MGB & GT Forum

I drove down to Ukiah last weekend to visit a buddy and get a healthy dose of MG. While I was there I decided it was time to swap on my rebuilt air of SU's. after some initial grief in mounting them we got the car running. but not smoothly. The car just wouldn't idle smoothly lots of splashy sounds coming out of the exhaust. Once on the road the car would stutter then accelerate nicely when you pushed on the pedal. the stutter would happen at any rpm seemed to be due to the throttle position. after a day and a half of t
We did notice some rust pitting at the top of each SU piston. as a shot from the hip we scavenged a set of pistons and needles. Suddenly the car runs better then it has ever run before!... puzzling results.. so the tuning issue has to be due to my pistons or my needles. right? I swap my new needles in to the old pistons and tune it. still some stutter but other wise ok. then I put my pistons in with the donor needles. we just couldn't get the thing to tune well enough to even take it for a test drive.

I'm running the car now using the donor pistons and needles with a minor amount of stutter still when I first touch the gas pedal


If these SU's sat then and there is pitting at the top of each piston then there will be pitting at the bottom of the bore in the bells and by changing pistons I only fixed 1/2 the problem. If that's the case I will need to replace my bells or fill the pits some how. Has any one thought of using plating to build metal back on to a wear surface? I was considering nickle for a filler metal.

I am not against spending money on a pair of brand new SU's but I would rather spend that money on a pair of 15 inch MGC rims and tires.

Jared,
HS or HIF? Are these freshly rebuilt? Rust on alloy parts? What metering needle do you have in which ?
The pistons should drop slowly in the dashpot when released.
Check it out a report back.

Bruce

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Life's most persistent and urgent question is, "What are you doing for others?"

Did you match the pistons to the bores of the domes? John Twist has a video that shows the correct procedure for matching the pistons to the correct dome if they happened to get mixed up.
It could be that they are just in the wrong domes.



Regards

Mark

stutter suggest too lean.

you can use this fact to get them set right.

First, (assuming you have valves adjusted right and timing right--if these are off, you won't be able to properly set SUs as the vacuum will be off and SU relies on vacuum) make sure the linkage is properly set up (throttle shaft clamps properly set, fast idle correctly set, etc.

Make sure float level is correct. Float level will directly affect mixture. Position jet 70 thou down from the bridge. This is starting point, not final setting. eyeball throttle clamps so each carb starts opening at same time. If adjustment is needed, remember that if clamps are tight, tweaking one will tweak the other as well, carrying on the maladjustment.

Start car and get engine warm. Drive down the street at 15mph in as tall a gear as you can get it run run in, probably 3rd. Very very slowly accelerate to about 25 mph, not changing gears. If it stutters, richen each carb by the same amount. If it does not, lean each carb bu the same amount and re-test. When it is stuttering and the richen phase solves this stutter, then go to a faster road and repeat at less than 2000 rpm in as tall a gear as you can. Probably around 30 in 4th. slowly accelerate to about 45. If it stutters, richen both carbs a little. If it does not stutter, don't lean it.

When that's done, synch the carbs with your favorite tool. Set idle at about 750 rpm (it will fluctuate if there's wear in the distributor and/or points are worn and if it can't hold a smooth idle that is likely your problem). Repeat the initial mixture test. It should pull smoothly at very slow acceleration. If it stutters at high acceleration, increase the viscosity of the damping oil in the dashpots.

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1973 Pale Primrose Roadster. A nice 10-footer!
SUs, Datsun 5-speed

Rebuilt SUs would include rehabilitating the suction chambers and the piston interface, if needed, checking the drop times and if satisfactory results were not achieved, sourcing new suction chambers and pistons, as matched pairs. Simply installing new needles and kits in worn or corroded carburetors may not yield anything close to a reasonable result. There are four moving parts in the SU carburetor which need to be working smoothly to achieve optimum results. They are the throttle shafts and butterfly discs; the floats and float needles; the enrichment device; and the suction chamber and the pistons.

Warmly,
dave

Thanks for your responses

Info on the carbs.
Freshly rebuilt HS4's over size shafts, new butterflies, Jets, needles (spring loaded type), seals, dampers and filters. linkage was nickle plated. My friend mike came down with lung cancer and passed while the plating was away I reassembled them my self. Needles are the spring loaded type. so I did not need to center the jet bearings when I reassembled them. I did vacuum balance the pistons they came up 2 seconds difference.

I'll have to look up the numbers on the needles.