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Running rich after carb clean

noah9988

Member
Joined
Nov 18, 2022
Total Posts
18
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0
Location
Maine
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.

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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.
 
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.
 
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:

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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.

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?
 
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
 
Are you talking about this piece?
View attachment 18671
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. IMG_20170325_134333870[1].jpg

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.
 
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.
 
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.
 
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.
 
I think I'll be able to find someone locally who can weld it. What's the best way to remove it? If I take off the throttle plate will I be able to take the whole rod out?
 
I think I'll be able to find someone locally who can weld it. What's the best way to remove it? If I take off the throttle plate will I be able to take the whole rod out?

Jim is the VB carb guy, he's on the west coast so he'll be on later.
 
I think I'll be able to find someone locally who can weld it. What's the best way to remove it? If I take off the throttle plate will I be able to take the whole rod out?
That's the safest way to do it, otherwise you have to drive the roll pin out which can get tricky and potentially bend the throttle shaft.
Take pictures so you get the linkages and springs set correctly. mark the throttle plate so it goes back in the right way.
On assembly use Blue LocTite on the throttle plate screws after you've center the plate in the bore.
Properly centered should give you an even smile when held up to light
Wrong:
201_4713.jpg 201_4714.jpg

Right:
201_4712.jpg
 
"The air cut off valves were taken out and polished with steel wool to make sure they don't stick."

I don't understand this. The air cutoff valves are rubber diaphragms. There's a metal cover. There are passages that need to be clear. There is a metal cover could be polished. I don't see why polishing it would be expected to make sure anything doesn't stick.

I'm concerned you may never have addressed the cutoff valve diaphragms. If they are cracked, you have an air leak. If they are frozen but not leaking, the only consequence is some lean popping/afterburning on sudden deceleration.
 
In the center of the diaphragm is a metal ball (more like a little rod on some models). In it's "normally closed" position it rests against a hole in the carb body. Upon deceleration the increased vacuum causes the diaphragm to pull that ball away from the carb body opening the passageway which allows extra fuel into the airstream. This creates a richer mixture which reduces backfiring. This little ball is the part that is polished along with the seat (hole) in the carb body.
 
J-T is correct. It's the ball end that gets polished along with the bore to be sure there's free movement.
If the ball were to stick in the closed position then as KenCM450 mentions there will be some decel backfire.
If the valve/ball were to stick open however you would have over rich condition that would be hard to figure out. Personally I block the vacuum source for the valves on my carbs knowing I'll get decel popping.
If the diaphragm is leaking you get an undetectable vacuum leak.
 
Alright, sorry for not updating but the bike is now running 100%. I took it to a local guy who knows his stuff and he got it fixed right up for me. All he told me was that I was missing the o rings in the fuel screws, and I somehow lost the spring where you sync the carbs. He went through them and synced it and now it's running like it should.

So now that everything is sorted I want to know how I can get this dead spot out of the throttle. The bike has always had it, before and after the carburetor mishap. I know for sure it's running lean in the mid range, over 1/4 to a bit over half if I were to guess. As I understand, it, the primary main (in my case it's a 68) and the needle mainly affects part throttle. Should I go up to a 70 or 72, or maybe put spacers under the needle? Let me know what you think.
 
I would increase the Primary main to a 70. Definitely do not shim the needle.
It sounds like the accelerator pump circuit isn't working. Could be a stuck check valve of clogged nozzle.
On the left side of the carb right above the float bowl mating line there's a letter/number combo, what's the letter after VB24?
 
I can assure you the accelerator pump is working. I put a new one in, filled the bowls and pressed on the plunger and saw a stream of gas squirt out of both nozzles. I set the clearance between the rod and arm according to the manual (0 - 0.04mm). Before the carb clean the diaphragm was petrified and the right size nozzle clogged. If I give the throttle a snap it seems fine, but roll it on slow and it's not happy.
20230721_084416.jpg
It says 24CASHJ
 
Going up one on the primary jet does make a difference. I was unsure, but gave it a try and now I'm a believer.

When you got the new accel pump did you check the length of the rod? They're usually too long and need trimmed down. Hope you saved the old one.

Also, did you remove the carb insulators at any time? If yes, the o-rings on them need replaced. You should replace them anyways if you're going to take the carb off again. They're likely a flat piece of plastic now. Part is still available direct from Honda.

You have solved your rich running condition, but also check the drain plugs at the bottom of the bike. Blowby crud always collects here and eventually into the air box and fouls the air filter. Get a mason jar or drain pan and pull the two plugs. A bunch of crud will come out if it's never been done before. You can safely leave the plugs out. It won't affect running. Your blowby will simple drip out like older bikes. Another advantage to leaving them out is that it will stop fouling your CV piston and other parts in the carb with a weird slime.
 
I did check the length of the rod. For some reason the old one was extremely short, there must have been a 1/4" clearance. I did trim down the new one and then bent the tab down slightly to set the clearance.

What do you mean by carb insulator? The intake boot? The guy that worked on it said he resealed them.

When I got the bike there was a ton of varnished gas in the air box, so I did drain it when I first got it. I did look in there not too long ago and didn't see anything in there.
 
"Reseal them". Man, double check that next time you take them off. I literally just worked on a bike where a professional RTV'd it. Unacceptable and may cause problems later. Give him a call, see if he replaced the o-rings or what he did to reseal them.

Leaving those plugs out is probably one of the easiest free mods you can do.

Did you replace the air filter when you drained the gas out of there? I'm assuming you already did.

Carb sync is important, but mixture screw adjustment is also important and so is idle speed. The hondmatics need 1250RPM at idle in neutral. If it's closer to 1100 it will run a bit funny. Especially when blipping the throttle in gear with brakes applied (i.e. creeping in traffic). You also need to get the mixture screws set so it doesn't "dip" under target RPM and then "recovers" when blipping the throttle.

Did you replace the tiny o-ring on the fuel accelerator cover and the o-rings for the crossover tubes? Here is a list of parts from MCM for these carbs:

1.5mm Wide x 4mm ID x 7mm OD - Accelerator Fuel Bowl - MCM PN: 1295N152
2mm Wide x 6mm ID x 10mm OD - Fuel Connecting Tube - MCM PN: 1295N224
2mm Wide x 4mm ID x 8mm OD - Accelerator Connecting Tube - MCM PN: 1295N222
1mm Wide x 2.5mm ID x 4.5mm OD - Idle Mixture Screw - MCM PN: 1295N113

Your carbs have the plastic floats that are non-adjustable. Did you use OEM float valves or aftermarket? Aftermarket ones are no good and will cause funny running.

FYI: it's not for some reason. The aftermarket ones are all too long. My guess is that it's a "pattern part" from another bike that the diaphragm happens to fit.
 
Thanks the C was what I was looking for, original jetting was 68/112 so 70/112 will help nicely with the effects of ethanol fuel. I would try the 72 as an experiment and it might be even better, or worse.
 
Air filter was replaced, yes. Noted about the idle and mix screws. O rings were replaced. I didn't replace the float needles because I had the incorrect aftermarket ones in the kit I got. All I used out of that kit are o rings and gaskets.
 
Air filter was replaced, yes. Noted about the idle and mix screws. O rings were replaced. I didn't replace the float needles because I had the incorrect aftermarket ones in the kit I got. All I used out of that kit are o rings and gaskets.

Did you find out what the mechanic meant by resealing the manifolds?

If the original float valve has ANY ridges on it you need to replace it. Only OEM. For real. Especially with the plastic, non-adjustable floats it's even more critical.

O-rings and gaskets are probably OK out of that kit. Just keep an eye on it and see if the crossover tubes start leaking in a year or two.

If you start getting gas leaks or the carbs look wet look very carefully. It can fool you into thinking it has something to do with the float bowl.
 
Is the jet I need called a 99101-393? It's 8mm in length, 6mm head diameter, and 4.9mm threads.
 
What's up with those manifolds? Did you ever find out? It's highly in your interest to do so and if it's wrong then fix it before you do anything else.
 
That's a real shame. Because if it was done incorrectly, which I almost guarantee it was unless he's a stickler for ordering the o-rings, then you have a real possibility of RTV being sucked into the engine and ruining it. Also RTV is not rated for constant exposure to gasoline... despite what internet lore says. If he, or you, taped up the outers of the insulators/rubbers with electrical tape/self-fusing silicone tape this is also incorrect and you need new boots.

I know you are running rich, and the insulators could be lean depending on what constitutes a "reseal", but if you can't get that right... how do you hope to solve the rest of the issue? Buying carefully selected jets in an effort to solve a performance problem is not right; unless you have modified your bike. By modifications I mean "deleting" the airbox and using "pods/velocity stacks/oiled air filters/k&n and their clones".

I repeat this again, if you are using an aftermarket float valve, it's wrong. It won't work. I'm telling you from personal experience. They are junk, all of them. If you want pictures of proof for comparison I will give them to you. If you're using an aftermarket float valve it's quite likely that it's pushing way too much fuel and now that could be why you are overly rich. If the float valve have currently is worn... if there is a ridge at all on it then it is past time to replace it. Any wear ridge, yes, for real.

Aftermarket Needle Jets/Jet Needle assemblies from a kit are all wrong. Tapers are different and made of inferior materials. They can, and will, cause overly rich running conditions.

Please, figure out what was done to the insulators, THEN get OEM float valves, and go from there and inspect if the rest of it is aftermarket from a kit. If it is, you're in trouble because the NJ/JN is unique to the Hondamatics and it changed every year. An $80-$100 mistake, if you can find them.
 
I sincerely appreciate the advice and knowledge, however I don't need to be lectured. The person I brought it to has been in business a long time and he knows these bikes inside and out. He does quality work and doesn't do hackjobs.

It's lean, not rich.

The airbox and exhaust are 100% stock.

I am not using an aftermarket float valve. They were not replaced. Nothing on the carb was replaced expect o rings and gaskets.
 
A source of lean condition that cannot be diagnosed is the air cut valves. When the diaphragm tears with age there's a direct air leak from the air bleeds below the piston to the vacuum port on the engine side of the throttle plate.
This is why I plug my air cut valve vacuum port.
 
A source of lean condition that cannot be diagnosed is the air cut valves. When the diaphragm tears with age there's a direct air leak from the air bleeds below the piston to the vacuum port on the engine side of the throttle plate.
This is why I plug my air cut valve vacuum port.

Sounds like a good idea. When I get to my leakers, I'll look into that (78 CB400T2).
 
So, final update for me I think. Put in some 72 jets and my god is it so much better. No more dead spot in the throttle, accelerates faster, and the engine sounds better (no more of that lean "blegh" sound). Couldn't be happier, it was so annoying with that dead spot in the throttle having to give it 3/4 just to pick up speed. So in conclusion I think the carb sync fixed the rich condition and the bigger primary mains fixed the dead spot in the throttle.
 
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