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MG MGA - SU Running rich - 1600

Today I substituted RO needle(s) for the standard number 6.'s According to the SU carby book RO is the rich alternative for a 1600. Looking at the needle profile data the RO is richer from stations 4 through 9. No other changes were made to the set up. The suction chamber and piston were perfectly clean on disassembly and reassembled with fresh engine oil in the damper chamber.

The engine has been slightly modified;

MGB headers
Larger valve head (late Austin 1800)
slightly longer duration and higher lift cam
Mallory distributor (duel point) with purple / orange advance springs

I changed to the richer "RO" needle because using number "6" needle the car seems to run slightly lean at higher speeds. OK up to about 1800 RPM then develops slight vibration through to 2700 RPM. Checking the plugs after higher speed (highway 100kph) appears slightly lean.

The "RO" needle is the same profile in the idling range.

Using the "RO" needle the engine ran much smoother and seemed to have more power, just from feel, no accurate measure. However after the test drive I noticed the idle was much lower and running rich to the extent I had to lean off the carbs by 4 flats just to get a smooth (still rich) idle.

I checked the actual profile of both the "RO" and "6" needles, and they are within 0.0005 of the published figures.

No other settings were altered, only the needle change. I am at a loss to understand why the engine would run so rich when the "RO" needle has the same profile to position 3.

I have since changed back to the "6" needle and moved the mixture down 4 flats and the idle is now smooth.

Any thoughts regards the "RO" needle please.


Russell

R J Goebel

A couple of thoughts:
Could the needle(s) have been fitted at a slightly higher position in the piston? Is the shoulder at exactly the same relative position on the needle? Are the needle lengths exactly the same?
Neil MG

...or did the RO needles centre properly ? It sounds like you need a rolling road - the engine changes may be quite significant - Peter Burgess might advise on the 1800 head -
Roger Walker

If everything else is set right (assumption) I can only say that they weren't in at the same level as the 6's.

I did exactly what you did with RO's today. Idle is unchanged. As it should be as the idle component of SU needles is pretty much identical for all needles. The RO's are are a bit richer in the middle, right where you want it.

To be honest, IMO the only way to be sure about mixture at idle is to use a CO meter. I have a fair bit of experience with SU's (own 3 E Types, MGA and have owned half a dozen B and A series engined cars in the past).

Listening to the exhaust, lifting pistons 1/16 of an inch etc is all bullsh*t. I have a gunson CO meter. You want about 5% CO at idle.

Check your jet heights with a vernier caliper and make sure that they're the same; mine are 60 thou below the bridge.

Once you sort out what went wrong, if you don't get enough richness with the RO's put in a set of yellow (8 oz) piston springs as well. your engine setup is very similar to my own. Mine goes like a beast!

Flick me an email if you want to chat about it by phone etc; I'm in Adelaide.
A Bowie

Earlier today (number 6 installed) I checked the position of the piston at idle using the "cats whiskers" from the SU tuning kit. It is ⅛" (3mm) above the bridge at about 800 RPM. That is at position 2 on the needle, the first 3 positions are the same so no apparent reason why the RO needle(s) would not work yesterday.

When fitting both the RO and 6 needles I used a steel rule to ensure the needle shoulder was flush with the base of the piston.

My thinking now is the piston must of hung up somehow......maybe the RO needle is bent.....will check tomorrow.

A.Bowie would like to get in contact don't have your e-mail. Mine is auslander2010@gmail.com

Russell
R J Goebel

I tried to find the old article that I had on setting needles in the carbs. Essentially it is this...

With the dome and piston out, loosen the needle so that it can be moved with a little resistance, and make sure that the securing screw does not sit proud of the piston, so the piston does not hang up.

Make sure the jet adjusting nut is all the way up, til it is right up against the jet tube. And the jet is all the way up to the adjusting nut.

Install the piston in its bore and press gently to seat the piston all the way down on the bridge. This should set the needle to the jet. It may have the shoulder a little proud, but it will set it to the jet. Tighten the retaining screw and re-install the piston, spring and dome, making sure the needle is centred so the piston clunks when dropped.

Back off the adjusting nut 6 flats and set as normal.

This may solve your problem. Its easy and it seems to work. If it doesn't, nothing is harmed.

... CR
C.R. Tyrell

I am running MME a similar engine without trouble for some years. Recommended by local SU expert

Mike
Mike Ellsmore

Got it running perfectly today using the RO needles. Engine smoother (than when using "6" needles) and seems a bit more power, jets are now at approx. 0.057.

After refitting the number "6" needles on Wednesday I measured the jet below the bridge at 0.090...idle was perfect and much more than a flat either way changed the revs. This was the position where the car ran ok, looking at plug colour after driving at highway speed showed up a bit on the lean side.


MIKE; I had a look at the profile of the MME, more fuel flow after idle, so might be worth a try. I did not mention earlier I am running dished pistons and CR of about 8.5:1 also flywheel lightened to MGB spec. Is your setup similar?

Russell
R J Goebel

This thread was discussed between 15/02/2016 and 19/02/2016

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