View Full Version : Twin Carb ~ Single Carb conversion
meach86
10-09-12, 06:35 PM
Im thinking of ditching my twin 40 setup and replacing with a single carb setup. Is this just a case of removing the twin 40's and manifold and replacing with a single carb manifold and carb? Also what single carb would be best to use for this? It's a 1.6 8v!!!
Any help appreciated :thumb:
meach86
10-09-12, 06:39 PM
Just the aggro of having to keep having the carbs set up every summer? It's only an option I'm looking into because as everyone knows you can't beat the sound of twin carbs.
I suggest you get the 40's rebuilt with NEW straight spindles and then take them to someone who knows what they are doing with regards to the 'setup' (also check your throttle pedal isnt straining the linkage on full bore.
I know an engine that had a set of 40's on it that was moved into a new engine bay, abused like feck and all sorts for 4-6 years and it never 'went out of tune' because the carbs were built right and setup right.
meach86
10-09-12, 06:44 PM
It's finding someone who you can trust though, I took it to one place and it was meant to of have new jets only to find out later that they hadn't been fitted. I'm currently using Aldon Automotive who seem ok, but I'm out of my depth when it comes to carbs.
paul james
10-09-12, 06:51 PM
Twin40's shouldn't need setting up all the time, get them done right and you'll love them and shouldn't need to touch them again. Why not put the injection system back on if its a 1.6 nova engine? Seems like a backwards step putting a single carb on, which would probably need adjusting too.
meach86
10-09-12, 06:54 PM
Good point Paul, I wanted to keep away from injection because of changing the fuel system. Like i say it's just an idea and the more advice I'm getting from you guys, the more I'm leaning to sticking with the twin 40's.
BRoadGhost
10-09-12, 09:15 PM
Ok what people don't tend to realise is jetting carbs is different to synchronising them.
You should only ever get the jetting set at a rolling road where the exhaust gases are measured at the same time so you're not leaning out at peak revs. You'll know if they're good if they turn the jets out by hand with a drill bit so it flows more fuel due to all the other things you've improved the flow within the engine. Ignition timing will also want fine tuning; advanced bit by bit until it makes less than the peak power it did the step before, then backed off to there. Ensure the tank is filled well with fuel no older than 2 days old too. Oh and try to get it done when the weather isn't either hugely hot or cold.
Beyond that, what makes people think it's all gone to sh1t is the fact engine vibration has a tendency to loosen screws & washers. Seems minor in your mind, but when it comes to very fine adjustment in things like idling & balancing screws it will make THE WORLD of difference to how well it "used to run" i.e just after the setup. The trick is to be able to measure the airflow at each trumpet so you can know if the airflow is out. A syncrometer will let you do this for the sake of £40 and then it's just a case of moving the bolt that sets the spindle openings so the airflow is perfect between each carb. The idle screw; IF you need to use one that is, should ideally be threadlocked too.
Expect to check carb balance as often as your oil if you want a perfect response, else it'll seem really rough & not feel right. Once you know what you're doing it's a 60 second job with the engine up to temperature so don't be shy away from it.
The only other thing worth mentioning is the airflow at the trumpets; if there's any uneven distance between trumpet end & say, heater blower, bulkhead or battery, then you wanna rectify that. I actually cut the end of my air filter off & bonded a flat piece in there instead, purely so the closest "wall" the trumpets would be equidistant and therefore balancing induction airflow.
Never had to measure the air flow at the trumpets other than 3 year checkups lol.
It's all in not bending the spindle with setting the throttle stops properly. Running a twin carb engine should be as care free as running an injection one.
BRoadGhost
10-09-12, 09:33 PM
What I'm basically saying is those screws / bolts, over time, due to the vibration move and that's what makes for a poorly engine.
As soon as you can set them yourself, it's not a problem :)
If a rolling road started taking drills to jets on a car I owned I'd unstrap it myself and leave. That's not even close to a way of deciding if they're 'good' it's a cowboy move to avoid paying £4 for a new jet.
If a rolling road started taking drills to jets on a car I owned I'd unstrap it myself and leave. That's not even close to a way of deciding if they're 'good' it's a cowboy move to avoid paying £4 for a new jet.
It is if they know what theyre doing, old skool tuners all do this, and your probably find yourself ending up being done over by someone who doesnt quite know what theyre doing, old skool tuners can tune an engine by ear :d
Seen far too many carb setups fixed by binning the drilled jets, they're never as precise as new, and have striations/burrs down them that affect the fuel flow. Plus the size is printed on the side of the jet and it just screws you over next time it needs rejetting as it's put a known size jet in and start again time, unless you keep record of all the sizes.
meach86
11-09-12, 03:19 PM
Thanks for the advice lads, just a bit daunting messing with stuff you don't really know about.
BRoadGhost
11-09-12, 05:47 PM
Dear god MK999 you make it sound like jets are randomly subjected to a rough old pillar drilling. The jets are turned out carefully by hand by the smallest bit[s] there's no way burrs and a like would ever be an issue. QQing about the jets then being wrongly labeled "for the next time" is utter nonsense; why would you ever be subsequently a] needing the jet size anything other than enlarged further due to even greater flow characteristics you've increased on the engine b] taking the car anywhere else once you've found somewhere proven over decades and offer a great service.
turbojolt
11-09-12, 06:13 PM
i swear ive read this whole drilling vs not drilling argument on here several times now lol
if you can afford a second set why dont you get a weber rebuild guide and have a go your self?
If you have to go back to a single carb you will really need to puyt on a twin barreled one to get any sort of performance.
BRoadGhost
13-09-12, 05:03 PM
It's as if what I've typed hasn't even been read.
Dont worry, no one ever reads advice about twin carbs properly.... Like the key thing of setting the throttle linkage up to NOT bend the sodding spindle, which is the cause of 99% of carb problems
Jon_nova1
13-09-12, 09:45 PM
if you really want a single carb they had an old Astra 1.6 (mk2/3?) running on carb, its the same fitting as the 1.3/1.4 inlet, you'd just have to find an inlet to match the head, not sure if the Astra one would fit
Novasport
15-09-12, 10:06 PM
Misab's and mounting rubbers have a big part to play. Perished Misab's can leak air and screw up the mixture & perished mounting rubbers can cause vibration which froths the fuel in the carb.
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