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MG Midget and Sprite Technical - Could overheating be due to exhaust problem

The mkiv sprite rebuild continues to take two steps forward and then one back. Spent the weekend flushing out rad block and heater. Fitted new water pump and thermostat etc. All back together and running with no leaks.

Problem is that after just a couple of minutes running from cold the top of the block around carbs gets incredibly hot incredibly quickly. Especially manifold (lcb). Then get hint of misfire. Have to shut down, for fear of doing damage.

I've got a theory that the old single box pipe might be partially blocked. It had'nt seen any action for ten years and I don't know what sort of life it led before that. Although it looks ok from the outside could it be kacked up inside enough to give these symptoms? Any other ideas?

Thanks for any of your thoughts.
M Adams

HT leads in wrong order?
N Atkins

Will go and check but pretty sure it would be running pig rough if that was the case. Mine starts easily and runs well for a while.
M Adams

>>pretty sure it would be running pig rough if that was the case<<
not necessarily

I've seen it done by an experienced mechanic, we can all make simple mistakes
N Atkins

Sorry 'pig rough' was not meant as a contradictory statement, honestly thought it would be difficult to even start engine if the firing order was wrong. I'm quite good at making simple mistakes and don't have loads of experience so its allways better to start with what you guys consider obvious when I have a query.

Had another look and am happy that leads are going where they should.
M Adams

Air lock maybe?
Graham P 1330 Frogeye

no worries I can't read well, right well, only have (many bad) experience rather than any mechanical knowledge or skill


>>honestly thought it would be difficult to even start engine if the firing order was wrong<<
not necessarily, see my previous post
"I've seen it done by an experienced mechanic, we can all make simple mistakes"

and it will get one area or plug very hot

not that I'm saying this will be the problem but it's best to go back to basics and recheck that replace good components (well most times anyway, sometimes you have to replace for the sake of sanity)

you need logical step by step diagnostics and allowance for faulty parts and aged wiring with many problems like this

a crypton machine will pick up stuff that can easily be missed too

N Atkins

I concur with air lock, what does the temp gauge say?

Dave Barrow

sorry I was going to add - unless your engine is more than standard (it will fire up with wrong firing order)
N Atkins

mine was doing the same last week, the timing and mixture was out. now runs alot cooler!!!
james gibson

air lock

when engine is cold check the radiator and expansion tank for coolant level (just in case)

replace rad plug but leave off expansion pressure cap

start engine and run, keep checking for hot spot and temp gauge

run to normal running temperature whilst keeping an eye on hot stop, temp gauge and expansion tank for air bubbles

run for 10 minutes or so after reaching normal running temprature with pressure cap still off

let the car fully cool down, check level in expansion tank and report back (to others)
N Atkins

Thanks for all of the advice. I think I was hoping it would be a dodgy exhaust mainly because I fancy putting a quieter standard double silencer on and could kill two birds with one stone. I think my wallet would prefer it if I have a better look at the air lock and maybe mixture. Thanks N Atkins for the step by step. Will follow it and see what happens.

M Adams

Yes, clogged exhaust can do it, and they are a favourite place for mousie nests when sitting, as well as rusted baffles etc. Very easy check with a vacuum gauge on inlet manifold vac: if the ex is clogged, vac will start normal c18"Hg, and then drop off as engine runs. If partial clog it will stay normal at idle but drop as engine rpm is increased. Either way it will cause overheat, especially of ex valves.

FR Millmore

Mark, make sure your heater valve is open too

Did you originally follow the owners Handbook instructions on refilling?

cut down version of it here:

1) refit drain plugs and open heater valve

2) fill system thro radiator orifice and refit radiator filler plug

3) half fill expansion tank and refit pressure cap

4) run engine at fast idle for 30 seconds

5) stop engine, top up system through radiator filler

6) refit radiator filler plug and run engine until normal operating temperature

7) stop engine and let it fully cool then top up expansion tank to half full

I've followed this to the letter 4 times with my present midget (don't ask why) and worked like a treat each time

bl**dy handy book that is
N Atkins


take it off at manifold joint and check and replace one section at a time, clip fit only will do for test

be pepared for potentially a lot of mess and noise so don't try it at night unless you want to meet the neighbours :)
N Atkins

Heater valve is def open. Top up was pretty much as you said except localised heat build up made me chicken out of runninng it up to temp. There was even some sort of vapour rising from somewhere around lower manifold area (sorry should have mentioned that before).

Am going to try the airlock solution hopefully tomorrow. Still got a gut feeling its exhaust related so will try and borrow vac gauge as FRM suggests.

Also, dont know if this is related but the carb air filters are the 'pancake style' and I can't find replacement sponge elements. Can I clean the ones that are there, they look pretty old and dirty.

Once again thanks for all help and suggestions everyone.
M Adams

sorry just found replacement pancake filters but they were listed for 1500 midgets are they not supposed to be on my 1275?
M Adams

will need a photo of these filters as there are different types of pancake filter with different filter materials

also remember owners Handbook for prevention and Haynes for repair (if it covers it or in any detail, I'm not a Haynes fan)

vapour could be leak or spillage so could be important
N Atkins

photo of filters

M Adams

Mark on that photo the foam and wire meshes look quite reasonable, unless the foam started off white :)

for now I'd just vacuum them whilst they are retained by the inner and outer wire mesh frames so as to not distort the foam out of shape

and start at low suction in case the foam starts to disintergrate

you need to keep the filtering on the carbs in case there is a nest in the exhaust and muck goes every where

for later fully cleaning see -

personally those filters are not the type I like, messy business oiling them and they look so narrow

there should be a gasket between the back filter plate and the carb body but don't worry about that for now

with the help of another paair of eyes (glamerious assistaint) go from point 2) of Handbook coolant refill but keep eyes on hot spot, temp gauge and looking for vapour rising
N Atkins

Thanks N A

Followed your anti airlock start up routine with the back box off of exhaust. When it was all warmed up and off the choke the tickover settled down at 1500rpm. Obviously need to reset that, however engine was nice and smooth temp gauge went up to 80 then stayed there, oil pressure read a steady 60psi. No noticeable hot spot, so well pleased.

I think that the vapour I mentioned was due to a build up of dust on the previously overheating manifold. there are bits that were difficult to get in and clean off after its long lay up. Had another go and no vapour now.

I like the look of the filters but as you say not the easiest to service.

Thx for all help everyone.
M Adams

Mark thank you for saying thank you

you'll save yourself such problems in the future if you get a paper copy of the owners Handbook (Ref: 0057) -

Haynes for repair, owners Handbook for prevention

not that I think the Haynes is that good anyway

electronic copies of Handbooks, (factory) parts Catalogues and fisches and factory Workshop Manual -

personally I much prefer K&N, fit and forget, service clean and reoil at 50,000 miles in normal use

they look red like this when new or on show cars but soon go grey with normal regular use

N Atkins

but soon go grey with normal regular use

N Atkins

This thread was discussed between 12/07/2011 and 14/07/2011

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