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MG MGB Technical - Tuning SU carbs

Hi,

I have a 77B that I retro-fit with HS4 SUs about three years ago.
I rebuilt the carbs (used) and sent them out to Joe Curto for a rbush job.

The carbs ran fantastically when I put them on and up until recently.
I orginally tuned the carbs using a gunson gas analyzer and a Gunson Colortune in tandem.

I took the car to a British car restorer who checked my work and said everything was really good.

Yesterday I was trying to get the carbs to balance...after checking them and finding them out of balance.

I checked the air flow and balanced it with my uni-sync tool...everything good.

The colortune tells me that the fuel to each is quite balanced.

But when I push the test rod up on the rear carb, the engine wants to die right away, while the front one will start to die but just lug along.
This is the case with both no matter how much I lean them or enrichen them.

Is this a symptom of needle wear, bad float level, dirty carbs?

TIA,
Erik.
Erik

Since the carbs are interpendant I don't see how a Colortune can be totally effective, and a CO meter is only any good for adjusting both carbs by the same amount in the same direction in order to meet an emissions requirement, once they have been correctly adjusted for air and mixture balance. The lifting pin is the correct method to use for mixture, after getting the air-flows balanced. Be aware that making an adjustment to the second carb means that the first has to be rechecked. If that is readjusted then you have to recheck the second, and so on until both are correct. You are aiming for the same effect when lifting each pin in turn. If one of them ends up a significant way from the starting point of 12 flats down and the other doesn't then there is probably something wrong with one or other of the carbs. You should find the lifting pin results moving from rich through correct to weak (or vice-versa) within just a few flats of adjustment. If you can wind one all the way up and down with no apparent effect then there is something wrong. The lifting pin has some free movement before it contacts the piston, you should only lift the piston 1/16".

When correctly adjusted they should hold their tune for many thousands of miles - as long as you only ever adjust them both by the same amount in the same direction.
Paul Hunt

Many people don't realize that to use the Uni-syn you must slacken the pinch-bolts on the throttle linkage, so that the carbs are independant. Once the flows are equal, retghten the linkage.
Since you've probably been all over the adjustment range on the jets, tighten them all the way up, then loosen 2 full turns (the 12 flats Paul mentioned) and start from there.
Ken Lessig

I had a problem where, as Paul said, I could wind the things all the way up and down with no affect. It took forever to figure it out - I replaced the needles and jets. Man, it was like having new carbs...

Curt
Curt

Using the lift pins to raise the piston may not be consistent between the carbs. I just looked at a set of new HS6s that are not mounted yet and the front piston raises by about 1/8" whereas the rear one raises about 1/4". So you should verify by looking into the carbs to check for equal movemen and that it is not exessive.
Gerald O'Docharty

I recall John Twist stating a a tech session once that the Color Tune was not reliable in his experience for the MGB. The advice given above should be followed for balancing. If you are really serious an O2 sensor (see the Moss Accessories Catalog. Holley used to make one too)could be installed pretty simply by welding a threaded flange on the down pipe in front of the first muffler.
Andrew Blackley

The colourtune works fine, but we aware that cylinders 2 & 3 run richer than 1 & 4.

I use 1 & 4 to set the mixture since running rich on 2 & 3 is less harmful than running weak on 1 & 4.

Don't just wind the screws or nuts al the way up - the jets will exceed the height of the bridge which is the true starting point. Take off the piston assemblies and use a finger to feel when the jet is level with the bridge, then adjust down 2 turns of the nut or screw.
Chris Betson

Thanks so much for your responses!

I agree with Paul and others that the colortune is not the greatest implement, but is better than nothing.
The gas analyzer...ditto Paul's comments.

Since the carbs were basically brand-new when they wnet in three years and 6K miles ago, I'm gussing that I may have knocked them out of adjustment.

If not, the needles and/or jets are probably worn on one or the other carbs, I'll just replace them...cheap enough!

Thanks again,
Erik.
Erik

Here's another thing to check...maybe one or both of the grub (set) screw(s) has backed off allowing the needle(s) to drop a little bit. The shoulder above the needle should be flush with the air piston surface. You can check this in-situ, just lift the piston some and check with a torch (flashlight).

Mike
'79B (SU equipped)
Mike Janacek

This thread was discussed between 11/02/2002 and 13/02/2002

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