Changing carb setup

If it's broken or just needs tweaked

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petewagn
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Changing carb setup

Postby petewagn » Wed Nov 21, 2007 11:41 am

Has anyone ever try building or using a 2 into 1 carb setup

Image

The attached photo is from a Honda 750, which the intake port are on an angle.
The carbs pictured are VM34 Mikuni's, I was looking at using VM30's or VM32's.

soos
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Postby soos » Fri Nov 23, 2007 12:33 am

No, but I do run cb750 carbs on my '79cb650.
Not too sure how that setup would fit in a cb650 frame, but it would look sweet!
A rigid, or custom frame no problem.
I have heard there is clearance issues with the stock750 frame.(unless you run without air filters)
The 650's frame is a bit narrower, but I don't know if it would add to the clearance problem, or clear it.


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-=≡ Soos ≡=-

Pinhead
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Postby Pinhead » Sat Nov 24, 2007 2:52 pm

Not sure about fitment, but it seems to me that the inside two cylinders would run richer than the outside two. The air/fuel has a straight shot out of the carbs into the inside runner, but has to take quite a turn to get into the outside cylinder's intake. I'd think the fuel would centrifuge out of suspension and end up in the two middle cylinders. I could be wrong, though (it's happened once before).

petewagn
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Postby petewagn » Sat Nov 24, 2007 4:38 pm

Thanks guys. Yes the frame is going to be custom, so I can make room where I need it.
As for the carb setup in the picture, it's being sold by a cycle shop, not sure if I can mention the name. But it's being working on my friends CB750 like a charm. No fouling of plugs, it takes a little in setting it up but once it's working it's much easier to maintain then the stock 4 carbs.

Still researching.

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Ibsen
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Postby Ibsen » Sun Nov 25, 2007 3:36 am

I have also seen pictures of a simular 2 carb set up on a CB750 where they have used 2 Weber carbs.

petewagn
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Postby petewagn » Sun Nov 25, 2007 10:36 am

They are using Mikuni VM34.

petewagn
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Postby petewagn » Sun Nov 25, 2007 7:00 pm

Image

This setup worked on a Virago.

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Buber
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Postby Buber » Tue Nov 27, 2007 8:22 am

That's all nice, but..... What for? Do I miss some special gains to be had by this setup? Or is it just for fancy looks?

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old gringo
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Postby old gringo » Tue Nov 27, 2007 4:25 pm

I agree, tho it DOES look way cool :)

petewagn
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Postby petewagn » Tue Nov 27, 2007 6:51 pm

Main reason for the thought in changing the carbs is I hate sync'ing carbs. Two are much easyer then 4. Also it's getting harder to find replacement parts.

Doea anyone know the ID of the carb insultor on a CB650 - 1979. My project bike is in storage until after Xmas.

Thanks

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Volker_P
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Postby Volker_P » Wed Nov 28, 2007 2:31 am

Welcome here, petewagn :D
I share a bit Bubers thoughts. Synchronizing carbs is not a real argument. I did it once 13 years ago or so and as long as it still idles perfectly at or below 1000rpm I see no reason to repeat it. Adjusting a new set of carbs is much more work than just balancing the old ones each decade.
Anyway, if you like the look, just do it for that and there is no need for other reasons. However one practical advantage might be that the original carbs are really hard to get out.
What do you mean by carb insultor? Carb boots? There are different ones at different lengths and diameters for the CV and the piston carbs in the different frames. For the Custom models there is 460 stamped on them, should be 426 for RC03. But I even have "460" stamped piston carb boots at different lengths so there may have been a change over the years.

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Buber
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Postby Buber » Wed Nov 28, 2007 4:19 am

Carb syncing - that's really easy, just you need to understand the process, and do it patiently. The only problem I found was proper locking of the screws without moving them. I went the easy eay, and painted them over few times, so they stay put :D
Carb boots - or those little rubbers between carb and engine. Once upon a time.... :D long time ago, in 1995 (i think) when there was no internet and such, and me from "not-europe" yet, i needed to find solutions, not parts :wink: . So you go to 1st motoparts store, and buy a similar diameter reinforced ruber pipe for hot water for car cooling system. Then you cut it to appriopriate pieces, OIL them before assembly - so they will fit, secure them with clamps, and bingo! Worked perfectly, till i sold that one last year.
yes, there may be slight mixture turbulence, but I didn't found any adverse effects. It just WORKED.

So, "don't worry, be happy" those bikes are such fun to fix that way - try to do it with a bike that has dedicated carb boots that are screwed in place... No way....

Good luck with any carb setup - but remember - "better is enemy of the good". if it works well - leave it alone, you won't regret it.

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Volker_P
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Re: Changing carb setup

Postby Volker_P » Wed Nov 28, 2007 4:45 am

Buber wrote: ... So you go to 1st motoparts store, and buy a similar diameter reinforced ruber pipe for hot water for car cooling system. Then you cut it to appriopriate pieces, OIL them before assembly - so they will fit, secure them with clamps, and bingo! Worked perfectly, till i sold that one last year.
yes, there may be slight mixture turbulence, but I didn't found any adverse effects. It just WORKED.

There is a good chance that a car cooling pipe is fuel resistant, but not all rubber hoses are. They may get much harder or softer after a short while. So one should keep an eye on a solution like that until one is really sure there are no problems. UV-light resistance is another point, it may end up in small cracks that suck air.

Buber wrote: Good luck with any carb setup - but remember - "better is enemy of the good". if it works well - leave it alone, you won't regret it.
Wise strategy if riding availability is the key requirement. 8)

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Buber
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Postby Buber » Wed Nov 28, 2007 8:51 am

Yes, I did had a thought about the fuel resistance, but i figured that if it withstands HOT coling liquid AND is exposed to whatever else under car hood, it should be fine. And since I'm usually the "lucky" boy - it was... :D

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Volker_P
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Postby Volker_P » Thu Nov 29, 2007 1:55 am

Buber wrote: And since I'm usually the "lucky" boy - it was... :D

Yes, this argument cannot be overestimated! :lol:


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