Carburetor mysteries

[Keith Andrews]

Butterfly bushes...lets take a large engine, say a small block chevy, 5.7 l. and it has a 4 barrel carb on it...thats 4 butterfly bushes....and the secondary bushes hardly wear as they are very rarely opened/used....so lets say 3 bushes to wear.
Now take a 1 L jowett engine that has 2 bushes.
These bushes are all damn near the same size and wear...
So comparing the above, the worn bushes in relation to thevolume of air/fuel mixture is huge compared to a 350Ci V8
And it doesnt take much to make a reasonable effect on a 350Ci engine....170K miles assuming some idiot hasnt put on a return spring that is too heavy....and that is very common.
That return spring being to strong..loads up the butterfly bush in ondirection, and since the shafts tend to wear more than the bushes, one has an ovail hole effect which can confuse tuning even more , espec when going from the carb idle circuits to power/cruise circuits.

Carbs are 3 separate independant carbs in one...a idle, a power and a cruise....get a vaccuum leak and one then has the carb runing on 2 circuits at once, say the idle and cruise at idle.

What Im saying here is a vaccuum leak...which on old engines more often than not is the worn carb butterflys really screw things up, AND the smaller the engine the more screwed up and the more compensation patch up backyard cowboy tuning has to take place.
Worn bushs , dont waste your time, fix and then go from there.

[george garside]

I cannot recollect seeing a zenith withthe copper tube etc & wonder if it is so;me crafty one off mod - perhaps somebody with a CA braddford can comment on this?

I am pretty sure the carb is off a aCA brdford as on the zenith application list I have nothing else uses the VEFH ( I athink the H is for horizontal - most zenith V types were vertical as on Jav & CB?CC Bradfords and of course other makes of vehicle.

AThe jet sizes in the ebay carb are a bit puzzling as aathe original settings were
choke tube 26
main85
comp70
slow 50
progression120

I wonder if the fat that the ebay carb has a much smaller main is something to do with having the extra jet & copper tube .Does the extra jet have a size stamped on it and exactly where does the copper tube etc open into the venturi - from the photo it looks as if it may come out in a position where the throttle valve blocks it when throttle closed???? \just curious!!

george

[AlanBartlett]

The copper tube, comes out of the top of the carb down around the side and on to one side of the ventri tube under neath, both ends of this tube, are open, the ventri tube leads up to through the middle of the carb, and comes out just in front of the throttle spindle, according to my book its part of the auto matic starting device, more problematic at the moment i must say. now looking at my carb and the ebay carb the hole for the copper tube in the top comes out of the side and the hole inside the top of the bowl area is also presnt on mine but doesn't exit to the outside. Ive taken the copper pipe off and the venturi pipe off, to see if i can run it alone, but seems not, though with both ends of the venturi blocked up the idle does slow down dramatically as it seems to be getting too much air, which i think could be a good thing. according to the manual its like a seperate carb, but with bits missing i fail to see how. ideally if i could get this one run properly id use this one as everything seems in better shape than mine.

[george garside]

have dug deep and discovered that the export CA van was fitted with a starting device rather than a strangler as used on home models (same as pre war). This could have been because the export version was fitted with some sort of air filter (as was the export CB/CC.( With the pre war jowett stangler an air filter could not be attached). The only snag with this theory is that apparently the export CA van apparently used a zenith 30 VEH rather than a 30 VEFH. ( I have no idea what the F stands for)

However if my theory is correct the copper tube & its bits & bobs will be the starting device which may well follow standard Zenith practice on other V type carbs.

george

[AlanBartlett]

On further investigations, many numerous differences have come to light, my plan was to use the good bits from this "ebay carb" and switch them onto my carb. but after stripping more of it down i have found more differences though i should have seen this coming but other than the added auto starting gear on the carb the overall appearance was identical to mine.
The throttle chamber are both different sizes, the studs which hold the bowel beaker assemble( middle of the carb if youve got the throttle flap at one end and the air strangler) are difference size, plus the casting holes for the studs are smaller diameter. So i cannot fit my longer studs to fit the air strangler on the end.

And Keith you were saying about the tapered point of the idler screw, im guessing that these were tailor made to suit each carburettor?

Also on the auto starting gear I'm wondering if it were possible to take it off? I guess this did the job of the air strangler?

Oh and George the thin other jet was numbered 06 or 60 as the screwdriver slot was a bit chewed so could only just make numbers out.

[Keith Andrews]

And Keith you were saying about the tapered point of the idler screw, im guessing that these were tailor made to suit each carburettor?

Yes sort of...They where mass produced by most carb manufactures in matched batch lots....The tollerances of the taper and the seats are measured in fractions of thou of inch.
What people tend to take for granted because they are tollerant, is that carburetors are very presicion 'instuments'
ie differnce in jets sizes are measure in fractions of thou of inch....sticking buts of wire in them changes flow charactoristics...and the sad part is all this poking drilling over the years so often accumicates into major headaches.

Bottom line, most carbs have the mixture screws and idle screws 1 1/2 turns out +/- little over 1/4 of a turn
If that is not right then there is a manifold leak, worn bushes or in the case of a carb with secondaries the secondaries are not adjusted correct....That is all there is to the idle circuit...and any influnce the idle circuit has on the cruise or power circuits is so insignificant even a cox or O2 sensor will hardly pick up.

The cruise circuit doest operate under idle, does operate under power

The power circuit doesnt operate under idle, but does 'suppliment' the lean running cruise circuit under med to heavy loads because a engine require a rich mixture then.
An more modern carbs for more powerful engines also have a pump to supliment the transition from cruise to full power to9 remove a lean /power spike.

What im saying is treat a carb like a precsion instument and the different parts /circuits act independant of each other, once one understands this, one understands and where and what may be at fault
But nothing can be adjusted right with a manfold leak or butterfly bushes worn, and if they are worn in a oval manner even weider things happen under load and cruise.