Looking for supercharer draw through carb options

[Bugster]

Hi.
I am a bit worried posting this on your forum, because this has nothing to do with land racing at all.
I found you guys when searching the web for info on selecting a carb for a supercharger set up on my Honda CB650 engined café racer project, and it seems like there is a lot of know how on tis site, so please forgive me if this out of line.
It seems like there is very little info on this to find, so I hope that if someone might be able to help, and that there might be something educational for others in this as well.
I have sent a few e-mails to some of the carb dealers that I could find, but no help so far.
Thing is, I am a bit of a noob when it comes to carbs. Have done some engine work in the past, but have never fully understood carbs.
I have not decided yet, because this is very early in this stage of the project, but I am considering an AMR300 or AMR500. (open for discussion here as well)
I don't want to do any internal work on the engine, so boost pressure will be kept at a level where I don't need to lower the compression. That's the idea so far at least.  
As mentioned this is a -82 CB650, 626cc, peak power @ 9000rpm.
One thing is to find a suitable carb, one other thing is something with vacuum stuff to ensure correct mix at low vs high rpm or at idle or flat out.
See, this is where I don't have a clue.
Hope you bear with me and have some sort of advice for the carb noob.
If I am totally off topic for this forum, I totally understand and will delete if needed.

Anders from Norway

[Seldom Seen Slim]

Anders, thanks for your concerns about asking a question that isn't necessarily strictly related to land speed racing.  Don't worry too much.  As for the information you're seeking -- the chances are that someone (maybe more than one) here will be able to offer some guidance or help to you.

And then -- maybe -- you'll decide that land speed racing might be what you've been looking for and you'll spend more of your time in the future with us.

Best wishes-

Jon a/k/a Seldom Seen Slim

[SPARKY]

 

[Bugster]

Anders, thanks for your concerns about asking a question that isn't necessarily strictly related to land speed racing.  Don't worry too much.  As for the information you're seeking -- the chances are that someone (maybe more than one) here will be able to offer some guidance or help to you.

And then -- maybe -- you'll decide that land speed racing might be what you've been looking for and you'll spend more of your time in the future with us.

Best wishes-

Jon a/k/a Seldom Seen Slim

Thank you Jon!
Land speed racing seems like a lot of fun, and would be a thrill to try, but unfortunately those salt flats are hard to find up here in Norway, in fact any kind flats at all, but thanks for your post!

[Duramax]

Just rambling.

When I turbo'd my '77? KZ650 DOHC 4 cyl Kaw, I used an off-the shelf Harley carb.  The kind with the adjustable jet knob on top?  Worked great, but I was running 2 bars of boost (30PSIG) and 717cc at 12,000rpm.  It ran excellent on the street, and scared the snot out of you when that boost needle moved. 

Seems a CB650 is 9:1 compression, so you should not have trouble running up to 15 PSI on good gasoline with the right tuning.  So now your engine is going to behave like 1300 cc single carb 9000 rpm engine.  Your jetting should be pretty close using a 74 ci stock Harley carb.  Even though your RPM is much higher, and your engine flows better, boost creates a lot of vacuum in the venturi. 

That's where I would start if you can't find someone who has supercharged a 650-4 Honda.

[Bugster]

Thanks Duramax.
I bet that KZ 650 was a lot of fun! I built a turbocharged VW bug engine about 10-15 years ago. It was a low budget build and I had much trail and error, but that -58 bug was wild when I got it working

I have been under the impression that I need some kind of special carb or have some mods done to a stock one to get it right.
Follow this link:http://victorylibrary.com/supercharger/super-carb6.htm
I don't understand much of it, but about half way down that page there is something about carbs in a draw through system

[maj]

Have a wade through this http://www.dlra.org.au/forum/viewtopic.php?f=18&t=1686
will give you an idea of what to look forward to
Tiny is a regular on here too and may comment

search draw through turbo info too , much of it will be relevant

harley type carbs are common in draw through turbo , but watch your fuel supply from tank to bowl, easy to underestimate flow requirements 

[Duramax]

Good point.  I had to have an electric fuel pump and low pressure regulator now that I think about it.

[generatorshovel]

Hi from down under Anders, at the risk of discouraging you, which is not my intention, I would suggest you are a brave man to embark on such a challenging project without a good understanding of how carbs work .
By adding a carb to a draw thru supercharged setup , you basically change the carb's ability to do the task .

harley type carbs are common in draw through turbo , but watch your fuel supply from tank to bowl, easy to underestimate flow requirements 

A problem I have encounted is the fuel dropping out of suspension in the plenum, air is like a male's 'ol fella (Oz slang), it'll go where it's pushed, but fuel is lazy and would prefer to go where gravity wants, rather than follow the air flow.
Iv'e been getting closer to a good setup for 2 years now, but I'm still not there yet ( wasting 4 months blindly following an O2 sensor BAD information hasn't helped)
Mind you, once I DO get it sorted, the past 2 years problems will instantly become just another non important memory,,,,
Good luck Anders,
Tiny

[Grandpa Jones]

Hi Anders, the Mikuni HSR 42 seems to be a popular carb for draw-through apps. I've
got the bits for a similar project, I may get to it one day! 

Cheers, Dave

[Buickguy3]

   Something to keep in mind with a drawthru system is that should you break a valve spring or float an intake valve you've got a perfect charge in the runners for a bomb. 
    Doug 

[Bugster]

Thanks for the replies guys!
I have been reading a lot about the subject, and think I am getting an idea of the carb size I need.
In that link I posted earlier there is a formula that calculates the needed CFM of the carb.
I used the data: 626cc, 0,9 efficiency, max rpm (red line) 9500rpm and 10psi of boost.
This tells me I need a carb @ 158CFM
The Mikuni HSR42 will according to this be too large, a Mikuni HS40 however would be more suitable in that mater.
http://www.nightrider.com/backup2/biketech/carbairflow.htm
I don't think HS40 is a carb that is in production any more, because I cant find new ones offered.
There is a few other Mikuni 40something offered, but CFM values are hard to find on them.
I wonder how much I can deviate from those numbers? (Sorry, is that the right word?)
Would a stock HD carb @ 185 CFB be to much?

Not sure how to connect a pulley yet. A solution outside the alternator cover will be to far out IMO, so I am considering other solutions. One of them is to completely remove the alternator and make a new flywheel with a pulley, and then use an external alternator somewhere. That is the problem, where? Much thinking and planning to do.
If some of you have an opinion on what the author of the article means by:

 "4. a draw-through installation must run downhill at a slight angle continuously from the carburetor to
    prevent fuel from puddling in the supercharger or manifold. "
(http://victorylibrary.com/supercharger/super-carb6.htm)

Does he mean downhill from the carb towards the engine?

[maj]

yes
Look closely at your intake runner size, you need to be small enough to maintain good air velocity, but not so small to be restrictive
see the old Mr Turbo kz or gsxr intakes for examples
also a trick i learnt from an old Carrol Shelby book Speed secrets back when i was playing with Chrysler v8s , a small sharp ridge on the floor can reintroduce puddling fuel to the airstream

[Bugster]

A friend of mine had a Dellorto PHM 40 ND laying around, and he gave it to me. I have no idea of the CFM values on this one, and there is nothing to find. Also sent an enquiry to a Dellorto dealer in UK, Eurocarb ltd, but they had no such values either.
It sat on a 1000cc Sportster he bought some years ago. I think that engine had been used in some sort of racing, so the carb is most likely an upgrade from the stock one.
I see that these carbs are still offered for sale new, and it is easy to get jets and parts.
Cold this be the carb to go for?
It looks to be in good shape, and it was still in full working order when it was taken off that bike.
I have had several vw bug engines with Dellorto DRLA`s, 40 and 45`s, and was at that time very good carbs, and might be still.

[Milwaukee Midget]

Anders -

This is from work that Fordboy did on my 45 mm DCOE Weber, which in a lot of ways, is a clone of a Dellorto.

We're running 1 liter normally aspirated on a head with shared intake ports, so it's not a deep breather.  However, I was really surprised as to how much flow was required of the carburetor in order to get the combination to work.  The engine required more CFM than I thought necessary, and in the case of a supercharged engine, the requirement might be higher.

Carb setup post dyno.

Weber 45DCOE Type 152   New Spanish Mfg'd with steel throttle shaft
Main jets                      185
Air jets                         190
Emulsion tubes              F11
Idle jets                        60F6
Starter jets                    85F9
Starter air jets               150
Pump discharge jets       55
Pump bleedback jet        00
Pump stroke                  8.1mm
Needle & seat                250
Float settings                 8.5mm & 15mm
Idle mixture screws        2 turns out
Idle air balance screws   closed
Main venturis                 40mm modified, 40.2mm net ID
Aux venturis                  4.5 modified
Throttle shaft screws      modified, MB design
Throttle blade angle        90 degrees @ WOT
Throttle speed screw       Adjust for best acceptable idle speed.   2500 rpm minimum for full race engines.
Air inlet horns                48DCOE surface mount, shortened to 28mm O/A length, 48mm ID

This carb flows 270 cfm/barrel @ 28" test pressure, as setup.

For anybody building a BMC 1000/970/1070, if you are using a Weber, and it flows this much air, the above settings would be a good starting point for your testing.
Happy tuning/testing.
 
Fordboy

I might also suggest an SU HS-6 or HD-8.  The metric version of the HS-6 is what I run on my supercharged MGB.  The SU has an advantage in that the fuel flow from the float bowl is regulated by the depression created by air flow through the venturi, which makes it, to a degree, self adjusting to altitude differences.