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

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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so what else does the primary main do beside limit fuel flow through the pilot jet, does it supply more fuel to the engine through another circuit? the manual from my Kawasaki only explains the slow jet and secondary mains, or maybe i just dont get it
 
Kei-Hin carbs use 3 jets, pilot, primary main, secondary main
Search around here and you'll find information on them (mostly by me)
Kawasaki use Mikuni or TK, maybe some had Kei-Hin before they went to fuel injection, I only remember them up to the late 80's

Joon-yah Bourelle said:
yep, I think the 3-jet were on the California bikes.
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They were on almost all of them not just California. I know some Canadian bikes had 3 jet but that may have been the early ones (78~79)
 
crazypj said:
Kei-Hin carbs use 3 jets, pilot, primary main, secondary main
Search around here and you'll find information on them (mostly by me)
Kawasaki use Mikuni or TK, maybe some had Kei-Hin before they went to fuel injection, I only remember them up to the late 80's

They were on almost all of them not just California. I know some Canadian bikes had 3 jet but that may have been the early ones (78~79)
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i cant find anything if you got a few mins and can link some info that would be great.

thanks
 
CB360floatchamberside.jpg
 
"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"

so your saying the pilot feeds from the main and then shuts off when the needle opens? Is it a vacuum issue that would starve the pilot?
 
No, the pilot keeps flowing but the total amount is only what the primary main can flow so in effect the primary main jet limits flow.
I'm not sure if you get better atomisation with multiple discharge ports but I think it's probable. The problems happen on the transition from primary main to secondary main, primary is flowing as much as it can then the secondary 'dumps' more fuel into the mix.
 
Primary Main > Pilot jet > Pilot air jet > Carb Bore
> Primary main jet > Primary Bleed pipe > carb bore.
so the MAIN system consist of Primary Main and Secondary main, so why when tuning these bikes people say you need to change your secondary mains, what about the primary mains? when should you size up or down on these?
 
I haven't found any need to go up on primary mains as the air box doesn't offer any restriction at low rpm and acoustic tuning isn't really relevant at low airflow (although almost all 'high' rpm - 8K+ engines have a 'problem' around 4,500rpm even when fuel injected)
Sometimes the pilot may need changing but that only happens when there was a problem even with stock airbox (usually due to EPA restrictions for OEM)
 
Okay so my jet kit showed up and I finally pulled the carbs apart... I have the 2 jet carbs. good thing I had read this thread earlier or id still be sitting around trying to figure out where my damn primary jet was... even my manual (US) has the three jet carbs in it. the bike is an 1983 CM450e in canada :)

here is a pic of how mine is. ill take the bowl off again tomorrow and get a real pic for you guys

cm450floatchamberside_zps5bfb4161.jpg
 
The thing that we tend to ignore/forget is that air jets play a huge role in how a carb works. In some carbs there is only one air drilling and no jets. In others there are two or three carefully sized air jets.

On the 3 jet Keihins the interaction between different circuits and different air and fuel jets is what makes them a nightmare to get right at part throttle conditions. There is a Honda fuel manual on the web somewhere that shows the three jet carbs. I didn't take a note of the link, so I can't help there.
 
my manual is usless when it comes to the carbs as it has the three jet ones in it and no add on for my canadian ones. But I shimmed the needle last night and saw 2 air jets, i didnt bother to pull the cover off and confirm or deny the other 2. Manual says my carbs have a secondary main and slow and a primary main and slow
 
You'll be OK with the 2 jet.
Primary main circuit isn't drilled so you can use Mikuni/AMAL type set up
You will need a slightly bigger pilot jet, probably 40~42.5 (assuming you have a 35 stock?)
Remove shim from needle, I've NEVER had that work on Kei-Hin carb
You also have a 3rd or 4th air jet under the air cut valve plate (side of carb)
Diaphragm must be in good condition and plunger free (they stick, a lot ::) )
You 'air mixture screw' is a fuel screw, out = rich, more than 2.5 turns, need bigger pilot
 
The pilot i'll have to order (its not a normal looking jet, it uses a needle jet?), the kit just came with normal style jets for the primary and main of US carbs. i'll take the shims back out. the kit said to set the fuel screw 2.5 turns out then adjust from there.. So they are 2.5 out right now.
 
D4N63R the canadian carbs have a supliment manual with the differences in the Canadian and US carbs the US Carbs have emmissions shit and stuff the Canadian market bikes didn't get. If I can find it, I will post it up for you. Not sure where I put it after I sold my CM. But you will have a lot less issues then the american folks with your tuning, at least I did with my Canadian model bike.
 
I have the shop manual but no supplement for our carbs. If you still have it that would be much appreciated!

I ordered a pair of 40 and 42 pilot jets after reading pj's post
 
D4N63R said:
okay here is a pic of one of my carbs

9eb2f15f-8407-44cb-bbcb-be182e262cfe_zpsec29a68f.jpg
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In this pic you have pulled one of the jets already right? It would have been a 75-80 35-ish, (I am gettting jets mixed up now between my CM and CX) is number stock I believe. you have a 122.5 main jet. my bike had a 118 and I went to a 135 0r 140 but left the other, lower number Jet alone. Notice the candian carbs have a mixture screw where the US EPA carbs have a cover over theirs you have to drill out to turn. The middle hole has a rubber plug, not used. These look like what was on my CM and really all I did to work with PODS and open shorty cans, no H-pipe was get the 122.5 jet sized right and play with the mixture screw. set float height and make sure slide were operating correctly and non of the air ports in the mouth of the intake were blocked.
 

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that picture shows the carb the way way it looked when i took the bowl off, nothing in that hole. The bike ran when I picked it up. All i had touched was the main (switched to the 122.5) stock was 120

this is the other jet. it looks like it has what i can only describe as a backwards 'C' then 38 etched into it

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OK, mine was a 2 piece, just changed the top I believe, may be because of the model year of the bike. I don't think I have any pics left but I will look when I look for the canadian tuning supliment.

edit wait does the 38 go under the 122.5? if so you are missing a jet in the other hole I pointed out.
 
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