110 cc engine in a 1969 CT 70

anotherct

New Member
Hi everyone, this is my very first post. I am not new to bikes, but first time owner of a CT 70 with a Nice 110 engine. It came with the Mikuni 22 mm carb and what seems to be a ~253 main jet (yes the hole in the main jet is 0.253mm). Don't know much about the history of the bike whether it ever ran with this setup. It runs ok upto ~45 mph at low to mid rpm, but then it starts misfiring. Plug is black so I would think it runs way to rich. Also throttle response from idle is bad. I have to slowly open it up. I haven't really checked anything like valves or ignition, but wanted to get of feel where I should go with my jet. Thanks!
 

69ST

Well-Known Member
Grab a magnifier and get the numbers from the main jet & pilot jet. Sounds like it's lean at low revs & pig-rich at WOT. This carb can be an S.O.B. to dial-in but, it is possible and only needs to be done once.

Ignition problems with the stock Honda setup are almost unheard of with this motor. Valve lash is easy enough to check...and I doubt that's what causing the high-rpm/WOT misfire.
 

anotherct

New Member
Will do, thanks. Al least it is so much easier to work on this one compared to my KZ with 4 carbs! Regarding main jets, what do you recommend where should I start, around 150-170? That's what I have read elsewhere and I want to order them as soon as possible.
 

69ST

Well-Known Member
Find out what's in there, first. If it's already flaming-out with a 160, sourcing another, plus a 170 would be a waste...
 

anotherct

New Member
I had measured it, the hole is 0.25mm, which means 250, or am I wrong? And I believe I read 255 on there.

Won't be able to get into the carb in the next days, but wanted to order some jets anyways. Thanks!
 

69ST

Well-Known Member
I honestly don't know if this series of Mikuni jets are numbered by actual orifice ID. That said, I don't believe they even offer a #255. Above #150 the sizes are in increments of 10. #145 is the biggest -5 jet in their catalog. Thus, it'd either be a #250 or #260. IF that's what's in the jet holder...it's beyond grossly oversized.

The number will be stamped into one of the flats on the hex jet itself. Those stampings can be quite poor and hard to read. Anyone over the age of 35 will need serious magnification to read it.:26:
 

anotherct

New Member
I agree with you, all I read tells me I should look at the 150-160 range. My eyesight isn't that good anymore and those tiny numbers suck...need my kiddos help to decipher!
Hopefully I will be able to take it apart again one of the next days and I'll let you know what exactly I find. Definitely a big thanks for now!
 

anotherct

New Member
Ok, learned something today. I disassembled the carb again to dip it in carb cleaner and voila, realized that the jet was screwed into the bowl... I confused the piece with the O-ring that is screwed into the bottom of the needle jet with the main jet. Had never seen something like this before, I am used to the CB750 and KZ1000 type carb setups.
Anyway, main jet is 160 and everything has been soaking for a while and looks good now. Tomorrow is assembling time! One thing I noticed, the o-ring at the bottom of the needle jet (the one that seals carb and to bowl) is really hard and dry, could that have caused the problems?



(I believe that's what it is
 

69ST

Well-Known Member
The remote-mounted main jet is a convenience, at a small price. That O-ring is critical to seal the main jet well to the emulsion tube. If it leaks, you'll get the fuel equivalent of "false air" that occurs from a vacuum leak, downstream of the carb. Only instead of running lean, it'll run richer. Replace the ossified O-ring and you'll probably be fine, maybe even a bit on the lean side with 2014 oxygenated pump gas.

As for what killed the O-ring, a combination of the fuel used and age. Your carburetor was placed into service almost a decade ago, now. Sounds to me like it spent many months sitting unused with stale, deteriorating, fuel in the bowl. Putrefying gasoline ruins carburetor parts. A Viton O-ring would be virtually a lifetime part, if you can source one. It's not really necessary; O-rings are cheap and easily replaced. I'm running this same carburetor and the factory-installed original is still in place more than a decade on. The motor fires on the first kick, every spring...same as it did in the fall.

A little...and I do mean little...storage prep should prevent problems from returning. If you're going to park the bike for more than about a month, drain the entire fuel system dry - tank, lines & float bowl. The main jet holder is a convenient drain. That'll prevent acidic compounds & varnish from ever forming and keep the bowl clean, indefinitely. If you want to go above & beyond, give the main jet well a light shot of fogging oil before reinstalling the main jet holder. That should keep the O-ring from drying out. Rubber parts tend to shrink when dried, after exposure to most pump gas formulations.

Use minimal torque on the jet holder, or you'll distort the dead-soft aluminum sealing washer and get a leak. I'd also run an inline fuel filter...cheap isnurance.
 

kirrbby

Well-Known Member
anotherct, if that old motor is giving you trouble I would be happy to swap you for a very nice coon skin cap. ;) Welcome to Lilhonda.
 

anotherct

New Member
So I have done some testing, have #150, #160, #170 mains, #20, #22.5 and #25 pilots available and tried most combinations. Here is what I found. Main jet does not have as much of an impact as needle clip. With the clip in the leanest (or second leanest) position, engine revs out fine, however there is some stumble with throttle response at low-mid rpm, especially when yanking the throttle open. The larger pilot seems to help a bit, but it still comes down to the needle, if I want responsiveness at low end have to sacrifice high rpm.

Currently at #25 pilot, #160 main and second leanest position on clip. Somewhere I have read that sanding down the needle (making it skinnier) may be an option? Anyway I will do some more trial and error but if anyone has a good suggestion, please let me know! Thanks!
 

69ST

Well-Known Member
#25 pilot is too big, I think you're going to load plugs at idle. With #20 the airbleed screw needs to be ~1/2-3/4 turn from seated; with #22.5 usually more like 2-1/2 turns.

Your main jet is right in the ballpark and may be the best for your situation. How low can you set the C-clip and still have the engine rev-out under WOT? The only other possibility is the #150, with the jet needle in the richest (or next-to-richest) position. The richer part-throttle mixture curve should give stronger throttle response but, may leave you on the lean side under max power, or close to it.

As for being able to snap the throttle open at low-to-midrange rpm...ain't gonna happen. You've got 49% more venturi cross-sectional area than stock and no accelerator pump, or vacuum-activated diaphragm (constant velocity carb, as used back in the day on larger bikes). At just beyond half-throttle, you've as much open venturi area as at WOT with the stock 18mm carb. You're going to have to learn how to modulate the throttle at lower rpm, as a CV carb would do automatically by vacuum. I do realize the psychological impact, it's second only to a slow-reading speedometer. Still, if it's performing smoothly once at normal cruising rpm and making full power, you're not losing anything...but the ability to snap the throttle open with lightswitch rapidity at any speed. Is this really all that important?

It is possible to re-profile the jet needle to get some part-throttle response improvement. I've done a number of these. However, this is a seriously tricky, high-precision, proposition. Only the exact section can be modified and, all it takes is going a few ten-thousandths of an inch too far to get a permanent, light-throttle rich misfire.

FYI...your jet needle is unmolested stock.
 

anotherct

New Member
I haven't really paid too much attention to the plug at idle, focused more on mid range and top end.

With the #160 main, needle needs to be in the leanest position and even then it doesn't rev out well, with the #150 main (current set-up), the richest I can go is the 2nd leanest position (2nd clip from top).

I guess I am used to my tiny 26 mm carbs with accelerator pump on my KZ 1000. It has the best throttle response I have ever experienced in a carb'd bike :)

As for driving the bike, I will mainly use it to let the kids putt around or for my occasional trip to the gas station to refill my beer supplies. I don't care much about long time driving at WOT. That being said, having a good response at low to mid-range is more important to me than max performance. On the other hand, I don't want to burn a hole in the piston either....

Let the testing continue!
 

69ST

Well-Known Member
I only set these up for optimal road usage. Funny thing is, by doing this, everything else pretty much falls into place, including gearing. The lone exception is ultra-low-speed use, such as a swap meets. For that, I drop-down 3 teeth at the C/S, well within the range of chain tension adjustment and you can virtually drop the clutch at idle...the bike will lurch away without stalling.

Sooner, or later, every rider is going to want to push a machine to its limits, or at least flex its muscles a bit. That's a universal trait among true gearheads. Your bike has enough motorvation in its boiler room to sustain mid-50s all day long, without breaking a sweat let alone hurting anything. Setting it up so that it can do so sacrifices nothing. I get it with the KZ comparison, it's like comparing a `60s era car with an equivalent from the EFI era...in a number of ways. The further one goes back with vehicle technology, the more rider/driver skill is required to get the most out of it. We're talking about 1960s-era vacuum-operated carburetion tech here and setup such that every throttle position is virtually the same as 1.6x that of stock. At the big end, you get completely unrestricted breathing, at the low end, airflow velocity is lower than stock.

anotherct said:
So I have done some testing, have #150, #160, #170 mains, #20, #22.5 and #25 pilots available and tried most combinations. Here is what I found. Main jet does not have as much of an impact as needle clip. With the clip in the leanest (or second leanest) position, engine revs out fine, however there is some stumble with throttle response at low-mid rpm, especially when yanking the throttle open. The larger pilot seems to help a bit, but it still comes down to the needle, if I want responsiveness at low end have to sacrifice high rpm.

Currently at #25 pilot, #160 main and second leanest position on clip.

anotherct said:
With the #160 main, needle needs to be in the leanest position and even then it doesn't rev out well, with the #150 main (current set-up), the richest I can go is the 2nd leanest position (2nd clip from top).

Between those last two posts, I'm confused; they're contradictory. I'm unsure what you meant by "doesn't rev-out well". The motor will either rev to redline, or go into rich misfire before it can reach peak power, at WOT, if it's overrich. Under that scenario, throttle response should be crisp, below the misfire threshold.

Let's go back to basics. At the very beginning, I baselined the VM22 as-delivered (#30 pilot, #100 main). This gave very easy starting and EFI-like low-speed throttle response. Unfortunately, the idle turned pig-rich once everything was up-to-temp and the main circuit was more than lean enough to burn a piston. It's always been a coin toss between #20 & #22.5 for the pilot jet. The pilot airbleed screw can cover ~one jet size. Where you're going to get into trouble, with an oversized pilot jet, is carbon buildup. Even if you're "lucky" enough to not get fouled plugs, heavy carbon deposits inside the combustion chamber can eventually cause problems...like hot-spot-induced pre-ignition.

When yours was setup, it ran best with #150 and the needle near its richest setting + high float level. That was unique, at the time...8 or 9 years ago, as best I can recall. Going by the rotted O-ring in the main jet well, I have to assume that it sat for a long time with bad fuel inside. Thus, something may have changed, in addition to pump gas, during the intervening years. Pump gas is a moving target. What's available depends upon location & time of year. If that O-ring isn't sealing completely, all bets are off. A substantial leak, at that junction, would be the same as running the biggest main available (and they go into the 400s if I remember correctly).

Assuming a proper O-ring seal, I'd try #145, maybe #140 + the highest jet needle position (clip in the lowest groove) working back, one position at a time, toward the lean side if it still won't rev-out...until you find the point at which it will. Get that sorted first, only then can the driveability be addressed.
 

anotherct

New Member
Are you Bob from Houston who set up this bike?

I admit I haven't kept perfect track of what I was testing so I will report back with a more scientific baseline in the next days, but it seems that with a #150 main, clip needs to be on the leanest or second leanest position. That allows me to rev the bike to the equivalent of 50 mph, which is about it's max speed.

Just curious, I am trying to calculate the rpms/mph, do you know the primary reduction and reduction for 4th gear? I used 3.72 (primary) and 0.96 (4th gear) and with sprockets 16/31 and tire circumference of 57 inches I come up with ~6,500 rpm at 50 mph. Where is the redline of this engine?

I don't quite get your comparison between 60s era carbs and EFI when talking about the KZ 1000. The KZ is a 1979 bike with carbs very much the same as this one (although it's a pumper). Same thing there, lots of testing to get it right :) Again, thanks for all the input!
 
Are you Bob from Houston who set up this bike?

I admit I haven't kept perfect track of what I was testing so I will report back with a more scientific baseline in the next days, but it seems that with a #150 main, clip needs to be on the leanest or second leanest position. That allows me to rev the bike to the equivalent of 50 mph, which is about it's max speed.

Just curious, I am trying to calculate the rpms/mph, do you know the primary reduction and reduction for 4th gear? I used 3.72 (primary) and 0.96 (4th gear) and with sprockets 16/31 and tire circumference of 57 inches I come up with ~6,500 rpm at 50 mph. Where is the redline of this engine?

I don't quite get your comparison between 60s era carbs and EFI when talking about the KZ 1000. The KZ is a 1979 bike with carbs very much the same as this one (although it's a pumper). Same thing there, lots of testing to get it right :) Again, thanks for all the input!
I had one of those I bought new in '79. A blue KZ1000 MKII. Had to sell it in '95, sold it to a collector in Dallas for $2,500. It was cherry, had only 6000 miles with a 1075 wisco bore kit with a cam and header. Put the stock cam and exhaust back on when I sold it thou.
 

69ST

Well-Known Member
I've never so much as visited Houston. However, if this engine's outer covers are painted cloud silver, as I suspect, then yes, I did the carb setup & testing circa July 2005. The bike was run up to a maximum of 62mph, I still have the 640x480 (VGA res) video in the archives.

Your numbers are a bit off. Primary drive ratio for this motor is 4.058. Assuming you're running 4.00x10 Trailwing tires, 57" is on the high end of the circumference range, but within 1% of expected...mathematically insignificant. Just from memory (so don't use this as a laboratory standard), 16/31 should give you ~7.1mph/1000rpm. Should be okay, but a little on the light side, it'll pull up to ~7.4. With a reasonably accurate speedometer, you should be able to max-out at close to 50mph in 3rd gear.

Peak power occurs @ 8,000rpm, according to factory rating. From extensive experience, the power curve extends into the low 9000s. Valve float sets in north of 10,000rpm but, spinning it that fast is really stupid. The stock rotating assembly weighs a ton and the power curve goes off the proverbial cliff at roughly 9400.

As for the EFI reference, the point was newer tech - accelerator pump and OEM development engineering input to get everything dialed-in, including relatively arcane details like airbleed sizing, emulsion tube configuration, slide cutaway sizing, right from the get-go. Today, there's nowhere near that kind of tuning capability available for carbs small enough to use on any CT70 powerplant. Even the VM26 tuning kits, widely available in the `70s, have vanished. Carburetor choices are few and mostly all involve big compromises. We're left with jet sizes and jet needle adjustments as the only tuning tools. Jet needle re-profiling, circa 2014, is as exotic as it gets. 35 years ago, OEM development engineers faced no such limitations and were able to dial-in every parameter to perfection, for each specific, known, application. As I recall, Kawasaki sold a few KZ1000s...:3:

Dunno if your KZ carbs are CV, Hondas of that era were. Kwakers are a little outside my experience. It is worth noting that they spec'd 26mm carbs for 250cc cylinders...especially for anyone running VM26s on sub-160cc engines. Just goes to show how small you'd have to go to seriousluy undersize a carburetor. Vacuum-only carburetion is decades-older technology and their lack of self-regulating throttle opening rate of constant-velocity-valving, or pressurized shot of extra fuel to bandaid-over the transient lean spot that accompanies overly-rapid throttle opening is a huge disadvantage.
 

anotherct

New Member
Love the bike, just took it on a 2,300 mls trip from CenTx thru Arkansas all the way to (almost) St Louis and back. Never let me down...oh wait, forgot the voltage regulator that failed and caused my battery to get cooked :)
 

anotherct

New Member
A little more playing around with mains and needles got me to a point where I decided to order a VM 20 carb. With a #160 main and the needle at the leanest setting I got the bike to over 60 mph, however throttle response in mid range was really bad. Since I will mainly use te bike to let the kids putt around and the occasional trip to the gas station I think lower max speed is a price I am willing to pay for better throttle response. What would be a good starting point for pilot and main jet for the VM 20 for my setup? Thanks!
 
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