Inliners International Forums General Discussion Engines multi carb question

I'm in the process of modifying a cylinder head (spare the details here) but the result will be three ports feeding two cylinders each. The plan is to repeat what I have seen done on two engines like mine and put three side draft carbs (Carter YH) on it without a plenum. More or less like the Weber type manifolds for the Chevy 6 type installation where each one feeds two cylinders without a plenum, although not that involved in terms of casting.
The question is... As I understand it from reading here when using a 3x1 manifold you do not use a choke on the two end carbs, or perhaps no choke at all. And also some accomodation has to be made for the poor vacuum signal regarding jetting.
I am assuming (not a good plan) that things are different when you do not have a plenum to allow sharing of one carb by all cylinders. Can I get some general info on how to begin as far as setup (mixture, jetting, choke)?

Thanks very much
Paul T

With out a plenum, its not going to matter which cylinders you choke. As long as one has a fast idle and a choke it should start. Using the center carb is easer to set up since the exhaust heat plate is close. A lot of multi carb setups run the center as primary and the ends as a secondary, thus the center choke. With multi carbs and one large plenum where all three run as one, it will not matter. You are going to be faced with more of a challenge getting three sets of cylinders to run equal.

In your case, you have to be 100% sure all three YH's are the exact same year and model number. With out the plenum to mix all the vapors together, you have to be right on with each mixture.

A flow meter like the Uni-sinc ( http://www.larryscorvair.com/Merchant2/m...egory_Code=LCTO ) will be needed to dial in each carb. Do a e-bay search for carburetor tools or balance tool or what ever you can think of. There are some nice tools used for multi-carb motorcycles that should work really well.

Jetting will be just a guess till you run it a bit, it won't take long to figure out if it rich or lean. Start with what came stock in the carbs or one leaner.

As far as vacuum goes, a properly set up multi carb unit will have just as much vacuum as a single carb set up if both are sized correctly.

Joe

If you are considering building a manifold without a plenum, I believe you will have MAJOR problems using YH's as they are nowhere near large enough for this type manifold on a Chevy 235.

I would highly suggest either building a manifold with a plenum, or why not pick up a 53-54 Corvette unit to start.

If you really want to build the manifold without the plenum, consider using 3 large 2-barrel carbs instead of the 3 single-barrel YH's.

Choking will depend on the type of configuration. For a no-plenum set-up, you cannot run a primary/secondary set-up; all carburetors must work all the time. Using this type setup, if you need a choke on one, then you need a choke on all. If you don't need a choke, then you can have no choke or chokes on any or all. If you have a manifold with a plenum, then you can set the carbs up either as a primary/secondary or 3 primary. If you use the primary/secondary, then a choke, if needed, would be used on the center carb only.

If you do the manifold without the plenum, I have no clue as to where to begin jetting. I really don't think it will matter if you try to use the YH's without the plenum.

Jon.

Dear Paul;

The 6 cyl Vette engine choked all three with a cable and they wouldn't start (cold) W/O it etc.

I ran one on a 252" engine for years in the late 50s early 60s.

Before I bought the cable, I stuffed a rag in the front carb. When it started I ran around & pulled it out before it died.

For some reason the YH won't/can't pump gas in with the throtle after sitting/cold. Was fine after being run a while.

Perhaps Jon can tell us why that is.

Happy trails.

PS: I guess a squirt of "starting fluid" would do it too.

Thanks for the replys.

Jon: No this is not a 235. It's a 161 inch f-head Willys motor. I've seen pics of this modification and am going to give it a try. The three times I've seen pics of it one used the Yh carbs and two used SU's. I've got three YH's so I thought I'd start there.
If It doesn't work I can always change it to fuel injection, perhaps motorcycle injector throttle bodies, and mega-squirt.
I'll know more about how to proceed as I get a little further into this. The machine work starts next week to open up the side of the manifold. (integral to the head).

Thanks again:
Paul T

Paul - on a 161, the (3) YH's may possibly be large enough with a plenumless manifold. And the 3 would probably be too large if you used a plenum.

As Joe stated earlier, all three YH's should be identical as to tag number.

Start with the stock calibration for a baseline, including float levels. Fuel valve orifice values must also be identical. Make absolutely certain that all carburetors are synchronized before making any calibration changes. "Test" calibration changes (NOT PERMANENT) may be made my raising/lowering the fuel level in the bowl from specifications plus/minus 1/16 inch (yes, fuel level IS that critical). These changes would be roughly equivilent to one calibration step. These test changes can help you see what a change might do, without purchasing costly metering rods. With the YH's, recalibration is done by changing rods, not jets; as changing jets will not provide a uniform percentage change (calibration is jet area minus dynamic rod area). YH metering rods are not interchangable with other Carter series, so calibration parts will require custom machining.

With an engine this small, leave the choke plates in place; whether or not you use them for a choke. If you opt not to use the chokes, affix the choke plates in the open position. The presence of the choke plates will act as a "straightening vane" to help eliminate eddy currents in the air entering the air intake; thus improving flow.

Sounds like an interesting project. Keep us posted.

Jon.

Jon:

Thank you very much for the information. I realize I'm in for an uphill battle, but information like that will certainly make it a little less of a struggle.

Thank you again. And thank you to all the board members here. This is a fantastic resource!

Paul T

Do not overlook motorcycle carbs when sourcing sidedrafts. Most Harley models are 80"-96" single carb on a common plenum. Many people change out the OEM carb for a performance aftermarket carb. Many bike shops will have these as take offs that are inexpensive. The 1991-2006 models used a very good CV carb.