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MG Midget and Sprite Technical - Uneven Tick Over

Over the winter I've carried out lots of work on my 1500 on brakes, suspension, bodywork etc and rarely ran the engine which ran well prior to the work.

The car has run poorly since the winter and after swapping plugs and leads and adjusted the timing which was strangely quite a way out, there was an improvement in starting and running but not smoothly when pulling away with a lack of power especially once the engine has warmed up and the choke in.

I have attached a link to a video of the engines uneven tick over with a warmed up engine, I have ruled out faulty plugs, leads and timing and refurbed the carbs last year. With the car running better with the choke out it points to needing more fuel but surely a slightly lean engine would not run so erratically?

https://www.youtube.com/shorts/KBPXtq209cs

Any thoughts?
Tim Lynam

2 possibilities spring to mind: are you sure you have got the carb gaskets the correct way round and not blocking the vacuum ports on the carb?
And
Erratic running like that could be an air leak on the inlet manifold. Possibly in the lower edge of the manifold obscured by the manifold itself?
GuyW

Guy, I checked the filters and they are on the correct way but did not check the gaskets, hopefully it is something as simple but it’s easy to miss the obvious so great to get others views, the manifolds haven’t been off for a couple of years but I can check for any possible leaks, thanks for your thoughts.
Tim Lynam

Before any dismantling, try spraying carb cleaner along the inlet manifold. An increase in revs shows a leak.
I only found this technique thanks to from people on this forum😃
Jeremy MkIII

When my 1275 suffered lack of power l found that the points carrier plate was sticking in the distributor body. With no advance possible at higher revs the engine ran like a bear. I also found that the dwell angle wasn't the same on each of the four corners of the cam. Using a meter is a better way of measuring dwell than getting drag on a feeler guage. Just my six pen'orth....
David Cox

Hi Tim, hope you are well - sorry to hear you are having more problems :-(

Nigel would always say to start from the beginning...

1. Compression?
2. Valve clearances correct?
3. Spark. Timing correct (you say you have checked this) and good spark?
4. Fuelling. When you rebuilt the carbs did you set the jets to the "default" 12 (IIRC?) flats down?

Are you pressing the throttle at all in that video? Or is it surging on it's own? A surging idle like that is normally a sign of lean running (I forget the how and why it does, but it does). Which if the carbs are set correctly, would suggest to me an air leak.

Some easy ones to check:

1. Vacuum pipes and vac pipe to dizzy connected?
2. The manifold nuts on the 1500 are a bugger for coming loose...
3. Carb gaskets/air filters as mentioned above.

Cheers,

Malc.
Malcolm

Thanks everyone for the items to check, several which should have been obvious but I always fear the worst and did not think of some suggested, so all comments very much appreciated.

Malc, yes another issue but hopefully it will make it to Curborough this year but it may be on a trailer to make sure!
Your question of was I touching the throttle in the video was no, it was ticking over on its own but after checking for any air leaks, the dizzy, vacuum pipes etc it appears the carbs were too lean which seemed to be the obvious but I'd partly discounted due to the carbs being set up with the normal flat adjustment after their rebuild last year and it had run fine since, so why the timing and carbs had drifted over winter is a mystery, but its now running far better after richening the carbs, not perfect but much better.


Tim Lynam

This thread was discussed between 03/04/2023 and 04/04/2023

MG Midget and Sprite Technical index

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