CT70 Carburetor Questions

Having completed restoration on three CT70s recently now I am trying to get them dialed in and I have a few questions about what carb to use. What I am interested in is determining the best carb for an 88cc big bore CT70 engine. I have rebuilt the 3 engines using big bore kits from Dr. ATV and they are running OK but I think there should be more top end and the idle characteristics on one isn't very good. Here is what I have on all three:
88cc big bore kit
high volume oil pumps
CDI ignitions
16/35 sprocket sets
Stock exhaust
Stock replacement CHP carbs in place of original Keihin, larger main jets(#60) and main jet needle clip set to lowest setting, richest mix. I am getting about 35 mph out of them am aiming for maybe 40-45 mph if I can because they are registered and licensed so I would like to take them on city streets safely. I have read on other threads that with the big bore kits to use a "VM20" carb. I am assuming this would refer to a 20mm diameter outlet leading to the intake manifold? The stock carbs are about 15.63mm:

Would a 20mm carb then need a larger intake manifold? The inlet end is only about 16mm in diameter:

Also, the insulating gasket between the carb and manifold has an opening about the same size:

The outlet of the manifold and the inlet to the head is approximately 20 mm so the flow is restricted through the current carb and inlet of manifold:

So my question is, what carburetor do any of you recommend and do I also need to replace the intake manifold along with it? Would this require a differant air cleaner? Is the stock exhaust adequate to allow sufficient flow through the whole system?

Any help/recommendations are welcome and encouraged.

Thanks in advance for any help.
John

You should think of the induction system in terms of bottlenecks. The largest ID should be that of the intake port. The intake can be no larger than the port and the carb can be no larger than the intake. Get any of that wrong and you'll spend a lot of time trying to polish a turd that will never run right.

VM20 is a Mikuni carburetor, it's enough to feed 65mph horsepower...a really healthy 100-125cc tune. That is absolutely the biggest I'd ever go with a 20mm intake port, regardless of displacement. With 88cc, it's the upper limit. If you do choose to install a VM20, you'll need a 20mm intake, port & gasket-matched to the head (it must be perfectly aligned to the port), and a mushroom-style open-element air filter.

You have the right sprocket combo, for a K1-later 88cc tune. Something's not right here. You should be able to easily get 45mph from that, wearing the stock carb & airbox. I'd want this tuned to perfection and broken-in first...before trying a carb swap.

racerx-Thanks for the feedback. In doing the research for a larger carb I also found Mikuni 16VM which are bigger but seem to be a closer match physically to the stock carb. Has anyone been using these on CT70s? I will put some more miles on these when(if) the weather ever warms up and see how they tune before I do any replacements.

I also didn't mention that the valve seats in the heads have been machined for new valves so I have great compression.

First thing I'd check is the airbox assembly. The foam can become obstructed from dirt, age or even too much oil. Next up, check the airbox boots. The inlet side can collapse, restricting flow. These also had a conical rubber insert, with a small opening...with 88cc downstream, that would be restrictive. The cone is just glued in there and is removable.

Find the limit of the point gap. We usually want to see a point gap of 0.14-0.16" with a 3-speed alternator (fixed timing). With some engines, 0.18" works better. Larger gap advances ignition lead (a.k.a. "timing"). Much beyond 0.18", the points won't close, killing the spark. It's worth a little experimenting, since it's easy and doesn't cost anything.

With an 88c kit, slightly hotter cam and a 16t C/S sprocket, I expect to see north of 45mph...stock carb & airbox still in place. For that matter, a healthy stocker that's optimally tuned should be able to pull low 40s, on the flat...and accelerate into the high-30s with some authority.

What exhaust are you running? If it's one of those cheapie Chinese knockoffs, your engine is being partially strangled.

The ignition is CDI so no points. The exhaust is stock on the two K0s(1971) and a cheap knock off muffler on the K1.

The cheapie mufflers are really quiet but that 12-15mm outlet is like trying to exhale through a straw. That doesn't explain all three bikes.

If you're sure that everything else is optimal, then perhaps it's time to try upsizing the induction assembly on one machine...you could always swap it between bikes, for comparison. I just wouldn't go all-in on all 3 bikes at once, expecting carb & intake upgrades to be THE solution to the missing mph; it's a little too early yet, imho.

Those two bikes with the OEM exhausts should be moving the speedometer needles past 40mph, in bone-stock form. With 88cc kits...easily past 40mph. First thing I'd want to know is why that's not happening...before throwing parts at them.

OK, thanks. They are real fresh as I just went through them over the winter. I'll put some miles on them and keep tuning to see what I can get out of them as they break in, but I'll keep my eye out for a larger carb to try on one of them to see what happens.

PurplePackFan said:

OK, thanks. They are real fresh as I just went through them over the winter. I'll put some miles on them and keep tuning to see what I can get out of them as they break in, but I'll keep my eye out for a larger carb to try on one of them to see what happens.

Click to expand...

Hi Purple Pack,
Are you using the older style dome pistons? If using newer style flat pistons they won't reach the top of the cylinder and that would explain your low speed at the top. For stock look, I use original Keihin AT7B carburetor with a 65 main jet which is the same if you have K0 3 speed. The K1 533A (3 spd) would have a 58 main so I would try a 65 in that as well. If you want to stray from the stock look, the VM20 with a 130 main jet works well along with a K&N RC1070 filter or similar. I really like the tbparts performance pipes. CHP also makes some that are stock look- alikes. Example photo Custom Lime green K1 88cc 4 speed semi-automatic

Looks like a really nice build so far vht.

I'm about to install the 88' kit on a stock engine too. I'm sure the Dr Atv 88' kit will be able to cruise at around 45 -50 without too much problem.
If your only seeing 35 mph, that seems way low to me.

A big restriction is the aftermarket china exhaust pipe. they look great but perform like crap on anything over 50cc.
The intake manifold should match, i will be using Tbolt Intake part TBW0273 fits a 20/24mm carb.The Air Filter should flow free, usually aftermarket flow the best. My carb will be the Dr Atv 20mm with 65 main and 35 idle jet.

vintagehondatrailbikes said:

Are you using the older style dome pistons? If using newer style flat pistons they won't reach the top of the cylinder and that would explain your low speed at the top. For stock look, I use original Keihin AT7B carburetor with a 65 main jet which is the same if you have K0 3 speed. The K1 533A (3 spd) would have a 58 main so I would try a 65 in that as well. If you want to stray from the stock look, the VM20 with a 130 main jet works well along with a K&N RC1070 filter or similar.

Click to expand...

Regarding the topic of pistons, there's a little more to it than just dome configuration. The primary difference is compression height; that's the distance between the wristpin centerline and the top of the first (compression) ring land, not the dome size. 12v-era engines have a longer connecting rod. Attempting to run a 6v "short rod" piston on a 12v crank (long rod) will give you a negative deck height, i.e. the piston will stand proud of the head gasket mating surface at TDC. That is not possible, since the piston would contact the head, preventing the crank from rotating. Running a "12v/long rod" piston on a 6v/short rod crank will leave you with excessive deck height, the piston never reaching the top of the bore...and abysmally low compression, as described. There is a third parameter that can affect compression...dome configuration and there are a number of different dome sizes & shapes out there, and they can be independent of compression height. The easiest one to miss is the 6v C70 piston; it has the right compression height but a smaller dome. 12v heads (identifiable by their ball bearing cams) have smaller combustion chambers than their 6v predecessors. When swapping 6v & 12v heads, the dome type should be matched to the head...as well as the compression height to rod length. Dratv's page has comprehensive details on this subject...and it can be overwhelming trying to mentally process all of it at one time. Fortunately, the way parts are marketed pretty much takes care of the details for you. If you sourced tbparts 88cc bore-up kits labeled "K0-1981 CT70" you're good to go, in terms of piston match.

When it comes to carb jetting, jet sizing recommendations should be thought of as starting points. They can be dead-on but that only happens when making "apples-to-apples" comparisons. For example, I agree with the VM20 main jet as a starting point...because Mikuni quality is consistent and there's only one model of VM20 available. (BTW, I'd start with a #20 pilot jet, also) You're still going to have to dial-in the tuning...and you may even have to use a different jet size on one, or both, circuits. (If that does happen, you'll probably be within one size/number) With OEM 16mm Keihin carbs that came with these bikes, jet sizing differs by model number. There are other parameters that affect fuel flow, in addition to jet sizing. The most prominent is emulsion tube configuration. That's why these carbs wore mains between #58 & #64 from the manufacturer. They all delivered the same air:fuel ratio to the engine. Note the number stamped into the original main jet in your carburetor. If it's #62, then the larger #65 main will likely work with a simple jet needle height adjustment. With oxygenated fuel available in 2020, upsizing the main jet slightly usually compensates for the difference. OTOH, if your carb wore a #58 main, there's a good chance the #65 will be too big to "tame" via jet needle height.

I got my K1 running good by replacing the main jet with a new #65 jet. I tried drilling out the other #65 jet with a pin drill and it was apparently too large as it would not run. I put in a fresh #65 jet and put the needle at the mid position and it starts and runs/idles great now. I still only get about 34 MPH out of it. I replaced the muffler with an aftermarket muffler I had in my stash and it seems to help on the low end but my top end still is about 34 mph.

I guess I'll put a few more miles on it and see how it improves.

The bike probably can't pull the 16 tooth front sprocket. How fast is it with the 15 tooth ''stock'' sprocket? My guess, faster than 34 mph...

You need a smaller counter sprocket, not a bigger carb. It's always best to wait to see how fast you can make the bike go with different ''light mods'' or 88ccs running the stock sprockets before changing gearing, this way you will know right away if a gear change actually helps, not hurts.

Another thing I just noticed in post #1 last picture, the intake port is BLACK meaning your valves are leaking. Without the valves properly seating you are losing power and compression.

Long time since I've seen a Gianelli pipe on a CT70.

When it comes to drilling jets...DON'T. This isn't a Holley 4-bbl carburetor. Minuscule changes to the jets, emulsion tube or jet needle taper can have huge impacts on these little machines. There is no substitute for OEM precision. If you want predictable results and a good chance of success, do yourself a favor and just source different OEM jets.