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CT3 Runs OK, But Maybe I can Fix That!


DaVinci856
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Hi All.

It's been quite a while since I've posted here, but last year I had a running good day/bad day chronicle of when I took my old CT3 out of mothballs and got it on the road again. (See "CT3 175 Runs but then Doesn't" from last summer.) I'm no expert, but consider myself more of a slightly experienced amateur. My background is in engineering, though (which my brother the mechanic never ceases to give me grief about - something about a tendency to over-analyze things). My hope is that by documenting my errors and discoveries, that I may help others.

I have lurked here once in a while, but unfortunately haven't had time to do much work related to motorcycles until recently. I have ridden both the 175 and an FZR600 I have since late last summer without much trouble, but both of them have issues that I'm starting to address now. The first candidate this summer was improvements to the CT3.

The 175 runs OK for what I've been using it for (teaching my 22 year old daughter to ride) but it's just not what it should be. It doesn't have the responsiveness that it should and burns way too much oil (I bought it new in 1973, so I know it should burn oil, but let's just say that people probably don't like riding behind it. Since I had replaced things like crank seals - which I documented in the other thread- I figured it was now time to deal with the cylinder issues.

Removing the head and looking into the cylinder didn't look horrible, but certainly not ideal. A compression test was actually humorous. 3 or 4 cranks with a wide open throttle yielded only 70 PSI! CompB4.jpg

I didn't even know a bike would run with such horrible compression, but it's a tribute to how forgiving 2 cycles are, I guess. Given those results, I removed the piston and jug and took it to a machinist to be evaluated - in my opinion it was a toss up whether just honing it would work or if it needed to be bored. After putting a micrometer on the piston, with the scuffing on the sides the variance between the top and skirt was out of tolerance by a little, and the walls of the cylinder weren't fantastic, so I just took the easy road and decided to replace it and go to the first oversize about a week ago. That way I know that any minor imperfections in the bore would be fixed and by giving the new piston to the machinist, a $60 bore job meant that the tolerances should be perfect. (And they were - if anyone needs a recommendation for an good, honest, motorcycle machine shop near Boston, send me a PM.)

scuffing.jpgCylB4.jpgCrosshatch.jpgDSC00770.jpg

It took a week after my decision to get all the gaskets/piston/rings from various sources, but I got the beautiful new jug back Friday evening (yes, he works nights as well!) and looked forward with anticipation toward today's assembly... little did I know what was to come...

First off - if you are replacing older rings, it's pretty common knowledge that they could be either rectangular cross sections or keystone. I'd taken pistons out before and removed and replaced rings in the slots they came from, but getting the new ones was a bit of a surprise. For one, the new ones came with little spreader springs that should obviously go behind the rings, but I never found a description in my Clymer manual that they existed, or in any on-line parts exploded drawing. Add to that the fact that carbon buildup on the original piston totally hid them from me and I thought maybe they shouldn't be there on a 2 cycle engine. That was until I was fooling around and saw a piece of the springs on the old piston almost by accident (glad I found that in time). Secondly, the new piston I got had keystone rings as I expected, but only on a very close examination did I find that only the top was keystoned. The lower one was rectangular both on the ring and the piston. I never realized they were mixed, but that's where inexperience takes one sometimes. Again- I found out in time, but wish I knew it early on.

Well, after cleaning off the bored jug again (just to be sure), I was all set to assemble things. I was expecting a couple hours and then I'd bee done...not in this Universe, however.

In the past I've pulled the head and cylinder off several times, and after some rather colorful twists of the English language, getting the rings back into the cylinder just by pushing with my thumbs always got it together. The rings were always the only bump in the road, so I expected a similar experience. However, the new rings (BTW, not chrome like the originals, but a medium grey metal - maybe an iron alloy?) were much more rigid than the originals. That turned out to be a major obstacle. (Of course, I first seated them in the cylinder with a piston and checked the ring gap, which was right in the middle of the acceptable range.) I tried it several times, both from the bottom up as I usually did and then after totally failing, from the top down (caving in to pride and using a real ring compressor) but still no luck. These new rings were just too hard to keep in close enough with my thumbs and were not cooperating getting into the cylinder no matter no much oil I tried.

The final disaster for the day came when I thought I just needed to push a little harder. After doing what i usually did with the ring compressor, which was centering the ring gaps around the stop pins and slowly closing the compressor, something must have moved in there. I had been slowly twisting the compressor back and forth around the rings to make sure that the stop pin was in the gap and had been successful at least a half dozen times, but every time I was only getting the lower ring in and the top one would get stuck. This last time I tweaked the compressor a little more once the lower ring had made it into the cylinder just to hopefully get something more on the top ring while lowering it into the cylinder, but then heard a soft "tick". Fearing what it was, I looked into the compressor and saw the expected two pieces of ring where one should have been! I don't really know if the ring had moved over the stop pin or if the compressor was too tight, but I was done for the day and ordered another set of 1st oversize rings to match the piston.

Oh well. Any suggestions on the rings would be welcome, but I think I'm going to just try the new set with my thumbs again. At least that method never broke a set!

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I really cant see what all the dufficulty is myself so long as you have

a: The top ring is keystone

b: The pin clearance cutouts the right way up

c: There is ONLY ONE expander ring to fit...in the bottom (parallel groove)

RingSections_.jpg

Possibly you will find it easier to fit the piston into the bore away from your bike??

Place wrist pin in the deep freexer for an hour or so

Place rags in the crankcase

Fit just one wrist pin clip into the piston

Fit the top of the piston / rings into the bottom of the bore

If you have a hot air gun or blow torch warm up the bottom of the piston (or even place whole lot in the oven for 15 mins)

Retrieve the pin from the freezer and quickly assemble to the con-rod

job done ;)

It is a concern you said spreader rings (plural) Definitely only the one in the bottom, you will notice the bottom parallel ring is narrower...to accomodate the spreader ring behind it ;);)

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I really cant see what all the dufficulty is myself so long as you have

a: The top ring is keystone

b: The pin clearance cutouts the right way up

c: There is ONLY ONE expander ring to fit...in the bottom (parallel groove)

RingSections_.jpg

Possibly you will find it easier to fit the piston into the bore away from your bike??

Place wrist pin in the deep freexer for an hour or so

Place rags in the crankcase

Fit just one wrist pin clip into the piston

Fit the top of the piston / rings into the bottom of the bore

If you have a hot air gun or blow torch warm up the bottom of the piston (or even place whole lot in the oven for 15 mins)

Retrieve the pin from the freezer and quickly assemble to the con-rod

job done ;)

It is a concern you said spreader rings (plural) Definitely only the one in the bottom, you will notice the bottom parallel ring is narrower...to accomodate the spreader ring behind it ;);)

Hmmm. Thanks for the reply, Paul. Very interesting. When I removed the original piston, there was a spreader ring behind each ring and the upper keystone ring was definitely in correctly. It would only fit in the groove one way and get (mostly) flush with the edge because, of course, the groove is tapered at the top to fit. However, your suggestion for the piston pin gives me an idea. The only things different this time than any other times was that the rings and piston were brand new and in a new bore. That would make the clearances slightly closer. The uncompressed new rings also spread out more than the originals at rest which complicated my squeezing them around the locating pins a little (maybe a lot if it had moved and that's what broke the ring). This was also the only time I've tried to put the piston head in away from the connecting rod. The wrist pin is snug, but fits. I have in other cases cooled off piston pins and heated the head to fit them together and maybe I could do something similar here by heating the jug (slightly) and putting the piston in the freezer for a while - even that little extra might be enough. Doing all this sounds like silly overkill for what has always for me been something that was a minor annoyance but relatively simple, but whatever works is often best.

Steve.

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Hmmm. Thanks for the reply, Paul. Very interesting. When I removed the original piston, there was a spreader ring behind each ring and the upper keystone ring was definitely in correctly. It would only fit in the groove one way and get (mostly) flush with the edge because, of course, the groove is tapered at the top to fit. However, your suggestion for the piston pin gives me an idea. The only things different this time than any other times was that the rings and piston were brand new and in a new bore. That would make the clearances slightly closer. The uncompressed new rings also spread out more than the originals at rest which complicated my squeezing them around the locating pins a little (maybe a lot if it had moved and that's what broke the ring). This was also the only time I've tried to put the piston head in away from the connecting rod. The wrist pin is snug, but fits. I have in other cases cooled off piston pins and heated the head to fit them together and maybe I could do something similar here by heating the jug (slightly) and putting the piston in the freezer for a while - even that little extra might be enough. Doing all this sounds like silly overkill for what has always for me been something that was a minor annoyance but relatively simple, but whatever works is often best.

Steve.

Minor update 7/30/11

Emotions at a a very low point yesterday when I received the new set of 1st OS rings from ebay. The rings I received had the correct part number on the box, but were definitely not the correct rings. The rings in the box were at least 1 CM less in diameter than even a standard ring set. One interesting point is that there was only one expander ring n the box as OG had expected, though. To say I was disappointed at the incorrect size is putting it mildly. However, the seller agreed to accept the return, so at least all I'm short is a a week of fun. I'll be waiting another week for a chance to try this again. It reminds me yet again that I should have been doing this when it was freezing outside and not in perfect riding weather. It certainly makes one yearn for the ability to just go to a nearby shop and get parts for this bike like I could in 1973...sigh.

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  • 3 weeks later...

Well, it looks like I've corrected the main performance problem, but mostly as a side effect of fixing a no-start problem and not from incredibly intelligent troubleshooting. It's been a couple of weeks, and I've been tinkering, but not writing.

Short story update - when replacing a piston into the cylinder, it's a heck of a lot easier just to do it with the piston installed onto the connecting rod. After having more problems trying to do it in what I thought should be the "easy" way (with it off the bike), I just went back to the way I had done it many times before and it was quick and easy. I don't know exactly why, but maybe everything was squared into the cylinder better, don't know for sure - in times of doubt, just go with what works.

In any case, the engine went together easily after that, I torqued everything to spec and was just about ready to go when I needed to connect the carburetor to the air cleaner. and the only real issue was that the stupid rubber duct between the airbox and the carb refused to fit. The rubber has gotten pretty hard at the carb end especially since 1973 and I'd replace it if I could, but I can't find one anywhere. I ended up cutting my right index finger pretty badly on the duct edge of the airbox when the duct slipped... lots of blood for a finger, but none got into the gearbox, so I didn't get a chance to see if it would improve performance <sigh>. BTW, if anyone has a good recommendation for a product that would soften old rubber I'd love to hear it. The research I did didn't yield much that was helpful as a permanent fix. (Drenching it with silicone didn't help, but I didn't try things involving petrol based things because I figured it would eat the rubber eventually. I did, however find a way to get it onto the carb easier... a heat gun (or even a a hair dryer) does a great job of softening the rubber long enough to get it connected.

Once everything was connected I turned on the fuel and kicked it over. Started on the first kick! I was thrilled. Of course it smoked like I'd just set an oil refinery on fire, but it ran. Besides everything inside the cylinder being new, the smoke was largely due to the fact that I had oil all over everything for assembly. Luckily the clouds subsided fairly quickly. I popped the throttle a bit and then got my helmet for a test ride. Unfortunately, I didn't really see much of an improvement from before the bore job. Also, I still had the uneven running between 4 and 6k RPM that I had previously. In general, it ran about the same, which was depressing, but it did seem to smoke a little less. After about a 10 minute test ride, I brought it back home to see what happened. I checked the compression and strangely, I saw just about the same 70PSI that I had before the new piston/bore/rings. Another puzzle was that when I tried to start it again it wouldn't start. That was odd, since it ran pretty much the same as before the work was done. In pretty short order I found that I had spark, but no gas getting to the spark plug. Normally I'd figure that the I'd gotten a piece of dirt in the carb, but I had installed a rather ugly fuel filter between the gas tank and the carb last year and thought nothing could ever get there. I'm still waiting to find a smaller in-line filter that will do a good job and fit in the space easily, but no luck yet. In any case, I took the carb apart expecting to find the clogged pilot jet...but it was clean! However, I was surprised to see the amount of grit in the carb. Not a lot, but I expected to find none. It probably only had 40 miles or so on it, but there was a reasonable amount of grit in there and I've no idea how it got there. I should have taken pictures, but didn't this time - sorry. There was a little grit in the bottom of the float bowl, but the real surprise was that it looked like the main nozzle had one of it's holes partially plugged with some sort of granular material that looked like some kind of corrosion - but I'd cleaned that carb thoroughly at the end of last year. A big puzzle, but I decided to soak the carb in cleaner for a while and try it then. After the cleaning, it started right up again and took a ride for a half hour (unfortunately it had to be short because I had work to do around the house). This time there were no bad spots at all up through the throttle range, which proved that the problem was simple dirt. I'm still shaking my head as to how it got there, but I'll be changing the fuel filter ASAP since it doesn't appear to be doing a good job. I'll check the compression again after I put a few more miles on it, but it seems to run pretty well right now.

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  • 4 weeks later...

Sorry - it's been a while since I've been here....

Yes, very weird that I had dirt in the carb again, but it was there. I found a new fuel filter that fits pretty nicely, so I should be good from now on.

I know what you mean about the compression test - I was expecting over 100 psi as well. (Yes, I was at WOT when I did the test.) It really would annoy me if it was bored too wide. I may have to get a caliper and measure it. I will check it again this weekend since I now have quite a few more miles on it. Oil consumption seems better, but it does appear to use a little more gas that I think it should. It seems to run just fine now, but I do remember it being a little snappier when it was new...but of course I was 17 then and I think Pink Floyd had just started singing "Money" or some such. As my kids would say, nothing that was a memory from a time when large reptiles had just stopped roaming around like that is guaranteed!

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