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MG MGA - New Running Problem

The car has suddenly been difficult to start when cold. When it does start it idles OK but when I give it gas it hesitates badly; bogs down under load.

It seems to do it with the choke engaged and disengaged. However when it warms up it goes away; starts fine and runs fine under throttle.

Anybody have a diagnosis?
Tysen

Could be lack of oil in the SU dashpots, or oil that is too heavy in viscosity.
Also, do you have the lifting pins fitted for the SU pistons? If so, lift the pistons and let them drop. Should have resistance to lifting, but a free drop (with an audible "clunk").
If no lifting pins, remove the air cleaners and lift and drop the pistons with your finger.

Mick
Mick Anderson

Test piston travel with the engine not running.

Mick
Mick Anderson

I would double check that the choke cable and linkage is fully enriching the mixture. The levers can wear so that even though the choke appears to be fully on, the jets do not move all the way. When it happens, try lifting the levers by hand right at the carbs.

Jeff
Jeff Schultz

I'll give it a check.

Here's a question. I have no idea what type of oil the PO used in the dash pots.

Is there a problem mixing them? If so how do you you drain the oil?
Tysen

I don't expect that there will be a problem mixing oils - it's only there to damp the piston movement.

You can use engine oil (in fact that is the recommended oil for the Metro carbs). Or you can try to get something lighter - Penright sell an oil specifically for SUs.

Draining is very easy. Unscrew the three screws holding the piston cover down and lift it off. Take the piston out and turn it upside-down to drain the oil out of the tube in the top of the piston.

Replace and remove the damper - I find that a spark plug spanner fits. Refill with oil to the top of the tube in the piston - not to the top of the cover as that will just drain into the carb and get burnt up.

The pistons and covers should be a set, so do the carbs one at a time to avoid mixing them up.
Dan Smithers

On most SU's the bellhousing which contains the piston can be fitted in different positions each time (the locating screws are equally spaced). As you don't know what the previous owner did, you should fit the bellhousing, if you remove it, and do the piston drop test as mentioned earlier. If it falls freely, with the audible "clunk", mark the bellhousing and carb body for future fitting. Also mark them "F" and "R". If it does not fall freely rotate the bellhousing and try a different screw position.
Look carefully as some SU's have factory alignment marks cast in the metal.

Mick
Mick Anderson

I wouldn't be messing with the damper just yet. I really don't see what the damper has to do with hard starting when cold. The function of the piston is to pull the needle out of the jet as the airflow increases, and the function of the oil is to keep this from happening too quickly. If the choke is on and operating properly, the jet is already pulled down from the needle. Also, the fact that it works fine after warmup says that the damper is probably working OK.



Jeff Schultz

The dampers could still be the culprits.
The piston has two functions. One is to alter the mixture by raising and lowering the needle. The other, just as important function, is to adjust the cross section of the airflow so as to maintain the correct airflow speed over the jet. This does effect starting, but allows it to idle correctly, and causes hesitation when cold, "to bog down when cold" as Tysen describes it.
I find that once heat reaches the carbs the piston movement smooths out. It is not only a damper oil problem, but a sticking piston caused by dirty surfaces inside the bellhousing. On the housing, the piston, and the locating sliding tube.
This should be carefully cleaned with a cloth and carb cleaner.
The SU company tried to reduce this problem by venting the area above the piston internally, instead of to the atmosphere through the damper cap.

Mick
Mick Anderson

Are you sure your chokes are coming on? This sounds like they're not, your fast idle may be ok, keeping it running. Since it runs fine when it's warm, I'd forget all the ignition and damper stuff for now and work on getting the jets to come down farther. Mine is going to get some more attention soon. I adjusted the cable up to get more choke travel, but the lever hits the sheath of the cable before it comes out far enough. So I have to loosen the cable again and draw the sheath back farther. Should have done that the first time. I also still have to adjust the connecting link between the jet arms to get the 2 to come down at the same time. Takes a bit of fiddling around. There's a lot of play built in to the system, very odd. I went to the trouble of soldering bushings into the holes that are way too big for their pins, then discovered that's the way they were designed (with holes that are really sloppy).

BTW, my car is a TR4, I assume yours is designed with similar levers etc.

My first statement stands, however. More choke!
Tysen

I used to have the same problem when it was very cold. I just pulled out the choke as far as it would come and start, then push it in so that it would lock at the knob.

The symptoms do sound like you need more choke.
dominic clancy

Here's an additional clue that I just remembered that makes me think Mick might be on to something.

A couple of weeks ago I was polishing the brass dash pot lids with "barkeepers friend" a powder polish. At the time I didn't realize that the hole in the top was a breather hole and went into the dash pot. A tiny amount of the powder collected in that hole and then ran into the dashpot when I rinsed.

Would that be enough to effect the operation of the piston and produce the syptoms I've described?

The chokes seem to be working as they should and I did adjust the cable travel over the weekend and still have the problem described earlier.

Tysen
Tysen

Tysen,

If you tink that abrasive material went into the damper cap breather hole, I suggest that you remove the SU bellhousings and thoroughly clean and re-oil.
Remember:
Do not clean inside with abrasive materials, use only liquid cleaners and cloth.
Mark bellhousings and carb bodies for alignment when re-fitting.
Do not mix up front and rear bellhousings.
Do not bend the needles, take care if lying the pistons on their sides.
Do the piston "clunk" drop test after replacing the bellhousings,but without the air cleaners (easier).
Re-oil dashpots.
Ensure breather hole in damper cap is clear.

Mick
Mick Anderson

Mick,

You were exactly right! Cleaned it and its running perfectly.

Thanks!
Tysen

This thread was discussed between 10/10/2005 and 17/10/2005

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