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MG MGA - MGa spitting back thro the carbs

I wonder if anyone out there can help me with an annoying mis-fire on my 1600. On light throttle the car spits back though the rear carb. I had a new camshaft put in last year, and it ran fine for a bit, but now whenever it runs on a light throttle, I get this spit-back. Once I put my foot down a bit, it runs ok. Power is also down though.

A valve problem and carb problem have both been suggested but I haven't got the facilities to investigate my self - so before I pay someone to take the head off or re-build the carbs - has anyone else experienced this??

Best wishes to all


Dave Hill
D J Hill

I don't want to make 20 suggestions about what might be wrong. So, .... Do a compression check. Check valve lash and valve lift. Check ignition timing, wobble in the distributor shaft, and operation of the vacuum advance. Check oil in the carb dampers. If you haven't solved the problem by then, get back to us with the results, and we may have some idea how to procede.
Barney Gaylord

Check your distributor cap, rotor, and plug wires very carefully. You may be firing one of the cylinders at the wrong time due to a carbon track in the cap or between plug wires.
John H

Dave
If you have checked the usual service items such as plugs, points, condenser, dist cap & rotor arm,
try checking the carburettor float chamber for dirt etc. Sometimes especially if the car is stored, water can get in through condensation in the tank.
Unfortunately the cause of problems is not usually obvious so everything needs checking!
Robert.
R J Collier

Sounds to me that this may not be an ignition problem as the problem goes away when the accelerator is opened further. It's still easier to check the ignition so change or substitute all parts just to be sure. On the assumption that the cam and valve timing etc is OK there could well be an induction leak to the rear two cylinders meaning that they are running weak at part throttle. Make sure that the manifolds are tight and also the nuts holding the carb to the manifold. If they are then change the manifold gasket and also the gaskets between the carb and insulator and the insulator and manifold. Make sure that all surfaces are flat, if not, correct this. Finally how much wear is there in the carb spindles and bushes as this is another source of induction leakage.
Iain MacKintosh

I agree with Robert and Iain, It needs a complete tune up - set tappet cleasrances - service/replace all ignition parts - check timing and then do a mixture check (I use the gunson Colour tune) and then balance the carbs - I use a Gunson Carbalancer.A very weak mixture on one cylinder could be a gasket or carb spindle air leak. Good luck - Cheers Cam
C Cunningham

Backfiring through the carbs is most often a symptom of a lean mixture. Check all the areas described above first, but I'll bet you'll find the problem is a vacuum leak.

Bill
Bill Boorse

Ref: Vaccum Leak

A good way to see if this might be an issue is to take some carb. cleaner and while the car is running at lower throttle, spray the carb cleaner around gaskets, throttle shafts and other places where aiir leaks may be present. This will confirm or rule out the vaccum suggestion. May bet is on a worn throttle shaft. At higher RPM's the fuel mixture compensates for slight air leakage.
robert maupin

I have to agree with Bill Boorse, you are describing the classical weak mixture case.
Whilst the exact symptoms you describe are generally associated with a weak mixture the same is possible with a rich mixture also. I don't see how it could be any of the other suggestions!
Bob (robert) I am coming out!! yes I once owned an MGB!!

Bob once owned a MGB!
Bill Borse may be right about the weak mixture, but assuming there are no vacuum leaks, you must first
check the points, plugs, timing and tappets etc before you can set the carburettor mixture.
All of the suggestions so far have an effect on the running of the car, so unless you find the cause straight away all must be checked.
Robert
R J Collier

OK then Rob myself and Bill are incorrect it has nothing whatsoever to do with mixture! I stand corrected and will seek your advice in future. Thanks for your time.
Bob (robert) I am coming out!! yes I once owned an MGB!!

By the way the key here is the light throttle info.
Bob (robert) I am coming out!! yes I once owned an MGB!!

Bob once owned a MGB!
I do not know the answer & neither does MGA guru Barney Gaylord. I agreed it may well be the mixture.
All suggestions are helpful, mine was intended to be.
Robert.
R J Collier

Well that is fine but why then did you decry mine and Bill's observations?
If you follow your logic then every time you have a fault you will be changing plugs ignition leads setting timing and so on. I have a midget running that has had the same dissy cap on for 18 years and equally the same plugs for 6 years. I do not change things in the hope it may be faulty I find the fault and cure it. That is perhaps why I can assure you this is a mixture problem and more than likely a weak mixture.
If you don't know the answer then why give advice? You could say clutching at straws so and so maybe the cause, at least in this manner the recipitant can assess the quality of responses.
Bob (robert) I am coming out!! yes I once owned an MGB!!

I reckon its the old problem of metric tappet clearances, if you re-adjust them to imperial clearances it should be OK. Don't use feeler's with dual markings as they will confuse things.

I've never owned an MGB, but have been offered a rusted wreck, so I may own one soon


Not much use Terry

I'm surprised Bob(robert) admits to owning a Midget, next it will be an MGF !
Terry Drinkwater

Don't waste time swapping in new parts. Always do the diagnostic work first to determine exactly what's wrong with it before you go wasting time and money on new parts. We only have people guessing in dark here. I refuse to guess. Where is the diagnostic information?
Barney Gaylord

Bob once owned a MGB.
I don't know where you are coming from.I do not decry your & Bills suggestions on carburettor mixture.
Mixture is usually set after the plugs,points, timing and tappets have been checked.
Your suggestion may be correct,but this is not certain
and other suggestions are also relevant.
Robert
R J Collier

Barney the diagnostic info is in the first post, it states "on light throttle opening the car spits back.
Now in 35 years of repairing cars I have never had this scenario happen with anything but mixture and 99 times out of 100 weak. It is as simple as that.
Plus it is a 2 minute job to set the mixture so it is what I would do.
Dave the choice is yours my friend you have heard all the evidence.I do not call myself MG Guru but that does not mean I have no idea about B series engines does it? Equally if I did it would not mean I did, would it?
I agree that sometimes a number of possibilities are correct, in which case then I would suggest that to be the case, but for example here, how on earth could timing cause this affect, ie idle and run OK but light throttle spit, of course it does not add up.
Bob (robert) I am coming out!! yes I once owned an MGB!!

Dave
If you know that your car is in a perfect state of tune,Bob once owned a MGB is right, check the
mixture first.
I don't advocate changing parts unless you know that these are overdue for replacement, although none of the ignition service parts are expensive.
For example I renew my points every year,cost £2.00

Timing would not cause the effect on its own, but it is no use setting the mixture if the point gap,plug gaps, tappets and timing are out or there are leaks
in the induction.
Robert
R J Collier

My thoughts

Everybody has a point but it is difficult to diagnose without listen/driving Dave's A, timing will cause spitting back but not through just one carb , I agree with Bob that it is probably weak, SU's dont like being weak. Has the needle slipped ? It could be an inlet valve

Terry
Terry Drinkwater

Good lively debate tonight guys. It's been more fun than watching Chelsea beat Everton (soccer) on the TV.

I'm not offering an opinion on this one, just eager to hear the outcome and add it to my knowledge bank.

Steve
Steve Gyles

Dave

Nobody has suggested cleaning the dash pots yet...
I cleaned mine, which stops a possible sticking up of the pistons (too lean), and my misfiring went away!

Happy hunting!

Pete
Pete Tipping

So far I only hear one symptom which indicates a problem, but no diagnostic information. A flat cam lobe, zero valve clearance, error in cam timing, badly misaligned ignition timing, or a variety of ignition faults can all cause the same symptom. Even when badly out of tune it may seem to idle okay if the idle speed is tweaked for the right idle speed.

I still don't have any clue what might be wrong here. So far everyone's efforts appear to be guessing and suggestions for diagnostic work.
Barney Gaylord

Chaps, we are arguing with ourselves here. Dave Hill hasn't dropped back into this thread at any point. Now with all due respect to him and from his opening post it looks as though he will get someone else to do the work for him so our efforts are through Dave as an intermediary. I would suggest that unless he comes back in on this we won't get much further with the problem.
Iain MacKintosh

Rob let me give you an example if someone were to say the car wont start. When I turn on the key and before pulling the starter I hear a rapid clicking that persists.
Would you tell him to do an ignition service or would you tell him to fill it with petrol?
Do you now see the point?
Rob if you want to change all your ignition parts yearly then thats fine, whether it is necessary is another thing altogether.
Bob (robert) I am coming out!! yes I once owned an MGB!!

Many thanks chaps for all the suggestions - I'll make a start on replacing all the obvious bits over the weekend, treat it to a new set of plugs, points etc!! Whip the head off and see if the valves are opening as they should. I had the compressions tested recently though and it seemed ok. The concensus seems to be carberation rather than valves, so i'll have a bash at that.

I'm open to further comment, and I'll let you all know how I get on - you never know, it might help someone else in the future.

As an aside, by 'modern' is in for servicing, and I have to go out tonight in the MG. The temperature is going to get down to -3, and as I NEVER have the hood up, I'm going to have to rely on a decent hat, and the wonderful heater. No - I won't be tempted to start a new thread on how you actually get the heater to work properly in the winter. (Mine only spews warm air between June and September).

Regards

Dave

D J Hill

Well one has to wonder if it is all just a waste of time and effort! :)
Bob (robert) I am coming out!! yes I once owned an MGB!!

Bob once owned an MGB!
We could go on arguing forever Bob.
I believe that £2 spent on points a year and £4 spent on a distributor cap every 3 years cap is money well spent. I don't change ALL ignition parts yearly.
If every fault had an obvious cause we wouldn't need a
BBS forum would we?
We are just going to have to agree to differ!
Cheers Robert
R J Collier

Is this a 5 minute arguement or the full half an hour

( Monty Python's )

I wouldn't join a club that would have me as a member

( Groucho Marx ) no help whatsoever but I thought I'd throw it in. Does Dave's car know that its impolite to spit especially for a female, have cars got a gender?


Terry
Terry Drinkwater

Dave, don't take the head off for goodness sake, that is a last resort. Go methodically through the other points that we suggested finally coming round to the induction on the rear carb which I'm pretty sure is at fault. You have confirmed that the compression is OK and you can observe the valves opening and closing by taking the rocker cover off so provided they are OK then no need at this stage to lift the head.
Weak mixture is the cause of spitting back and this can be caused by either an induction leak or a fuel delivery problem, but I don't think its fuel delivery because it only spits back on light throttle and if it was delivery it would be probably get worse as the throttle was opened. It must be a significant problem though as you say the car is down on power. How about contacting the expert Chris Betson who is not too far away from you.http://www.octarine-services.co.uk/service.htm

Let's know how you get on Dave.
Iain MacKintosh

Once again, many thanks guys. I think I've got enough to keep me amused over the weekend. I've done a few of the things mentioned already - changed the H.T leads, new dist cap and rotor - but there's a few other useful suggestions.

As I mentioned, once I get to the source of the problem I'll let everyone know - so when you get some other idiot asking daft questions, you'll all be wiser and know the instant solution.

I was in the MG last night - it starts and ticks over ok - but the misfire is still there. Most annoying!!


Regards, Dave
D J Hill

Sorry if this was already mentioned - my head is spinning from reading through this thread - but have you thoroughly checked the vacuum advance on the distributor? Is it the proper unit? Is is functioning properly? Does it leak? It could be throwing off the part-throttle timing and messing with the mixture.
Jeff Schlemmer

My vote is with the sticking carb piston as Pete indicated above, which as you well should know will affect mixture. This is the only time I remember such a symptom as described here. It is a normal maintenence task anyway, costs nothing but a little time and carb spray cleaner, but can only result in better operation. While you're at it...make sure your damper oil is at the correct level. Then you can move on to verify mixture correctness and vacuum leaks. As Bob wrote (I can't believe he owns a Midget): "the key here is the light throttle info".
Cheers
Jon Bachelor

This thread was discussed between 07/02/2006 and 11/02/2006

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