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MG Midget and Sprite Technical - Weber miss-fire?

midget 1973
1430 stage 2
Weber 45
Lumenition optronic ignition

OK, here we go again.
This problem has not worried me for a while as I will explain.

I only get this problem above 4,000 rpm, I recently fitted a Frontline 5 speed box so my cruising rpm has dropped to 3,500.

If I'm say cruising down the motorway and I increase the rpm to abouve 4,000 for more than a few seconds the car missfires VERY badly. I can rev through this missfire but as soon as I drop the revs to a steady rate it starts again.
Once I have induced this miss-fire it will not go away until I come back to the car the next day!
I am convinced that this is carburettor related although I may be wrong.
The fact that the problem will not go away leads to to think of some kind of flooding???
I've had this problem for many years, getting the 5 speed box has made the car driveable on the motorway again but I really would like to get to the bottom of this.
On another note, thanks Anthony for your advice re the ballast resistor. It was this that was causing my starting problems. All sorted now with a new resistor.
Mallorcaben

It could be lots of things.

I'd find a quiet road, induce the fault, engage neutral, switch engine off and pull the spark plugs followed by the top cover of the DCOE.

The actual spec of the cam and the complete calibration spec of the DCOE would help.

Of course it's possible the optronic is on it's way out.
Daniel Thirteen-Twelve

"The fact that the problem will not go away leads to to think of some kind of flooding???"

The fact that the problem will not go away leads me to think of muck or crap in a jet that comes afloat inside the float chamber after a certain pitch of revs is induced, gets into a jet or emulsion tube and after settling overnight doesn't get "aroused" again until or unless you do the same revs juggling about again.

And as Daniel warns the Optronic might be preparing for a long holiday on a shelf in your garage (or... do you actually throw things away that pack up? I have an awful habit of saving these things "just in case"...)

The advice Daniel gives about finding the fault is very good
Bill sdgpm

In my experience, this sort of a misfire is often caused by loose chokes or veturies in the carb bodies. There is a little screw under the body of each choke that secures these in, if this comes loose (which it can) the chokes will vibrate at certain engine rev frequencies and cause a misfire....

Mark.
M T Boldry

Mark makes an interesting point that I haven't come across before. On my car I have lockwired those securing screws since they came with a ready made hole.

Bill, anytime you want to trade dead/dying electronic ignition systems let me know - I have a small collection including really old stuff (Mobolec??) that came with s/h distributors.
Daniel Thirteen-Twelve

Daniel

some place I have an old Ford/Lucas unit (both the dizzy and the amp) that I started to adapt for the Midget by fitting a dog drive instead of the Ford bevel drive

Yours when I can dig it out of storage

Still has the chunk of inner wing that was on it when my dad scavenged it first many years ago from a reluctant scrapyard donor.
Bill sdgpm

Ignition fairly recently replaced, did not change the problem.
No, Im fairly certain that its the carb. Anything else specifically to look for, or are we talking removal & strip down. Not looking forward to that! I was going to order a Weber service kit.
Any recources on the net re servicing Webers?
Mallorcaben

try
www.teglerizer.com/dcoe/weber_service_manual.pdf

I googled 'DCOE service' :P
robnrrugby

The last time you enquired about this problem was a while ago wasn't it Ben? At that time I was not aware you could "drive" through the misfire and thus it did sound like the choke retaining screws had come loose. If the chokes move then the main jet is effectively blocked and with only pilot jets fuelling the engine 60 MPH would be about the limit!!

However if you just get a misfire and can then drive through the problem and enjoy power at higher revs then the chokes moving and blocking the main jets is not the problem.

It is possible that the weber is not jetted correctly and is suffering at mid RPMs.

If you could remove the jets and note the values and also look at the chokes for the sze of the chokes then I could tell you if the fuelling is good enough or suggest which jets you should use. The jetting of a weber is very much determined by the size of the chokes so that is important to know.
Robert (Bob) Midget Turbo

This thread was discussed between 17/02/2009 and 18/02/2009

MG Midget and Sprite Technical index

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