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MG Midget and Sprite Technical - Carb piston height

Q. When under load, does the carb piston sit at the same height at a certain fixed rpm when compared to a no-load situation or does it rise/sink?

Trying to fathom out tuning using wideband lambda reader and looking down the carb throat to work out where to shave the needles.


Thanks for any links or pearls of wisdom.
rob multi-sheds thomas

Rob,

I think the piston height can be different at the same rpm depending upon throttle position/load.

The throttle position will change the vacuum without changing the rpm, so I think the vacuum section of the carb will respond accordingly.

However, I have not tried to observe this. I am only trying to reason it out.

Charley
C R Huff

Slightly different to tuning the aeroengine. Lots of throttle position changes.

I can see where you're coming from. I guess the usual method on a dyno is to tune it a zero vac with the throttle wide open?

I'm getting about 12.4 AFR at idle and rising to 13.5 during acceleration until 3000rpm+ when it goes to 14.4.
rob multi-sheds thomas

Rob,
i've got an Innovate which i am going to plug into a Megajolt for my SC conversion, what set up are you using & any pictures?

cheers
Brad
Brad (Sprite IV 1380)

The piston rises and falls according to the demand for air. At WOT and max revs, it should be at max height so that the carb mouth area is maximum.

It's fascinating to stare into the SU at WOT/,ax revs to see a cone of fuel streaming from the jet into the manifold.

A
Anthony Cutler

Is the 'Demand' or 'Consumption'? If I stamp on the throttle pedal at 5mph does it make the piston rise to the top (allowing for the damper to slow it) or does it rise to an intermediate height?

Not easy, is it. :o(
rob multi-sheds thomas

Indeed.

The piston is raised by vacuum on its top side; it's movement is restrained by the piston/oil damper.

So if you WOT at 2K, the piston will rise slowly (this delay causes richer mixture). If the revs are restrained, then it might not ever reach the top; if it goes to max revs (well max power revs) then it should achieve its max movement (to make carb throat round). This max point is determined by the spring; some people would cut a coil off the spring if the piston doesn't reach this point at max power.

But I think you should concern yourself with throttle steady-state operation, ie. watch the lambda reading as the revs rise for WOT and take note where weak/rich. If over-rich anywhere you need to raise the jet (since you can't add meat to the needle!) and start again. Then when the mixture is fine or weak, you can focus on filing to eliminate the weak spots (also taking note position of needle in jet) and checking improvement with another run

This should allow you to set the needle for correct operation towards the pointy end, and ensure the needle runs fine for WOT - normally considered the most important.

Then you can turn attention to the thick part of the needle, and check (say) steady part-throttle revs for rich/weak operation at points until the piston is reaching a low point.

If you can identify throttle position and revs for cruising that might be a good area to try to focus on for good fuel cons.

It goes without saying that once you start to file the needle, you shouldn't move the jet except maybe to correct over-indulgance with the filing...

I think the SU is a great design of carb, since it allows precise calibration if you have the time and patience (and maybe ££) for good torque and good fuel cons.

A
Anthony Cutler

Rob,

Here are a couple quotes from Peter Burgess on the MGB Tech board that might be of help. If you want to read the whole thread, it is still on the current page and the thread is titled, Exhaust Gas Readings.

"… ideally the piston should reach max lift at full throttle 75% max rpm."

And:

"I am looking for max lift at 3/4 rpm, which I suppose is between peak torque and peak bhp. That is what I was taught by a couple of people who worked at SU and came to me with cars to tune and passed on some of their knowledge to me."

Charley
C R Huff

There is a SU carb book around that charts all of the different needle diameters at various heights along the needle. This seems more efficent and accurate than filing the needle. Maybe you can file first, then measure and replace with a machined needle. You might also google to find the chart.
J Bubela

J

The problem is: once you've worked out where you need more/less dia and identified the correct needle, the man on the RR is already bored; and he def won't wait for one to turn up in the post...

Filing is best done for single SU cars...


A
Anthony Cutler

CR : peak torque and peak power are at different revs; for a road engine, peak torque can be 3-4K rpm and peak power at 5.5-7K rpm.

A peak torque, the engine's breathing efficiency is at its best. At peak power, the engine is consuming the most air. Hence my statement below about the piston reaching the top at max power revs (WOT).

This might be 75% of max revs; maybe 80-90% (depends on cam, head...).

BTW - I define 'top' as when the piston body has disappeared; the piston may have a little more headroom above that, but the throat of the carb won't get any bigger as it rises.

Some years ago, BRB was at Aldon, and they spent ages trying to stop the mixture leaning out at max power (6.5K) - cost me a fortune!

I booked a session at Mech Repairs in Cheltenham soon after - they allowed me to be part of the team (which wasn't Aldon policy at the time). We discovered that Aldon had been filing the needle to a point near the end to try to cure the lean-out. So I put the piston back, and raised it to where we'd seen it at peak power, and marked with tippex where it appeared from the jet. We found that Aldon had been filing some way from the active part of the needle. IIRC I gained around 3 bhp with the needle filed in the right place (based on earlier Mech Repairs run).

Must say that at the MGCC midget Register RR session at Aldon last year, the operators were much more open, and even let me use my laptop to tune the Kseries cars, which was great; so maybe their policy of 'stay in the waiting room' has changed.

I'd recommend the next session in Oct to anyone who wants more torque/power/mpg from their Midget - or even just a set of curves; the Aldon guys and the MGCC reps will recommend improvments should you ask.

A
Anthony Cutler

This thread was discussed between 19/08/2009 and 23/08/2009

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