twin carb tuning

arthuy

arthuy

Well-Known Member
I have a little problem with my carbs, I failed the MOT for emissions so tried tweaking them down a bit but no joy and failed the retest.

I had it running ok the idle was about 800 but by the time I ran it a few miles it was idling at 1500. I found it difficult getting the idle to the 700-800rpm without the car dying.

I am looking for a few hints or tricks to getting the car running sweet. I have a carbalancer and colour tune.
 
Arthuy,
Remind our dear readers of the model and year of your car as this may help in deciding on adjustments.
Regards, John.
p.s. tweeking down worries me as down means richer mixture and you need weaker if you have failed on emmissions.
Regards, John.
 
Thanks John,

My tinkering hasn't yet extended to the carbs so I have only the theory from books.

Car is a 1977 2200 tc
 
I've spent many an hour tuning my 74 2.2TC so I know where you're at !

2 Usefull tools, one is a carb-ballancer, you need this to make sure the carbs are both taking the same amount of air in, Gunsons do a cheap one that works. The second is the gunsons colour-tune, this replaces a spark plug and allows you to see the combustion while the engine is running. This helps the get the mixture the same on both carbs.

1st get the airflow balanced, you do this by releasing the linkage between the carbs, the adjusting the idle scews on each carb until you get the same flow with the carb-balancer, and your idle is approx right.

Then you can adjust the mixture to get the correct blue colour combustion from both pairs of cylinders.

Finally you need to lock the linkage back so that both butterflies open together with the throttle.

You now have the carbs set up basically correctly, from here any adjustments should be carried out to both carbs identically. So if the mixture is still too rich, you need to adjust each jet equally about 1/4 turn at a time (the screw on the side of each carb)

As you weaken the mixture, you should find the idle speed increases (to a point before it stalls) so will beed to adjust the idle screws, again make sure you do both the same, or you'll knock the airflow out of balance.

It all sounds complicated, but its all about working methodically. If you can lay your hands on the Haynes manual, it covers most of the above.

Richard :)
 
Oh, and I forgot, make sure everthing else is ok, before you start messng with the carbs...

So do a service, new air filter, spark plugs, points, condensor, rotor arm and dizzy cap.
Also check the dizzy is adjusted correctly ( you'll need a timing light for that )

Richard :)
 
Richard,
I nearly agree with you! (Or you've missed a vital step) I set the mixture with the Colourtune first and balance the airflow last (assuming all the ignition settings are correct, as you state) Mixture, airflow and hence tickover are all inter-related but if you are starting from scratch than the fuel mixture is set first by adjusting the main jet up(weaker) or down(richer) or am I just considering my old SU's with the large hexagonal jet adjuster underneath?
Regards, John.
 
To be honest John, I think its just the way you work best. On one occasion I was so sick of playing with the bl**dy things on the car that I took them off, and into the house.
Then I used the vacuum cleaner as if it were the engine sucking through, and balanced them seperately like that, this way I could strip the tops off, and stretch the springs so they both lifted exactly the same.

After this they were certainly in the best tune ever !
But still not perfect, mostly due to them being very worn and having had the bodies pistons and tops mixed up on several occasions. Then there's the bi-metal temperature compensating bits in the float chambers......

I am of course talking about HIF6 later carbs......

Give me fuel injection and a laptop anyday !!

Richard :)
 
Spent the day with the bonnet up.

I have managed to get them balanced and the colour tune to give a nice blue.

the car runs and then dies after a few minutes of running, there is a hiss so presume this is why. I replaced the exhaust manifold so this is more than likley why.

Going to try for the MOT this week so will know for sure soon enough.
 
Its worth checking that you haven't got a leak somewhere between the carbs and the head, quite common on the tc for the large O rings (between carb and manifold) to split or get sucked in and create a leak.

A good trick for testing this is to spray some wd40 in the area and see if the revs rise / fall as the wd40 fills the leaks.

Richard :D
 
Sorted the carbs out right enough.

I took it for a test today, I didn't pass but didn't pay either. The pesky old rear calliper was to blame.

previously there was an imbalance, caused by the lefthand pad being wedged by a shim. now don't ask why I put it on, it was on my spare set maybe the pad retainer was worn or the likes. removed and the problem was solved except for they could have been better adjusted, my neighbours will be thinking Ireally love the car when I am yanking the cable.

The leak is nothing sinister either, there is a gap between carb and air box due to a missing stud.
 
The carbs fitted to the 2200tc were the first to have a bimetalic strip on the jet.the jet and the needle(sping loaded and directional) MUST be set up as per manual if not the needle/jet bind and do not let the jet
eedle adjust correctly for fuel temp and engine temp
also engine MUST be tuned prior to carb adjustment.Balance can be done with air cleaner removed,and front carb should have one eighth inch lead over rear carb else flatspots and poor performace will happen
also the engine breathers must be clear and all hoses in good condition
 
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