<VV> LM carbs

[Tony Underwood]

At 05:54 hours 05/02/2006, NicolCS at aol.com wrote:
>Someone suggested that...
>
><snip> Or, plug that venturi air bleed with JB Weld.    That will help a
>lot.  (with leanness)  The lack of a power enrichment circuit can be lived
>with, jet
>up a notch if you worry about too lean. <unsnip>
>
>I doubt that plugging the venturi air bleed would have much if any effect on
>things.


It does for casual "around town" driving where the throttle isn't 
opened much.   The venturi air bleed leans just about everything down 
except secondary metering.    I'd plug it in any event since I'm not 
interested in meeting any smog regs particularly on a car that's 
registered as a vintage vehicle, sees limited use, and doesn't have a 
full compliment of smog pump gear aboard the engine.


>The venturi air bleed affects only idle fuel and off-idle transition.


...which means it influences off-idle slow speed driving.


>  By plugging the jet, you would simply increase the % fuel in the idle and
>transition air/fuel emulsion.


...back to what should be a better mixture so as to avoid hesitation 
and "flat spot" throttle response.

The idle mixture is corrected easily enough with the mixture screw, 
no biggie.


>To compensate and get back to a correct idle
>mixture, you would have to tighten the idle mixture screws and you 
>would be right
>back where you started from.  (That's why smog carbs idle mix is 
>about 3 turns
>out where non smog carbs are only 1 1/2 turns out).
>
>The writer's suggestion to "jet-up" if you are worried about leaness is right
>on the money.
>
>But who says the smog carburetor is "too lean"???


It's not that it's too lean.   It's that the extra air bleed doesn't 
much help the off-idle throttle response in slow-low speed driving 
with a manual gearbox, seems to contribute to jerky idle-off-idle 
driving.   The carbs just don't seem quite as smooth in low throttle 
operation.


>Who says engine
>temperature goes up when mixtures are on the lean end of the 
>"normal" range???  I think
>both of these notions are nothing more than folklore.


The '68-up carbs *will* tend to make the engine run leaner and in 
some instances hotter.   Been there done that.   Of course, some of 
my observations could well have included carbs with worn throttle 
shaft bores which were leaking more than their fair share of air 
etc.  Other factors could have influenced engine operation and temps 
etc but it still seems that if you plug that venturi air bleed you 
tend to resolve some of the idle issues as well as a bit of the 
occasional off-idle stumble that seems to be evident in the later carbs.


Got a couple of '68-'69 Vairs and the original carbs tend to be a 
little bit troublesome...  not a lot but noticeable and 
distinguishable from the '67 and earlier carbs.    I'd find myself 
under the deck lid piddling with the idle mixture screws more often 
than on the other Vairs.

I've plugged that venturi air bleed on the late carbs often in the 
past and not felt bad about it.


>  FVIW, I can't make either one of my EFI cars get hot (or even 
> warm) and they
>are running mixtures FAR leaner than anything GM ever put into a Corvair.

Are they air cooled...?   ;)

The venturi air bleed was designed to coincide with the smog gear on 
cars fitted with the later vintage HV carbs.   It does not serve a 
useful purpose as far as smooth running is concerned, and as such it 
is IMHO useless for my purposes so I dob them with epoxy or JB Weld 
or whatever is on hand and I remain satisfied.    In some instances, 
when I had extra carbs on hand, I'd just replace the late carbs with 
non-bled variants to eliminate the whole thing.    I'm partial to 
'65-'67 carbs in any event, try to collect as many of them as I can, 
not so easy anymore since they're getting kinda scarce.

Again, it's me doing this to my own stuff and such, so I'm not 
advocating everybody with a '69 Vair go to the hardware store and 
start buying JB Weld.



tony..