carburetor issues CT70

[Larry Navarro]

I bought a knockoff carburetor for a 1980 CT70. Once installed, it idles nicely however at approaching mid-throttle, the carb bogs with a lean "pop" sound.
The carb has 3 slots on the needle and the clip is positioned on the lowest slot which would place the needle in the main jet with richest mixture relative to throttle position.
Now upon looking at the original carb, the needle only has one slot and clip, no adjustment at all. I'm going to rebuild that one and see if a similar problem persists.

Let me ask another question, this particular engine was built with a TBparts 88cc top end kit. I have no experience in this area and I am thinking that maybe the stock carb is too small for the application.

Wondering if others have had the same experience and what they did for proper engine operation.

Any recommendations would be appreciated.

 

[Old Guy Too Many Bikes]

Two questions, what size (venturi) is your replacement carb. Also, how does it run at full throttle (takes needle position out of the equation).

 

[Larry Navarro]

Two questions, what size (venturi) is your replacement carb. Also, how does it run at full throttle (takes needle position out of the equation).

It's a stock replacement. The engine bogs when approaching half throttle so I can't even get to full throttle.

 

[Old Guy Too Many Bikes]

Something's not right. Check to make sure the main jet is not clogged and the needle raises correctly. Also check for vacuum leaks.

 

[Gary]

I have the stock carb with an 88 kit on my H motor,runs great. A small carb on a bigger motor is better than too big a carb as the air velocity stays high. It is exactly what VW did with the air-cooled beetles

 

[Old Guy Too Many Bikes]

I have the stock carb with an 88 kit on my H motor,runs great. A small carb on a bigger motor is better than too big a carb as the air velocity stays high. It is exactly what VW did with the air-cooled beetles

It's funny when you think about it. A 1200 cc air cooled 40 horse VW engine had a 28mm venturi carb. My 140cc CT70 has a 26mm venturi carb. My 1340cc Harley has one carb with a 40mm venturi. My 2110cc air cooled VW has 2 two barrel carbs, each of the 4 venturis are 45mm. But you're right, a smaller carb and intake promotes higher mixture velocities and smoother performance at lower rpm. Larger carbs provide more fuel at higher rpm for more top end power at the expense of bottom end performance. Way back when, my one cylinder Honda XR350 had two carbs, a small one for lower rpm and a larger one that kicked in as you cranked it up. The power on my CT70 comes on pretty abruptly and is not the smoothest off the line. I tried a 24mm carb which ran smoother, but it lost about 7 or 8 mph of top speed. But Larry's problem isn't the wrong carb, it's that there's something wrong with his carb. If it craps out at half throttle, it's either getting not enough fuel, too much fuel, or some operational problem. At least it idles, so there's fuel in the bowl and the idle jet isn't clogged. So it shouldn't be too hard to diagnose.

 

[OLD CT]

The 1340 Harley carb, or CV series only lets as much fuel as it needs with a vacuum diaphragm for smoother quick full throttle ''all at once'' response.
The 440 six pack carbs, 1350cfm Holleys had 2 end carbs rated at 500 cfm each and the center was 350 cfm. Also, you can nail it all at once because both of the end carbs open with ''vacuum'' so as not to ''dump'' too much fuel all at once. CT carbs are not, and need a slower roll on for sure.

 

[Old Guy Too Many Bikes]

I agree, case in point, the center 350 cfm two barrel has small venturis for normal driving and when you put your foot in it, the other two carbs open up. But if I remember correctly, the other two carbs operated using linkage, not vacuum. There'd be a hard spot on the gas pedal halfway down. But getting back to Larry, if you're still there. Have you rebuilt. your stock carb and tried it. Have you taken apart your replacement carb and found anything wrong? I've had really good luck with those cheap Chinese ebay carbs

 

[OLD CT]

Here is the vacuum diaphragm style six pack I had. No hard spot, just a super long burnout for as long as you want to stay in the gas. It came on the auto trans cars. Pretty sure 4 speeds had the manual linkage, like you said. On to Larry!

 

[Larry Navarro]

I have the stock carb with an 88 kit on my H motor,runs great. A small carb on a bigger motor is better than too big a carb as the air velocity stays high. It is exactly what VW did with the air-cooled beetles

okay, that gives me some hope. Given the knock-off I have already, I was thinking about putting a plastic shim(s) under the clip of the needle to raise it some, just to see if it makes any difference.
Like I said, it idles nicely up to about half-throttle then the lean bog.
This '80 model was emissions hindered with all the tubes and vapor recovery canister. The carb needle has only one slot, so there's no adjustment other than placing thin shim under the needle clip to alter the fuel curve to richen it up a bit.

 

[Larry Navarro]

I agree, case in point, the center 350 cfm two barrel has small venturis for normal driving and when you put your foot in it, the other two carbs open up. But if I remember correctly, the other two carbs operated using linkage, not vacuum. There'd be a hard spot on the gas pedal halfway down. But getting back to Larry, if you're still there. Have you rebuilt. your stock carb and tried it. Have you taken apart your replacement carb and found anything wrong? I've had really good luck with those cheap Chinese ebay carbs

I'll take the cheapee carb apart to check it for any problems.

 

[Old Guy Too Many Bikes]

I've done that on several 80's bikes that were jetted too lean from the factory, but that was to quicken warmup and cure lean surges, but regardless of the needle position, at full throttle, the needle position has little effect because you're running on the main jet. The needle sits in the main jet. fuel flows around it, through the donut shaped passage created by the needle in the main jet. As the needle goes up, it gets thinner and the donut shaped passage gets thicker permitting more fuel to flow. Changing the needle position only makes minor adjustments at part throttle. At full throttle. the needle is all the way up pokes a very small hole in the donut. The bike is essentially just running on the main jet. That's why it's surprising to me why it won't take full throttle. I once had a bike where the main jet unscrewed itself and the bike wouldn't take any throttle. Also remember that it's the vacuum created by the incoming air that draws the fuel up from the bowl through the main jet. And a bad vacuum leak will also lean the mixture to the point the bike will barely run. Check your carb's innards, check your gaskets, and try your stock carb. If it does the same thing, you may have a bad magneto or points issue causing a weak spark that gets weaker as rpm increases.

 

[Larry Navarro]

I've done that on several 80's bikes that were jetted too lean from the factory, but that was to quicken warmup and cure lean surges, but regardless of the needle position, at full throttle, the needle position has little effect because you're running on the main jet. The needle sits in the main jet. fuel flows around it, through the donut shaped passage created by the needle in the main jet. As the needle goes up, it gets thinner and the donut shaped passage gets thicker permitting more fuel to flow. Changing the needle position only makes minor adjustments at part throttle. At full throttle. the needle is all the way up pokes a very small hole in the donut. The bike is essentially just running on the main jet. That's why it's surprising to me why it won't take full throttle. I once had a bike where the main jet unscrewed itself and the bike wouldn't take any throttle. Also remember that it's the vacuum created by the incoming air that draws the fuel up from the bowl through the main jet. And a bad vacuum leak will also lean the mixture to the point the bike will barely run. Check your carb's innards, check your gaskets, and try your stock carb. If it does the same thing, you may have a bad magneto or points issue causing a weak spark that gets weaker as rpm increases.

One thing about the ignition system on this bike is that after installing new points and condenser, it produced a nice blue strong spark. I was real happy when I saw that. Looks better than the spark on my XR75.

 

[Old Guy Too Many Bikes]

Spock once said "once you eliminate all the obvious possibilities, the answer, no matter how unlikely, has to be the one that isn't readily apparent." or something to that effect.

 

[Larry Navarro]

Well, getting back to this "bike from hell".......I installed the rebuild kit on the original carb and the engine is doing the same as the knockoff carb. Revs nicely with full choke but when it's dialed back, the engine bogs while trying to keep it running. It couldn't be a timing issue if the engine revs with full choke, right?? Gotta be a carb adjustment. I read up here on the post that someone uses a stock carb on a 88cc big bore kit no problems but I don't don't know what else to do but maybe try a larger carb designed for this big bore kit. Like the 20mm mikuni's that came on SL70 or XR75? I'm so far under in this build, i'm doing it for free now. The customer has be very patient but I gotta get this bike to him after having for a year.

Let me add that when I rebuilt the stock carb I noted a 38 pilot and a 55 main jet.