CT70 K0 runs strong, but smokes a little- top end questions

[Meangreenhk0]

Good Evening All,

A couple years ago, my uncle and I brought back to life a gold K0 three speed after 20 years sitting in the back of a mechanic's shop. The PO put some Mystery oil in the head due to the length of time it sat. It had no spark but the engine turned over and we happily purchased it for a low price. We quickly brought it home and put a new coil, points, oil, condenser, new chain, steering bearings,rebuilt the carb, new tail pipe, etc. Eight hours later fired it up for the first time in 20 years, on the second kick.

To our surprise, this little guy was and is fast and smooth! It's one of the strongest original bikes in the bunch. (maybe it has been rebuilt or bored over in it's life.) It easily hits 46 mph with my 175 lb body, looks great, stops on a dime, and shifts great.

However, I noticed riding behind the bike over Christmas that it smelled bad enough to not enjoy riding behind it when it was in the front of the pack. It smells of exhaust, or oil, or lawnmower... you know the flavor I presume. It does visually puff smoke, just a bit, on first start up then stops quickly. The smoke is not noticeable when riding or idling, just the strong smell that can make you a bit nauseous over the course of 15 miles with it leading the pack.

I assume new rings and piston are in order, but I hate messing with it because it is such a nice original bike. I have never gotten too deep into the engine, but I have done clutches and stators more than a few times for our fleet. I would love to eventually rebuild an engine or do an 88 upgrade. I have watched many, many hours of how-to videos and I figure a top-end rebuild is may be the first step toward this goal. This bike would be kept near stock, as it is mostly used by more novice riders when we go out as a group.

I have a few questions and hope to gain a better understanding of what my thought process should be.

1) Would it be better to just buy a whole new 72cc top end from TB ,or the like? I assume this is the easiest to just plug and play, everything fits as it should, and everything is new including valves, rockers, etc.

Would I need to set the rings and gap them, take it to a machine shop to do, or are they ready for installation?

2) Or, since it runs so well, could I literally just do new rings and piston? I assume I must have the Honda jug machined. Is the Honda top end better quality and worth saving over buying new?

Do you bring the machine shop the piston and rings when they hone the jug? .50, 1.00, or 2.00 over? Will they set they set/gap the rings for you?


Thanks for the input, as always.
Christopher

 

[69ST]

That's running very well for a bone-stocker. Top speed is reliable enough to be "the poor man's dyno". Assuming reasonable speedo accuracy, that motor has to have very good compression...inconsistent with poor ring seal, or leaking valves. While it's possible that there is some cylinder wall pitting, it's unlikely with this kind of power. If it does turn out to be poor ring seal, I'd have the cylinder mic'd. If it's within wear limits, free of pitting/scoring, you may be able to just have it de-glazed, then install new rings.

Oil can get past the valve guides; you might need nothing more than a new valve seal, on the exhaust side. If the intake port is blackened, time for a valve job and a new intake valve...at least. If it's still clean, then test the valves by spraying solvent into the ports, looking for traces of wetness inside the combustion chamber. Obviously, the head has to be on the bench to do this.

It's also possible that oil is getting past the head gasket, or one of the O-rings that seal the oil passages. No machining necessary to replace those.

Now, all of that having been said, I'd want to verify that oil control is an issue in the first place. If it is, there should be clear signs...black deposits on the sparkplug & oily threads, the exhaust port should be heavily sooted, too. Actual, wet, oil inside the exhaust port, or/and headpipe would be clear indications of excessive oil consumption. Wet, oily, deposits at the muffler outlet would be the proverbial smoking gun...no pun intended.

IMHO, the machine is running too well to expect a seriously worn top end, thus I'd want to begin by verifying that there is a problem, then tracking it down, starting with the easiest/least invasive stuff. Once you determine what's wrong, then we can discuss the best way to fix it.

 

[hrc200x]

Seems like you have some mileage on the bike so this is unlikely the problem but is it possible some of the oil the previous owner put in the head or cylinder ran into the head pipe of the exhaust and needs time to burn out? Have you had to add oil?

 

[69ST]

IMO, you would have been better served by including your RTV/oil leak here, rather than in another thread. RTV is a strong wrong...

 

[Meangreenhk0]

IMO, you would have been better served by including your RTV/oil leak here, rather than in another thread. RTV is a strong wrong...

Since it was a different bike, I kept them apart as to not confuse the symptoms/diagnosis. The gold bike on this thread runs great but puffs a bit.

The other thread red bike does not smoke but I found the RTV and oil around the head gasket.

We have over a dozen CT70s between friends and family and essentially 2 of us are responsible for the maintenance. I happen to own 3.5 CT70s, 2 HK0 and 1.5 K0s!

 

[69ST]

OIC...now.

As you can tell, I'm sometimes easily confused.

 

[Meangreenhk0]

Yeah,

Seems like you have some mileage on the bike so this is unlikely the problem but is it possible some of the oil the previous owner put in the head or cylinder ran into the head pipe of the exhaust and needs time to burn out? Have you had to add oil?

we have run it for 2years and probably put 500+ We have added some oil here and there.

 

[69ST]

Yeah,

we have run it for 2years and probably put 500+ We have added some oil here and there.

Based on that, it's probably inhaling more oil than is optimal...as you thought. Time to inspect the top end assembly.

 

[OLD CT]

If you do not already use 20/50 4 stroke motor oil with a little shot of Lucas oil stabilizer, I recommend you do. I wouldn't take it apart just yet if it will hit 46 mph stock.

 

[Meangreenhk0]

Based on that, it's probably inhaling more oil than is optimal...as you thought. Time to inspect the top end assembly.

If you do not already use 20/50 4 stroke motor oil with a little shot of Lucas oil stabilizer, I recommend you do. I wouldn't take it apart just yet if it will hit 46 mph stock.

This is an interesting suggestion. I was hoping for something like this to buy some time, since it runs so well. What’s the thought process of the heavier weight oil and Lucas oil? Would this interfere with the clutch?

I wondered if maybe a pinched/clogged vent tube could be the culprit.

 

[69ST]

I wondered if maybe a pinched/clogged vent tube could be the culprit.

I'd check that before breaking out the wrenches, for sure. FWIW, I agree with Pat and would be reluctant to open an engine that runs this strong.

The theory is that thicker (20W50) oil won't get past the rings as quickly as thinner (10W40)... IOW, a bandaid. That said, I go by peak operating temp and also am strictly a fair weather rider, typical MI winter temps & riding are outside of my wheelhouse. Below ~65F ambient air temp, oil temp is unlikely to ever reach ~165F (minimum), needed to get it to flow freely. I've not used the Lucas product (strict diet of Honda GN4 oil, 20W50 flavor only, in my machines) but, AFAIK it won't contaminate the friction discs.

 

[hrc200x]

Plugged air filters or heavily restricted air intake can cause oil to be sucked past the rings, air filter clean and the rubber that goes up inside the frame not pinched?

 

[Meangreenhk0]

I'd check that before breaking out the wrenches, for sure. FWIW, I agree with Pat and would be reluctant to open an engine that runs this strong.

The theory is that thicker (20W50) oil won't get past the rings as quickly as thinner (10W40)... IOW, a bandaid. That said, I go by peak operating temp and also am strictly a fair weather rider, typical MI winter temps & riding are outside of my wheelhouse. Below ~65F ambient air temp, oil temp is unlikely to ever reach ~165F (minimum), needed to get it to flow freely. I've not used the Lucas product (strict diet of Honda GN4 oil, 20W50 flavor only, in my machines) but, AFAIK it won't contaminate the friction discs.

Is marvelous mystery oil the same principle as the Lucas?

I have used that in my 1950 pontiac (flathead straight 6) with 20W/50 as well as using it as a gas additive. It would be a shame to break into this strong running bike (although, I am curious if this is original or 1.0 over). We also only use GN4, 10W40, in all of our CT70s and buy it by the case. Since we live in Florida with very mild winters, I would think the 20W50 could be used most of the year. We do ride our fleet all year long and there are definitely some cold mornings below 65F, and during the winter we could ride well below that temperature.

Do you like/use the thermometer oil dip sticks to verify temp?

BTW, I know I post a lot of questions and I am told I can be a bit long winded. However, I am a true CT70 enthusiast and an addict. With over a dozen of these bikes, there always seems to be some quirk or gremlin we are chasing, and that is half the fun of owning them. I am extremely interested in not only the "what" to do, but understanding the "how" and "why". These are elegantly and fairly simple machines; a brilliant design that has stood the test of time. We have grown in our confidence and skill in the past few years and I hope to eventually do a top end and a full engine rebuild in the future. I constantly watch videos and have made it through reading 150 pages of questions on this forum. I certainly appreciate all the knowledge shared. I hope that my questions, at very least, stimulate some good discussions that might help someone else.

 

[Meangreenhk0]

Plugged air filters or heavily restricted air intake can cause oil to be sucked past the rings, air filter clean and the rubber that goes up inside the frame not pinched?

The air filters are newer, the snorkels have been removed on all our bikes because they always get pinched. do you think a K&N style would be a better bet and better performance? I use one on my 88.

 

[OLD CT]

You live in a area where you are best served using 20/50 not 10/40.
I would only use 10/40 when riding in under 50 degree weather. Marvel Mystery oil is good but not for a wet clutch. Lucas stabilizer is! It ''clings'' to parts. They used to have a little clear display at the auto parts stores with plastic gears and a little hand crank on it to show how Lucas works. I do add a shot here and there of MM oil in the gas only, of my CT's.

Even my fresh rebuilds get a small shot of Lucas at oil change time.

MM oil is more of a cleaner and top end lubricant. It is great for sticking lifters in vintage cars.