has anybody modified linkage on motorcycle carbs to use in a multi-carb setup? i have a '53 buick straight 8. i wanted to do something a little different, and build an intake for 4 single carbs from a Harley. the motor has 4 siamesed intake ports, so i am going to try to hang a carb on each port basically. it will look like 4 side-drafts! if it runs awful, i'll pull it and sell the carbs on ebay! lol. i'm not asking about sizing, but rather looking for some ideas on how to modify the pull-push cable linkage on the 4 carbs to work with a single pull cable (or something else?) i'm using a spoon-style pedal with a cable linkage currently.
The push-pull linkage won't be a problem; the 'return' cable was added as a safety feature in the early 80s and virtually all motorcycle carbs have built-in return springs. And where multiple carbs are used, hard mechanical linkage is used to 'gang' the carbs together and a single cable is attached to that. But where you will have problems is fuel pressure. Motorcycle carbs are designed for gravity-fed fuel only, and applying fuel pressure to them will easily overwhelm the float valves. If you're looking at Harley carbs, there is an outfit that sells modified Harley carbs as an 'upgrade' for Suzuki Samurais. The carb has a special float valve, along with a special fuel regulator to keep pressure low enough. A web search should turn up more info, but they're not all that cheap IIRC and are useless on a bike after being modified.
I'll also mention that using multiple CV-type carbs will require that you synchronize them or you'll have issues with poor/erratic idle among other problems. You'll need a manifold vacuum nipple at each carb, a set of 'carb stix' or vacuum gauge with multiple connections, and some way of adjusting each carb individually to each other carb. This is why they have hard linkage between the carbs...
I'd put in balancing tubes between the 4 intakes to even out the pulses, and help with the synchronizing. ( see pics ) And for the fuel pressure, I wonder if you could make a float bowl that sits over the carbs ( out of a Holley 4BBL floatbowl, for example ) that gravity feeds into the bike carbs. For the linkage, I'd use a rotating shaft with adjustable pull rods ( with a left and right heim joint ) to each carb, similar to what they use on twin Weber Bug engines. Seems to me, if you do this right, the 'ol Buick could really benefit from this...
My Plymouth with Suzuki TBs for linkage thoughts. And a intake I made for a Buick. Close up of master rod linkage as used on the Buick.
The 'balance tubes' aren't a help, they make it harder to sync the carbs. I've got one motorcycle that has a factory 'balance tube' between the four carbs machined into the head, and you have to plug it with a special tool in order to do a carb sync. Most owners just buy the tool, install it and leave it in... Makes no difference in how the bike runs. The manufacturer abandoned the idea a few years later. And I thought about some sort of 'surge tank' set-up, but you'd have to make sure it can't pressurize.
looks like the tank would be the same as recommended for use of Hilborn injection on the street.Pressurizing is not an issue. Here is yet another linkage i did using the same master rod and links deal. It's cheap. Easy to do and does not require everything be perfectly in line.
I guess I'm less concerned about the "gravity feed" nature of motorcycle carbs than previous posters are. I don't claim to be any expert of Harleys, but I've rebuilt dozens of Japanese, Italian and German motorcycle carbs. While they may be designed for gravity feed, the needle and seat that comprise the fuel shutoff valves are pretty much identical to the needles and seats on every automotive carburetor I've ever worked on. I doubt the difference between gravity feed and pumped fuel will make any difference. Just don't use a high pressure fuel pump meant for fuel injection systems and you should be fine. Oh, and don't forget to post up photos of your project!
If you are proposing to use the HD Keihein CV carb be aware there are several different versions of that carb.The one you want is the 40mm version that was used on big twins from 1990 to (on a few models) 2006.Try and get carbs with matching part numbers.They are IME as good a carb as you will find and will work fine as is with 3-4 lbs of fuel pressure.Be advised that they have a plugged tower in the bottom of the carb body.Under the plug is the pilot fuel screw. EPA required it to be nonadjustable.Dont buy any carbs that have this plug pulled out.Nine times out of ten the screw will have been run in hard and the outlet in the carb body deformed.YOU pull the plugs and start the setting at 3 turns out.
The Jalopy Journal
