new to Honda engines but hopefully it provides some insight.

20250221_140715.jpg

As for the piston, I have to agree with you there. And I was wondering why the sprocket and only had two bolts, when in the manual it had 3.

At the very least, the head gasket and o ring need replacing as there was oil leaking into the cylinder overnight, but I don't want to fix it just for it to die again, and I need to order a puller to get the flywheel off and rectify whats going in behind there... and noone stocks the pullers where I am. I am also 99% sure the cam chain and sprockets are worn out. Pain!
 
Ok looks like a honda head so maybe they just did a piston and cylinder swap and a modern head? That head if in good condition is fine but would need to see cam and combustion chamber to get you tge right piston.
 
Not from a 74. Pull the intake and check the intake port for black soot, it's a dead giveaway of a leaking intake. Looks dirty/black from my screen. Measure the cylinder bore. Sure seems like more than a stock 49cc. also looks like the intake valve has a ''big'' leak. Leak down test both ports with brake parts spray or wd40. I would do a Trailbikes complete stroker kit or whatever you can buy in NZ from the crank up as a kit with a matching head. Lots of stuff for the scrap pile there.
 
Last edited:
Looks like a crf50 head with an aftermarket cam. If you want to stay under 88 cc it will be ok. Confirm what old ct suggests on valves and make sure you replace the seals. Not sure what is available where you are but tb does sell correct piston to match your 6v crank, 12v head at 88cc kit size
 
Thanks OLD CT and fatcaaat. @OLD CT I measured the head gasket and cylinder with a ruler (I don't have anything more precise, sorry.) and the bore looks to be roughly 5cm (49mm), so you're right, not stock. What do you mean by leak down test using brake clean/WD40? After removing the cam I put a bit of water in the intake and exhaust ports, none made it to the combustion chamber. In the photos both valves are slightly open due to the position of the cam. @fatcaaat I will have to pass on replacing the valve stem steals for now as I don't have a valve compressor. I checked in the ports now and it is very dark on the intake side. I wouldn't describe it as soot, since none would come off on my finger, however it's definitely not great. I think I will probably buy a crf50 wrecking engine and install a CDI, this engine has been molested and I don't want to spend the money to get it back to normal right now. It will be cheaper and require less effort to go with this option, I think.
 
Hi from Aus,

Okay, so your st50 engine is the same crank, and top end as a 74 j1 and is 6v

Classic Honda 50's or groovyguru on eBay might be able to get some parts like cam chain and gears over to you.

The piston and head are incorrect and will not work.

Kitaco make a 6v 75cc cheat kit that says 49cc, that comes with bore, piston, ring and gaskets for about $100. You'd need to find a 6v head which I'm sure won't be too hard. It comes in black or silver finish (webike)

The CRF motor is an option but you'll need a takegawa Street rotor, 12v CDI, coil, switches, etc etc, it's an expensive exercise if you want the street gear

Also, do you want to sell those bars? They are from a gorilla and I have a gorilla for them. Haha

But aside, you've got a cool bike. There's a tail light bracket extension that you're missing but reproductions are available on eBay

Matt
 
Last edited:
Also, a different option if you want a little more power is a tb parts/tb racing top end 88cc kit

It comes with bore, piston, head etc and will give you more power but you'd also need a heavy duty clutch and upgrade the oil pump (and drill the oil passage)

I personally don't like these kits, only for the fact that the head kind of looks like a pitbike head with bigger cooling fins.

I guess, you have to choose where to stop
 
Hi @power6994 and others. Thanks for the info. The dude who sold me the bike has lent me a 2000's C50 engine to test fit. It appears similar to the crf50 engine but it comes with a lighting coil and a slightly different stator cover. I'm considering either sticking that engine in my bike or rebuilding the one I have. It would be cheaper to rebuild the one I have, I think, but I would always be nice to have a modern engine in my bike which I can easily find cheap parts for. I found this listing online for a loom and switches, would all I need to be that + a CDI? Cheers. https://www.ebay.com/itm/FOR-HONDA-...HARNESS-ASSEMBLY-CONTROL-SWITCH-/132104089461

As for selling the bars, I was considering putting a pair of replica monkey bars on this bike as the new c50 engine has an auto clutch, however I want to concentrate on getting the bike road ready first.
 
As an eBay Associate we earn from qualifying purchases.
I've not used one of those harnesses before but I think you will have a time trying to get it to work.

My advice to you is keep the 6v engine and harness, put a kitaco 75cc piston and bore kit, change the bars to monkey style bars that suit the bike. Ride and enjoy
 
Unfortunately I can't go overbore as if I want to get it road legal in New Zealand the engine must be stock displacement, apparently I can only modify it and get a low volume vehicle certification after getting it certified for the road, and they check everything. After some thought I do think I will keep the 6v engine, as it will be cheaper and less effort, the original owner of the bike has a 6v head which they may sell/give to me but if they don't I'm not sure what I'll do as there's no way in hell I'm shelling out $300 for a reproduction or crusty original 6v cylinder head.
 
The kitaco kit is a cheater kit, which means is says 49cc on the bore but actually it's 75cc piston. They're really good and very affordable compared to new piston, getting it bored or honed etc
 
I will consider it, but they may check the top speed. Another possibility I was wondering about is if there are any points assemblies that will fit in the 12v engine? That could be an easy option for me if it's possible. I'd appreciate any input if someone knows about this. Edit: For example stator assemblies from 1982-87 bikes. They seem to be the most similar engines to the modern 12v and they use points, but I can't find any info.
 
An 82 CT70 has the stator and the ''first year'' for the ''6 volt long rod crank'' you need for points. The crank taper is different for points and they both need to be changed to work in a `12v case. Not going to be any cheaper with this option.
 
Okay, new question. Can I buy a long connecting rod and get it pressed into my existing crankshaft? It would be incredibly cheap but I'm concerned Honda could have changed the dimensions of the crankshaft over the years. If this method works then I could use a 12v top end kit, correct? Sorry if all my questions are becoming bothersome, I'm just trying to figure out the best (most dirt cheap :whistle: ) way to do this. Even if noone knows the answer I may get desperate enough to try it out as knockoff conrods are inexpensive.
 
I got hold of the 6v head I mentioned earlier, and I've also bought all of the other parts I need, unfortunately for my my bank account. I'll post an update when I get it back together. New Chinese made cylinder, CF120A piston, 0.5mm thick copper head gasket and Chinese made cam chain kit are the main things with a few extra bits like seals. Should have a bit more compression than stock while retaining stock bore. I won't be rebuilding the stator for now even though it was contacting the flywheel, I'll just file it down to fit. This is because there is a big hole on the inside of the stator cover (yay!) and anything I buy to go in there will probably just get ruined, so when I have the funds for a new cover I will rebuild the stator.
 

Attachments

  • 20250318_213238.jpg
    20250318_213238.jpg
    1.6 MB · Views: 27
Back
Top