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MG Midget and Sprite Technical - Summer problem 3

Summer problem 3

Summer is over, evenings are cold, so I can drive my sprite on top speed for more than minutes. If I close the heating though, it will run uneven after 3 minutes and give less hp in the high rev range.
I went to a rolling road: The PO thinks it is the valve train. Why:
The air fuel ratio is good (between 12 and 17) At high speed there are sudden HP losses while unburned carbon comes in the exhaust smoke. Most likely unburned fuel from cylinder 3 and 4
Ignition All new 123 (excellent!), new leads, new spark plugs BP6 Es, new Bosch coil
I use a standard 948cc motor 30.000 miles from new, a standard cam,
standard valves, standard valve springs, Peter Burgess valve guides. Compression warm is even and very good

What gives the carbon and uneven running? What part of the valve train will I replace first?


Flip



Flip Brühl 948 frog 59

On cast/forged rocker assemblys it is my understanding that the shafts and bushing tend to wear alot...most likely this would be the cause of your valve train problems...might take alook at the bottom of the rubbing pads of the rocker arms for any grooving


prop
Prop

Prop,
I renewed the rocker assemblys shaft and Bushings, it did not help, and why is it hesitating only when warm? ar it the springs?
Flip
Flip Brühl 948 frog 59

Hi, spridget detectives,

I have been back to an other rolling road, the problem is independent from the motor (water) temperature! It happens only with the hood down, so it could very well be vapour lock. (Although the labda stayed the same when it happened)

What will I do first?

Replace the mechanical fuel pump for an electrical one?
Make a custom heat shield?
Make a hole behind the radiator so cool air comes to the H1 Carburettors?
Any suggestions welcome

Flip
Flip Brühl 948 frog 59

Flip,

Peter Burgess guides - bronze? Bronze guides can 'nip' the exhaust valve stems, particularly during warm-up. How old/new are the valves/guides?

Do you still have the by-pass in place?

Do you have valve stem oil seals on the exhaust valves? Inlets must have, exhausts not.

Is it a 'hard' misfire, like a lead come off, or a gentle reduction like running out of fuel? Also any mechanical noise at the same time?

Try opening up the exhaust tappets to 0.016" (0.4mm) and richen the mixture up a little.

Been there done all of this!!

Richard
Richard Wale

Richard,
Yes Burgess guides, 30000 miles the problem is since 30000 miles, no valve stem oil seals on the exaust valves, it is a gentle loss of hp, but the exaust sound changes first it is harsher sharper, I tried opening up til 15 thou, mixture is rich enough I changed the GG nedle for a richer EB.

Thank you for the response
Flip

Flip Brühl 948 frog 59

I didnt relealize you had an old engine with the mech. fuel pump that rides on the camshaft....Id certianly give that a go and change to an electrical fuel pump maybe with a fuel regulator as well...and see if that does anything.

prop
Prop

I have noticed a similar problem in the summer time with my 1971 1275. When the ambient temperature is over about 30°C and the engine is at normal running temperature (90°C), the car misses and stumbles at 4,000 RPM especially when under a load.

It is my conclusion that it is related to modern fuel formulations that are incompatible with the S.U carbs that sit right above the exhaust manifold. When the missing occurs, it is normally accompanied by a strong fuel odor which would be indicative of flooding or vaporizing of fuel in the bowls. I have changed needles and seats and floats and the problem persists. A rich or lean mixture makes no difference as the problem seems to occur upstream.

My theory is that ethanol and other additives to modern fuel make it less temperature stable than 98 octane leaded fuel available when the cars were designed. Modern fuel injection designs would clearly be less prone to vapor locking.

My car is running hotter than it should be in the summer (due to cylinder wear I believe) and this has accentuated the problem.
Glenn

Glenn,

I just installed an electrical fuel pump, it looks like it solved my summer problem . I am not sure about it, as the air in Holland now is 14 degrees Celsius. The hot spot! exhaust manifold generates so much heath that I cannot tooth the intake manifold. The float chambers are so hot I can hardly touch them. I use the rubber distant pieces on the left side of the heat shield. Will it help if I wrap the outlet manifold with heat tape or will it crack?

Flip



Flip Brühl 948 frog 59

Flip, ive wrapped my exhaust manifold to "cool" the underbonnet tempeture.
It did help a lot(no actual mesherments) and was very easy to do.
I bought the heatwrap from Biesheuvel in Holland for about € 20,- and used RVS hoseclamps to tighthen it.

My exh.manifold is steel but I guess it doesnt have a bad effect on a cast iron manifold either but do inform to make sure.


Arie de Best

Thank You Ari,What power plant is this?

Flip
Flip Brühl 948 frog 59

Rover 1.8 K-series (MGF)
Arie de Best

Beware of the heat wrap!! where is that heat going if it can't escape from the manifold?? Mine stayed in the head near the twin exhaust ports (1500) New studs and headgasket required. The problem of heat is made worse by removing the engine driven fan. Improving airflow over the carbs helps, ether by ducting air to them or by allowing the hot air out somehow helps! A good fab shop can press louvres straight in to a standard bonnet, this can look great and very period. IMHO!

Carl
C Bintcliffe

might be better wrapping the float bowls instead

Airflow through may be improved if you run without the fill-in panels between the rad and the inner wings I suspect

I dont have those on my car (they'd been removed when she was an Arkley and I didnt fit replacements)

I have not yet had any carb overheating issues even though the HIF sits right above one of the branches of the Peco 'zorst pipe

I put this down to less flow restriction across the compartment
W Mohan

So I'll make a custom heat shield. And not wrap. Run without the fill-in panels is easy to do but what about mud and so on.

Flip
Flip Brühl 948 frog 59

Flip,
I had the fuel vapourization problem with my 1500.

Cured it by re- fitting the belt driven fan (like Carl says) which at low revs at least allows some cooler air into the engine room. Not the case with electric fans.

The problem only accurred while the car was stationary or travelling slowly. It never happened when the car was seriously on the move.

Strange that you should have the problem around 4000 rpm.

Dave
Penwithian

Flip

Lara has been on the road in all sorts of weather conditions since 2000-ish

I have never needed to wash mud away from the underbonnet area yet

I don't have those panels on either side and don't have wet coil problems either

(excuse name change the BBS had reverted to base setting today and I became HIM again)
Bill

Flip,

i wrapped my LCB manifold, it lasted a year before it was cracked in half.
Brad
B Richards

Heres A photo of it,

cheers
Brad


B Richards

Brad, you must have wrapped it to tight! ;-)


Ive had the same hapening many times on my unwrapped 1500 Peco exhaust manifold.
At the end i couldnt even weld it any more... never had that problem with the Peter May manifold tho.
Arie de Best

I came across this old thread: Peter Burgess solved the problem: a cracked head. Loose valve seats causing detonation when hot.

Flip


Flip Brühl

This thread was discussed between 21/09/2008 and 18/10/2018

MG Midget and Sprite Technical index

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