twin carbies with adjustable mains

[clivetitchener]

hi all i have a twin carbie set up , standard stromberg bxov-1 but these have been fitted with an adjustable main jet (a brass cylinder with 2 adjusters and i would guess a tappered pin ) question 1 are these adjusters common  and question 2 how do i set the carbies up and do i still have to use a suitable jet size i hope some one  can give advice regards clive

[Harv]

Clive,

I'm in the process of pulling together some info for a carburettion document ( ), so will comment from the info I have to hand.

Adjustable mains are common, and you can still get them from places like American Auto Parts in Sydney. The main jet and main jet plug are removed. A special jet (that comes with the adjustable main) replaces the main jet. It has a tapered seat (shoulder) to suit the needle of the adjustable main (standard mains are square shouldered). The adjustable main (a glorified brass needle valve with a rubber o-ring) replaces the main jet plug.

Most material  I have read indicates that adjustable mains in twins and triples are not recomended. The hard bit is trying to get the mixtures equal in both carbs, which is why most enthusiasts recomend non-adjustable mains. To set up adjustable jets, the following may help:
a) tune them the same way as fixed jets, but "screw them in" the same amount on both jets. This is pretty rough but, as you are adjusting a poofteenth of a turn on each one.
b) tune them and then measure the exhaust gas (for CO) on both front and rear exhaust runners, then adjust for even CO. This can be bloody hard as few exhaust runners (extractor pipes) allow you to "tap in" - a hole drilled/nut backwelded may provide a sampling port, with a short plug screwed in when the port is not in use.
c) tune them and then measure exhaust gas temperature and tune for even temperature. Again, hard to measure if the front and rear exhaust runners are not sampleable, but an infrared temperature gun may give a rough idea.
d) I've heard some guys disconnect the carb linkage, screwing one jet in until the motor runs rough at cruise throttle, then back it off, then do the same with the other carb before reconnecting. I'm not sure I like the science of doing this, as it sets the engine up to run off one carb (and should be overly rich when both are reconnected.

If they were mine, I'd recomend fixed jets.

Cheers,
Harv

[Johns]

Don't agree.

I suspect that as delivered from the factory as a completely unmolested 132 ci the fixed main is fine. From my experience, as soon as you do a rebuild everything changes and trying to tune out flatspots becomes difficult.

I have rebuilt both a stock grey and warm one with twin carbies. In the case of the stockie which is simply a 40" overbore we couldnt tune out a serious flatspot. Same deal with the warm grey I built for my FC wagon with twin strommies.

Ultimately, I put both cars on the dyno and we used adjustable mains to fine tune them. This cured the flat spots, particularly the twin carb car. Problem is, I think, there simply isnt enough adjustment with a fixed main once the engine has been rebored/played with.

Cheers
John

[Harv]

John,

I guess some of the difficulty in tuning mains for a twin-carb setup is that the standard 0.051" jet is the smallest Holden offered other than the 0.050" on humpies - to go lower (which you need to do on twins) you can't pull them from another Holden - they have to be aftermarket. Having said that, places like American autos stock 0.040", 0.043", 0.044", 0.045" and 0.049" mains. Was the lack of adjustment you had with the fixed mains because of the above sizings being wrong, or were you tuning with just the one sized jet?

Flat spots on twin carbs are often associated with the lower venturi pressure. By adding a second carburetor, the “hole” to allow air into the engine has effectively doubled in size. Whilst this is good for air flow, it can do some interesting things inside the carburetor, where pressure is critical. By doubling the “hole”, the pressure in the venturi has reduced. Of note:
• The idle system, which operates below the venturi, still sees the same pressure and acts similarly to a single carburetor.
• The main metering system takes vacuum signal from inside the booster venturi, which in turn takes signal from the main venturi. With lower pressure means that less fuel is taken per carburetor (though there are now two carburetors feeding). The slower air speed also means that the fuel delivered is not atomized as well as a single carburetor.
• The accelerator system sees no difference in having two (or three) carburetors, as it is purely mechanical and not vacuum driven.
• The power system sees no difference in having two (or three) carburetors, as it takes it’s signal from the manifold pressure (just like the idle system).
• The choke system sees no difference in having two carburetors as it operates above the venturi.
As can be seen from the above, running two carburetors can have an impact on the main metering system. This is often seen as good idle, then moderate initial acceleration (as the accelerator system works), then very sluggish acceleration (flatspot) while the main metering system gets the engine up to speed, followed by decent performance as the main system takes over. If the venturi diameter is reduced, the pressure issue goes away, and the sluggish performance is removed. Holden recognized this when it fitted twin carburetors to the HD and HR Holden red X-2 motors – the single BXV-2 carburetor with a 15/32” venturi was replaced with twin BXUV-2 carburetors with smaller 13/32” venturis. Unfortunately, for the smaller capacity grey motors the BXOV-1 carburetor has no locally available smaller-venturi alternative to use when fitting twin and triple carburetors. Were you running BXOV-1 twins (bad enough reduction in venturi signal) or the larger BXV-2/BVUV-2s (even worse reduction)?

One way however to improve the sluggish performance is to fit venturi sleeves (sometimes referred to as venturi restrictors), Venturi sleeves increase air speed through the venturi, getting the good vacuum signal back to the main metering system and helping fuel atomisation. This removes the sluggish low-down acceleration. Venturi sleeves were once anecdotally made by an FX/FJ enthusiast to fit the BXOV-1 carburetor, though enquiry to find the person has come up empty. Venturi sleeves however are available for the Holley 7448 carburettor (the “Holley 350” discussed later in this document). Redline Performance venturi sleeves are available from American Auto Parts (part number 14-35) and Barnes Performance (part number BP14-35). It must be recognized that the sleeves are made for a venturi diameter of 13/16” , and must be filed back (so they close up more) when fitting to the 11/32” BXOV-1 venturi. The filing appears to be extensive – almost ½”needs to be removed from the circumference. The Redline Performance venturi sleeves are 0.035” thick (~1/32”), and if gapped correctly will change a BXOV-1 venturi diameter from 11/32” diameter to 0.96” diameter (~31/32”).
Note also that in fitting venturi sleeves, the signal sent to the main metering system is much stronger (i.e. the main metering jet gets “sucked on” harder. This can lead to the main metering system running rich. When fitting venturi sleeves, the main metering jets will need to be decreased in size to account for this. Were the twins fitted with venturi sleeves?

When the engine was tuned on the dyno, what process was used to ensure both carbs were running equal mixtures in the main circuits - I assume it was by "turning the screws out the same number of turns" on the adjustable mains, but could be wrong.

Apologies in advance for the epic novel above - I'm just really curious to understand the overall system you had for the twins.

Cheers,
Harv

[clivetitchener]

harv and john thanks for the replays what an interesting subject i think i will end up having a go at both i have started the engine and have got some reasnable results with the adjustable  mains a steady tick over and good response i guess it will have to be a trial and error period for me , i know have to source some jets and the caps that cover the jets  thank you both for your replays and i will let you know how i get on