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MG MG Y Type - Running rough when Hot

My YA runs terrific on short trips and on days of moderate temperature. But after running in slow traffic on a hot day, it starts running a bit rough, i.e. when accelerating it spits or misses a bit in the torque band, and then runs smoothly at higher RPMs (idles OK as well). I have a temp guage and it starts running this way when the water temp gets about 80 degrees C. The temp doesn't get much hotter than this ever, so I don't think it's actually overheating. No rust in the radiator and the water is clear and circulating well.

Any ideas? Has anyone seen this before? I am thinking of starting with new high tension cable hoping it's electrical in nature, but wondering if anyone has seen any carberator related prolems with the Y in the past.

Ben
Ben

Ben

Are you running unleaded? If so, have you laid back the ignition a degree or two? You should as U/L runs hotter than Leaded.

Also get a copy if the Y Sheet "Fitting a Heat shield". With modern high (higher than Pool petrol of '47)octaine fuels vapourisation and carb. hot spotting may be your problem. Common to Ys, and easy to cure with the help of this wonderful little sheet - see the regalia page of the website to get a copy and book review for a summary.

Happy New Year.

Paul
Paul Barrow

Paul,

I am running what is called "Lead Raplacement Petrol" (LRP) in Australia which is unleaded 96 octane with additives designed to protect valves etc as requred by older cars designed for leaded. It was an environmental initiative put in place 2 years back and is commonly available (you can't get leaded anymore obviously). Here is a link from BP that describes it, but it is available from most petrol companies.

http://www.bp.com.au/products/fuels/lrp/lrp.asp?menuid=ed

Hadn't thought of this -- in fact I purchased the car the same month as leaded petrol became unavailabe and LRP was made mandatory, so I would be the first owner to notice if this was the problem, and I have observed this ever since I bought it.

I will order the fact sheet and make the suggested adjustments. Thanks for the suggestion.

Ben
Ben

Ben

Pleased to be of assistance. We have the LRP here too. Fortunately for me our local garage also sells re 4*. I believe it can be detrimental to mix LRPs ie BP v Exxon (Esso) v Shell etc. so stick to one brand as much as you can. Well worth having a proper lead free head done sometime - careful who you choose to do it though. My Y is Lead-free but the ZA still runs on 4*.

You will still benefit from dropping the ignition timing back about 1 - 2 degrees though.

I have also fitted an isolation spacer block between the inlet manifold and the heat shield. I believe the early 1098 midget one is the one you need. You will also need longer bolts to hold the carb. body on. All the details are in the Datasheets.

Regards

Paul
Paul Barrow

Paul, Can you give me guidance on how to go about laying back the ignition timing a degree or 2?

Also, do you think it is worth running hotter spark plugs to minimize cabon buildup -- this is recommended for some cars by the oil companies when switching to U/L fuel. I am currently using the standard plugs recommended by the Y manual.

Thanks again!
Ben
Ben

Rough running when hot, and an apparrent loss of power, is due to the fuel vaporising in the carburetter and manifold. Modern petrol has a lot of bnzole in it, and easily vaporised additives; both in leadfree and so-called 4 star. If the carburetter body is permitted to get hot, fuel in the float chamber, and the fuel line, turns to a gas. This makes the carburetter's work almost impossible. The 'air-lock' in the fuel lne will force petrol away from the carburetter, so reducing and feed.

A quick cure is to pour cold water over the carburetter body to condense the vapour. This is only temporary, but lets you start the car again. The cure is to insulated the carburetter from the inlet manifold, to route the fuel pipe AWAY from the exhaust, and to fit some sort of heat sheild between the inlet & exhaust manifolds.

Any 'A' series engine Paxaline insulation block will fit the XPAG of the 'Y' Type. But it is too thick, so needs carefully cutting in half and rubbing down flat. The studs on the manifold are too short to cope with a full width insulation block. ( Blocks found on Mini's, Morris Minor's, Austin A30, A40, and MG Midgets.) Also, if you fit a full thickness block the air filter elbow can hit the bonnet side.

Run the fuel pipe away from the engine, then up along the radiator stay in front of the carb, then to the float chamber. This stops the pipe from absorbing the heat from the exhaust manifold, as it does in the normal position.

All this is in my boolet on the 'Y' type.

Other causes of rough running when hot is a condenser about to fail. These have a finite life, and should be changed every time you renew the ignition points.

Perhaps going a bit too far, rough running is a sign the valve seats are getting burnt out, but a compression check will soon alay this worry. People who adjust the valve clearance too-tight suffer from rough running, keep to the 19 or 12 thou, ( depending upon which camshaft your engine has.) Set the ignition timing correct, to retard it is NOT a good idea, as this will just cause the engine to run even hotter. The correct idel RPM igntion timing is TDC. The automatic advance inside the distributor takes care of rises in RPM and the need to automatically advance. If you have a late SC/2 engine with vacuum advance, this is only used on the overrun, to ignite the weakened mixture.

Check your spark plugs are of the correct grade and heat range. Fitting the wrong type will cause problems when the engine gets hot.

Good luck.

Neil.
Neil Cairns

Thanks Neil for your detailed response. I think this is definately my problem as your "water-soaking the carb" trick does provide a temporary fix (saw it on the Hint & Tips page earlier and tried it!).

I bought an isolation spacer and intend to install it my next free moment. I got one intended for the MGB (PN AEA586) and after unbolting the carb from the manifold it looks like the right one, made for the 1-1/4 inch carb with the bolt holes in the right places. It's about 7/8" thick. However my carb isn't mounted with studs on the manifold as you described, rather it just has 2 free standing bolts and threads cut directly into the manifold casting. So I will have to locate some longer bolts to accomodate the spacer, and am hoping to avoid having to cut it in half. It looks like the bonnet clearance is sufficient with no problems.

As for fitting a heat shield, this will be my next move if the isolation spacer doesn't fully cure the problem. Do you know of one from another car that will fit the XPAG, or do I just have to craft one from from some sheet metal? I have ordered the "Fiting a heat shield" information sheet from the Regalia page, but it will be few weeks before it arives I expect -- this may answer this question for me as well.

Thanks again, Ben
Ben

Ben

My recommendation would be to wait while the datasheet arrives. It is a very comprehensive sheet and when mine was made up to the drawing on the back page it looks very neat indeed.

Sadly I no longer work where I was when we did that as I had thought about commissioning a batch of them and then selling them.

They are very custom built really and when I acquired the car it had a very "heath-robinson" affair which I scrapped very early on!

Patience - good thing come to those who wait - and it will be worth it - I promise.

Paul
Paul Barrow

Hello,
We have been using Un-Leaded fuel for many years in this Country, and the concensus amongst old car owners and machinists seems to be that if a car has been run on regular leaded fuel in the past, and has not had the head off, then there is sufficient residual lead in the combustion chambers to lubricate the valve seats until the next overhaul.
Scott Barrow
Scott Barrow

This thread was discussed between 29/12/2002 and 07/01/2003

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