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MG MGA - Carb Tuning Tip ?????

A friend was making some adjustments to my carbs and I saw he undid both plungers, of the carb pots,...lifted them so that he could lay them (on an angle but still in the carb pots)towards the engine. Then when he flicked the throttle,you could see the plungers rise and fall.One was rising & falling faster than the other,so he adjusted the jet on the slower carb till they were in sync. Now there is no more hestitaion and the idle is much steadier.

Question: Is this a good way to adjust carbs or is lifting the plunger pin a better way?
Gordon
Gordon Harrison

That's how the little su adjusting kits with the wires work.
Fred H

Gordon I don't think he will have adjusted the jet to get the rise in sync! He would have adjusted the throttles to achieve it. I have no difficulty hearing the suck, but if one can't then it would be a good technique I think?
Bob (robert) I am turning? yes I once owned an MGWasp!!

As has already been said, this is a way to sync the throttle. Lifting the pin under the piston is the technique used to adjust the jets first.

The SU kit works but a hose at the entry to the carbs works better.

It has been so long since I've adjusted mine.... Adjust the carbs and forget it for 5-10 years is my motto.

Chuck
Chuck Schaefer

I could swear he adjusted the jet nut (the large brass hex nut) at the base of the carb. He did not touch the throttle screws at all.
Previous to this I had had my carbs rebuilt and set by Mr. Bryant(a pro rebuilder in Maine ). Then when I started the engine I did a full adjustment (as per the MGA TECH BOOK) and with a carb guage. They were pretty close, but this final "teek" made them great.

I know about the "pin" at the base of the carb and the lifting of the pin to change the RPM ...therefore the mixture, but is the rise and fall of the plungers not a result of the same thing the "pin" would do.
Gordon Harrison

It is a bit complicated to explain, but you have to understand the difference between "cause" and "effect".
The lifting of the dashpot plungers is "caused" by the opening and closing of the throttle, and used to set throttle position.
The "effect" of lifting the piston pin is to change change the velocity of the air over the jet, and hence the suction at the jet. This changes the mixture and gives the characteristics of no RPM change, higher RPM, or Lower RPM, depending on how accurate your mixture (jet settings) are?
Make sense? The English language does have some limitations.

Mick
Mick Anderson

Just a bit of lateral thinking. If 1 needle was slightly rubbing against the jet, it could cause the pistons to rise and fall out of sync. Turning the jet nut slightly might just rotate the jet enough to clear the needle. Crazy but perhaps true.
Henry

For H-4s, jet centering is important to eliminate needle drag/contact. The nut which holds the jet's slide in place, is mounted on the bottom of the throttle body above the actual jet adjustment nut. If your mechanic was adjusting the jet for centering, perhaps it was this "nut" he adjusted? Could be.

After the jet is centered, the next two important settings will be the initial jet setting (adjusted by the lower, spring loaded nut), and the needle position in the air piston. It is considered by some to be more accurate an initial setting to make sure the top of the jet is level with the bridge BEFORE lowering it the number of initial flats at the adjusting nut. Simply running the adjusting nut up until it stops doesn't always yield the same height from carb to carb, but starting a level with the bridge can give a truer starting point for any further adjustment.

The needle position simply needs to have the shoulder's lower edge even with/flush with the bottom of the air piston. You can ensure this by taking a metal straight edge/ruler, carefully setting it against the shoulder and pressing the shoulder until the edge of the ruler lies across the face of the air piston and then tightening the set screw.

After doing this, you have some assurance that your starting points are mechanically the same for both carbs. After that, any variance/adjustment is due to the dynamics of the engine and the carbs while it is running.
Bob Muenchausen

Hello, I'm late to this talk but have comment/question regarding needle position. As Bob said, needle shoulder should be flush to bottom of piston. When I rebuilt my carbs 10 yrs back, I couldn't get them to lean out. My new needle/jets apparently weren't well matched to each other so what I did was lower the needles about .025" and ran the car that way for 10K miles. I'm now performing a quick rebuild to replace leaky cork jet seals and wondering if my needle position should be corrected to "flush". Anyone have good/bad experience? thanks for any advice,
JoeM
'56 MGA
JoeM

This thread was discussed between 29/06/2006 and 08/07/2006

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