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Old 02-08-2012, 02:30 PM
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6 Motorcycle carbs on my Pontiac OHC 6... any ideas?

I'm building a hot rod from the frame up. I'll be using a Pontiac OHC 6 for power. I thought it would look sweet having 6 MC carbs on it. I've done a ton of Googling on the subject and it's been done, but it's a tricky thing. Nobody I've talked to has actually done it themselves, they just have seen it done succesfully by somebody else. It seems the main problem is MC carbs don't have accelerater pumps and aren't designed to run with much fuel pressure at all. It's a budget build, so I can't just go buy 6 new Mikuni Harley Davidson carbs at $500 each.

Has anybody here done this, or know somebody who has? Any Ideas? They wouldn't have to be MC carbs, I'd just want 6 1 BBL carbs.

Thanks!
Chris

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Old 02-08-2012, 02:52 PM
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What are you going to use for a manifold?
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Old 02-08-2012, 02:57 PM
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I'll build one. I have the flanges made up already... just in case it will work. I'll use 6 "tunnel ram" tubes. The manifold is the easy part. Getting the right carbs is the issue.
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Old 02-08-2012, 03:19 PM
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I take it when you say 6 tunnel ram tubes, that you are going to have one carb feeding one cylinder......Just my opinion but, I think if the 6 carbs fed into a single chamber, then into the 6 ports, you would have a lot less trouble tuning it
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Old 02-08-2012, 03:26 PM
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Yeah, 1 carb feeding just one cylinder... just like on a motorcycle. I think that has the highest cool factor. I just need to figure out how other's have done it successfully.
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Old 02-08-2012, 03:30 PM
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You will need to have balance tubes between each carb usually in the manifold.. The expense of the carbs has been the stopper for most guys to do this but once you get it built it can be real cool to look at..

Actually a better bet would be to go efi with a meagasquirt system and individual runners like a Hilborn inj ..you get better performance and less cost but there is the mental overhead of learning the efi system but we are hotrodders are we not..?/

Sam
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Old 02-08-2012, 03:41 PM
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This may be the most difficult part of my project. I'll run with a 4bbl 'til I get to the point of putting the 6 carb thing into action. I have to prove it possible, though. I've had too many people tell me it won't work. I may have to save my nickles longer than I thought! Haha

'til then, I'll keep working on it.
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Old 02-08-2012, 03:59 PM
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Actually it will work just fine with considerable thought and investment..

Sam
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Old 02-08-2012, 04:48 PM
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I have been thinking about a swap like this for a while. If you watch eBay for used carbs off sport-bikes like Ninjas you can find sets of four pretty cheap. I'm doing a 4 cyl. so that's all i need but I got them for $30. You should be able to get a couple sets for under $100. I haven't had time to do anything on them yet. With 6 carbs you will have to make some small things like linkage to run them all and a bracket to hold them but I would think it is do-able. There are kits out there for some cars like Triumph TR6 but they are pricey. I don't see why you couldn't do it yourself and save a ton of money.
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Old 02-08-2012, 05:59 PM
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It would be cool, maybe the velocity stacks pokin up through the hood...
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Old 02-08-2012, 06:14 PM
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Quote:
Originally Posted by chrisinestes
I'm building a hot rod from the frame up. I'll be using a Pontiac OHC 6 for power. I thought it would look sweet having 6 MC carbs on it. I've done a ton of Googling on the subject and it's been done, but it's a tricky thing. Nobody I've talked to has actually done it themselves, they just have seen it done succesfully by somebody else. It seems the main problem is MC carbs don't have accelerater pumps and aren't designed to run with much fuel pressure at all. It's a budget build, so I can't just go buy 6 new Mikuni Harley Davidson carbs at $500 each.

Has anybody here done this, or know somebody who has? Any Ideas? They wouldn't have to be MC carbs, I'd just want 6 1 BBL carbs.

Thanks!
Chris
Ak Miller did this on 6 cylinder Mustangs back in the 1960's. Problem then was finding M/C carbs with enough venturi area to support a cylinder. These days with 40 plus mm carbs that's less of a problem.

Like you said 6 Mikuni flat slides for a Harley would set you back about 3000 dollars. that would get you carbs with accelerating pumps. However when the carb is mounted up close to the head an accelerating pump is less necessary.

The carbs don't have to be logged together, there is no need to balance cylinder to cylinder vacuum except to run vacuum accessories like power brakes. Still if you log them you can somewhat take advantage of all the venturi area per cylinder. Note Yamaha's Ex-Up system that opens all the carbs to a log at high revs so any cylinder can breath through all, this system was used on the V-Max.

The fuel pressure can be managed with a regulator to get the pressure down to a couple pounds. The best way to do this is either a bypass regulator or two regulators one between the pump and a reservoir than another to the carbs feed from a fuel log. Either of these takes the pump pulses out which makes the carbs usually fed by gravity a lot happier.

The throttle linkage gets complicated as each carb has to be actuated and adjusted individually, you'll be using a lot of small Heim joints with ball bearing supported shafts for smooth operation.

For this kind of money you could put individual runner EFI on it which would be less trouble in service and look every bit as cool. you colud build this around the system off a junkyard Chevy Colorado with the inline 6.

Bogie
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Old 02-08-2012, 06:17 PM
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i dont see why it couldnt work might be a ***** to get it fine tuned but after that hell i am still looking for a 2x6 manifold for my engine keep checking on ebay
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Old 02-08-2012, 08:24 PM
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Not to rain on your parade, but there are things you need to consider when using motorcycle carbs for an engine that has a fuel pump. Most motorcycle carbs are gravity feed, and wont handle fuel pump pressures over maybe 2 lbs. at the most. They also don't have accelerator pumps, so getting past idle will be an issue with most.
If you look around at some cycle shops, or motorcycle salvage yards you can find a few carbs that are mounted on bikes that use both fuel pumps, and accelerator pumps. The older Honda Goldwings had the tank mounted below carbs, so they used a fuel pump, and would be a good choice for this project. They used 4 per engine, so you'll need to find a couple bikes to get 6 carbs.
There are also carb kits available for these older carbs to cheaply and easily rebuild them, so getting old ones running is not a problem. Saber Cycle sells a kit for all 4 carbs for under $50.
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Old 02-08-2012, 08:32 PM
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Its really not that big of a deal, you'll need to make a new mounting system and a shaft for linking the throttle plates. Rejetting will be necessary but again, nbd. You can also use bike throttle bodies to do it efi.
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Old 02-08-2012, 10:34 PM
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2x4`s can help

Hi Chrisinestes

I love this idea.Ok let`s start first off your ponti is 38.3 cubs per cyl.A 500cc engine is 30cdi.A motorcycle engine will run at almost 2x the R.P.M. of the ponti.So I think 200-250 singel carbs will work real well.If you can get some used quad carbs that match you will have way more cfm than a stock engine can use.The impornt thing is that are mounted at the same angel of the engine that they came from. Most quads have acc. pumps in the carbs and you can pin drill the distcharge nossile to just the right size.Normaly opening the hole .003 at a time and testing till it is just right.Also most have cables so making a pull block should be simple.Low speed jeting will be the challange as most low speed jets are .015 to .021 so wonce again you will be drilling I am thinking .035 to be a good starting point.If drilled incorrect no big deal soder closed and start again.You should have someone around with some carb know how to help with the tuning.This will be the hardest part of the job.The rest is simple like the guy`s have told you will need a pressure regulator.If You can make the manifold you can easly do this job. GO For IT



2x4`s
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