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MG Midget and Sprite Technical - A summer problem?



When warm the motor starts to hesitate at 120 km/h. When relative cold I can easily reach the top of 138 km/h. When warm (after driving in top speed for 2 – 3 minutes) I cant reach the top and I have to go back to 100 km/h. The problem starts with a change in the tone, it is sharper higher. It aggravates when I shut down the heater radiator, so I made a by pass (see photo). I thought I got rid of this problem with Peter Burgess valve guides and taking away the valve stem seals. But after 2 years it is back. I also tried to get rid of it wit a cold thermostat. Water temperature is never higher than 170 F. I use Mobil 15W40. The kind of fuel does not influence the problem. It is a 948 frogeye no changes to the original motor but KN filters. Everything blue printed. Do you have any more questions any diagnosis or any solutions?

Thanks Flip



Flip Brühl 948 frog 59

Im no Einstein, BUT Im guessing your problem is "Its a stock 948...speeding along at 75-80 mph""".....LOL

I have to ask...do you still make your little blue haired grandma run 5 miles at a 6:30 mile time before breakfast every morning also?....lol


sorry for eing so mean....but thats just funny!!!

prop
Prop

K & N filters usually require a change in needle to account for the extra air they flow. On the other hand if you didn't change the needle and it was running lean as a result I would expect it to be worse when cold.

Once cause of your symptoms can be spark plugs too high in heat range, causing overheating of the tip and consequently symptoms just like you describe.
Check the heat range is correct for the engine. If it is, I know you say the engine is standard but over the years successive encounters with machine shops can nudge the compression up quite a bit. You can get a good idea of the current situation by doing a compression test and comparing it with standard spec -if it is higher, it usually requires a cooler spark plug.
Paul Walbran

Flip,

Peter May's bronze guides and unleaded fuel? Does it happen when first driving the car from cold?

It could be the same problem that I had on our 1380 with one or more bronze guides nipping the exhaust valves when under load.

Do you have the standard front by-pass hose in place still? It defintiely helps with this problem to prevent local overheating around No.1 exhaust valve during warm-up.

Peter's suggestion was to open up the exhaust valve tappet clearance slightly (2-3 thou (0.05/0.08mm) to allow the valve to get more 'seat time', and thus cooling.

Also make sure that the timing is correct, definitely not retarded, and the mixture is never leaner than it should be. As a test try slightly richer mixture (runs cooler).

What cam are you running and what tappet clearances?

I had huge problems with another supplier's bronze guides, with sticking and one burning out, so went to Peter and had his guides fitted. Slight problem during the first few hundred miles, but none after he removed the exhaust stem seals (inlets remain) and opened up the exhaust tappet clearance - now back to Kent's recommended.

Richard
Richard Wale

This weekend I will first set the outlet clearance to .015 (Now .012) then try cooler spark plugs (now NKG BP ES 6). I don’t have a bypass hose, the compression is low, I already tried richer needles, it did help a tiny little bit. I am running a standard cam, and my granny died in 1978... see photo.

Flip


Flip Brühl 948 frog 59

>>> K & N filters usually require a change in needle to account for the extra air they flow. <<<

True, when applied to the later fiber air filters; but the original Frog filters were basically wire screens that couldn't have offered much resistance to flow.

Just a thought,

-:G:-
Gryf Ketcherside

Richard and others,

I use Peter Burgess instead of Peter May bronze guides and unleaded fuel.
Does it happen when first driving the car from cold? No only when warm?

Why is local overheating around No.1 exhaust valve during warm-up prevented by the by pass?

What cam are you running? Standard. And what tappet clearances? 0.012 now 0.015

Peter's suggestion was to open up the exhaust valve tappet clearance slightly (2-3 thou (0.05/0.08mm) to allow the valve to get more 'seat time', and thus cooling. This only did the trick!!!!

Tank you

Flip
Flip Brühl 948 frog 59

Flip,

Sorry, error, my second set of guides are from Peter Burgess and all the references to 'Peter' are to him, including the opening up of the exhaust valve tappet clearances!!

Unfortunately I changed two things at the same time, twice!! I went from cast iron guides + by-pass to supplier No. 1 bronze guides + no by-pass, then to Peter B's bronze guides and back to a by-pass. BUT, without the head to water pump by-pass there is less circulation around the front of the head, even with some small holes in the stat. The Metro does have a by-pass, but just not one using the pump to cylinder head hose.

Unleaded fuel burns slower and hotter than leaded and can lead to overheating, especially in a modified engine. So anything that can be done to help keep the valves, particularly exhaust, at their correct temperature has to be a good thing.

The same applies to the back, which is why the recommendation is to leave the heater tap open all the time, or fit a by-pass, as you have done.

Richard
Richard Wale

This thread was discussed between 15/05/2008 and 19/05/2008

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