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MG Midget and Sprite Technical - Too lean or not too lean?

I took my new engine to the Aldon rolling road the other day to be set up. The spec is 1330cc, 270+ Peter May billet camshaft, MG Metro head (polished), K&N filters, LCB and HIF44 carb. I set the carb up initially with a BDL needle as that was the standard MG Metro setup and my old engine ran well that way. After the run at Aldon, the tech guy said it was running too rich and fitted a more lean needle.

I'm puzzled by this as I expected it would need a richer needle. Unfortunately the car also ran very badly on the way home until I richened up the mixture. Is it plausible that a bigger capacity, hotter cam engine would need a leaner mixture than before? If the car had run great after the RR session I would have just accepted the diagnosis but as it ran so badly my faith in Aldon is a bit dented. Any views anybody?
Chris H (1970 Midget 1275)

did Aldon's tell you what needle they fitted? can you remove it and check to see if they actualy fitted what they think they fitted? easy enough to get lots of needles in compartments all mixed up !
David Smith

Aldon fitted a BFW needle to replace the BDL. According to my WinSU programme, BFW is consistently leaner (i.e. thicker) than BDL throughout the rev range. I have not yet had a chance to confirm that they did, in fact, fit a BFW.

Chris
Chris H (1970 Midget 1275)

agree BFW is leaner throughout; did they change the spring in the dashpot by any chance? Winsu suggests richer, BDK or BDD is an appproximation to your given spec. Think I'd be on the phone, and back there as soon as they can fit you in - they deserve a chance to put it right, FOC of course.
David Smith

that is odd.... to go more lean

did he check that the old needle was in good unworn shape and was working properly

I just think if the old needle was badly worn or sticking or even then dash pot oil was to thin perhaps the first run test was a false positive it was running too rich


could even have been his equipment if he was using 1 of those tailpipes sniffers and there was a lot of exhaust smoke in the air with the door closed ....then I think that would give a false positive for richness also


I wouldn't be so fast to say it needed a lien needle .

Prop
Prop

So Aldon changed the needle after the run but didn't do a run with the new needle to show the car was either running smoother or developing more power or both?
Daniel Thirteen-Twelve

They changed the needle to (an unknown) lean needle but decided they had gone too far lean and picked a needle inbetween. They did do a run after that but how it can have been right after the poor run home I don't know. The engine kept missing and running rough. I did the usual 'lift the SU piston' method of checking the mixture and it seemed way too weak (and the plugs were white). I richened the mixture at fast idle and it seems OK now but I have no way of knowing what the mixture is at high revs. I'm not the only person to have had an experience like this - but I know others who swear by Aldon.
Chris H (1970 Midget 1275)

Simple answer, take it back!
Cheers John
HALL JOHN

Chris

Larger capacity and/or (especially) higher compression can result in stronger pulsation in the inlet tract which draws proportionately more fuel than before (with the same jetting), so sometimes changes can result in the need for leaner jetting to conteract this effect.

Did Aldon check the mixture at full power only, or did they do some runs under light to medium throttle openings? If only full power, it would be quite possible to get the mixture there right without it being OK under cruise conditions.
Paul Walbran

Agree entirely with Paul. I have seen on a couple of occassions where people have been tuned on a rolling road and then the car became a pig to drive, this simply because he RR tuner did not pay any attention to low throttle running. At low throttle running the mixture is crutial to smooth operation.

I would suggest that you contact Aldon and ask them to sort it for you. By adjusting the mixture for good low throttle work may well have made the mixure elsewhere to be off and should be checked by those who you have given your hard earned cash!
Robert (Bob) Midget Turbo

Depends who's on the 'road' at Aldon. A couple of years back, I went to Aldon with my HS6 and BBL; after most of a day (when I wasn't allowed near the road) it was pronounced OK and I paid the bill (gasp!).

Some weeks later, I went to Mech Repairs in Cheltenham - they encouraged me to take part in the session. We found it was running weak at WOT/7K. We marked the needle position in the jet during this condition, and then pulled the piston out to inspect.

It appeared that Aldon had filed the needle at the very end to a sharp point... far beyond the point where the needle would have been emerging from the jet.

So we filed at the correct point and did another WOT run to find it was now running ~stoich and was giving another 3-4 bhp.

It seemed the operators failed to understand how the SU worked - they were 2 youngsters.

The guys at Aldon on the yearly MGCC midget Register sessions in Oct (or is it Nov?) are much more on the ball.

All I'm saying (in several paras!) is that it's the RR operator that determines how well your engine is analysed and set up.

There is at least one highly recommended road often mentioned on this BBS.

A
Anthony Cutler

If it really is the case that it needed a leaner needle then could this be the explanation? An earlier RR session with the old engine and the BDL needle found that the SU piston was not rising to its full extent even on full throttle. With the engine change, could it be that now there is more induction 'suck', the SU piston now rises to its full extent and gives a richer mixture with the BDL than before.

I said I would take it back to Aldon when run in after 500 miles and that is not long now so I will wait for that anniversary. I was just disappointed that Aldon let the car leave in such a state but feel that it is running quite well now after my own adjustments. I just worry that it could be running lean when I really drive it in a spirited manner (that's what RR sessions are supposed to put your mind at ease).

Chris
Chris H (1970 Midget 1275)

I've never used Aldon myself, although I have used Hi-Tech at Cradley on more than one occasion.
Dave O'Neill 2

This thread was discussed between 16/05/2011 and 17/05/2011

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