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MG MGA - Sputtering problem

This problem won'y go away. The acceleration sputters for a second after upshifting at high RPM, indicative of low oil in carb (HS4) dashpots, or too light oil in dashpots. But I've tried everything from 3in1, 20w, 30w, 50w, and ATF with the same effect. This is an acquired problem (didn't used to do it). Any suggestions.
thanks,Tyler
TYLER

Tyler - The old wisdom is that when having a problem with the SU carburetors, the first thing to check is the Lucas ignition. How is the timing on your car? What about the dwell angle (points gap)? The rubbing block can wear quickly if not well lubricated, causing the dwell angle to increase (gap closing up), which will cause drivability problems. Finally, try changing the condenser. Good luck - Dave
David DuBois

I side with David. The number of problems I have had that at first sight suggest fuel/carb related issues turn out to be electrical.

I have yet to have condenser problems in 40 years of driving. Nearly always points (dwell angle) and timing - in that order.

Steve
Steve Gyles

I agree with Steve and David. Most problems are ignitions related. On a previous car, I ran almost 10 years without touching the carbs. On my present one I'm only up to 5 years without touching them. I find that they are about as rock-solid a carb there is.

Not mentioned yet is the distributor rotor. I have had one fail on me as have others. Also the plugs ahould be in good shape along with the high tension wires. Lawso look at the internal grounding braid wire in the distributor. This sometimes fails and only makes contact depending on the mainfold vacuum and RPM conditions of the car.

Chuck
Chuck Schaefer

Hi Tyler. You state that the sputtering occurs at high RPM's. If you are using Lucas points and condensor, and cant track down the problem, you might want to switch to an aftermarket brand. I have found Lucus points to generally be substandard compared to aftermarket points. Lucas points have miniscule contacts that tend to burn and pit quickly. Aftermarket points often have points with at least twice the surface contact area! Variably, it also seems that the Lucas points springs can sometimes lose their tension and often become weaker over time, and allows the points to "bounce" at higher revs. In my personal experience, these flaws can and do cause misfiring at higher revs. I have also had Lucas condensors that went bad over time, which caused "interesting" starting and running symptoms! Dirty spark plugs can also cause severe running problems, even plugs that look good. Plugs are cheap, so try new ones first. Best of luck, Glenn
Glenn

I'd give the ignition electrics a work through, new points, condensor, rotor and cap. If the HT leads are old, replace those too.

Mark the current distributor position with a dab of correction fluid overlapping the distributor and the clamp (saves hassle getting the timing set afterwards). Remove the distributor and set up in a vice - this makes getting the point gap setting correct much easier. I would guess that the shoulder on the points is worn away and that the dwell angle is completely wrong because of this.

Replace distributor and use the dab mark to put the distributor back in exactly the same position as before. If you want to check the timing with a lamp, that's a good idea, but at least it should run this way.

I think you will discover that the carbs are fine. I never touched my SUs once they were setup, apart from topping up the dashpots with light oil as part of routine servicing.
dominic clancy

Hold it! I forgot to mention the car has an electronic ignition, and I really think this is a fuel problem. The problem is the movement of the piston in the dash pot. It is either too high or too low in its position when the accelerator is pressed and fuel enters the carb. If the piston is too high, a lean mixture results (yes/no?). HS4s rebuilt 5 yrs ago and piston action is okay when testing the piston lift button. If that is the problem, what is causing the fast piston lift? Or am I way off base here?
Tyler
TYLER

Tyler,

This gets a bit complicated. Your comment about the piston being in the wrong place when the accelerator is opened seems to indicate that you think that the piston position and the accelerator position are related. They are not. The piston position is related to vacuum. Vacuum is a good measure of the air requirement under various engine loads. The piston adjusts the air flow, and more importantly the air velocity, irrespective of the throttle position. This in turns creates a venturi effect of suction at the jet.
Your question of "is the engine lean if the piston is stuck up" is also not a simple matter. The jet would be in an excessively rich position, but the air velocity (and venturi suction at the jet) would be too low.
Do you have an SU tuning book? This will give graphs of the mixture condition when the is piston lifted higher than its proper running position.
See other comments at your thread "New Running Problems".

Mick
Mick Anderson

Sorry, read as "the needle would be in an excessively rich position", not the jet.

Mick
Mick Anderson

Is this sputtering only when it's cold?
Tom

I put the answer in your other post of the same question on the B board.
FRM
FR Millmore

Tyler,
Does the "sputtering" occur at the high RPM before you upshift, or only at the lower RPM after the upshift? Any way, I have to agree with the others, this sounds more like ignition. An electronic ignition does not automatically cure all ignition problems. You still have the coil, distributor, cap, rotor and wires. Any one of which is a more likely cause of this problem than the carbs.

Jeff
Jeff Schultz

Tom: sputters when it's warm.
Jeff: at low RPM right after shift when I hit the gas. Sputter lasts only for a second before full acceleration resumes.
FR: Answered your reply on MGB page (my engine is an 1868cc B engine). I'll try tearing into the pistons to find the problem. Seems the likely solution. Then work back to electrical.
thanks folks!
TYLER

Tyler, These same guys fixed a carb problem for me last week. Turned out to be a cracked dist cap. Picked up a cap at Advance auto parts for six bucks. Good luck, Neal Turner
Neal Turner

Well, heck! I'll do that too.
thanks
TYLER

MG Owner/Shady Tree Mechanic since 1962. Lots of good dope here! "Smokey Yunick" (race car driver/mechanic/owner) used to say: "98% of the CARBURETION problems can be repaired by fixing the IGNITION!" On my MG midget 1275 I used to use the "MINI-COOPER S POINT SET" which had 6-8 ozs more tension and worked far better at over 4,500 RPM. Don't know if these fit MGA/MGB or not, or if they are even still available. ELECTRONIC IGNITION, (which I've used since 1957!)if it was PROPERLY DESIGNED, has filtering circuits that ELIMINATE any possibility of "POINT BOUNCE" of weak point springs. Also would like to point out that S.U. CARBURETOR PISTONS & DASHPOT ASSEMBLIES are, or at least should be, "MATCHED ASSEMBLIES"! If these got switched around at "CARB BOILOUT TIME", you ALMOST CERTAINLY will get UNEVEN PISTON LIFTING! (I've seen people who were incredibly lucky where the "mismatch" made NO DIFFERENCE, but this would be very much an exception!)
OLD BILL-67

This thread was discussed between 10/10/2005 and 18/10/2005

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