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MG Midget and Sprite Technical - Misfire under load

Hi,

I'm hoping someone can help me with a poorly 1275 A series. When I accelerate briskly or am travelling up hill then there is what feels (and sounds) like a misfire combined with much increased vibration.

The engine is fairly standard (stage 2 head and fast road cam being the only modifications) and rev's freely with no load. The ignition system is an Accuspark dizzy fitted with a Pertonix Ignitor. I have the stock air filters fitted.

So far in an effort to figure out where the problem is I have:

Changed the coil
Changed the plugs, leads, rotor arm and dizzy cap
Checked the compression (all good, 170, 180, 180, 180)
Checked the valve clearances (16 thou as per the cam spec)
Checked the mixture
Changed the air filter elements (getting desperate now!)

The only thing 2 things left I can think of that are relatively easy to check are:

Try a different brand of fuel
Rummage in the shed for the original (and rather worn) distributor and refit it.

Is it worth checking the fuel flow? Could this be a fuel starvation issue?
TonyJH

Getting weak on 1 carb will cause that. I had a bit of rubber stuck in the float valve and it did the same thing until it eventually stuck the valve open, flooded and leaked out. Also had a sticky piston on one which did that for a while when accelerating but cleared after a several seconds
Alan McKewan

yes the the problem does have a fuel sound about it but it might not be

check for proper venting of full cap, collapsing or deteriorated rubber hoses, full fuel flow to carbs and through carbs

electric side -
usual question is does the rev counter twitch with misfire?

is the car running unusually warm?

>>Accuspark dizzy fitted with a Pertonix Ignitor<<
that's different

delicate wires chaffing inside or outside dissy cap or as Guy reminded me recently delicate wire trapped by cap
Nigel Atkins

Excellent suggestions! I will check on the fuel flow next. I feel the purchase of some new float valves may be a wise investment.

On the electric side, no, there is no twitching of the rev counter, and no, the engine doesn't run warm. It hovers between 80 and 90 degrees.

Shall check for chaffing also of the pertronix wires. I'm happy again as I will have more stuff to try this weekend :)
TonyJH

obviously don't be changing parts just for the sake of it but I don't know if you've seen the new 'SU carburettor stay up floats', I've no idea if they're the eight wonder of the world or will prove to be problematical but here's the details (other provides are available) - http://www.mgocspares.co.uk/pdfs/hlarchive/hotlineJun14.pdf

ETA: don't forget to look for signs of trapping of those delicate wires - when you take the cap off the wire might move so not appear trapped
Nigel Atkins

Throw some dry gas in the tank. Sounds like water contamination. It may not be but its a cheap fix and you can cross fuel off the list. You should look at you spark plugs and see if they give you any clues!
Steven Devine

Im interested in the dissy

You said you replaced the old dissy with the new that has a pertronix chip

When you made this conversion did the problem start at that point ... how many miles between the old dissy and the new did this start

Im leaning towards a dissy timing curve problem its advancing to fast...or the fuel is to low of octwne and its burning to fast before the dissy can do its job

hmmmm, a cam that requires 16 thosands valve lash and no hi lifters... thats alot of lash for just a basic cam and engine, .012 is normal...valve bouncing ???

Prop



Prop and the Blackhole Midget

OK, since its a guessing game, my offering is that it is a small bit of crud floating in one of the fuel jets. At low revs / tickover, the needle drops down into the jet, pushing the "floater" down and allowing the fuel to flow. When the engine is under load the piston & needle rises and the flow of fuel up through the jet carries the crud with it where it partially blocks the jet orifice, causing fuel starvation and a misfire.

Prop - 16 thou is not at all unusual for a performance cam without high lift rockers. Pretty standard for a "fast road" spec cam. And I think if it were the dizzy, then the tachometer needle would be flickering.
Guy Weller

Guessing games are fun! I'll look for debris at the weekend in the float chambers. Dry fuel sounds like a good plan too.

The dizzy has worked fine for some time (18 months or so) before this problem appeared. In fact when I first fitted it I was amazed at how much more smoothly the engine ran, especially considering it cost less than half the price of a reconditioned Lucas part.
TonyJH

Tony, for my theory, the debris won't be in the float chamber. It would be in the jet itself, or in the little tube that feeds it.

Either slacken the compression nut where the tube exits the float chamber and flush the tube out. Or there is a technique that may clear it: With the engine running and air filter removed, rev the engine as best you can and hold the palm of your hand partially over the carb mouth and then withdraw it quickly, Repeat this a few times and the suction created should clear any debris from the jet feed pipe.
Guy Weller

Yepp... dissy off the table, not an issue

But ... have a look at the 2 wires that come out of the pretronix chip, right at the chip and see if the insulation has broken down and exposing the silver wire inside... you may have to use a magfy glass to see the damage...or not


.016 on a full hot cam, I can understand... but on a mild fast road cam? .012 to .016 is a 30% increase ...but thats not the cause of this problem, so its a dead herring

As guy says... because we are just guessing, what about a split in the fuel pump diaphragm

To follow on guys clean out method...pull the dashpot, spring, and piston/w needle attached then pull float chamber cap off, get some carb cleaner with the small red straw push straw into hole that held the needle and give it a few good blast...just remember to add oil back into dashpot on reassembly

Prop
Prop and the Blackhole Midget

May just be an incorrect fuel mix. Have you changed the needles in your carbs? (I'm assuming twin 1 1/4 SUs). It's easy to be fooled into thinking your mixture is ok using a colortune as it's a static point your measuring but a session on a rolling road would probably highlight a fuel shortage at the points your experiencing. Most sources suggest that with a modified head and camshaft you'd need 'AF' needles (if using fixed needles) and red damper springs.
graeme jackson

I recently had similar symptoms. Misfire when running on full throttle for sustained period (going up a long hill, started to splutter near the top). Needle too lean. No problem under normal running. That said, my car has a Shorrock so was running about 6psi boost at the time.

Couple of needle changes later and problem solved. Just have slightly lumpy idle now...

Simon
SA Wood

I've used outboard fuel tanks with an electric pump in the line as an alternate delivery system to eliminate / indite anything behind the carb(s)
Bob Ketcham

This thread was discussed between 30/07/2014 and 08/08/2014

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