An unexpected turn of events leads to a memorable ride

[fatcaaat]

Today I had a real interesting set of events that lead to a pretty decent ride on my retro-build 1982 CT70. It started off with my wife needing to go back to Salisbury PA for a viewing. She loaded the kids in the car and left from here in Fairfax Station VA. I would come later driving the beat up GMC Sonoma with my CT in the back. I had been using this truck to cart refuse from my housing renovations to and from the dump and it was really on it's last leg. The transmission was slipping, it had a hard time in and out of 4wd, had a whirring coming from the rear end, three body mounts were completely rotted off, ac didn't work, the frame had holes in it and the shackles looked like they were ready to come off. The idea was to take the truck to the farm and use it there to haul brush and stuff.

So, I'm about 1/3 the trip and the truck starts shaking violently...all of a sudden...BANG...and bits and pieces start flying out the back. The drive shaft had come off and the rear differential exploded, busting bits of the pumpkin all over the road, and the gears blown out. I get out, assess the damage and throw the driveshaft in the bed. I put it into 4wd and limp to the next exit...the gears grinding as I drive along.

I pull into a burgerking in Myersville MD and a guy pulls up to talk about my CT...within 5 minutes, I have the truck sold to the guy for scrap, unload my bike, and get ready to embark on a long journey, on a relatively UNTESTED bike. I hope on and leave out on Old State Rt 40...eventually it gives way and I have no choice but to run on I70 for 8 miles...i get off in Hancock and fill up. Next I tackle Sideling HIll I68 and I actually pass a Semi struggling to get up and over it. I run I68 for 20 miles and get off on the byways on Rt144 into Cumberland. Right at the bottom coming into Cumberland, i ran out of fuel..and low and behold 3 guys were at the bottom working on a front end loader...i pulled in and asked if they could spare some gas...and I was back on the road again in just a few minutes. Then again Rt40 for the remainder of the miles.

The motor here is the 88cc hammerhead top end on top of a 4-speed, takegawa with 51mm TB stroker. I had this thing wringed out for pretty much the entire ride...

Now for the ride, take a look at the Topography. I started at 672 Feet and had several up and down passes with two of them topping out to 2900ft...very steep inclines over and over again. This little bike did it and got me to where i needed to be, on time.

It needs a little help with the jetting, but beyond that this little bike proved that it can hang. I had this thing pinned at about 55-60 for miles and miles at a time. and on the hills running 40-45 up the hills in 3rd. I had never ridden a bike that I ragged out to that degree...just never wanted to or thought it would last...but I was very impressed here.

 

[Enginedoctor]

quite the story. sometimes things just happen that way... it's good you got to put some real miles on your bike... even though the terrain was inconsistent, what were you averaging for miles per tank?

 

[cjpayne]

That's the second great story I've read where a CT70 saved the day. You definitely built it VERY WELL!!! I'm impressed too.:41: I bet you were very relieved when you got to where you were going:18:.Glad you made it. Especially with an untested bike.

 

[fatcaaat]

using rt40, I logged about 125 total miles. I did 32 miles before I had to hit the reserve.

 

[kirrbby]

Cool story fatcaaat. That's as it should be. A good ol mini trail. Can't beat em, can't stop em, can't get enough of em. You should have just towed that truck back to the farm. Now that would be a story!

 

[Tripod]

Sounds like a fun day. I cant imagine many people wanting to give out free gas for anything with a larger engine than a CT.

 

[OLD CT]

Cool story for sure,i had my 124cc covered with a blanket in the backseat of the Taurus ready to go for a whole season. Never had to use it in a pinch like you thank goodness,but it was ready if needed. That was the running joke when friends co workers asked WHY?? I like to getaway to some choice street riding spots on the weekends with mostly 50mph roads in the mountains with not a lot of stopping and cross streets.

Sounds like it's fully broken in/tested tough now.

 

[69ST]

Isn't that how things go? The high-impact stuff happens in an instant, then the lasting effects are realized.

Looks like you got a "twofer"...a nice ride and the answer to your questions regarding minimum engine output. Talk about turning (mechanical) failure into victory.

 

[fatcaaat]

I have to admit I was white knuckling this bike. Trailwing tires, stock suspension and brakes...the design of the bike was to ride 40-45mph and spend time on gravel roads..certainly not to do any type of touring. I'm of the opinion that this bike would be perfectly suitable, powerwise, if it were ridden it's life in the flats...certainly not powerful enough for large mountain passes. However I did get a lot of thumbs ups as I was on the interstates running 55+.

What I was most interested in was the gas consumption. It actually consumed more gas than I was expecting...quite a bit more actually. This thing only has a vm20 carb on it. I'd say my TRP176 uses less fuel as does my Nice 127+4R both with VM26's on it. I'm assuming that having it pinned probably was the reason for this. I know that the 176 would have gone over any of those mountain passes in 4th gear at 55mph or better and achieved a better fuel efficiency.

I'm just happy that I decided to take this bike along rather than the CT110. While I would have been much more comfortable on the ct110 physically and fuel-wise, top cruising speed is really only 40mph and I probably would have been in 2nd gear over some of the passes. That would have made for a very long, uncomfortable ride.

Before I crack into the bike, next time I'm at the farm, here's the symptoms I've experienced...maybe you guys have some thoughts on how to address.

1. It won't idle. Stalls out. I'm assuming that the pilot is too small based on the fuel screw. And the fact that if I lean up the needle it performs terribly. I'm leaning the needle up because
2. It has a dead spot just before WOT. Moving the needle leaner improves it. Richer makes it worse. But moving it leaner makes off-idle worse.
3. Wide open runs pretty good but really dies out and bogs if you hit a big incline. I'm moved to think this is the main jet being too fat. Right now the jets are 20/160. I have run similar motor combinations with this carb anywhere from 17.5-25 on pilots and 145-170 on mains.

 

[OLD CT]

Lean pilot would do the tell tale 'high idle' right after letting off,and the lower and die. Check for a low float level and kinked fuel lines also.

 

[Enginedoctor]

hmm. sounds like up top, it's really rich, and down low, yeah a bit lean. if there's 'dead' spots just near WOT, it must be getting way too much fuel. i've definitely had flat spots up top on bikes before, but not one where it wouldn't accelerate. that seems to be the difference between lean and rich.

like you suggested, try fattening up the pilot and leaning the main. i dunno how much fuel it would require to stay idling.... then once you get it to run okay at WOT and okay at throttle full closed, play with the needle to get the off idle right. i'd say you're close.

and yeah, it surprised me too how much fuel you used, but when you're really hitting it hard, the most volume of air is passing through the carb, and taking fuel with it. there's something to be said for mid range power and torque, if your speed and driving style doesn't change, i've seen vehicles with LARGER displacements actually do better on fuel... namely 2005-2008 grand cherokees w/ the v6 getting worse mileage than the v8, even though the epa sticker would lead you to believe otherwise... This is certainly comparing apples to cinderblocks, but it's the theory i'm interested in.

 

[69ST]

Works out to ~65mpg, far worse than anything I've run across and, imo, not commensurate with the power levels needed to sustain 60mph. Lowest I've ever seen with one of my road bikes has been 74mpg, that was before I dialed-in the jetting on the purple bike. The red bike typically delivers 84-92mph running mostly flat-out at 60-ish speeds. Best, educated, guess is carburetion but, not necessarily just a matter of jetting. Right from the get-go, I'd say that the carb is marginally too big for the displacement but within the right range for the peak hp of this tune.

You might try dropping down to a 150 main jet and raising the jet needle 3-4 grooves higher. That ought to cover the part-throttle lean spot. Hopefully, it's actually running fat at WOT in which case that combo could be the magic bullet. I sometimes get the same symptoms OLD CT mentioned, at idle...but mine is set to the rich limit, another enigmatic hiccup that needs a tech department to definitively sort out.

Over the years, I've noticed that intake manifold temp, ambient air temp and differing fuel blends can all have significant effects and can interact with each other. I'm sure that significant changes in elevation can be added that list. However, with the highest point in this state being about 2000ft above sea level, I'm not going to be the one testing out that theory. Your intake manifold has to become progressively colder as you ride at higher speeds. That can have a dramatic impact, especially in lower ambient air temps. I've seen extreme cases where a bike became totally intractable due to low intake temps killing fuel atomization. You may be a teensy bit over-geared, as well. I'd imagine that this tune has a fairly hairy cam profile with lots of overlap...and a fair bit of reversion under certain conditions. None of these is a "smoking gun, taken individually. That said, any two of them (or more) in combination could be giving your Hammerhead motor occasional indigestion.

I'd verify that the carb, intake, gaskets and port are all aligned, with no undesired overhangs upstream, break out the IR thermometer, and try the jet change/jet needle height adjustment, then maybe a 1-tooth sprocket change. If you get no joy from those, might be worth trying a VM18. That'd certainly tame the lower-rpm & light-throttle operation.

FWIW, I've not been able to completely tame the VM22 on my stocker (Nice110) and the hiccups have gotten worse as pump gas has been with. There's always been a light-throttle lean spot but it was tiny, a quick overtwist on the throttle would cover it (still will). However, right at the light-throttle limit, as when descending a long, steep, grade, the throttle response has taken on the old, dreaded, "light switch" quality over the past couple of years. High (above 80F) ambient temps and ethanol-free gas make it go away...that combo doesn't happen along very often. Idle quality is actually stronger & smoother when cold. And, if that weren't sufficiently counter-intuitive, above about 3,500rpm the thing takes throttle like EFI. My point, here, is that my setup is actually a little more conservative than yours in terms of venturi size/displacement ratio and the carburetors are siblings.

 

[OLD CT]

I never really considered a vm 20 to be too big for a 108cc. I totally agree that major altitude changes would possibly have an effect and running wide open for great lengths can cause the intake to go too cold to keep the needed heat in the intake and change the way the bike reacts to throttle. A stock CT70 that putts around on trails will not run into these intake icing conditions.

 

[Adam-NLV]

Cool Story FatCatt.
Here's a lil story not as dramatic as yours but a true one...

My first year here in the desert my Honda Prelude blew it's thermostat badly. Antifreeze all over the road. I shut her down and had it towed home. Needles to say it was hot as hell and 110F was the norm and 117 was on it's way.

Not Knowing anybody here and money tight...my Ct70 K0 was my primary mode of transportation. So made sure it had plenty of oil and not sure how it would take the heat, I headed back and forth to the auto supply store. Round trip 8 miles but had to do that a few times (wrong thermo) for a couple of days before the car was road ready. It's no fun riding in that kind of heat but imagine how that stock 72cc motor felt... It performed like a trooper. These things are bullet proof.:16:

 

[fatcaaat]

I certainly can agree that the stockers are bullet=proof. when i was a kid I had a blue KO that I completely ragged out every time I threw a leg over it from age 10-14. I am not sure if the oil was ever changed during that time period...I had to bump start it every time, but outside of that the bike ran great. It was beat up and mangled from being dumped or falling over when going up the side of a mountain, but it took all the licks I could give it and all that my brother gave it before me.. Funny thing is that it was beat up from the day my dad brought it home with 2 others in the bed of the pickup.

Back to the bike in question...I am slowly beginning to agree with Bob on the carb thing. I used to think that every bit of HP was the optimal goal and top speed number was most important. Over time, I have shifted that way of thinking. If you only spend about 1% of your riding time at top speed, why is that the goal? The goal should be to configure the bike to be most ridable in the speed range you plan on spending the most time. And to do that, a smaller carb than what I normally recommend is more likely to achieve that.

On this bike, I'm going to try rejetting and messing some more with the needle. As I said, this bike was pretty much untested other than a few laps around the neighborhood to make sure the jetting was not completely off. I might consider swapping the carb out if I can't get it running the way I want it to, but that will be the last option really, since I don't want to throw more money at a bike that will see such limited use. However, the gearing is another thing. I think for the flats, the gearing was pretty good, but it will never work where the bike is going to be spending its foreseeable future, so a drop on that front sprocket is going to be necessary.

Additionally, the rear shocks have to go. They are downright dangerous at anything over 35mph and will be near useless for the riding it will be used for.

 

[69ST]

Have to begin this with a correction and it's a biggie, the VM20 should be spot-on for this displacement. Yesterday, my fatigued brain locked-onto "88cc". IOW, you're absolutely right, Pat.

Now, that having been said, there's still the matter of sorting-out the driveability of this machine. Something is causing the hiccups and we lack the tech resources of a major OEM. However, their R&D, plus the real-world experience we've collectively amassed over the years, can be used as a base. The short version...the 12v CT outperformed the 6v iterations and the stock 110 Nice makes similar power to your 108 tune. (Yes, I fully understand the differences in their respective power curves) The point is that both have smaller valves than the 6V ct head and, further, the Nice came fitted with an 18mm Keihin carb. I thought that was ridiculously undersized, until recently. Now, I'm thinking that it may be slightly small and that it's the stock airboxes that present the restriction, until displacement/tuning is taken far beyond stock.

The common thread is flow velocity. Given two ports with the same CFM capability, the smaller one will be more efficient...manifesting in smoother throttle response, higher "below the curve" output, and more mpg. The biggest difference between your 108 tune and what I run is the rpm range. There's a far bigger differential in flow rates, through the induction side, with yours...since it's a (comparatively) short-stroke/high-rev/hp tune. IOW, your carb will need to have a bigger vocabulary, if you get the weird metaphor.

Your bike seems awfully thirsty however, I'd really like to see some more fuel consumption numbers, over more miles...as the motor breaks-in and over flatter terrain, before reaching for the gavel and passing judgment.

Okay, I'm not going for the "captain obvious" POV, just thinking out loud. You're getting inconsistent air:fuel ratio and the question is why. There's nothing to do but go through the old trial & error process, until you get acceptable results. One idea that only came to mind this A.M. is swapping-in a 20mm intake. A few less CCs in the air column might just help keep fuel from condensing out of suspension...

FWIW, I went through the same thought process evolution with carburetion, just some years earlier...another gearhead's rite of passage, I reckon.

As for suspension upgrades, I couldn't agree more. The stock shocks are deathtrap dangerous for road use above 30-35mph. I'm not wild about the stock fronts either. However, the K1-later can be improved quite a bit with "oil tuning". With a 40mph+ road bike, real rear shocks are mandatory and should be the first suspension change made. With decent rear shocks it's possible to live with a K0 pogo stick front end...sort of...over really well-maintained pavement. With the stock rear shocks, you'll spend too much time either bottomed-out or with the tire not making solid contact. Damaged shock mounts are highly likely.

 

[fatcaaat]

Here's a picture of the bike as soon as I arrived at the destination.