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Discussion Starter · #1 ·
Has anyone else had a sticking Keihin carburetor where the piston momentarily sticks in position as it is sucked up into the dome? This would be the vacuum operated style (without spring) as used on a Honda CL450 1974.

My symptoms are between idle and 3000 RPM beyond which operation of the engine is smooth and free. But below 3000 and attempting to accelerate, it jerks.

I've tried several attempted repairs that didn't work, such as:
1) Oiling the machined and polished guide tube of the piston that lines it up with the dome.
2) Building up the worn slot on the piston's side with JB Weld and then sanding smooth.
3) Soldering the worn area in the brass insert of the dome and filing smooth.

Several things that helped were:
1) Rotating the domes 180 degrees from where they should be.
2) Polishing the inside of the guide tube of the dome and the polished tube of the piston.
3) Measuring the force to slide the piston up into the dome using a postal scale to see if I could get it down to 3.5 ounces.
4) Cleaning out the idle jet nozzle ultrasonically.
 

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Have you actually observed the piston stick? You didn't happen to swap pistons with the other carburetor while servicing them, did you?
 

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Discussion Starter · #3 · (Edited)
There were various ways to see if the pistons stick.
1) Most obvious and easiest on the right side (exhaust pipes on scrambler don't allow air cleaner removal on the left) is to have choke open, air cleaner off, rev the throttle and perhaps the piston will rise (Keihin carb piston). On a dyno would be even better. Need lots of air volume to happen.
2) During cold moist weather operation they stick due to carburetor ice. Atmospheric air inside the dome which is below atmospheric pressure freezes on the machined and polished guide tubes.
3) The jerking phenomenon on acceleration is the best way to know it happens. Idle screw position determines idle speed, main fuel jet high speed, but coming off of idle is the constant velocity, increasing volume of the piston that should run just smoothly. In 3rd gear, low speed, slowly twisting the throttle, the motorcycle jerks and falls away, jerks again, falls awy. Since there are two carbs, each does it at its own will, so that makes it even more interesting.
4) Remove the right carb and empty all its fuel, place on a vacuum cleaner hose and with the electric vacuum cleaner running, hold choke open, open up throttle plate, and note the piston action. Can take the dome/piston of the left carb, place on right carb body, observe its behavior.
5) Attached photo shows pressing down on dome with piston on a postage scale. Piston weighs 2 oz. on its own. Pressing down on dome adds some force to the 2 oz. If it is more than 4 oz, too much friction. I've seen 6 oz before, not good. The R on the piston means right piston.
6) I've turned the dome around 180 degrees relative to the original position to get new material for the piston to work on and got a bit better results. Remember, incoming air is pushing the piston forward as well as up which cocks it on the guide tube. The top of the piston then pushes against the rear of the dome's mating surface.

Does anyone have other means to show the pistons seize on the dome guide column?
 

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I noted there is an R on the piston in that photo; do I assume you marked them on dis-assembly, so that you wouldn't swap them?

I also note what appears to be a polished area near the top of the piston, which would be an indication of wear from vibration. If that is true, the piston could be tilting ever so slightly, making it bind more than usual. Polishing out the surfaces the piston slides in would also increase the gap, reducing the vacuum in the dome, while at the same time, increasing the room for the piston to tilt, making it bind more. I'd say that carb may be worn past the point of recovery, unless you can resurface it to make it a better fit.
 

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Discussion Starter · #5 · (Edited)
The bright spot on the piston lands is from sunlight coming in through the picture window on the right. The camera focused on a reflection of a planter or the fish pond beyond the window so items of interest are fuzzy. And yes, the R stands for right carburetor/piston.

Attached is at least one more photo of measuring clearance around the lands with 0.001" aluminum foil. No binding found. Note polishing dome's tube and a wasted effort of blowing dome off of piston whereas in operation the dome sucks up the piston. You can try it, doesn't take much air to keep it floating, plus tilt at an angle like it is in the machine.

This amount of polishing required is very minimal, much less than 0.001" in the dome's tube. It removes oxidized brass as compared to polished chrome on the piston which doesn't wear off, but should be polished, none-the-less.

I didn't want to buy someone else's worn pistons so am trying to improve what I already have. One could recommend, "Buy another pair", after all the work I've put into this, but they look serviceable and I'm in the repair mode.

The last photo is the best and simplest way to check for smooth operation. Dome in left hand, right thumb pushing piston forward and up. It should move as smoothly up as if one were just allowing it to center it up pushing straight up as machined in the factory. I presume in the factory the white plastic plug in the dome is still out, they've finished the polishing, then insert the plug. The fit of tube to dome is as close tolerance as with hydraulic valve lifters I suspect.
 

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Maybe it isn't binding in the dome, but in the body of the carb. When the airflow tilts, or cocks, the piston within the body it can bind, especially if the surface within the carb body is worn into a slight ellipse. In that last photo, it still appear that there are uneven wear marks on the piston. Plus, any tilting of the piston in the body will increase the force in the guide bearing above it, in the dome. Try putting the cylinder into the body, and see if there is much rocking.
 

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Discussion Starter · #7 ·
I used to think there was rocking which was why I actually tried soldering the bore where I thought it had ovalized. Probably 0.004". See attachment. I had dimensions of the width fore and aft vs. left and right. Then I filed the solder down to get the dimensions I wanted. I applied black felt pen to it and pushed the piston up into the dome until I got enough solder removed to slide it in.

Well, some of the solder came off onto the shiney piston tube. Also, some of the felt tip pen slightly disolved, I thought, adding to friction. What a mess I was in. So I think I'm past that stage now and by removing oxidized brass way up inside the tube, where it is still machined (go high enough the tube and it has a larger ID) I think I have it right. I polished this weekend and will know tomorrow if it worked on my drive to work.

By the way, I've had the 450 for 30 years, bought it as a basket case for $25, have put $8000 into it for gas, insurance, license, fuel, and parts since then. Annual costs are $400 for license, insurance, gas and parts. Mileage is 48 mpg and I drive 60 MPH to work.
 

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Discussion Starter · #8 ·
I was completely wrong about my Keihin carburetor sticking, causing jerking sensations below 3000 RPM. What was causing the problem was the ignition points plate as point gap was 0.007" instead of 0.014", the advancing mechanism behind the plate was also sticking which I'm working to resolve.

I can't retard the spark anymore because the arc cutout on the right side of the plate that is supposed to miss the left exhaust valve adjuster bolt is hitting the bolt. Can't rotate it CCW any further. I've never replaced this plate and the points are worn. I'll grind away on the plate a bit more to retard. Also, advancing range is too far so the stops for the two throw out feet will need to be modified to reduce range. So my work is cut out for me.

Sorry for making rocket science out of a problem without having putting more basic troubleshooting in to the issue. Struggled for months on the CL450. At least I have smooth bore alignment tubes on the Keihins.
 

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Did you test the advance range with a timing light? The spark should move from the basic timing marks to the second set of advanced marks, reaching them at about 3000 rpm. I've found several good quality used and new advance mechanisms for the CB/CL 450 on eBay, and more sets of points with plates, if yours are too worn to clean up properly.
 

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Oh, and I have taken the advance mechanism apart to run the parts across a buffing wheel, with a Dremel to buff the insides, just to make sure the parts don't bind.
 

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Discussion Starter · #11 ·
Yes, a timing light was the answer.

I had lost one of those small C-clips that keep the feet on the advance mechanism in place, so had placed a loop of wire in the clip's groove and twisted the ends together. Over time, as the foot moved in and out it wore against the wire. Then resting over the wire it would jerk into the retarded mode or jerk into the advanced mode. Also, the range of travel was too far because I had replaced one of the advancing springs so that it had some slop in it. Needs to always be snug, even with the advance mechanism relaxed.

I'll try correcting the "cutout in the plate" problem that won't let me retard to specs plus tighten the spring of the pair.
 

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Discussion Starter · #12 ·
My spark advance was at 4500 rpm rather than 2500 rpm. I measured the spring coefficient of the pair of advance springs and then made one to replace one of the pair that was less and got it to advance sooner, around 3000 rpm.

Another problem is that it is advancing too far, beyond the two marks that center on 40 deg BTDC. I'm seeing 50 deg BTDC which is my concern. I need to weld some stops on the plate to prevent the feet from advancing as far.

I also used a Dremel to sooth out the corners where the springs attach to the advancing feet.
 
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