Carburetor Advice

[ben23f]

Oh man had to do some research on carburetors after all that info but its great thanks heaps!
By looking at the comparison numbers above the larger venturi suggests higher cfm rate?? So maybe the Hitachi is better rated for the P Engine...

[Johnny Roadkill]

opinion only here - reasonable minds may differ...

yep, the venturi is the flow restrictor that ultimately limits maximum power in the carb - and all else being equal a carburettor with a larger venturi will outflow a carb with a smaller one. This gets a bit complicated since all carbs differ in their design, and also in they way cfm is measured varies between manufacturers.

Flow rate is dependent on pressure difference, and while most seem to use 3" of mercury ( about 0.1 bar ) to flow rate their carbs, I believe Holley uses 1.5" of mercury - which means the 350 Holley will flow more than 350cfm if measured using the same standard as the Stromberg - but not 700cfm, since the flow-versus-pressure curve is not linear. Without getting into nerdy discussions about boundary layers and reynolds numbers, this is why the carb needs to be able to outflow the engine's requirements - the venturi "appears" to shrink as flow rates increase simply because of the fact air doesn't like being forced down a tube. How much headroom you build in depends on the character of the engine. For a lazy P motor there's no need to go overboard here i.m.h.o., but for a high revving race motor you don't want the carb(s) appearing as a flow restriction as you approach maximum power, so bigger is often better - you don't care about fuel economy or low-end driveability in a racer anyway as long as it can pull itself out of the slowest corner on the track.

Tuned properly, the BX would probably work fine until you want to punch through a mud puddle, climb a sand dune or hit a steep hill on the open road - then the motor may feel a bit lethargic and be slow to climb the rev range. But you'd probably have a more responsive, economical engine around town. I figure the Hitachi VHA should flow around 300cfm based on the venturi size and comparing to similar sized carbs, but that's really a guesstimate only, and the cfm rating doesn't say anything about the carb's ability to accurately add fuel across the rev range.

Getting back to your initial post, basically driveability and performance are kinda working against each other, so the carb choice is always a compromise between the two. A twin venturi carb goes some way to improving that compromise since each venturi is in a happier place at low rpm / low flow than a single big venturi with the same cross sectional area because of the non-linear flow curve thing. A twin venturi carb with a vacuum-operated secondary is nice because it behaves like a small carb until the engine "pulls" the secondary open under load. If you don't want to limit top end power, then it's probably safe to say don't go for a carb smaller than the original Hitachi VHA.

I think experience is everything here, so maybe others who've tried various carbs would be better able to help out choosing the best option...anyone ???

cheers,
D

[RiverPatrol]

Okay, so I have a bit of experience to add to this topic now.

My '79 ute has a Stromberg carb on it. I'm not sure which model it is - if ferdy would chime in, he knows. Here are pictures of it:





I had the chance to really drive this today for the first time. I had to get the wheels balanced because it shook so bad at 45, so I had never driven it any faster than that. Coming back from the tire shop I got a real feel for the Stromberg carb; it is pretty gutless when climbing, just like Johnny Roadkill said. Getting back to my place is a very gradual climb, not even visible. I had the accelerator to the floor and at times I was actually losing speed.

So, now I think I am in need of a VHA carb that has the connector for the cable type throttle linkage, or even just the connector itself.

[250g60]

Hi Ben,
Here is some Stromberg info i have, hope it helps. Ive had BX & WW Strombergs on my HD Holden, had better power with the WW (two barrel) and economy was about the same as the BX (single barrel) except when you were in to it. XC Falcons also had single barrel Stromberg that should be jetted similar to a G60

[sleepyzzz]

Okay, so I have a bit of experience to add to this topic now.

My '79 ute has a Stromberg carb on it. I'm not sure which model it is - if ferdy would chime in, he knows. Here are pictures of it:





I had the chance to really drive this today for the first time. I had to get the wheels balanced because it shook so bad at 45, so I had never driven it any faster than that. Coming back from the tire shop I got a real feel for the Stromberg carb; it is pretty gutless when climbing, just like Johnny Roadkill said. Getting back to my place is a very gradual climb, not even visible. I had the accelerator to the floor and at times I was actually losing speed.

So, now I think I am in need of a VHA carb that has the connector for the cable type throttle linkage, or even just the connector itself.

did you find a carb yet? seems like i might have gotten a spare with my Patrol. will dig through the parts box out in the garage today and see.

[RiverPatrol]

No I haven't, I'm still looking. Post up a picture when you find it, then I can see if it has the correct type of linkage.

[sleepyzzz]

No I haven't, I'm still looking. Post up a picture when you find it, then I can see if it has the correct type of linkage.

Leslie, if you don't mind, pm your cell and i will send it to you in a text sorry for being lazy but this is quicker and easier for me.

[RiverPatrol]

PM sent.

[sleepyzzz]

PM sent.

pictures sent just a little bit ago hope it is what you need

[RiverPatrol]

PM sent.

pictures sent just a little bit ago hope it is what you need

Got the pics, thanks Butch! The carb won't suit my needs tho, so I'm still looking for a VHA carb with the type of linkage to connect to a throttle cable instead of rods. If some has just the linkage I may be able to put it on an early VHA carb.