Jump to content


carb balance question


Recommended Posts

Hey I got a carb synch tool and worked on balancing my carbs on my 71 this weekend. I think I did ok, the car is running pretty good , although it wasn't that far off, but i have a question about setting the carb mixtures.

When I lifted a piston in one carb to check the mixture, the engine continued to run ..roughly, so i'm told that means my mixture is a bit rich.

OK, so when i went to raise the piston in the other carb she started to die, so that means Im running a bit lean.

Does this mean when i raise a piston in one carb, the engine symptoms point to the condition of that carb, or the one I'm not rasing the piston in??

Link to comment
Share on other sites

The opposite carb. When you raise the piston in one carb the engine is only running on the other one. Obviously you want them to run the same. I adusted mine during warmup and then again after the engine warmed up I shut the motor off for 10 minutes and restarted. I had to richen slightly but I know I have it right where I want it.

Link to comment
Share on other sites

Hi 1971rb26dett

"When you raise the piston in one carb the engine is only running on the other one"

are you sure (how do single carb installations work then)

I always thought that buy lifting the small plunger an 32nd or so you were artificially leaning out the mixture (reduced vaccum across the venturi) so making a weak mixture weaker or a rich mixture closer to the norm. That carb is then adjusted so that the effect is minimised then the same is done to the other

Quoted from a Jag based site, corroborated by an MG owners site

4) Lift the piston of one carb (there doesn't seem to

be a good reason to do either the front or the rear carb first) 1/32" Use

the lifting plunger or a penknife blade. The 1/32" isn't critical but the

piston only need to lift a gnat's whisker. 1/4" is way too much. What you

are doing as I think Bruce said is to reduce the Veturi effect of that

carb. You are trying to examine the effect of weakening the mixture.

Because the needle taper is slight a very small lift weakens rather than

richens but obviously if you continue to lift the richening effect of the

needle taper starts to interfere with your adjustment. 5) If the engine

speeds up when you lift the piston and stays speeded up then the mixture

was too rich (because weakening it slightly improved it). Conversley, if

the engine speed decreases immediately then you started with a mixture that

was too weak. The happy state that you are aiming for is a momentary

increase in revs and then a drop back. When you get this the mixture is a

tiny bit too rich but that's not a bad thing. For those interested in

emissions this will give a CO of about 8% provided all else on the engine

is A1. 6) Adjust the mixture on the carb whose piston you are lifting by

turning the mixture adjusting nut one flat at a time - up to weaken, down

to richen. It sometimes helps to blip the throttle open and let the engine

settle before repeating the lifting test.

As I said if lifting the plunger disables the carb how do single SU installations get tuned up (they still have the plunger)

Link to comment
Share on other sites

Yes you are right. I was very unclear. I was responding to the question

Originally posted by deadflo

Does this mean when i raise a piston in one carb, the engine symptoms point to the condition of that carb, or the one I'm not rasing the piston in??

and yes it does indicate the condition of the opposite carb. when you lean out the one carb it serves as your control (albeit not a very good one) to test the other carb. Unless one is running very rich. I assume that they are pretty close based on statements about the synch tool. I also assume that idle rpm is being matched to spec.

I plan to install 2 O2 sensors and a A/F ratio gauge in my car, hopefully I will have some spare time (and money) soon. . I am not a big fan of that test. My throttle shafts are leaking anyway so the carbs are already artifically lean. My friends dad is an old time MG racer and he showed me that test. I had already balanced them with the Unisyn and they 'passed' the test.

With mine I warm up the engine and lean out the carbs to the point where they will need to be choked to restart. Then I richen half turn and verify synch. If the pistons are moving the same and the float level is the same then I really fail to see where this test is beneficial if you have a synch tool.

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Create New...

Important Information

By using this site, you agree to our Privacy Policy and Guidelines. We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.