Engine knock

[JohnnyT]

Have a 1973 CT70H. It has 6200 miles on it and has not been rebuilt. It starts right up. Runs great, has plenty of power and speed, but when idling there is a knocking sound coming from the motor. It doesn't have a real rhythm or pattern to the sound like a ticking, but a kind of rattle like something is loose. Kind of goes away when motor is reved and sometimes I don't hear it after riding it for a while. My first thought was the cam chain, so I attempted to make the adjustments, but no change in the sound. Any thoughts?

 

[69ST]

I would have started looking in exactly the same place, too. The most likely problem is either lower end rod knock or a problem elsewhere in the valvetrain.

6K isn't a necessarily a ton of mileage, unless the oil spinner is filled with accumulated crud. On the other hand, 38 years is plenty of time for acidic oil to erode the big end rod bearing. Far more often, dirty oil results in a scored cylinder; needle bearings & the rod journal are hard steel alloy.

I've run across engines with unevenly worn rockers that caused noise. Those can be diagnosed by progressively tightening valve clearance & testing until the noise goes away. If it does, you've only isolated the problem. The offending rocker(s) should be replaced if there's insufficient clearance. If the noise remains unchanged, then it's either piston clearance or a lower end issue.

Other possible causes range from relatively uncommon to rare: loose cam sprocket bolt(s), excessive camshaft endplay, loose clutch, primary drive slop, bad crank bearing (very rare and should manifest as other problems & noises), warped flywheel making intermittent contact with a stator coil lamination (I've seen a couple of these). You could still have a cam chain-related issue. A worn idler or roller, stretched chain or a binding oil pump (which will wear the drive sprocket teeth quickly) could all cause, or contribute, to the rattling.

HTH

 

[Mark Pearson]

Hey Jonneyt, I rebuild alot of the little guys, and 6000 miles to me its ready for a rebuild and the little guy has done his job I think. Most are ready at the 3000 to 5000 mile mark with 40 years sitting around not being used. That 6000 mile ride is coast to coast twice and taking 40 years to do it. I think its ready for a transplant. But thats just me.

 

[69ST]

I also rebuild a lot of `em and agree with you that most need open heart surgery before 6000 miles. I've seen sub-2000-milers that needed major repair work, 5000-mile examples needing no more than a light honing (std. bore diameter, which shocked the hell out of me), plus the ceremonial intake valve replacement, and two examples (in ST70 Daxes) that made 30,000 miles(!) before needing the first rebuild. Neither needed a crankshaft rebuild...go figure.

The "head-scratching" material here is that he says the motor runs well, makes decent power and doesn't smoke. Seems a tad unusual for a motor with enough internal wear to have a rod knock, doesn't it?:39:

I'm going for 2 points in this post. First off, as you say 6000 miles of few oil changes, zero filter cleanings and 40 years spent mostly sitting is a lot of neglect and that's why most of them die young (in terms of mileage). It's not a weak design, but neglect/abuse, that prevents them from getting well into 5-digit mileage.

Second, fewer than 1 in 5 need a crank rebuild. I'll never cease to be amazed by how many totally up originals still have crank assemblies well within OEM specs. The journal end of the rod usually must be a lot looser than factory spec before you'll get a rod knock...but it does happen and sometimes in very few miles.

 

[JohnnyT]

Well, I guess you all are saying what i figured you would. I just am in a bit of a denial and not wanting to rebuild the engine, but probably should before it gets worse. The bike has been in the family since around 1976. Dad bought it for me when I was about 14. It had yearly oil changes for a while, but then I'm not sure what happend to it in the last 20 or so years. I know it sat a lot probably with dirty oil in it. When I got the bike in April, it had the motor out. Brother in law was going to replace the coil, but never got to it. I replaced the coil, did a tune up with new points and condenser. Cleaned the oil spinner. I'm sure it was never cleaned before. Put on a new muffler and put the bike back together. Started right up and shifts smooth with no slipping. Only problem is the periodic knocking. I figured it was due to the age and mileage. I probably have put on 30 miles since putting it back together. Well, I guess a rebuild is in order. What do you guys think of the new Lifan motors? Have seen some for around $200.00. Would like to keep it original, but, hey probably will spend more for rebuild parts than a brand new motor.

 

[69ST]

It's a case of "you get what you pay for" and your priorities. A bike that's been in your family going on 35 years doesn't sound like a candidate for a cheapie resotmod. You could source a PRC clone motor on the cheap. Converting the 12v isn't difficult; modifying the wire harness might be, if you're unfamiliar with electrics and, the mod would be more, or less, permanent. The rest of the conversion would be, essentially, bolt-ins and easily reversible. I'd have to at least ask how important things like running, shifting, overall smoothness/durability and broad parts support are to you; that's where the original Honda motor has it in spades. Plus, it's, well...original and all that comes with it.

If you're going to do the rebuild, then figure on roughly $350 in parts, plus machining (bore/hone & valve job). A crank rebuild, if needed, will likely add $175; you could source a better used crank from another 6v 3-speed CT70, or a 6v Z50...which would be a lot cheaper. The rest is time and elbow grease. If you farm it out, plan on spending $700-800 to get it back in show condition, ready for the next 35 years. The total cost spread will be less than $500, probably A LOT LESS. Sounds to me like your clutch & transmission are in decent condition. It's up to you to decide which way is best.

 

[P.C.]

If it is a H model the 6v 3spd or z50 crank wont work.dr atv sells a complete ready to go HKO HK1 4 spd crank for 89.95

 

[69ST]

You are correct, I be an brain damaged:35:.

That's a decent quality aftermarket crank, at a bargain price and one of the big breaks for H-model owners. It's the 3-speed cranks that have become unobtainium.

 

[JohnnyT]

Thanks, racerx for your thoughts and estimates on price for re-build. Got some time over winter to think and decide what to do. It's a matter of time and money. I also have an air-cooled VW that I am working on. Just not enough time and money to go around. Would be great to just have someone re-build it if I had unlimited funds.

 

[darrel gunderson]

When you cleaned the oil spinner did you have the clutch off? Did you properly retighten the spanner nut that hold the clutch basket on? That may be your knocking sound!!!

Darrel

 

[JohnnyT]

Did not take off the clutch only the cover to clean the spinner. The motor made the knocking sound before I took off the side cover.

 

[Epic_username]

The rod bearing probably just slipped on the bearing cap. This happened on a few 3 year old honda power washer engines i was working on. I know theyre not the same but still...
It might also be a badly damaged main seal. By badly damaged I mean partly deteriorated (saw that happen on a suzuki ts75)

Or it may be something stupid, like a loose motor mount bolt (happened on my pocket bike)

Whatever it is, hope you find the problem

 

[69ST]

The rod bearing probably just slipped on the bearing cap. This happened on a few 3 year old honda power washer engines i was working on. I know theyre not the same but still...
It might also be a badly damaged main seal. By badly damaged I mean partly deteriorated (saw that happen on a suzuki ts75)

Or it may be something stupid, like a loose motor mount bolt (happened on my pocket bike)

Whatever it is, hope you find the problem

Can't be a rod cap/bearing insert issue. The Z50-based horizontal engine has a one-piece rod and an assembled crankshaft. The big end needle bearings and/or crankpin can wear out, usually the end result of galling caused by grit in the oil.

 

[ANDY11]

Just a sugestion here. I built three of these motors over the last year or so all being earlier model ko. The problem I found in all tree of these motors is that the backing plate on the backside of the primary driven gear has cracked or even came completly off, like the one in the picture. I did'nt hear any of these motors run with these gears this way, but i'm sure they had to make some noise. I know honda changed the design of this gear later on. If you geat your left case cover off again take a peek at that gear. Hope you get it figured out.

 

[OLD CT]

ive never had that happen.that primary gear is under the right side cover,not the left..

 

[tom0762]

johnnyt, its sounds like a headache to me! i think i would take racerx & mark pearson's advice, if you have any extra cash?? both these guys are pretty darn slick with these little guys.... soon as i get some extra cash,mine engine will be shipped to mark.. asap

 

[ANDY11]

ive never had that happen.that primary gear is under the right side cover,not the left..

You are correct. That would be the right side.

 

[69ST]

Open enough of these little motors and eventually you'll see one of every possible failure/type of breakage/damage/etc. Of course, that may take more than a single lifetime. Some problems nearly defy explanation...like this primary driven gear...:13: