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MG Midget and Sprite Technical - Funny Noise

Just counted up the threads against my name, 1/3 of the technical threads are mine! I wonder what that says about the state of my car? :-D

Anyway, blatting down the M5 today I had to go through a 50 mph average speed section. 50 mph, 4th gear, 3000 RPM, very light throttle just to maintain momentum, I was getting a noise that I could only think sounded like a noisy popcorn machine!

Foot off pedal completely, it goes. Push the pedal harder and go up to 3200 or so RPM, it goes.

Temperature was OK. My lambda readout seemed to say I was a touch rich, maybe 14.0 AFR.

Thoughts? Pinging? Something resonating?

Thank you again everyone!

Malcolm
Malcolm Le Chevalier

Loose valve

loose exhauste

piston making a prision escape... mmm probably not

nor would i think beaings

although.. maybe the beginnings of thurst washers ... common in 1500... easy to fix i understand

maybe loose rear suspension hardware .... that can bang and pop

tranny crossmember secure? And the tranny attached to it securely

is the beating sound random and non repeatable or rythmic with a pattern based on what you do with the gas peddle

prop
prop

Pinking ?

Try disconnecting your vaccuum advance and see if it still does it...

(advice given to me recently by Peter Burgess)
Andy Phillips (frankenfrog)

I had forgotten about this...

Yes Prop it's a very repeatable effect. Just get it in the sweet (unsweet) spot and the noise comes back. I thought due to this it probably is engine related and maybe pinking.

I could test it by disconnecting the vac. assuming it is, what is the long term solution? Could a slight mixture alteration help?

Cheers,
Malcolm
Malcolm Le Chevalier

This has made me go and dig out my info on my distributor.

In reference to the attached:

- The 41449 is the standard dizzy for the Spit and midget 1500, it comes with a 3-18-7 vac can.
- In terms of the the mechanical advance, my plate is stamped 8, thus giving 16 crank degrees total, opposed to the standard 41449 which only has 14. I assumed the max advance speed is the same, this is how I have derived the assumed mech. advance curve for my dizzy.
- The vac advance is as per the numbers on my can (5-12-6) vs the standard 41449 can.

At the problem point, 3000 rpm, I could have around two degrees extra mech advance. Plus, assuming pulling around 10 - 14 inHg of vac, 2 - 4 degs of vac advance. Totalling up to 6 degrees more advance.

Standard 41449: ~ 31 degs advance
Mine: ~ 37 degs advance

So... too much advance seems likely, right? What do you think I should do short/long term? Is there a way to limit the vac advance somehow?

Cheers,

Malc.


Malcolm Le Chevalier

Try again with graphs, things work better portrait on here.


Malcolm Le Chevalier

37° @ 3000 rpm

yepp id call that high... i want to say normal is around 38° at 4500 rpm

here is a good PDA file artical that describes a little more indepth

prop

www.firstfives.org › faq › timing_for_m
prop

Sorry malc

i cant figure out how to trasfer a PDA file .

i googled ... bmc advance at 4500 rpm

and that gave several good articals including a pda file just for the lucas distrubuter both 25 and the 45? And accordingly

prop
prop

Malc,
The number of threads says absolutely nothing about your car and everything about the amount of time on your hands :-) Get a bloody job you layabout and stop sponging off your wife.... ha ha

It does sound like a bit of pinking and, as said, disconnect the vac advance as a quick check. You could try it a little richer also but I think the long term solution is a mappable ignition unit.

I'll do you a deal... you buy it, test it and if you don't want to keep it I'll take it off your hands in lieu of storage costs for the B. What say you?

Best of....
MGmike
M McAndrew

PS. AFR of 12.5 to 13 would be better (according to Garret Turbo artical :-)
M McAndrew

"""And everything about the amount of time on your hands :-) Get a bloody job you layabout and stop sponging off your wife.... ha ha """

When i grow up i want to be just like my hero... (( Malc ))...haha

prop
prop

14 Isn't rich - anything below 10.5 might be and at 7-5 the choke is stuck. There are always certain speeds that resonate and you may have found one.

If your speedo is 100% accurate your car has a 3.9 diff and 70 profile tyres.
Daniel Stapleton

Prop, thanks I will have a rummage around the internet.

Mike, I am working on finding a job. Stories of shortages of skilled and educated workers appear to be grossly exaggerated!

Daniel, I meant richer than stoiwhatsit at least. yes tyres are 155/70s, diff is 3.9 and speedo is accurate.
Malcolm Le Chevalier

With my new Weber 38DGAS i get 10.5 if i floor it from a standstill and between 12 and 16 when driving normally.
Andy Phillips (frankenfrog)

This thread was discussed between 06/05/2016 and 11/05/2016

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