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MG Midget and Sprite Technical - Fuel coming back through the carbs...

First time working with a cam as hefty as in this engine and thought might as well pick some brains here before she goes off to "a man that knows" some time soon.

Got the 948 (649 cam) running today but as she comes down the revs shes kicking fuel back out through the carb intakes. Think the oil in the dampers is a bit thick as they return too slowly currently and guess probably this doing it (had to stop due need to get a train) but before do anything silly is this problem caused by anything obvious?

http://www.youtube.com/watch?v=3TNzQpEuSMo&feature=youtu.be
D Paul

Lean mix would be my first point.
And I think you could use a bit more advance.
But other than that it sounds good on my phone.

Lean mix can account for the carbs blowing back (worse with a cold engine) and it sounds like it is not that willing to rev that is where more advance would come in to play.

What kind of advance is it at at max advance?
Onno K

Hmmm... perhaps put a vac gauge on the intake manifold and consult the internet for what the reading means


Thats alot of fuel being shot out the carb

What about a blown manifold gasket...exhaust gas are reverse pressureizing the intake...yeah, very unlikely

But i can certianly see where Onno is commig from, its a head scratcher

Is it out of both carbs or just the rear carb

Prop
Prop and the Blackhole Midget

I might be talking rubbish BUT are you certain an inlet valve isn't sticking closed every now and again?
Daniel Thirteen-Twelve

I'm with Onno , I think I remember had similar thing years ago after changing heads and the needles were way to lean , it coughed and wouldn't rev, changed needles problem solved.
Ed H

I would suspect delayed cam timing, or perhaps valve clearances.

If you're using 1.5 ratio rockers remember the valve clearances need to be a bit wider than when using standard rockers, or you'll increase cam duration.

Then again, a cam with lots of overlap will often spit back a bit at low revs, especially if you're using a short inlet manifold.
Growler

How hot did you get it? I'd let it warm up to something approaching normal and the see how it is.

It sounds like what was described to me as " cylinder wall dumping"
d cusworth

Well i guess Paul blew himself up while smoking around it...i wish pual would come back,,, its an intresting dilma

Rest in peace Paul D.....it is halloween, so happy hauntings

Prop
Prop and the Blackhole Midget

This is out of both carbs, so unlikely any one cylinder problem. Are you sure the valve timing is right?
Art Pearse

I've thankfully not blown myself up :)

Just haven't got time to get in the garage apart from the weekends so will be back with some updates over the weekend.

Cam timing:
The cam unfortunately is unknown, but from all the figures I could work out it appears to be of the 649 profile and lift. The engine had been built over 25years ago by some BMC garage owners. The valve timing appeared appropriate when I checked it (sorry was over 6months ago and can't remember for life of me the specifics but was happy with it at the time)

Ignition timing: hasn't been played with at all yet.

Fueling: Hasn't been adjusted yet. The needles in the carbs are unknown as they are a very early set of HS2s off an unknown application and so could be miles out.

The fuel was Originally only being ejected from the front carb. But on that last run up that videod it seems to have been coming from both, will double check as was me in the car that time and had been me observing (fire extinguisher in hand) on previous run ups.

Yes was run up to operating temperature and good oil pressure (65 ish). Unfortunately revs are all on the ears at the moment as appears to be an issue with tacho: not registering until I would guestimate around 3000rpm when starts bouncing from 0-500. And if the cams as hefty as I presume then only just about getting it onto cam to be honest so most of that video is it off cam.

Will get the tacho sorted, check the ignition timing, and stick a colourtune in over the weekend I think. I think shes running really lean as the exhaust silencer isn't remotely coloured/a bit of tissue held over the end didn't pick up any black at all....

D Paul

Well we will hear when you checked it but that does sound very lean.

Don't worry about all the possible faults with cam timing etc.
It is easier to check all the simple things first and only worry about the difficult things when the simple things don't work!
Onno K

But the simple things dont explain back pressure!
Art Pearse

Never worked with a high overlap cam with poorly adjusted carbs right
Onno K

http://www.youtube.com/watch?v=St44TDWIxLk&feature=g-upl

Sounding much better, pushed her up to 5k rpm now. Oil pressure 60 - but think got an air bubble in the line.

Timing was miles out
Fueling was miles out

Fuelings better now, probably a bit rich, need some different needles, will get it mapped on rolling road to give some direction there.

Bit smokey (thought had blown head gasket or a ring but compression tests are ok) but that seems to be getting slowly better so presume just the rings and valve guides bedding in.

Few bits and bobs to finish off, time for an MOT and then some road work me thinks

Thanks for the help guys.
D Paul

This thread was discussed between 28/10/2012 and 03/11/2012

MG Midget and Sprite Technical index

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