motorcycle carbs on 250?

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i have mildly discussed this in another thread but i dont want to impose on someone elses thread so i made this one.

i am planning on modifying my intake, but would like to get better performance and mileage out of the deal. I was wondering how 2 motorcycle carbs would do on a 250.

1. the 28mm motorcycle carb has about 141 CFM so if i used 2 i would be getting bout 280, which is more flow than my stock 1BBL carb wich has bout 190 CFM

2. would the jets need to be resized (would it help if i found the jet size of the 28mm?)

3. would gas mileage be better with the bike carbs?

4. ive heard of 6 bike carbs, 1 per intake, and that would be great for performance but with that many carbs wouldnt gas mileage be bout the equivalent of 2 2BBL carbs?

5. anybody try anything like this yet?
 
Probably let Jack take point on the 6 carb deal, but here's something I can tell you-

The MGC and Austin Healey 2.9L sixes used two HS6 SU carbs, with
1 3/4" throttle blades (44 mm). Suggested sizes for Weber DCOE's for the same engine is a 40mm throttle w/ a 36mm choke, or a 45mm w/ the same hhoke size for each cylinder. Note that this isoltated runner needs greater choke area than multiple cylinders pulling off a plenum.

I think Jack is using six 36mm CV carbs (probably Mikuni's off a GSXR)- he's probably near the bottom end of carb size for an IR setup. he's also running these on a 200

Jetting will probably need to be tuned, but it depends on the type of carbs these are. Are they earlier needle/slide type, where the throotle cable actuates the side, or are they CV type, where the throttle actuates a throttle valve and the slide moves in response to the flow requirement?

If these are needle/slide type, well, I was running four 28mm Keheins on a 590cc mtotrbike- I'm guessing you may be undercarbed with this setup. If these are CV carbs, I've got to believe your pretty seriously undercarbing. Note that there's nothing wrong with undercarbing, but it will limit your ability to rev. Probably still a bunch better than the sinle barrel.
 
The reason independent runner inlets need great carburation is that the atomised spray never touches the sides of the walls. It is very responsive to throttle movement. Any mixture forms a nubulae-like cloud which streaches out like a tongue from the jet. Large openings make a well atomised spary which makes bulk horspower.

A 250 I6 has 682 cc per cylinder, and revs to 5000 rpm. It needs between 35 to 40 mm of carb venturi at wide open throttle to run right.

A 200 I6 has 545 cc per cylinder, and revs to about 5000 to 6000 rpm if modified, and needs between 30 and 35 mm of carb venturi at wide open throttle to run right.

LinkPhoto



On a 1-bbl feeding 6 cylinders, the wetted perimater is much greater, and any large carb just wets the walls of the intake manifold. There is no way a well atomised cloud can form, no hope of it gliding through a selection of 90 degree bends without drop its load, and raining on the intake walls, forming puddles which prevent power.

If you use six very small carbs on a 250, the venturi will be open most of the time, and the needle won't discharge the fuel properly. You'll need to run a very stiff spring to make sure the venturis stay small, and the air speed will be very high, too high for good power.
 
Are there any benifits to having bike carbs though. sounds like I'd run into a few problems along the way. I'm pretty much thinking about the bike carbs for there simplicity and (not sure) maybe better mileage.
I have heard of buying plans for high mileage carbs, but something like that i would need a shop to build it for me, but would it be worth the money if i could get around 80mpg? it might end up costing over 1000.00 bucks (which i would be willing to fork out if performance and quality and that type of mileage were to be had). anyone ever think about these high mileage carbs. supposedly y can buy the plans, but u cant distribute the product.

I got a good true story actually. A guy in town bought a caddilac and was wondering why his gas needle wasnt going down (had a V8) he had it checked out and it turns out he had a experimental carb of something on it which got around 100 MPG's. Anyways, The company that sold it to him wanted it back cause he wasnt actually supposed to have it (bet the guy who sold it to him is getting a whooping) and they are giving him a brand new caddy every year (trade in type deal) for the next ten years. I would love to be him.
 
Bike carbs will probably be too small in any configuration you can think of on a 250.

If you want to try something with side drafts, you should look for a pair of H8 SU's (2" throttles) or H6 SU's (1 3/4" throttles). Triple H4's (1.5") might would work also, but are probalby smallish.
 
what type of vehicles would those carbs come off of though? Definitely dont want to buy new, but rebuild is ok.
 
thanks Jack, i'll see if i find any at the wreckers this weekend. over there tey're only 20.00 each
 
"A guy in town bought a caddilac and was wondering why his gas needle wasnt going down (had a V he had it checked out and it turns out he had a experimental carb of something on it which got around 100 MPG's. Anyways, The company that sold it to him wanted it back cause he wasnt actually supposed to have it (bet the guy who sold it to him is getting a whooping) and they are giving him a brand new caddy every year (trade in type deal) for the next ten years."

I first heard that story about a Studebaker in 1955.

No.
 
Here's a pair of HS-8s: http://www.sumidel.com/images/c20.jpg

The "IF" suffixes are to be avoided if possible: HIF-6, HIF-8. They are integrated fuel bowl carbs and can trap/hold the heat more in the tight confines of an engine bay. Also the separate fuel bowls of HS series carbs can be swivelled so that the bowl is level even if the carb is tilted.

Everybody says "SU's are a simple carb". This is physically true, but the unit is so different to conventional downdraught that you will be learning a new subject, in essence. There's a good little roundup of background here.

See what you find in the yards! Might be a pearler.

Cheers, Adam.
 
Bike carbs can work very well even if small by using the log as a comunicator between cylinders. Do it right you have the best of both worlds.
 
I don't think it's likely you will find any SU's in the junkyard. If you do find a yard littered with Jag XKE's and old Healey's, let me know, I'll be on the way.... :)
 
Motorcycle carbs (and some Webers) have no high-vacuum cruise circuit. This means that you have 2 choices:
1. rich jetting and poor mileage
2. lean jetting and poor response + high chamber temperature
IR carbs are intended to be used without a plenum, and rely on tract reversion pulses for enrichment, so the fuel delivery curve is:
1. fixed to a greater extent than appears
2. self-correcting to some degree in proper apps
3. not going to work well on a log or common manifold

Mcycle slide carbs are also very expensive to jet unless you get the right size slide - sildes are $35 or so EACH, and you may need to try more than 1. The other big jet (NJ) is about $10. each, and you'll definitely need more than 1 size. The small jets are about $3.
CV (diaphragm) carbs are much cheaper to work with, since the slide isn't a tuning component.

If I were going to consider multiple side-draft carbs I'd try SU, since the original apps are very close - follow Austin-Healey Mk II specs for a start (3.0L = 183" I6). TR4 (2.2L) is probably also close - would be 201" if 6 cyl.
There really good info around on these (Vizard), and the parts are cheap once you have the carbs.
 
SU's will be difficult to find in a JY, but there are some vehicles out there that have sidedraft Strombergs that would work. The CD175 (1 3/4" throttle) is pretty common and can be found on a variety of Volvo's, Triumph's, and 70's Jaguar XJ6's, among other things.

They have a couple of idiosyncrosies that do not endear them to some tuners, like the temperature compensators and the floating needles, but basically they operate much the same as SU's and being a constant velocity design, are somewhat forgiving in terms of size and jetting.

They turn up on ebay all the time.
 
Ebay is where I found my set of SU’s. They were removed from an early 240Z car and included the complete set of parts for install. This includes the manifolds (which I have not found a way to use, yet), throttle linkage, air cleaner box and crossover tube. Keep in mind that the 240Z is opposite of the 200 so this setup is installed on the drivers side of the block.

I think I have this solved but will need a good machinist to make this work. If you guys are interested I will post my ideas on this solution.

Have fun, Ric.
 
yeah Ric, post your ideas, it might spark a new fire, anything new is good for discussion. Thanks everyone for the help, and if i cant find any good used sidedraft carbs I'll figgure something out then.

Cheers, dave
 
Ok, here it is. This is a huge file so I can only post it for a while. The drawing is done in Visio which is now a Microsoft product. I was surprised to see that IE would read the file .vsd type so it should open in Exploder. Page 3 and 4 are what I have in mind for the mod to the log. This dosn't show the carb casting it is just a concept drawing.

Let me know if you can't view it and I will try something different.

http://home.comcast.net/~tuckerjr3/200intakelog.vsd
 
It says I cannot view XML input using XSL style sheet. watever that means.
 
The Hitachi SU's are slightly different than the Brit SU's, but jets and needles pretty much interchange. Be careful (if you go for carbs off a Datsun Z car that you get round top and not tuna can type. The differences are obvious
 
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