Topic: cb750 carbs on cb550

[cashoes]

hello all got my 750 carbs cleaned and rebuilt and on the 550 i was wondering if anyone has some 1st hand experience with this mod and if you have any advice you could share, jet sizes and what not, also i have to take some knotches out of my 550 tank so that the outer 2 carbs have room to travel, is there a tank i can swap for with the proper clearance? i really dont want to raise it so if i cant get another tank to fit cuttings the only way i see. thanks

[Bill/BentON Racing]

What you have can /will work.69 CB 750 carbs use a 4-1 cable arrangement instead of linkage.They will fit and easier to access jetting etc.Also look @ different intake boots.Good Luck,Bill

[Steel Dragon Performance]

I had a set on one ,I don't remember if it was a 550 or 500 , it was punched to a 651 ,the carbs worked great ,I got the bike from a friend, he built it 25+ years ago ,it was a lot of fun to ride .  Have some fun!!!!


Mike 

[svenD]

I used them last season on my mildly ported 550,pods,streetable 4inone exh.(classic k style carbs w links)
 125 mains,40 idles,needles in the middle pos.
 used carb runners of k3 model 550,(gave the best spacing)using new 500 intake rubber. Mounted carbs, rubber and runners on workbench(tight fit! but o.k) then fitted complet assy to cylinder head,
 raised tank slighty for clearance(could not bring myself to ding tank!)
 would do 195 km/h on freeway,reving 10k,never quite solved the clearance issues though.
 carburation was VERY sensitive to crosswind/leg position
 great sounding and looking mod.HAVE FUN.
 

[cashoes]

thanks for the information , modding tank this weekend hopefully be able to post an update.

[leaderbean]

I'm looking to revive this thread.  Sorry if that's annoying but I'm having a hard time finding one authoritative source in the forum about the compatibility between 750 carbs on 550s.  Piecing it together. 

I have a '76 550k with a top end rebuild going on.  I'm working with a local bike builder/mechanic for labor and expertise while I've been sourcing a lot of the parts.  Specs are: 

-bored .5mm over
-new pistons
-new OEM valves
-Webcam 358a
-Delkevic 4-1 exhaust

Carbs are the issue right now.  He simply cannot get things dialed in.  It's back together/running but he couldn't get the stock 550 carbs running smoothly throughout the whole throttle band.  Then he tried a rack of '80 650 carbs and he cannot get those to work either.  I'm thinking about trying 750 carbs.  I don't wanna spend the $$ for a new system from Murray's https://murrayscarbs.com/product/honda-cb550-carburetor-kit/ if i don't have to.   I know where I can get a set of rebuilt 750 carbs from a very trustworthy source.

Info here helped me understand that if I go the 750 Carb route, I might encounter interference with my stock 550 tank and 750 carbs.  I wish I could find more info on the different years, though. Or can anyone confirm that years don't matter and that all 750 carbs will interfere with the tank on all 550 years?

The Carb FAQ page here:  http://forums.sohc4.net/index.php/topic,5410.0.html helped me confirm how close the spacing is between carbs on stock setups between 77/78 550 PD carbs and "7A" '73(?) stock cb750 carbs.  But again... I wish there were more years/models specifically covered.  Listed below to save time:

1977 or 78 CB550 PD series carbs

depth (front to back)               -100mm
OD (engine side)                     -32.9mm
OD (airbox side)                      -38.1mm
#1&2 and 3&4 (c to c)         -75.8mm
between 2&3  (c to c)               -81.1mm
Venturi: 22mm (unverified...)

The only 750 carbs listed on that page are:

"7A" '73(?) stock cb750 carbs:

Type of carb                  - Mechanical linkage
depth (front to back)       -88.26mm
OD (motor side)              -36.95mm
OD (airbox side)              -40.08mm
#1&2 and 3&4 (c to c)     -76.3mm
between 2&3  (c to c)      -81.5mm

Venturi: 28mm

The origin of that FAQ page is this thread: http://forums.sohc4.net/index.php?topic=43161.0 , in which Bwaller says "I've seen several racing 500's with 750 carbs. It might help to use the 77-78 550 intake manifolds. I remember mounting some on a buddies stock 500 in the early 70's and never really got it well sorted."

[leaderbean]

I used them last season on my mildly ported 550,pods,streetable 4inone exh.(classic k style carbs w links)
 125 mains,40 idles,needles in the middle pos.
 used carb runners of k3 model 550,(gave the best spacing)using new 500 intake rubber. Mounted carbs, rubber and runners on workbench(tight fit! but o.k) then fitted complet assy to cylinder head,
 raised tank slighty for clearance(could not bring myself to ding tank!)
 would do 195 km/h on freeway,reving 10k,never quite solved the clearance issues though.
 carburation was VERY sensitive to crosswind/leg position
 great sounding and looking mod.HAVE FUN.
 

This is a short, but very helpful comment here.  Thank you.  From this one post I have learned that

a) it's possible
b) confirmed tank clearance issue
b) might be a good idea to use a K3 550 runner to improve spacing
c) might be a good idea to use intake boots off a 500
d) potential jetting once it's together

@svend could you help me understand your use of the K3 runner and the 500 boots?  How much better did the spacing get due to the K3 part and are 500 boots bigger than 550 and therefore work better on the 750 carb setup?

[leaderbean]

Found this comment:

 ---'79 & '80 cb650 intake ports will mate with SOHC cb550 and cb750 carbs

in the first post here: http://forums.sohc4.net/index.php/topic,40414.0.html

[newday777]

I used them last season on my mildly ported 550,pods,streetable 4inone exh.(classic k style carbs w links)
 125 mains,40 idles,needles in the middle pos.
 used carb runners of k3 model 550,(gave the best spacing)using new 500 intake rubber. Mounted carbs, rubber and runners on workbench(tight fit! but o.k) then fitted complet assy to cylinder head,
 raised tank slighty for clearance(could not bring myself to ding tank!)
 would do 195 km/h on freeway,reving 10k,never quite solved the clearance issues though.
 carburation was VERY sensitive to crosswind/leg position
 great sounding and looking mod.HAVE FUN.
 

This is a short, but very helpful comment here.  Thank you.  From this one post I have learned that

a) it's possible
b) confirmed tank clearance issue
b) might be a good idea to use a K3 550 runner to improve spacing
c) might be a good idea to use intake boots off a 500
d) potential jetting once it's together

@svend could you help me understand your use of the K3 runner and the 500 boots?  How much better did the spacing get due to the K3 part and are 500 boots bigger than 550 and therefore work better on the 750 carb setup?

He hasn't been active since 2011

[scottly]

I have a '76 550k with a top end rebuild going on.  I'm working with a local bike builder/mechanic for labor and expertise while I've been sourcing a lot of the parts.  Specs are: 

-bored .5mm over
-new pistons
-new OEM valves
-Webcam 358a
-Delkevic 4-1 exhaust

Carbs are the issue right now.  He simply cannot get things dialed in.  It's back together/running but he couldn't get the stock 550 carbs running smoothly throughout the whole throttle band.  Then he tried a rack of '80 650 carbs and he cannot get those to work either.  I'm thinking about trying 750 carbs.  I don't wanna spend the $$ for a new system from Murray's https://murrayscarbs.com/product/honda-cb550-carburetor-kit/ if i don't have to.

If you and your "mechanic" can't make it run correctly with stock carbs, tuning 750 carbs will be even harder to do, and Murray's are a step down from stock. You need to fix your stock carbs, in my opinion. Also, have you verified the ignition timing is correct and the advance is working properly? Often perceived carb problems are actually ignition related, and vice versa. 

[flatlander]

i agree with scottly. with those mods, there is no need for any other carbs than stock. my own 550 has more severe mods than yours and runs perfectly on stock carbs. of course jetting needs to be adjusted.
so don't complicate things trying to fit other carbs. but do make sure everything is correctly set and adjust the carbs last.

[leaderbean]

Thank you very much scottly and flatlander, very helpful!  Your confident statements give me hope.  My "mechanic" is indeed a legit motorcycle mechanic but my belief in him is going down.  I had a really hard time finding somebody to help me with this bike and it's kind of a "beggars can't be choosers" situation.  When the stock carbs were on, he said he had a flat spot that he just can't get rid of. Either on the low end or the high end, but he can't seem to eliminate it.

At this point I just want the bike back.  Ideally... in running order   But if I need to pull the plug and pay him for the work done so far, fine.  Then I'd be left with the carburetion issues on my own, which is specfically something I'd like to pay a pro to figure out.  I have other large projects going on right now that prevent me taking this on.

[leaderbean]

@scottly, I'm curious why the Murray's carb setup (dual Mikuni VM34s) is lower quality than stock.  Can you please help me understand your point of view?  Curious if it's the 2 carbs vs. 4, manifold design, Mikuni vs Keihin, etc.

[Mark1976]

   Your in the proverbial 'rock and a hard place ' situation. Do you pay someone else (and yer gonna pay, who works on bikes of this generation these days), or do you invest your time and money learning how to do it? Lets see, cost of paying someone else......... Or learning how to do it yourself?? Real question becomes, 'how mechanically inclined are you?'  This isn't that difficult of an issue unless, you've never ever in your life tied your shoes, dressed yourself or made yourself a meal, all things you had to learn in order to live. Ya want to ride a old bike, ya better learn how to maintain it, lest you have a few bucks to spare. Your better off learning how repair and maintain the original rack, it's just not that difficult, whether you've done it before or not. Start a thread, post pic's and away you go.... just an opinion.....

[willbird]

@scottly, I'm curious why the Murray's carb setup (dual Mikuni VM34s) is lower quality than stock.  Can you please help me understand your point of view?  Curious if it's the 2 carbs vs. 4, manifold design, Mikuni vs Keihin, etc.

IMHO it is because of the attempt to run a 1:2 intake manifold where there were originally 1 carb per cylinder. Every video I have seen that was promoting that setup the bike looked and sounded like it ran like crap afterwards :-). The guy doing the video always says "it is running GREAT now" LOL.

Bill

[denward17]

My '76 550 needed a needle clip adjustment to get the bike to run well throughout the RPM range.  It has the 087A carbs and was running rich down low (~1/8 throttle).