SU HS float bowl location...

[Nunyas]

When I say float bowl location, I'm not referring to the angle of the bowl relative to the carb. Rather, I'm referring to the location of the bowl fore or aft of the carb relative to the car. I was reading through my carb manual and it made reference to the bowl placement for certian installations. In particular, it mentioned placement of the float bowl forward of the carb body to prevent "starvation" (over leaning) on inclines or under hard acceleration, and to prevent over richening on declines or under hard deceleration.

I've notice that the "typical" twin SU setup positions the bowls both ways. The front carb has the bowl up front and the rear carb has the bowl behind. I suppose this evens out the changes in richness thanks to the balance pipe/tube.

Any of you have any experience with this stuff? I'm planning to switch to a single HS-6, and noticed that the carb in my posession will end up with the bowl aft of the carb body once installed on the car. I've thought about relocating the float bowl to the other side of the carb body, but the throttle and choke linkage looks like it might be in the way, and there doesn't seem to be any provisions to move the linkage to the opposite side. I do drive some relatively steep roads on the weekends, and am somewhat concerned on whether or not this might become a potential problem to contend with.

I appreciate any and all SU knowlege you folks can contribute to this topic.

 

[Bruce74B]

I don't think you need to worry about the inclines causeing fuel starvation on a paved street. Now I have had some problems with Jeeps on a couple of hills that I had no business being on...if you keep the RPMs up though there should be no problem even then. And I think that the acceleration of a B would require high enough RPM's before it would starve!

Bruce /ubbthreads/images/graemlins/cheers.gif

 

[DrEntropy]

Moot point AFAIK. Bowl supplies fuel to the jet from the bottom, needle and seat comtrolled by float. which is at an approximate 45 degrees to angle of car direction... if it fuel starves due to any attitude but *inverted,* ya get yer MONEY BACK!!! /ubbthreads/images/graemlins/jester.gif /ubbthreads/images/graemlins/cheers.gif

 

[Nunyas]

but but but... doesn't the design of the HS line of SUs set the fuel level in the jet to just below the top of the jet... and if going up hill with the bowl aft of the carb wouldn't that allow the fuel level in the jet to drop... there by causing a leaner condition? and the opposite for traveling down hill?


Or is it that the changes in richness due to pitch on paved roads are well within acceptible mixture range that it shouldn't really matter?

 

[DrEntropy]

That thing is siphoning fuel past the needle from a venturi with enuff force to suck a golf ball thru a garden hose! The needle rarely rises further than half way... I really don't think it'd be an issue unless climbing Pike's Peak straight up at 80 MPH is in your plans.

 

[Nunyas]

hmmm... Pike's Peak... now there's an idea... /ubbthreads/images/graemlins/grin.gif

Ok, ok, ok, point taken. /ubbthreads/images/graemlins/smirk.gif

I will have to upgrade the piston spring to an 8oz unit though for this single carb application. In my cross referencin' lastnight I found the spring that came with the carb should be a 4.5oz unit... I guess that would let the piston ride too high at any given steady engine speed making for a mixture that's 'too lean'. So, in what I've learned about SU HS model carbs so far, is that the mixture you get is determined by more than just the needle selection. Piston spring selection can be used to enrichen or weaken the fuel mixture. /ubbthreads/images/graemlins/smirk.gif

 

[DrEntropy]

Ahem... if the piston rises "too high" it enrichens the mix. Needle is narrower at higher piston travel... allowing more fuel past the jet/needle interface. Yeah, the spring is a contributor, but more in the "fine tuning" range than the needle. By all means get the "stronger" spring, and try it to do some comparative "plug cuts", but from what you've learned and posted so far, needles are more an issue.

Remember: it's "all-of-a piece" with the silly (SU) things. /ubbthreads/images/graemlins/wink.gif

 

[jlaird]

/ubbthreads/images/graemlins/lol.gif Yep.

 

[Nunyas]

true doc, but the theory of the piston height versus mixture bit as explained by the documentation I've seen thus far is almost contrary to what you've said. Yes the mixture *is* richer when the piston is higher, *but* the vacuum in the venturi varies with piston height, butter fly position, engine speed, and let's not forget the amount of flow through the filter.

The way I've read the documentation I've available thus far is; when you mash the loud pedal this allows manifold pressure/vacuum to reach the piston and venturi. Because the piston is still sitting at it's lower position the vacuum in the venturi area is stronger and draws even more fuel through the jet (allowing SUs to fore go the need for "accelerator pumps/jets"). As the piston rises to the proper height, the vacuum felt in the venturi weakens, and the amount of fuel drawn lowers. However, given the design of the needles more fuel *does* flow at speed than at idle, but it is less than the amount of fuel drawn before the piston settled into its new height.

So, if one wishes to increase the fuel richness in the mixture there are two ways to go about this.

1) If the given needle profile desired (identifiable by the mixture being too lean throughout the entire engine speed spectrum), then one can use a stronger spring to increase the velocity of the air (and therefore vacuum) across the venturi area. A greater vacuum in the venturi would in effect cause it to be richer than before.

2) Change the needle profile if the piston spring in use is the desired. We all know different needles can be chosen for richer or leaner mixtures.

After reading what part the spring played in the functioning of the SU carbs, I saw the light in how the same carb with the same needle could be deployed in a twin configuration with a 1800cc mill (your shed) AND in a single configuration with a 1800cc mill (Morris Marina).

In the twin carb configuration a 4.5oz spring was used, because the each carb was feeding 2 cylinders, and therefore a lower overall manifold vacuum was being felt by each carb. Therefore a weaker spring to allow the piston ride higher at any given engine speed.

In the single carb configuration, the carb is feeding all 4 cylinders. Resulting in the carb feeling a greater manifold vacuum. So the stronger spring was deployed to prevent the piston from rising too high. However, since the vacuum across the venturi is greater (with the stronger spring) the fuel richness will also be greater at any given speed, when compared to the same carb setup using a weaker spring in the same application...

Yer right, it definitely 'all-of-a-piece', but being able to change the spring out for different strengths (I believe there's 4 to 6 different strength springs to choose from) allows for a greater amount of tune ability than just changing out needles... without the right spring even the richest available needle the mixture could still be too weak.

Setting a SU up could easily get as complicated as setting up a DCOE if one doesn't have the know how for either.

'course I'm just now learning this stuff (should have picked it up for the ZS... hind sight's 20/20 and all that), so I'm no expert and will certianly give way in the interest of learning right /ubbthreads/images/graemlins/smirk.gif

 

[jlaird]

That was good stuff, thanks.

 

[DrEntropy]

Nunyas Wrote: [ QUOTE ]
...In the single carb configuration, the carb is feeding all 4 cylinders. Resulting in the carb feeling a greater manifold vacuum. So the stronger spring was deployed to prevent the piston from rising too high. However, since the vacuum across the venturi is greater (with the stronger spring) the fuel richness will also be greater at any given speed, when compared to the same carb setup using a weaker spring in the same application...

[/ QUOTE ]

No argument as to effect overall, BUT again: "fine tuning". It's necessary fer sure, to get it as close to 13:1 as possible across the range, the spring will get you to that 10/10ths of the process. Depression on initial throttle opening will be whatever vacuum is available both to venturi throat and chamber, needle position/profile governs fuel volume, and yes the flow rate will increase/decrease with velocity of the demand through the throat... which will also continue to raise the piston. Spring affects piston final "rise" mostly, and transition rates to a lesser extent than the damper and viscosity of the "oil" used: With S L O W throttle position changes the spring's effect is almost negligable. At each "steady" state the spring is the final determinant as to needle "stage" in the jet. You *can* get one to run "passably" without the spring, with some repositioning of the jet height, needle position in the piston and juggling damper oils. Not *great* but it'll get ya home /ubbthreads/images/graemlins/wink.gif

This is great stuff to bandy about, the theory far "out-finesses" the visceral kick of "jus' put yer boot in it!" and has us actually ON TOPIC!

And as for bowl location, I'll still revert to : Moot point. /ubbthreads/images/graemlins/tongue.gif

 

[Nunyas]

sooooo... what yer sayin' is I need to be doing 10second 1/4 mile times for the location of the float bowl to even matter... right? /ubbthreads/images/graemlins/grin.gif

And since I'm hard pressed to get under 10 seconds for 0-60MPH times, the point is even more moot... right? /ubbthreads/images/graemlins/grin.gif

 

[DrEntropy]

...uh... pretty much.

I DO expect your acceleration times to 60 will improve. I'm MUCH more concerned as to how you can vent the float bowl, frankly. Check around for Devcon Plastics products. They're DaBomb for epoxies... AND they're English!

 

[Nunyas]

nice.... I'll peel me eyes for them/it /ubbthreads/images/graemlins/thumbsup.gif

*was* beginning to wonder if good 'ole JB would be good enough... heh