How to tune CM400/CB450 carbs for pods and a less restrictive exhaust.

After seeing many posts about these carbs not being tuneable I decided to try and help people out. I've had a 81' CM400 and another with a 82' 450 in it. I ran pods and a really less restrictive exhaust on the 400 and pods and open exhaust on the 2nd. I've had no problems with either one. The trick is simply swapping the main jets and raising the needle location with shims. I don't claim to be a carb guru or anything but i've tuned a few and never had any problems. I don't have any magical calculations I use,i just use the basic teachings I got from old hot rod guys. These engines really come alive once you open them up a lil and add some fuel. So in short feel free to let me know you're setup and i'll help you get her running great.

Hello Junior nice to hear that. I will put velocity stacks in my 450. Do you know which size is the stock jet? I mean internal and external? Cause I want to purchase some from the web. Im planning to buy a couple of different sizes and just try them out. Where do you put the shims in the needle? Tks for you help!

That doesn't work, you may think it's OK but it will be running real rich mid range.
If your running head pipes only it won't be as bad as running silencers, the too short pipes have other issues and need a richer mixture to prevent engine damage
If your only ever flat out or braking it will be OK but if your trying to cruise at 40~50mph it will probably be horrible
Mid range needs leaning out not richening up (which is what your doing when you shim needles)
Your 'old hot rod guys' were not talking about 3 jet carbs, what works for Bing, AMAL, Delorto, Jikov etc doesn't work on Kei-Hin
Someone who understands GM Quadrajet could possibly get them working?

Nealox said:

Hello Junior nice to hear that. I will put velocity stacks in my 450. Do you know which size is the stock jet? I mean internal and external? Cause I want to purchase some from the web. Im planning to buy a couple of different sizes and just try them out. Where do you put the shims in the needle? Tks for you help!

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Depending on the model it will either have 112's or 118's for the mains, you won't always have to raise the needle. Depends on what you do to the motor as far as exhaust goes. If you don't have a manual it would be good to get one so you can study the carbs before you pull them apart. You'll also have to get the bike running as it is before you can tune it,we have to find out the problems before we can find the answers. No prob,it's my pleasure.

Junior Burrell said:

After seeing many posts about these carbs not being tuneable I decided to try and help people out. I've had a 81' CM400 and another with a 82' 450 in it. I ran pods and a really less restrictive exhaust on the 400 and pods and open exhaust on the 2nd. I've had no problems with either one. The trick is simply swapping the main jets and raising the needle location with shims. I don't claim to be a carb guru or anything but i've tuned a few and never had any problems. I don't have any magical calculations I use,i just use the basic teachings I got from old hot rod guys. These engines really come alive once you open them up a lil and add some fuel. So in short feel free to let me know you're setup and i'll help you get her running great.

Click to expand...

crazypj said:

That doesn't work, you may think it's OK but it will be running real rich mid range.
If your running head pipes only it won't be as bad as running silencers, the too short pipes have other issues and need a richer mixture to prevent engine damage
If your only ever flat out or braking it will be OK but if your trying to cruise at 40~50mph it will probably be horrible
Mid range needs leaning out not richening up (which is what your doing when you shim needles)
Your 'old hot rod guys' were not talking about 3 jet carbs, what works for Bing, AMAL, Delorto, Jikov etc doesn't work on Kei-Hin
Someone who understands GM Quadrajet could possibly get them working?

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Just trying to make it seem more simple,each bike will get different advice. If people can relay what their bike is doing than changes can be made to correct them. FYI I have been rebuilding Quadrajets for about 20 years along with the Holley on my hot rod. I'm just trying to help people out and make them love their bike a lil more. Have you had any experience with these carbs? If so maybe you could help some people out instead of just trying to be negative about it.

I've been working on them since 1978.
I've done a couple of modifications on a few so know what doesn't work
I'm exclusively motorcycle mechanic although I 'play' with cars I don't pretend to be any sort of expert there
I'm trying to be helpful, the 3 jet is the most difficult to do anything with, it really needs internal modifications including blocking some passageways and fitting internal piping
Carb is virtually identical to CB750/900 DOHC, when racing Superstock they needed an incredible amount of work to operate correctly as it wasn't possible to fit the more normal CR carb conversion

I understand and as i stated I am not an expert. But I have a 82, 450 thats been ported and polished with pods and an open exhaust with a x pipe. She runs pretty damn good, I was able to tune the stock carbs to work great. What I meant by the hot rod guys is the principles for tracking down and finding the tuning issues, where its rich or lean then finding where to correct it. I'm just not willing to buy into the fact that its not doable because I've done it more than once with 2 different motors and set ups.

I don't know what Junior did to his carbs but it runs like a bat out of hell. I've put a few hundred miles in the saddle riding next to him and his bike surprised me often.

Junior Burrell said:

I understand and as i stated I am not an expert. But I have a 82, 450 thats been ported and polished with pods and an open exhaust with a x pipe. She runs pretty damn good, I was able to tune the stock carbs to work great. What I meant by the hot rod guys is the principles for tracking down and finding the tuning issues, where its rich or lean then finding where to correct it. I'm just not willing to buy into the fact that its not doable because I've done it more than once with 2 different motors and set ups.

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I did it too and rode the bike all over the place in all situations in all weather. I also had the CND market bike which did not have the emmissions controls on the carbs so I could adjust more. Although the emmissions crap is removable. I hade emgo pods, stock headers with 2-3" of pipe then shorty cans. They were loud so i put some drilled out B&S lawn mower baffles in them to keep the neighbors happy. It got a bit rich but not bad enough to foul plugs. I did ride that bike around at WOT a lot cause it was more fun but I could and did tour on it at sustained 55-65 MPH for hundreds of miles at a time, like 5-6 tanks of gas in a day. Sold it to a guy who has been riding the crap out of it since. Was it perfrect...no. but it was faster than stock, lighter than stock and that made it more fun. I went on one trip where it rained so hard I couldn;t see more than 10 ft in front of me and with the pods the bike ran along without any hiccups. Long story short. I re-jetted, changed mixture settings and did plug chops and the plugs were never lean and slightly rich at a few spots but all in all the bike was fine. I didn't write it down however and now I can't remember what settings I had everything at but I think the jets ended up at 128???? 2 up from factory but foggy now. Oh and my motor was an 82 450 with the carbs from the 81 400. Bike would hit the ton + about 10mph more with a small screen one it or if I laid right down on the tank, I am 6' and 210 pounds, I think it would be faster with a smaller lighter person on it. Cruise all day long at 75 on the freeway but was most fun at 50-60 on twisty back roads.

Junior, I hope to contact you in the next few days/weeks when I get my CM back together. I have UNI pods and a 2-1 open exhaust. I'm willing to throw in some baffles to create some back pressure. My stock main jet was 112... I was able to get a 116 locally but I still need to get a bigger slow jet because it won't idle without the choke off. I got a 75 slow jet but it was a different size. Where should I look for jets to fit these carbs?

Seems like this thread has sort of died, but I figure this is as good a place as any to seek some advice.

I've got an 81 CM400t with the stock exhaust (in bad shape- probably many little leaks... i'll get to it)
and pods. It runs fine with the jetting I have now, but I feel like it could be much better.
I've been playing around with a ton of different jet combinations, and some have worked better than others, but nothing has really jumped out at me. I was running 138-78 for a while, and it ran pretty good but I think that was a bit rich. I'm back down at 135-75 right now I beleive, and it runs ok, but not quite as nice as 138-78; i'm getting a bit of a flat spot in the mid range which sounds common from reading elsewhere. Since both of these settings seem much higher then the 128 that the previous poster suggested, I might try to lean it out further just to see what that's like. I obviously would rather have it too rich than too lean, but I think it might still be rich now. But that doesn't explain why it ran better at 138-78..
I also need to replace my carb boots- sprayed some carb cleaner down there while it was idling and it seemed to perk up a bit when I hit the left boot. So those will be replaced probably next week. tomorrow is payday for me ;D

So if I may, what is everyone else running with any success?


Also, would anyone suggest needle shimming? If so, how exactly do you do that? I've fully disassembled and cleaned every milimeter of these carbs so i'm pretty darn familiar with them. Do you just squeeze a tiny washer under what I beleive is the float needle (rests in the hole between the two floats attached to the bar connecting them so that it rises when the float level rises?) or is there something more to that?

Also, Junior - thanks for starting this thread. I follow retro on Instagram and it's always a pleasure when that CM pops up on my news feed. Makes me want to hack off that stamped-steel section of my frame in the worst way.

Found something pretty interesting last week.
CM450 has 2 jet carbs, the passageways are not drilled for the primary main jet.
That probably explains why your having such good results?
Anyone trying to modify needs to check if they have 3 or 2 jet carbs

Thanks Travis, sounds high on the jets n prolly from the leak. Get ya some new boots n see where she's at. once you've done that send me a pm n I'll help ya out.

No one ever mentions in these post about jetting to big of jet size (or over carbing) other than it seems to be running rich. When you way over jet it will cause you to lose top end or bog at top end of rpm range. This would be like going from a 108 main to a 180 main. Sometimes its easier to start big see if you lose top end and start going down from there. Don't worry about needle height, that is for fine tuning unless you are running on a drag strip and looking at an E.T. it wont matter a lot. I've seen some of these old bikes completely stock get so lean they wont run just from age and need to go up 10 points on the main to fix the problem. Another thing is when you go bigger on the slow jet it increases through out the throttle range. Get the slow jet to where it idles good and takes throttle then start going up on mains.

Sorry, your trying to relate apples to alligators
3 jet Kei-Hin carbs have fuel flow through primary main jet to pilot/slow jet so total fuel doesn't increase with increase in pilot jet size
On Mikuni it does but this is Honda CB/CM which are usually 3 jet (450 may be 2 jet)
Needle isn't 'fine tuning' it will control low/mid range, most are not adjustable (except in the 'wrong' direction by shimming)
Old bikesdon't 'go lean' they always were lean, from 1976~77 onwards
The fuel formulation changed, anything with alcohol in is leaner than 'pure' gasoline, it's normal to have to up-jet by 7%~10% even on a stock motor/airbox/exhaust (alcohol test kits were available in 1970's to find out percentage when bikes had running/overheating/seizure issues)

crazypj said:

Sorry, your trying to relate apples to alligators
3 jet Kei-Hin carbs have fuel flow through primary main jet to pilot/slow jet so total fuel doesn't increase with increase in pilot jet size
On Mikuni it does but this is Honda CB/CM which are usually 3 jet (450 may be 2 jet)
Needle isn't 'fine tuning' it will control low/mid range, most are not adjustable (except in the 'wrong' direction by shimming)
Old bikesdon't 'go lean' they always were lean, from 1976~77 onwards
The fuel formulation changed, anything with alcohol in is leaner than 'pure' gasoline, it's normal to have to up-jet by 7%~10% even on a stock motor/airbox/exhaust (alcohol test kits were available in 1970's to find out percentage when bikes had running/overheating/seizure issues)

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you should listen to him!!!

Well I now know why I never had running issue on mine. I HAD canadian 2-jet no emmissions carbs!

Sent from my MB525 using Tapatalk 2

That explains a lot, I never considered they could be non-EPA carbs with only 2 jets

crazypj said:

That explains a lot, I never considered they could be non-EPA carbs with only 2 jets

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yep, I think the 3-jet were on the California bikes.

here i am all in my happy little air box world, okay not really.... try as i might i cant think of how to make that POS look pretty. But im in canada, which i guess means 2 jet carbs (I havent pulled them apart yet but will do immediately and let you guys know what i find), which means using advice previously received from the man with the coolest fork brace ever, i can run my uni pods....