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MG Midget and Sprite Technical - Gentle popping sound

- - - from the exhaust, when going downhill. A soft raspberry noise. I would have thought weak mixture but if anything the plugs are on the blacker side of a healthy grey.

It's also misfiring at low revs when cold, requiring a fair bit of juggling with the choke until warmed up. Not what I would expect at this time of the year although we have been having chilly days lately.
GuyW

Guy, Here's my two pennyworth.It suggests a fuelling issue. A popping on overrun usually suggests a leak in the exhaust system. If the system is gas tight is the manifold/ gasket joint ok and the nuts tight. Bob
Bob Beaumont

A raspberry on the over-run, sounds like you've got a touch of the Morris Minors!
C Mee

Interesting Guy as mine is doing the same atm. A very gentle popping on the over run and a misfire on start up. I'm using BP super unleaded with Millers VSPE additive (thanks Nigel).
The latter I suspect is a plug/s (NGK 5BPES) as it clears with a blip of the throttle and only reappears the next time the engine is started from cold.
Earlier popping was as Bob described, a leaking manifold gasket so hope it may that again.
Definitely not in the Morris Minor league though Colin!
Jeremy MkIII

Ah Jeremy, it must be that there EEEEE TEN stuff then!

I know the Moggy raspberry, accentuated by the skinny exhaust they have. It's not like that though, it's a softer putt putt noise. I was wondering if it's a leaky valve but I will check for manifold leaks first. Compression test time as well.
GuyW

Hadn't thought about a compression test Guy. I did the tappets a few months ago so will copy you there and see what it shows.
Jeremy MkIII

Well Bob was right - again! The manifold nuts were a little loose, about half to one full turn on each. Tightening them and the popping noise has gone and it is running more smoothly as well. Thanks. Easy fix!

Its not the end of the story though as I also did a compression test. Its a low compression engine, at least compared to the other car. I am getting 165 readings , both wet and dry , on 3 clinders, but down to 155 on one. As that doesn't increase with oil it suggests a slightly deficient valve I think. Not yet critical for a low tune road car but something to keep an eye on. I usually do a compression test every 3000 miles, just as a routine check. You build up a picture after a while. The other car which is somewhat modified, consistently gets 125psi readings.

This is all still shake down for the frog. Its only just gone over 1000 miles since restoration. But the engine was just installed as it was, after a brief inspection. I bought it some years ago from Nick Veuger (remember him?). He was good with engineering and had said it was ok.

Now I do have a noisy n/s wheel bearing. As the bearings were new I wonder if it is loose in the hub. That does need checking.
GuyW

Guy
Why does the other, higher performance, car have lower compression figures?
Bill Bretherton

Because its a mistype!
225! Its around 10.5:1 CR using a skimmed MG metro head. 😁
GuyW

😂😂😂
Bill Bretherton

Guy

Glad it was an easy fix!

Not sure about the bearing but nipping it up a tad may do it. Years ago I put in new bearings and one made a noise after a couple of months use. I pumped more grease in (even though I coated them carefully beforehand) and voila the noise went. They are still in the car now!

I have got some NOS rhp bearings if needed!!
Bob Beaumont

Bob, with rear wheel bearings I usually set them in with some bearing locktight but think I had run out when I assembled the axle this time. I do know about getting the correct thickness paper but it seems a fine balance between gripping the bearing or sealing the hub. I prefer to add some locktight to secure the bearing.
GuyW

Ah sorry, thought you meant front ones! I have a stash of original hub gaskets and have not experienced leaks with careful assembly and the use of Hylomar. I find progressivly tightening the countersunk screws helps snug it all down. I use the wheel nuts with large penny washers to pull the hub up tight to the halfshaft and drum. The countersunk screws often go up a further 1/2 turn!
Bob Beaumont

This thread was discussed between 14/06/2022 and 18/06/2022

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