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Thread: Running rich after carb clean

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    Running rich after carb clean

    1981 CM400A. 100% stock air box and exhaust, new spark plugs gapped to .026", new air filter, Oil change + oil filter using honda gn4 10-40.

    The problem: When I got this bike it didn't run quite right. With the engine fully warm and the choke off, it would stutter at very very low throttle, and would hesitate when given more throttle. Wide open seemed fine. In general it would kind of skip or stutter while cruising. Turning on the choke remedied this. It always starts and idles perfect when cold, but had a high / and a bit erratic idle when warm, which is because it wasn't tuned properly. The plugs looked pretty good with the choke on, nothing I would worry about. The carbs did leak out of the bowl gaskets if left on the kickstand.

    I suspected that the carbs have never been serviced. It had about 6400 miles when I got it, so you know it was sitting for much of its life. I suspected the accelerator pump wasn't working because of the hesitation with throttle. I replaced the pump diaphragm and it didn't make a difference. I also wanted to replace the fuel screws. I got an aftermarket kit and found when replacing them that the tip was broken off in the right carb. After doing that minor work I continued to ride it (with no change in how it ran) until recently.

    With the cold weather finally arriving I figured it was time to take a crack at cleaning the carbs. I found that the accelerator pump jet was clogged on the left side, but the right side was working. I got it unclogged and it works flawlessly now on both sides. I also popped the broken fuel screw tip out of the right carb. I blew brake clean and compressed air through all the holes and passages I could find. I took out all the jets with the exception of one of the primary nozzles which is stuck and I stripped it out. The carbs are the vb 24c which has a pressed in pilot jet. None of the passages or holes in any jets were clogged or had any debris in them. The air cut off valves were taken out and polished with steel wool to make sure they don't stick. Both float needles sealed and they have the same level (checked using the clear tube method).

    So, after that I put them back on and it ran extremely rich. It started and idled beautifully (but the exhaust did smoke more than usually). However, the problems come when giving it throttle. As soon as I move the throttle it bogs down bad. It's so rich that if I let off the throttle it will stall the engine. I tried riding it but it just would not rev out even holding it full throttle. I checked the plugs and they were both carbon fouled, but the left side was worse than the right.

    Ok, so I probably didn't get it right the first time. I found out after going through it a second time that the needle jet holder / collar thing that protrudes into the venturi was missing from the left side. I found it on the floor so I went through the carbs yet again and reinstalled the collar. This time I didn't bother to but the intake boots on, but it did not want to start and would not stay running. At this point I was extremely frustrated. I looked at the spark plugs again and found that the left side looked the same, extremely rich and carbon fouled, but the right side looked normal...

    I have since gone through the carbs for the third or fourth time and cannot comprehend why it's running rich. I have blown compressed air into the back of the carb to verify that the slides are moving. I compared each part and jets of the carb to make sure they're the same.

    At this point I need advice on what to check for on these carbs if they're running rich, I'm at my wits end. If anyone needs pictures of anything in particular on the carbs I can get that to you, just let me know.

    20221119_111536.jpg

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    Benevolent Dictator ancientdad's Avatar
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    I don't know if you've seen this yet, but hopefully it will help. The author of it (LDR) should be on the forum soon, he lives on the west coast.

    https://www.vintagehondatwins.com/fo...VB-Carburetors
    (move along, nothing to see here)

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    Senior Member dougrun's Avatar
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    hows your mixture screws? can you lean it that way?
    1981 CM400A

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    Quote Originally Posted by ancientdad View Post
    I don't know if you've seen this yet, but hopefully it will help. The author of it (LDR) should be on the forum soon, he lives on the west coast.

    https://www.vintagehondatwins.com/fo...VB-Carburetors
    Yes, I have read through that. I'll double check everything though.

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    Quote Originally Posted by dougrun View Post
    hows your mixture screws? can you lean it that way?
    Yeah, when I had it back together for the first time and running, I could adjust the mixture screws in and out and on the extreme ends it would start running bad.

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    Administrator LongDistanceRider's Avatar
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    A couple of things come to mind, actually more.
    The primary and secondary jets can be swapped so make sure they are in the correct spot. Primary is the smaller numbered and goes closest to the mixture screw. Secondary is the largest number and screws into the jet holder.
    The black rubber plug has to fit snugly.
    Only use the original jets, nothing aftermarket.
    You mention checking the float level but is it actually set at 15.5mm? Too high, less than 15.5mm, will cause a rich condition.
    Once the plugs have been fouled they need to be replaced, not cleaned.
    Jim O'Brien
    1979 CM400T aka the Roadbike, 1978 CB400T1 semi restored, 1972 CL350K4 restoration and the 1971 SL350K1 disaster zone.
    Plus 2 SL350K0's , 2 SL350K1's, 1 CL350K0 and 1 CL350K1 waiting for space and time
    Contact: 408-239-9580 or [email protected]

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    Administrator LongDistanceRider's Avatar
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    Have you run thru the ignition system yet? Old saying is "95% of carb issues are electrical"
    https://www.vintagehondatwins.com/fo...rans-1978-1983
    https://www.vintagehondatwins.com/fo...-test-from-FSM
    Jim O'Brien
    1979 CM400T aka the Roadbike, 1978 CB400T1 semi restored, 1972 CL350K4 restoration and the 1971 SL350K1 disaster zone.
    Plus 2 SL350K0's , 2 SL350K1's, 1 CL350K0 and 1 CL350K1 waiting for space and time
    Contact: 408-239-9580 or [email protected]

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    Quote Originally Posted by LongDistanceRider View Post
    A couple of things come to mind, actually more.
    The primary and secondary jets can be swapped so make sure they are in the correct spot. Primary is the smaller numbered and goes closest to the mixture screw. Secondary is the largest number and screws into the jet holder.
    The black rubber plug has to fit snugly.
    Only use the original jets, nothing aftermarket.
    You mention checking the float level but is it actually set at 15.5mm? Too high, less than 15.5mm, will cause a rich condition.
    Once the plugs have been fouled they need to be replaced, not cleaned.
    I doubled checked the primary and secondary main jets. The smaller 68 is by itself and the bigger 112 is in the emulsion tube. I did get new passage plugs and they do fit snuggly. I'm using the original jets. I checked the float level with calipers just now and they're actually a bit low at around 16-16.3m.

    These are the plugs:



    Attached Images Attached Images
    Last edited by ancientdad; 11.19.22 at 4:06 PM.

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    Administrator LongDistanceRider's Avatar
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    Run thru the ignition system checks, doesn't take much time and that will give you a solid baseline if it's all good.
    Jim O'Brien
    1979 CM400T aka the Roadbike, 1978 CB400T1 semi restored, 1972 CL350K4 restoration and the 1971 SL350K1 disaster zone.
    Plus 2 SL350K0's , 2 SL350K1's, 1 CL350K0 and 1 CL350K1 waiting for space and time
    Contact: 408-239-9580 or [email protected]

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    Quote Originally Posted by LongDistanceRider View Post
    Run thru the ignition system checks, doesn't take much time and that will give you a solid baseline if it's all good.
    Ignition system checks out. Is there anything I should check with the carbs before I put them back on again? I noticed the plastic gasket at the top has a little semi circle on it, would it throw something off if it blocked on of the holes?

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    Administrator LongDistanceRider's Avatar
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    Are you talking about this piece?
    201_4708.jpg
    If so the bottom must be flat to seal the 2 air bleed port cavity. Those ports are actually jets that meter air to the different circuits. They should not be plugged
    Jim O'Brien
    1979 CM400T aka the Roadbike, 1978 CB400T1 semi restored, 1972 CL350K4 restoration and the 1971 SL350K1 disaster zone.
    Plus 2 SL350K0's , 2 SL350K1's, 1 CL350K0 and 1 CL350K1 waiting for space and time
    Contact: 408-239-9580 or [email protected]

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    Quote Originally Posted by LongDistanceRider View Post
    Are you talking about this piece?
    201_4708.jpg
    If so the bottom must be flat to seal the 2 air bleed port cavity. Those ports are actually jets that meter air to the different circuits. They should not be plugged
    No, I'm talking about the plastic gasket that goes around the slide piston thing. It's under the cap on the top.

    Anyway, I put it back together and it actually runs pretty good. The only thing I did was sand down the air cut off plate, accelerator pump plate, and those 2 black plastic pieces flat. It will start and idle and rev up pretty good but still stumbles a bit. Maybe the accelerator pump is out of adjustment.

    Unfortunately I broke one of the hook looking pieces that the pull throttle cable rests in. This picture isn't mine, but the same piece broke at the base of the hook.

    Is there a chance I could buy one of those? I can't find the part number or what to type in order to find one.

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    Administrator LongDistanceRider's Avatar
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    Ah, the piston stop.
    Add some SeaFoam to your fuel, 2x recommended dosage. and run it. That'll clean/clear fuel passages that may have some dirt or an air lock in them.
    The only way to get that cable bell crank is from another carb, it's held in place with a roll pin. I'm out of those.
    Jim O'Brien
    1979 CM400T aka the Roadbike, 1978 CB400T1 semi restored, 1972 CL350K4 restoration and the 1971 SL350K1 disaster zone.
    Plus 2 SL350K0's , 2 SL350K1's, 1 CL350K0 and 1 CL350K1 waiting for space and time
    Contact: 408-239-9580 or [email protected]

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    Dang, that's what I was afraid of. Would it be possible to reattach it with some sort of epoxy? There's not much surface area there, I was thinking an epoxy putty might be better.

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    Administrator LongDistanceRider's Avatar
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    Quote Originally Posted by noah9988 View Post
    Dang, that's what I was afraid of. Would it be possible to reattach it with some sort of epoxy? There's not much surface area there, I was thinking an epoxy putty might be better.
    I don't know of a repair and never heard of anyone attempting it. Can't hurt to try.
    Jim O'Brien
    1979 CM400T aka the Roadbike, 1978 CB400T1 semi restored, 1972 CL350K4 restoration and the 1971 SL350K1 disaster zone.
    Plus 2 SL350K0's , 2 SL350K1's, 1 CL350K0 and 1 CL350K1 waiting for space and time
    Contact: 408-239-9580 or [email protected]

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    Senior Member ballbearian's Avatar
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    Precision braze or welding. Know any jewelry makers?

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    Unfortunately not. I'm taking it to a local expert this spring to have it gone through. It needs tires and probably the valves and counterbalanced checked. I'll ask him what to do with the broken piece.

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