Topic: Honda CB550 Mikuni 29mm smoothbore carbs?

[Terry in Australia]

Well I was hoping these would fit my CB750, the spacing is right, but they're a loose fit in the OEM rubber insulators. The PO said they came off a CB550, are the 550 carbs the same spacing as the 750's?

I didn't think to measure the "spigots", but I'll do that tomorrow. Pretty cool carbs though, I didn't know Mikuni made CB550 smoothies......... 

[troppo]

Damned nice looking carbs terry

[Terry in Australia]

Thanks Trop, they are in nice condition, and I'm surprised that the spacing is for an SOHC4, I've never seen that before? Cheers, Terry.

[troppo]

Insert mission impossible music

Your mission Terry, if you choose to accept is to get these carbs working, including a full write up with detailed pictures and a report on performance.........


 

[cafe750]

mikuni didnt make 29s for the 550   the prevoius owner did   they have been re racked and the idle screw does nothing and the linkage probly wont open the slides fully before hitting on the outside   with the idle screw and linkage betwwen 3 and 4 it will probly hit the frame when installed in the cb750

That being said, I think that you should send those to me, so that I can dispose of them properly...

[Terry in Australia]

mikuni didnt make 29s for the 550   the prevoius owner did   they have been re racked and the idle screw does nothing and the linkage probly wont open the slides fully before hitting on the outside   with the idle screw and linkage betwwen 3 and 4 it will probly hit the frame when installed in the cb750

What do you mean "Re-racked"? The rack is a factory item, not a modified one, the idle screw appears to work fine, but that's as much as I know so far as obviously, I haven't tried them on a CB750 yet. Do you really know, or are you just guessing?

Anyway, welcome to the forums Annie, and don't forget to introduce yourself!

[paulages]

mikuni didnt make 29s for the 550   the prevoius owner did   they have been re racked and the idle screw does nothing and the linkage probly wont open the slides fully before hitting on the outside   with the idle screw and linkage betwwen 3 and 4 it will probly hit the frame when installed in the cb750

What do you mean "Re-racked"? The rack is a factory item, not a modified one, the idle screw appears to work fine, but that's as much as I know so far as obviously, I haven't tried them on a CB750 yet. Do you really know, or are you just guessing?

Anyway, welcome to the forums Annie, and don't forget to introduce yourself!

terry- the spacing is more or less the same on the sohc4 550, 650 ('79-80), and 750 models. they are off by just a hair, but can be made to interchange. the mouths on the 750 are a little larger than the smaller siblings, so i'd guess those would in fact fit a 550 or 650 if they are loose.

[paulages]

Insert mission impossible music

Your mission Terry, if you choose to accept is to get these carbs working, including a full write up with detailed pictures and a report on performance.........


 

great... thanks for getting that song stuck in my head. 

[Terry in Australia]

Thanks Paul, I need to "aquire" a set of 500 or 550 "insulators" so I can make adapter spigots to mount them to my 750 engine, have you got any spares lying around?

I don't really want to buy new ones until I can confirm that "Annie"s assertion that they won't fit a 750 is incorrect. Cheers, Terry.  

[kayaker43]

Mikuni or someone sold adapter manifolds to fit those to a 750. They were popular on the Z1's and GS100's back then like the CR's were popular for Hondas.

I used to run Z1 carbs on my 550 race bike and they were close enough to fit but I made different manifolds anyway. Should work fine if you sleeve down the manifolds somehow.

[Terry in Australia]

The trouble is that the "Mikuni to Honda" insulators are angled for normal Mikuni carb spacing, (GS Suzuki's and Kawasakis) i.e., instead of the carbs being an equal distance apart like ours, the normal Mikuni rack spacing is a much bigger gap between #2 and #3 carb. Because of this, the aftermarket "Mikuni to Honda" insulators won't work.

These carbs are almost exactly a straight fit onto the CB750 insulators, (i.e., even spacing between carbs) but the spigots being a slightly smaller OD, are a loose fit. My intention is to get some CB500/550 insulators, mount them on the carbs, then machine up some special "spigots" that will mount the CB500/550 insulators to the OEM CB750 insulators.

Sounds complicated, but it's not really, and will only add the length of the CB500/550 insulators to the length of the inlet tract, which shouldn't have any adverse effect on performance etc. Cheers, Terry.

[bryanj]

Terry 500/550 insulators are straight not angled like the 750 and are still available from honda for not that much 16211-323-000

[Terry in Australia]

Terry 500/550 insulators are straight not angled like the 750 and are still available from honda for not that much 16211-323-000

Thanks Bryan, you're quite correct, and that's what I want, straight insulators.

No biggie, I just had a thought, I've got some Suzuki "small port" GS1000 insulators that should work fine with these carbs, I'll use them to make some adapters to fit my OEM CB750 insulators.

I know, I know, I've got a mental picture that no-one else can see, but I'll find some alloy tonight or tomorrow to machine up one so you can all see what I'm talking about. Cheers, Terry.

[754]

Perhaps get Peter Graves & Martin Landau to help out..

if the experiment goes awry, Peter & Martin will disavow any knowledge of such carbs.. They will also start smoking & then disappear..completely..


This posting will self-destruct in 10 seconds....

[Terry in Australia]

The reason the idle screws aren't touching anything "Annie", is because the slides are somewhat "sticky" after the PO cleaned them out with water, and in the pics you can see, the slides are fully open.

The rack hasn't been "modified" at all that I can see, but maybe your eyes are better than mine? I see that you still haven't introduced yourself, and all your details are hidden, I don't suppose you have a brother in Canada do you? A rather verbose fellow who says a lot, but doesn't seem to achieve anything of worth?

Anyway, back in the kitchen with you m'dear, and leave technical discussion to the men............ 

[bwaller]

I have an extra set of 500 adaptors if you need Terry, and even hard enough to machine to fit I expect!

[Terry in Australia]

I have an extra set of 500 adaptors if you need Terry, and even hard enough to machine to fit I expect!

Hey thanks mate, I'm gonna try my small port GS1000 adapters today, if they don't fit I'll get back to ya! Cheers, Terry.

[MCRider]

Thanks Trop, they are in nice condition, and I'm surprised that the spacing is for an SOHC4, I've never seen that before? Cheers, Terry.

I have two sets of these carbs, one I ran on my CB750 streetbike for several years. Assuming the "rack" is for a CB750, you can get the proper manifolds here: http://www.dynoman.net/carb/inMani.html
Properly dialed in they performed flawlessly on an 888cc engine. Idle, response and all.

[turboguzzi]

interesting, I run 28's mikuni's (not smoothbore, regretfully) on my 500 and they have this nice gap in the middle which results in very straight manifolds.

I think they are from a GS750

Talking about carbs, does anybody know whatever happened to dghsicher (daniel) the great carb guy? his name has been deleted.

he should know where these were from originally

TG

[paulages]

i believe i do terry, but i'm on the road for the next two weeks. There were differences between the early and late carb insulators but i can't remember off the top of my head if the carb mouths themselves were different. Twotired would know.

[turboguzzi]

hijacking here....

hei paul, did you get the pics i sent you?

TG

[paulages]

hijacking here....

hei paul, did you get the pics i sent you?

TG

yeah, sorry. been on tour a lot and didn't a proper chance to write you back. thanks! think i'm gonna build up a proper chassis to house the 718. it'll live in my current bike for now, but it deserves a top notch frame and suspension.

[Terry in Australia]

Okey Dokey, so today I made one "Mikuni to Keihin" adapter just to see if it'd work, and (so far) it looks like it's gonna be fine.

Although it adds another 38mm (including the mikuni insulators) to the length of the inlet tract, the advantage over normal "Mikuni smoothbore to CB750" adapters is that with mine, there is also scope to balance the carbs with vacuum gauges.

The carbs have been soaking in a mix of diesel and gasoline all week to free up the sticky slides, and the good news is they are all as free as a bird! They open right up Annie, and the idle screw works great! 

Here are the pics so far, I'll take some more pics with the carbs mounted on a CB750 engine tomorrow. Cheers, Terry.  

[troppo]

you do some damned fine work terry, i didnt read machinist in your long resume lol

[Terry in Australia]

Ha ha, thanks Trop, it was a bit of work, but once I made sure it was right, the other three didn't take long at all! Cheers, Terry.