1979 Honda CM400A does not like mikuni carbs.

thathondamaticgirl

thathondamaticgirl

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i have a 1979 honda cm400a that i’ve been rebuilding for about a year now. my main issue is that it’s all crank, no turn over. brand new battery, ignition coil, valve seals + engine gaskets + timing adjusted … but i believe my problem lies in the vm28 mikunis i installed. i have to set the idle screw to extremely rich, and even then it will only turn over on choke after about a minute of cranking. and if it is idling, any gas kills it instantly. i have no access to the oem carbs anymore (they were unsalvageable anyways) and i’m at a loss. yes, they have been cleaned and thoroughly inspected. are the mikunis a mistake? my friend threw an amazon carb in his ‘88 sportster and it runs like a dream, considering doing that …
 
How was the initial setup of the carburetors determined?
  • Specifically for the 400A?
  • A generic vm28 kit?
The idle adjusting screw is used to raise/lower the stop position for the slide. There is another [air] screw to adjust the idle mixture. The slide cutaway, needle profile, and needle clip all affect performance just as the pilot and main jets do. It can that a lot of tinkering to get to a good setup from a generic kit.

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thank you for the diagram. i haven’t taken the carbs apart and really messed around with the jets, but i bought them as a kit from a “cafe racer parts” website and were advertised as plug and play. i was told that everything was already adjusted to where it needed to be based on my altitude and bike specs, but my bike doesn’t really like turning over. hahah. i’m not very familiar with adjusting jets, and i don’t know where they should be set. any advice?
 
I'm sure these carbs can be made to function correctly but they will require a lot of trial and error. Don't bother wasting money on the cheap Amazon carbs, your A model is rather finicky about the carbs. Moreso than the manual versions.
I'm not knowledgeable about the Mikuni's so I'll watch and learn.
In the meantime you might do a eBay search for the original VB24 carbs and save the search so you're notified when any get posted.
 
It can't hurt to download and read this tuning manual produced by Mikuni. It would also be helpful if you could report back on what components are in your carbs, especially the pilot and main jets. Everything should be labeled in tiny markings on the part itself.

If you haven't adjusted the air screws, you can gently turn one in until it just seats, counting the number of turns as you go. Here, 360 degrees counts as 1 turn. Then return it to that same position.

As for the idle screw, I would try to set it at the point where it just starts to lift the slide. If you back the screw almost all the way out and then start turning it back in, you should be able to feel when it starts to lift the slide. It will be slightly harder to turn as it pushes against the spring and weight of the slide.

When you crank the motor to start, have you given any throttle at the same time or no throttle at all? Have you tried with and without the starter plunger being depressed (same purpose as a choke, but not a choke)?
 
thathondamaticgirl said:
bought them as a kit from a “cafe racer parts” website and were advertised as plug and play.
Click to expand...
Did you use the original carb boots to connect the carbs to your 400A or did the carbs come with new boots?

LongDistanceRider said:
In the meantime you might do a eBay search for the original VB24 carbs and save the search so you're notified when any get posted.
Click to expand...

Do you know if 28mm carbs are compatible with the original boots?
 
stl360+450 said:
Did you use the original carb boots to connect the carbs to your 400A or did the carbs come with new boots?



Do you know if 28mm carbs are compatible with the original boots?
Click to expand...
the kit came with new intake manifold boots but the boots to the airbox are stock. i had to 3d print some adapters/reducers to correctly fit the carbs in the boots without any gaps.
 
stl360+450 said:
It can't hurt to download and read this tuning manual produced by Mikuni. It would also be helpful if you could report back on what components are in your carbs, especially the pilot and main jets. Everything should be labeled in tiny markings on the part itself.

If you haven't adjusted the air screws, you can gently turn one in until it just seats, counting the number of turns as you go. Here, 360 degrees counts as 1 turn. Then return it to that same position.

As for the idle screw, I would try to set it at the point where it just starts to lift the slide. If you back the screw almost all the way out and then start turning it back in, you should be able to feel when it starts to lift the slide. It will be slightly harder to turn as it pushes against the spring and weight of the slide.

When you crank the motor to start, have you given any throttle at the same time or no throttle at all? Have you tried with and without the starter plunger being depressed (same purpose as a choke, but not a choke)?
Click to expand...
giving it throttle usually does help it turn over, but the throttle has to be pinned wide open. and as its idling anymore gas i give it will kill the bike. and do you know where would the starter plunger would be located on my bike? i’ve never heard of that before and i’d love to try it.
 
thathondamaticgirl said:
the kit came with new intake manifold boots but the boots to the airbox are stock. i had to 3d print some adapters/reducers to correctly fit the carbs in the boots without any gaps.
Click to expand...
Okay, so you made adaptors for the airbox and the carb boots to the head are new. That should be fine.

thathondamaticgirl said:
giving it throttle usually does help it turn over, but the throttle has to be pinned wide open. and as its idling anymore gas i give it will kill the bike. and do you know where would the starter plunger would be located on my bike? i’ve never heard of that before and i’d love to try it.
Click to expand...
The starting lever is what you referred to as a choke, so I think you've been using it. Does it make any difference if you don't use it?

If you have the throttle wide open to start the bike, how do you give it more gas once it's running? I'm not understanding something in your description.

Have you checked your jets or the air screw setting?
 
thathondamaticgirl said:
do you know where would the starter plunger would be located on my bike? i’ve never heard of that before and i’d love to try it.
Click to expand...
He meant the type of choke that Mikunis use, different than the traditional butterfly-type choke on a stock Keihin carb. It's more of an enrichener, elevates the idle a bit and feeds a little raw fuel in. Do your carbs have flat levers for them, or the knob type that you lift up?

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stl360+450 said:
Okay, so you made adaptors for the airbox and the carb boots to the head are new. That should be fine.


The starting lever is what you referred to as a choke, so I think you've been using it. Does it make any difference if you don't use it?

If you have the throttle wide open to start the bike, how do you give it more gas once it's running? I'm not understanding something in your description.

Have you checked your jets or the air screw setting?
Click to expand...
oh yes haha, i have been using the choke everytime i start the bike. what i mean by the throttle is that pinning it open allows enough gas to get the bike started, and i can release it and *sometimes* it’ll idle. by sometimes i mean it only idled by itself once. if i tried to give it gas while it was idling it died instantly. i’m pretty sure it’s just getting way too much air, but i can’t locate the source of where and why it’s happening.
 
There's no harm in trying to start it without the choke. You could try to open the throttle a little bit while cranking to see if it will catch and possibly hold the throttle to keep it running if it does start. This could provide more information for further testing.

Carb tuning is a process of trial and error and, as such, the carbs will likely need to come off and go back on a few times along the way. If you're able to remove and install them, it would be helpful to know how they are currently setup. That will give a base point from which to suggest changes. At the least, you want to have the idle screws and air screws in a good starting position. It would also be good to document the needle clip position, main jet, and pilot jet.

Is there a reason you suspect it's getting too much air? The mixture won't burn if it's too rich or too lean, so I couldn't say without more information. Trying without the choke would mean trying with less fuel and that could tell you something, one way or the other. Have you skimmed over that tuning manual?
 
Can we back up a second? Were these carbs take-offs from another model? For instance, the Yamaha RD400’s are 28 Mikunis but they used a crossover tube for the enrichener so unless that tube is there, the enrichener won’t work properly, the bike will always be in a lean condition. I suggest the RD as that’s a good set of carbs to use in a 4-stroke application.

In general, the Mikuni design is adjustable for every throttle position so that is no issue - but likely it’s jetted wrong for that engine in today’s configuration.

Start point jetting advice:
-review the OEM carb jetting and use the same number slow vs pilot jet number. If 25 Keihin, then use a 25 Mikuni. Same for the Main jet. This will get you started and idling “close enough” Note the Keihin is based on diameter while that series Mikuni is a flow standard but for the pilot circuit, that will fix a too lean condition from Honda. The main should see you with enough fuel to ride and start tuning.

Being somewhat proficient in the 3D print world, porosity is an issue for an application like this.

Two printing questions:
-what material was used?
-what is infill %?
-drying is critical…

Something to think about is the ignition CDI box on these bike are marginal these days so don’t try to fix an electrical/ignition prob through the carbs. Ensure your sparks are strong and reliable first. Only then tackle the carbs.

We’ll help.
 
Can you post a photo of your installed carbs?
And, a list of how th carbs were jetted from the kit?
 
Hey @thathondamaticgirl, welcome to the crew!
The pit crew here are really good, me not so much ;) I have wondered about possibilities of using after market carbs on these bikes. I guess you are a case study. As you probably know, buying old original parts for these old bikes is a challenge. 3D printing, cool! I just ordered a basic one to get started trying some of that, an Ender 3. Actually I ordered 2 for like $120 total, listed as 'unrepair', which is like a used/return get what you get thing. So the original carbs on the bike were an 'unrepair'? ;)

​

 
Yes, where did you go @thathondamaticgirl? With that tag name, you can't just leave us! There were some other threads around here with someone using alternative carbs.

While we wait for hondamaticgirl, you can watch tomboy_a_bit, here's a vid I liked about chopper show in UK, some cool looking bikes.
Now this is mostly harley, but they look very cool.

I did get my 3D printers up and running and having great fun learning how to abuse them. I did make some plastic washers I am trying at the base of my mirrors. And made some rotors for the hard drives I am using as ignition jigs.

Jay, did you get your 3D printer fixed that you used to help design your oil pump?
 
Hey! Yes, its up and running nicely. Turns out my nozzle was burnt out. (Something a guy never wants to happen). Of course I got a more capable one and Nylon is my next challenge. That will allow an engine runable component. Still researching.
 
thathondamaticgirl said:
i have a 1979 honda cm400a that i’ve been rebuilding for about a year now. my main issue is that it’s all crank, no turn over. brand new battery, ignition coil, valve seals + engine gaskets + timing adjusted … but i believe my problem lies in the vm28 mikunis i installed. i have to set the idle screw to extremely rich, and even then it will only turn over on choke after about a minute of cranking. and if it is idling, any gas kills it instantly. i have no access to the oem carbs anymore (they were unsalvageable anyways) and i’m at a loss. yes, they have been cleaned and thoroughly inspected. are the mikunis a mistake? my friend threw an amazon carb in his ‘88 sportster and it runs like a dream, considering doing that …
Click to expand...


hello everyone! thank you all for checking in and being curious. :,) i’ve completely ditched the mikunis. grabbed some cheapy amazon carbs and they actually got the thing running! got to ride around for a bit and it was awesome, but eventually after a day or so those gave up as well. i accepted defeat (and also the sunken cost) went to a local motorcycle junkyard near me and picked up some vb21 carbs off of a 78’ cb400a hawk. closest i could find to stock for my bike. rebuilt em, threw em on the bike, unfortunately… no luck. i know the jetting and venturis are different between the vb21 and vb24, i was hoping i could pick up a jet kit and get pretty close to my stock jetting. any advice?
 
Hey! Thanks for the update and it is very puzzling what you're experiencing. We sadistically love a problem and this is a real (dare I say) puzzler!

Some things come to mind that are similar to my own experience, albeit on a different model bike ('68 450) and with the idea that a generic (i.e. cheapy amazon carbs) may not be jetted properly. My experience is if the jetting is on the edge of rich/too much fuel, the spark plugs simply die a slow death from soot fouling the electrodes...

Same for the original Mikuni installation.

To answer your question on kits, @LongDistanceRider is our resident expert/VHT Mod on those carbys. However, the kit jet(s) calibration may be suspect so most folks here save the original brass if possible, and use those. That doesn't help you, sorry. But yes, the rebuild kits will contain new jets/brass.

My fear is you're chasing an electrical problem through the carburetors. That is, if the passages are clear with the proper calibration, they work. But, you've introduced some (absolutely necessary) variables so sorting it out may be a bit more involved. Let's start with you've not been able to make any of the carby options work to me, points to a non-carby issue.

If the bike has been sitting, perhaps the petcock may be occluded down to only provide a trickle fuel supply? This could fit a longer ride where shorter rides may not be affected.

-at first startup, is the enrichener/choke needed for it to fire and continue to run?
-from there, how soon is the enrichener turned off/disengaged from startup?
-at riding, are there any backfires through the carburetor? Pops through the muffler? When does that happen (assuming it does)?
-when the bike dies, can you take and post a pic of what the spark plug insulators look like?
-when the bike dies, what is the bike doing? does the power just decay to nothing or is it abrupt?

Just so you know, too much information is perfect - especially with this bunch! And, feel free to PM any/all as needed.
 
Glad you tossed the Mikuni's, very doubtful they could ever be made to work with the A model.
The A model uses VB24 carbs(28mm venturi) along with a different head that has smaller intake runners. Going to larger carbs will create unsolvable problems. 1st being the there will be a lip between the carb and the head where fuel will accumulate and puddle causing dead/flat spots and stalling, the intake runner from the carb to valve has to have smooth transitions the entire way.
 
i worried that spark plugs might be an issue. maybe also the boots/spark plug wires. tried to start with starter fluid through the air filter and once again nothing but crank. i’ll go ahead and replace those parts and see where i can get.
 
LongDistanceRider said:
Glad you tossed the Mikuni's, very doubtful they could ever be made to work with the A model.
The A model uses VB24 carbs(28mm venturi) along with a different head that has smaller intake runners. Going to larger carbs will create unsolvable problems. 1st being the there will be a lip between the carb and the head where fuel will accumulate and puddle causing dead/flat spots and stalling, the intake runner from the carb to valve has to have smooth transitions the entire way.
Click to expand...
i ordered and installed the correct intake runner boots for the vb21 carbs so that it is a seamless fit.
 
So you have a full set of working VB21s now? I have a set of VB24A bodies I don't need, with one boot - would be willing to mail them to you with the original #72/105 jets, and venturi needles, which are shorter than in the 21s to accommodate the smaller diameter, if you want to give them a shot. Everything else should be essentially the same and can be swapped over. Won't be identical to the VB24Bs that would've come with the bike (#65/110 and different needle profile) but darn close.

On my '78 400 I did upgrade the ignition coil to a Duralast C849 to great effect, can definitely recommend that if the ignition coil ends up being the issue. Did have to melt some holes, solder a bit, and turn some bar stock into a mounting bracket to make that work, but it seems to have been worth it.
 
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