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MG Midget and Sprite Technical - Missing under power

I had the opportunity to spend the day at a local car show yesterday about 40 mile away yesterday. Not much of a crowd, though there were 2 midgets for sale.
On the way home I decided to take the highway and as I was under power heading up the hills at around 70mph in top gear, I noticed a small amount of hesitation as the car started to miss. rpm around 4200.
The car is a 1973 1275 pretty much standard. I have just but stacks on the HS2 and K&N filters.
Its electrics are standard and plugs points and condensor were replaced 3 years ago and I believe the coil is considerably older. I am leaning towards the condensor and is it a good time to move to electronic ignition.
Thanks,
Allan
Allan Jacks

Allan

Thoughts:

Was the misfire immediately apparent when you got to 70mph up hill, or did it take 10-20 sec running under those conditions to appear? If immeditate, it could be ignition like you say, or it could also be mixture related.

Mixture - Have you still got the std carb needles fitted?
K&N's usually require a thinner needle than std.

If the misfire took the 10-20 sec, check your spark plugs for correct heat range - if it is a little too high, under these circumstances the plugs gradually get everheated and when hot enough cause a slight misfire. If this is your symptom, check the plugs are the specified ones, and even if they are check the condition of the insulator adjacent to the electrode for signs of overheating.

Paul Walbran


Along the lines of what Paul said, putting stacks on the carbs (I assume the low profile stacks ala Vizard?) could mean you are getting a little bit of ram effect at that rpm, which means more air, which means it needs more fuel that it's not getting. You may be leaning out.
OR--if the A/F ratio is still correct, then a rammed charge is creating higher cylinder pressures and starting to challenge the spark--again, per Paul's statement regarding the spark plug heat range.
JM Morris

Yes it did take about 10-20 seconds running under those conditions. It seemed to pull up the start of the hill just fine. I do have the standard carb needles fitted. I am going to take a look at the plugs today to see what they look like. I believe the plugs are either NGK or Bosch recommended ones.
Thanks for the quick response.
Allan
Allan Jacks

just slightly increasing the fuel mixture could help both sugested problems

it compensates for the increased airflow due to the addition of the sub stacks and freeflow filters

and a slightly rich fuel mixture burns cooler thus not overheating the plugs

worth a try as it is free
Onno Könemann

Agree with Onno - and note that conversely a slightly lean mixture can overheat plugs, so a slight richening will work if that's the case too.

Generally, when an engine is modified to produce more power, the spark plugs will run hotter as a result, so a change in heat range is often needed.

Even though the engine is basically standard, if the engine has been overhauled in the past (as is likely with most of our cars) then the various machining operations can combine to result in an small increase in compression ratio. (For example, bored to +020 and 010 off both head and block to true them up would raise CR to 9.4:1) Together with the K&N and stacks, this could be enough to trigger a need for a grade cooler in spark plugs.

By way of example, we once had a 1275 which increased CR to 9.6:1, fitted an LCB and a mild fast road camshaft. Mixture set on RR. Spark plugs had to drop 2 steps in heat range.
Paul Walbran

I honestly did not think such a small change would have made such a difference to the engine. I do know that the engine is still in original condition as the car has only covered 19,000 miles. I just thought I would make some of the bolt on options as per vizard. I had taken off the air pump, then put these on. Next will be manifold. At that time I am expecting to be looking at other needles.
On a seperate note for Paul. I was at a Car show yesterday and one of the people in the market was selling AA emblems and they had about 20 or so from your neck of the woods. I was thinking of getting one as I use to live their and thought it would be a nice touch.
Allan
Allan Jacks

19000 miles! Cool - it must drive like a new one!

Paul Walbran

Alan,
I would also check the valve clearances. If they are a little tight it would produce a similar effect.

Guy
Guy Weller

Paul,
Not sure if it drives like a new one. It still has its problems, but it is very tight.
Guy thanks I will take a look at that also.
Thanks
Allan
Allan Jacks

If you richen the mixture and reset the exhaust valve tappet clearance (lash) to 0.015", instead of the standard 0.012" (leave inlet standard), and the problem goes away, it would generally indicate that the additional heat being generated under power is causing a slight 'nip' on one of the exhaust valve stems - more common with bronze guides than the standard cast iron ones.

Also make sure that you leave the heater tap open permanently to give good water circulation around the rear of the engine.

Richard
Richard Wale

Pop the dizzy cap and check the inside of the cap and the rotor.
tomshobby

This thread was discussed between 22/05/2010 and 24/05/2010

MG Midget and Sprite Technical index

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