MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG Midget and Sprite Technical - Misfire - where to look next?

I've trawled the archive but haven't found a solution so I'm hoping someone here can suggest one.

The other weekend my Sprite was running well at a Loton Park hill climb until it started to misfire towards the end of one run. I would not pull when I tried to accelerate.

Frustratingly, it will rev freely with no load but struggles again when I lift the clutch.

Here's a video of the run when it happened followed by the beginning of the next run which is too painful to show in full.

http://youtu.be/YxXuIQRVKVQ

Since then I've checked tappets, replaced the coil with one I had in stock and put in new plugs, new distributor cap, new plug leads, new DD condenser, new DD points and checked the ignition timing. Each replacement has seen an improvement and it will wizz along merrily on a light throttle but grumbles and goes uneven when I try to accelerate.

I wondered whether it might be stuck bob-weights on the non-vac 23D4 distributor but the timing light shows it advances when the throttle's pressed.

I recently had fuel flow/starvation problems with the Weber which turned out to be a stuck float valve after the whole fuel system was drained, cleaned and blown through. The fuel was not old and the tank almost empty so 10l went in today but that hasn't cured it.

What would you suggest my next step should be?

Colin
C Mee

I'd do a compression test.
Daniel

Thanks Daniel

I forgot to say that I did that and they were all lower than before at around 120PSI - previously 150ish.

The water level looks OK and there's nothing obvious coming out of the exhaust and the temperature has stayed in its normal zone.

Sounds like I need to have the head off.

C
C Mee

It’s unusual for all cylinders to be down by the same amount, unless you have the same problem affecting each cylinder.
Dave O'Neill 2

Was the previous compression test with the same carb and air filters, or when you still had the SU?

And the old check - when it is misfiring does your rev needle twitch, or just go lower?
GuyW

Colin--It sounds ok at idle and light throttle but going off song under load to me from here.
Have a look/feel down under the side of your weber and make sure one of the locating screws for one of the Aux. venturies hasn't disappeared--If one of these falls out there won't be any fuel for that barrel of the carb on throttle if the venturi has turned in the housing.
willy
William Revit

Good call Willy. I have also seen loose main jets cause the same problem.
I have also seen a set of DD points wear the heel really fast and cause a bad misfire, I think the material may need constant greasing the same as the old fashioned fibre heel points we used to have.
How did you get on with the rolling road session Colin?
Peter Burgess Tuning

Guy, the previous compression tests were with the SUs. Not sure why or whether that should make a difference.

Willy, I think you get the prize: the rear venturi is loose and the retaining screw is missing - probably finally rattled out by running over the rumble strip. Replacement screw on order and I'll make sure they're wired this time. I'm glad I saw your post before I started removing the head.

Peter, thanks for the advice about the points, new ones didn't make any difference but I'll keep them lubricated. The rolling road session did not go as planned. It wouldn't run at high speed, which was something new. After loads of checking of jets, floats etc. it was realised that the carb kept running dry which was eventually put down to fuel supply or pump issues so, as it was now late afternoon, the test was abandoned without a proper power run. The next day, after an awful test run, I realised we'd never checked the float needle valve as I'd put a new viton one in for the session and not given it a test run. Swapping it for the old one cured it and the car went really well on the Saturday and Sunday morning until the misfire appeared.

C



C Mee

Ahh, very good-
just be aware that with that venturi, although just looking down the hole at it,it looks like it can go in two different spots, half a turn opposite each other---it can't---It's drilled through for fuel on one side only, the other side is only part drilled for the locating screw----IF you get the screw in the wrong hole--no fuel.
Them little metal tabs usually hold the screws ok, maybe locking the tab onto the flat of the screw might have got overlooked when it was put together last time.
willy
William Revit

A mate had that same problem on his Caterham, Willy posted before it thought to post about it. I have a Dellorto on my Sprite and they retain the auxiliary venturi differently so that can't happen.
David Billington

Thanks again Willy. I went to the rolling road guy to see if he had any screws going spare. No luck there but he gave me a very clear description of how things are arranged and how to line things up correctly.

He suggested putting a dab of coloured ink in the indentation and then use a mirror to see that the coloured spot aligns before inserting the screw.

Just waiting impatiently for the replacement screw to arrive now.
C Mee

yep, or you can poke a little screwdriver in the hole, if it goes in an inch or so you're in the fuel hole, spin the vent. around half a turn and then you'll be able to feel the short hole with the screwdriver and wriggle it round to centre it up then fit the screw.
Whatever works best for you-
William Revit

Well done Willy.
Colin , don't put too much grease on the heel or the excess may let the points burn out
Peter Burgess Tuning

Cheers Peter -- and on a sad note I see we've lost David Anton (rip) Lori has gone back to the business and selling stuff, not sure if she'll continue the business or not ,but if there's any of his tricky bits you want/need it might pay to get in soonish rather than later.
willy
William Revit

David passed away mid May Willy, I only met him once but he was one of the good guys.
Peter Burgess Tuning

Yep.
William Revit

That's not a drift, it's a sideways lurch - still, glad the thread provided an opportunity for passing on that piece of sad news.

Back to the thread and my Sprite is up and running properly.

Willy, your small screwdriver suggestion worked a treat. The new screw is now firmly in place and tab-locked, though I still need to wire it in a belt and braces approach. I've just given my Sprite a decent run-out and there wasn't a hint of a misfire.

A shame it happened when it did as it cost me a place in the club championship. It happened on a practice run and just one half-decent competition run would have given me second place - that's just my sort of luck, such is life.

My competitive season's over now so I'm beginning to think what I need to do over the winter to be ready to start all over again in March.

Thank you for all your help and advice, it would have taken me a very long time to find the problem without your help.

Colin
C Mee

Sorry for the thread 'lurch' Colin - I grabbed the chance while Peter was about just in case he wasn't aware. Cheers
willy
William Revit

Colin. You noticed a problem. (Believe it or not, not everyone does until it becomes a problem of failure rather than loss of performance.) You thought about the problem and sought advice when you could not figure it out yourself. (Good research skills.) You tested out the various theories, observed the results, and seem to have found the solution. Then, you communicated your results so that others could learn from then and identified possible actions that could be taken to ensure the problem should not recure. Yes, I can certainly perceive of you as a "Loser!". But, perhaps, you should dwell on what is to happen next year, what you have learned this year that will improve your performance next year, and simply list this year as a "good learning experience". The mere fact that you tried and came close to actually being in the top three is worth remembering. In truth, the only true "loser" is the individual who would like to do something but lacks the courage to try. "Better to not try than to risk failure" is the motto of the masses. You tried, did not go quite as far as you had hoped, learned from the experience, planned improvements--personally, I would consider that to be quite a victory myself.

Les












Les Bengtson

Hello Les

Thank you for your thoughtful and thought-provoking comments.

There are several positives to take away from this saga. I have learnt more about Weber carburettors and my wariness of tinkering with it has reduced (but not gone completely). I've had some useful practice working on the distributor - again, something that I'm usually very wary of touching. Just having the presence of mind to mark it's position before removing it made re-setting the timing a doddle!

I try my best to find solutions to problems by reading, googling and combing archives before resorting to creating a new thread on this forum but I can almost guarantee that someone on here will have encountered my problem before and has a sensible solution to suggest and it is just typical of this group that the answer should come from the far side of the globe. In this case, the loose venturi was one my local rolling road guy had never encountered, so he learnt something from it too.

As for the competition, I have had the best season for a number of years and set a couple of new PBs along the way. It is the first time for a good number of years that I have been in contention for a top three place in my class of the Club Championships, and it could have been second. I know that most of the enjoyment of hill climbing is in pitting yourself and your car against the challenge and I am nearly always up against much faster opposition, so it is quite rewarding to take home the occasional trophy and I will be quite content with my third place trophy for which I beat one of my best hillclimbing chums by 0.01 in the scoring system.

With the ever rising cost of everything, especially motor sport, I am not sure how many events I'll be able to afford next year, so this year had an element of a last fling about it.

Colin
C Mee

This thread was discussed between 05/09/2024 and 10/09/2024

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.