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MG Midget and Sprite Technical - Idling rough only when warmed up

My 948cc sprite (original engine etc.)idles reasonably Ok from cold, but after I have driven for a while and engine heats up, the idling is very rough. If anyone has any suggestions where I should star looking, I would be very grateful. I haven't checked anything yet as have only had the car a few weeks, but thought I should look at plugs first and
also clean air filter. Bit nervous to start messing with the carburettors.

Many thanks
g Victors

Hi, do you have the heatshield between the carbs and the manifold as the heat could be affecting it.
Thanks
Mark
Mark Whitmore

Sounds like you may be running rich. There are several possibilities, none of which are too difficult to diagnose and fix

1. Maybe one or both chokes are sticking on (at the bottom of each carb their is a short tube about the dia of a pen (the jet); if you put a slight pressure on it, does it move upwards? Try both sides with the choke off. Compare what it looks like with choke off and on, and you'll get the picture. You may have to repmove the air-filter to see easily)

2. Maybe your mixture is too rich (and the chokes are releasing to their normal postion OK. Take out the plugs and describe the electrode and surrounding. After a run, the electrode should be white-ish and the surrounding part of the plug (above the threads on the body, adjacent to the electrode) is normally black. Tell us what you see.

A
Anthony

both great points...but please varify 1st that you have a heat sheild....cause it will run like trash if you dont have one....if you do have one then take up Anothonys thread next.


let us know what you find.

prop
Prop

Thanks for your help.I have only shortly got home from work but have now checked. I do have a heat shield and see when I pull the choke on, the tube definitely drops down and returns when I take choke off. The spark plugs look quite sooty black. Certainly both the centre and outer metal electrodes are black. Strangely the inner insulation (of the centre electrode) is black on one side and white on the other (checked a second plug and it was the same).

Perhaps I should mention that the engine starts reasonably well without any choke and am wondering whether that is normal (i.e. should it be that I should need the choke?)

Thanks
g Victors

The black/white nature of the centre electrode is evidence of uneven burning - not uncommon.

It seems you're running slightly rich. Assuming your air-filters are in good condition, you should weaken the mixture by a known small amount on both sides, and check the result over the next few days.

You've already seen that pulling on the choke drops the tubes / jets; this exposes more of the tapered needle in the carb, and lets more fuel in. So you need to do the opposite of this, to a small degree.

The 'rest' position of the jet is set by its adjustment nut. Often, you can turn these by hand, esp when the filters are removed.

You should turn each these nuts one flat upwards (if you're looking up at the jet from below, that's clockwise). The jet will move up by a fractional amount accordingly.

Then give it some miles to feel the difference; and remove the plugs as before after the car has been run to hot (>15 mins).

HTH

A
Anthony

Id say take it to a good british classic car machanic....NOT A MODERN day mech. this is going to be just a simple adjustment, and will be fairly in-expensive, I say this option cause you said your a bit frietful of digging into these carbs....


If you decide to take it on your self then get a haynes manual and read up and drop by one of the local mg pubs where mg guys hang out...they will be able to clarify what haynes leavess out.

prop
Prop

Thanks for your very helpful advice. I will try that out as it sounds straight forward.
I assume I should I clean the plugs before I weaken the mixture but will they show the results after just a shortish run?
Is it right that the car starts easily with no choke?

I have booked to go to Gaydon for the 50th. That will be my first long (75mile) drive and I just want to make sure I make it!

Thanks so much
g Victors

I find I only need the clutch during cold rainy days...other then that she fires right up....

look at it this way....if its still not right by the time you roll...(hopefully not push) into gaydon, your going to have around 960 people there that will be more then happy to help sort it out....Not a bad place to be with a fidgety midget....


something you might try, that I did when I wasnt certian with mine, Is drive it in the evenings or free time,,,,with in 10-15 miles of home...but drive the full 75 miles.....sort of a 7 lap 10 mile coarse....do that a few times and you will find your bugs....just take some susspect parts, tools, manual, cell phone (with friends #) and a tow strap...that way if your only 4 miles you can just tow it without getting busted by cops...most likely....and if it works great....hey you can belive in the car.


prop
Prop

You should run the engine for at least 5 miles (10 miles if cold). Plugs don't need cleaning after mixture change.

In the winter, you should always need choke to start. In summer, on a hot day, may start without choke (esp if run earlier in the day), but should be 'fluffy' when the throttle opened at low rpm until warm.

Bring your plug spanner to Spridget50 and we can take a look there.

Running slightly rich will not make your car unreliable.

A
Anthony

Thanks for all the responses - I will leave it alone until after Gaydon.
Lets hope the sun shines!!
G Vickers

This thread was discussed between 12/05/2008 and 15/05/2008

MG Midget and Sprite Technical index

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