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MG Midget and Sprite Technical - wont rev past 3500rpm

Thanks to help from the is forum I've manged to get my 1972 midget back on the road after an 8 year lay up.

Recommissioning included replacing a valve and guide as the value was stuck and overhauling the carbs. One of the jets was seized.

After this and with some fresh fuel and some carb fettling it ran beautifully and ticked over smoothly.

A few short trips later and now it wont rev past 3500 rpm. there's a slight miss and for want of a better description it feels as though it's being held back. It still ticks over smoothly and runs sweetly up to 3500rpm.

Judging by the colour of the plugs the mixture is fine. I must admit timing has only roughly setup. Turned the distributer until the best tick over obtained and then backed off slightly.

Carbs have been balanced and flooding float bowls fixed.

My first though was vacuum advance and I thought I'd cracked it when I found a leaking vacuum pipe. Replaced this and no improvement.

Plugs and rotor arm replaced. Leads and cap were replaced just before the car was laid up.

Any suggestions as to what to try next?

Thanks

Bob
Rob Storer

bob,

does the replacement rotor arm have a rivet?
If it does that maybe the culprit if it doesn't then a blocked fuel filter if fitted?
Jeremy MkIII

Hi Jeremy,

Good suggestions. I'll check both of these out. TBH I don't think there is fuel filter which isn't good.
Rob Storer

It sounds more like an air leak into the inlet manifold. Check manifold nut tightness.
Rob
MG Moneypit

My 1967 Sprite was revving freely enough before I dismantled it but I had a surprise yesterday when I cleaned the distributor and found that the outer contact point was completely loose and slopping around in its mounting. I would have expected that to cause some sort of misfire or loss of performance. It just might be worth looking inside your distributor to check that everything is in order.

The points were from what I thought was a reliable source and had only done 3000 miles. Replaced with genuine Lucas in a sealed pack that I found in my Morris's glove box, I must have had them for over 40 years!
C Mee

Has the dist. shaft seized limiting mechanical advance
Grab hold of the rotor button and twist it and see if it moves and is spring loaded back as well, it could have broken/rusted springs , it should move 'rougly' 10degrees and spring back
William Revit

Are you sure the throttles are opening fully?
Chris at Octarine Services

Thanks everyone, lots to look at. I'll report back when I've tried the suggestions.
Rob Storer

The title of the thread ('Won't rev past 3500rpm) reminds me of a Tommy Cooper joke, to which in this context the reply might be 'Rev to 3400rpm' then!

Interesting to hear what the problem is.

Cheers
Mike

PS The Tommy Coper joke is 'Went to the doctor the other day, I said 'It hurts when I do this' (raises arm) - he said 'don't do it'!

M Wood

😂
Greybeard

>>Are you sure the throttles are opening fully?<< (said Chris).
Probably nothing to do with the problem but I suddenly remembered seeing similar on somebody's Mini two or three years ago. Turned out the throttle cable adjustment had slipped through the clamp screw.
In that instance I ended up making up a Venhill cable for him.
Greybeard

Lol - that why I suggested it - since he has had the carbs off for fettling, the cable may not be correctly adjusted.
Chris at Octarine Services

and the correct answer was...........

Intel manifold air leak - thank you MG Moneypit!

I had loosened off the carbs to fit the p clips that hold the fuel line in place and hadn't fully tightened them back up. Fault diagnosed by spraying carb cleaner on all the joints.

Thanks to all of your for the suggestions.
Rob Storer

This thread was discussed between 19/05/2020 and 22/05/2020

MG Midget and Sprite Technical index

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