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 MGA - Misfiring Woes 3

Happy New Year to all!

This is an update on my MGA in Malaysia, for those helpful folk that offered suggestions and support to my "Misfiring Woes 2" in December, and to anyone else for whom it might be useful...

I rechecked all the carb rebuilt areas, found nothing too obviously wrong, tightened a few bolts etc. So I seriously revisited my distributor and vacuum unit. I knew the pipe from the carb was "loosely" connected via a rubber pipe to the vacuum unit. Nothing changed from before (when it was running okay) but anyway I sealed the connections as a stopgap (have ordered a new pipe assembly from Moss for a proper job).

Then I dis-assembled the distributor (dodgy - hadn't done this for at least 30 years...) and found....what I hoped was the culprit. After having to get rather brutal to free the screw securing the cb points, I noticed the upper plate (with the post, that the points are mounted onto) looked to be at an angle to the cam...strange). Took it all apart, and found - apart from all the ancient grease and gunge - one of the 2 nylon tips (separating the upper plate and the base plate and ensuring smooth movement) had come loose (I just finished off the job in my disassembly) so the upper plate was no longer parallel to the base plate, hence the points and upper plate and all were flopping around, no doubt the points sometimes opening, sometimes not. Put fast-set araldite to its normal invaluable use and reset the nylon tip, cleaned and refurbished it all, put it all together, and hallelujah...the car was running again, with no sign of the misfiring. Just think - all that grief due to a tiny speck of nylon that was loose....

So current action is to - again - rebalance the carbs (2 absolutely indispensable items needed here - the jet adjust spanner & SU carb sync tool), and try and follow the SU tuning procedure. I am NOT putting on the air cleaners until I feel the carbs are balanced more or less! But I am getting better at this.....

The saga continues, but we are slowly getting there, the car is running quite sweetly now, just needs a bit of advance/retard adjustment and probably jet tweaking to finish the job.

Thanks again to all

Andy
Andy Drinkwater

Andy

Another case of "carb problems" turnung out to be electrical. The SU is extremely reliable when correctly set up.

You can make a simple carb balancer yourself. It's called a piece of flexible tubing, and you stick one end in your ear and the other end in the throat of the carb. You first slacken the clamps on the carb linkage, and adjust the idle screws till you hear the same rasping note from each carb, then they are balanced. You need to have the free end in about the same place in the throat, as otherwise you will hear quite different things. Then tighten the clamp screws again when done and adjust the idle speed till it is correct (the carbs stay in balance as long as you have the clamps tight.)

If you are wanting to adjust the mixture on each carb, start from the settings in the book and run through the process as described. You can usually do this with finger and thumb on the mixture nut unless you have sausage fingers. Although I have the spanner, I always used fingers instead (one thing less to drop!)

When you put the air cleaners on, make sure that they have all the holes correctly aligned, and that the backplate is not upside down.
dominic clancy

Hi Andy. I agree with Dominic that an SU balancing tool is not really necessary. Listening to the carb intakes hissing works very well. When both carbs "hiss" the same amount, they are balanced. I have used this method for many years with excellent results. A unisyn or other carb balancing tool just gives a visual indication of vacuum instead of sound. Good luck! Glenn
Glenn

Andy. I would weigh in on the opposite side of Dom and Glenn's argument. I have little use for the UniSyn tool, having experimented with it some 35+ years ago and found it wanting. The German made SU tool set, sold by Moss and consisting of two rods which fit into the pistons (with the damper rods removed) and having two wire pointers attached, is an excellent tool. Not only does it allow you to set the carbs at idle, but allows you to "blip" the throttle and watch that the pistons rise at exactly the same time.

"Balance" of the carbs does not consist, exclusively, of setting them up at idle. The linkage between the two must also be exactly the same and must cause exactly the same throttle shaft movement with acellerator pedal movement. I have had people bring me their cars because they were having "problems" with the carbs. They were balanced at idle, but when I reved up the engine, one of the pistons did not move as far, nor as fast, as the other. With the linkage properly adjusted, the pistons moved insync and the "problem" went away.

IMHO, money spent on the German SU tool is a wise investment. I have been using mine for over 35 years now, on a number of cars, and the initial investment was paid for long ago.

Glad you got the dizzy sorted and things seem to be going better. Might I also point out, the German tool can be used with the air filters in place and can be used to diagnose a problem relating to the filters?

Les
Les Bengtson

Les, I looked for the kit you are refering to in the Moss online catelog and couldn't find it. Do you think they stopped selling it?

Tom
Tom

Tom. My Moss catalog shows "SU Tool Kit", part number 386-300 at a cost of $23.95 in my current MGB catalog. I still have my original kit, purchased for about $8.00 back in 1968 and have used it on the H-2, H-4, HS-4 and HIF-4 carbs. I also have a newer kit, purchased several years ago, which seems identical. My original kit, along with my Uni-Syn device, was left at my parents home when I went into the service in 73. I found them in my father's workshop several years ago when I was cleaning it out after his death. Took all of his tools and the SU Tool Kit as a reminder of the good times we had together. My older daughter and I have used his tools when working on our cars. Thus, Theresa and I share both "father/daughter" time together (now father/daughter/son-in-law time), but we also share a little time with my father who taught me so much about taking pride in one's work and doing the job right.

The Uni-Syn device? I through it in the trash rather than ship it to Arizona. A decision I have never regretted. It was "OBE-ed" (Overcome By Events) in 1968 when I found a better tool.

Les
Les Bengtson

I agree with Les, this tool (set) is continuously on ebay UK

http://cgi.ebay.co.uk/ESSENTIAL-SU-CARBURETTOR-TOOL-KIT-NO-RESERVE_W0QQitemZ300065715555QQihZ020QQcategoryZ27380QQrdZ1QQcmdZViewItem

Regards,

Neil
Neil McGurk

I have to confess to being totally bemused by Les' comments.

If a pair of SUs is balanced at idle, and the clamps are done up tight, they remain in balance all the way through the operating range. If both pistons are filled with the same / correct oil, they will (when balanced) rise at the same rate unless something is wrong with the pistons and dashpots. I don't have any idea at all what Les means by "adjusting" the linkage - Apart from the clamps and the idle speed screw, there is no other adjustment.

If you follow the instructions in the manual (summarised in my last post) the carbs are dead easy to set up, without extra tools - thousands of cars have been set up by thousands of people for years using this technique. The steps are simply to balance the carbs, and adjust the idle speed and set the mixture using the simple techniques described. Despite the myths, it's really not very complicated if approached in a methodical fashion and if you follow the instructions exactly.

If there is something wrong with the carbs, then I accept that this toolkit will perhaps help you to diagnose it (although $20 for a few bits of bent wire does seem extortionate to me), but for a simple tuning of a pair of correctly operating carbs it is completely unneccessary.
dominic clancy

Dominic,

I know exactly what Les means. At idle the throttle spindles rest on the adjusting screws, however when the throttle is opened the spindles are moved by the cable via the linkages. Ideally, you are correct, the two spindles will turn exactly in unison. In practice the slack in the linkages is not always taken up in exactly the same amount and so invariably one moves before the other. With great care and a keen eye this can be adjusted without a special tool, however, (IMHO) being able to see the piston movement makes this much easier and confirms that this is correctly achieved.

For anyone with a lack of experience or confidence I thoroughly recommend this tool, although I agree it is not essential and you could probably make something similar from an old wire coat hanger - probably already on Barney's site somewhere. Just kidding, no offence Barney!
Neil McGurk

My experience is with TR3's and 4's, but they have the 2 carb shafts locked together, no play whatsoever, so I agree with Dominic. I guess I still don't need a UniSyn or a bent wire thingy. I think I'm about ready for some new cork washers, though.
Tom

I could see the value of these as a routine health check as, I presume, they can be used without having to take the filter boxes off.

I ran for a month or two with the spindle bars running out of synch because the clamp had somehow slackened. The carbs had only slowly got out of synch and I did not notice the slow deteriation in performance. It was only my monthly fuel consumption check that highlighted a problem, warranting further investigation.

With this bit of kit I guess I could include it in my weekly maintenance check list.

Steve
Steve Gyles

A similar looking kit to that on Ebay is available as a direct purchase from the MGOC (MG Owners Club) for £12.95 plus postage:

http://www.mgocspares.com/acatalog/MGOC_Spares_TOOLS_220.html#l23

Steve
Steve Gyles

Dom. Perhaps I can sort it out. Two independently balanced carbs, at idle, are in sync with each other. If the carbs are perfectly set up, and the linkage is perfectly set up, they are, in theory, in sync throughout their entire range of throttle openings. The method by which this theory is tested is the tool that I have indicated. If it demonstrates that the carbs are actually in sync, you have a visual indication that all is correct. If one does not use some form of measuring equipment, all that remains is a belief that they conform to the theory. But, it remains an untested theory.

Such a tool can also indicate that there are problems with the suction chambers and pistons which does not show up when the system is tested at idle, nor, may a slow rising piston show up in steady state testing with a device such as the UniSyn. It does, however, show up with the tool I have described.

Les
Les Bengtson

I now understand, and it seems that Les and I are saying the same thing! The difference is that I use an ear to check the balance, and he uses a tool. What is clear now is that you cannot use the tool to make any check on the linkage, it just tells you if the pistons are rising at the same rate or not. I accept (as I did before) that if you have problems, this may help you eliminate or diagnose a dashpot problem, but that's the only advantage over the tubing.

Neil, if the linkage is not taken up at the same rate, then either the linkages are loose (simple remedy is to tighten the bolts) and/or the throttle shafts are worn (in which case setup is nigh on impossible anyway). Again the tool will only tell you that the pistons are rising at different rates, but doesn't tell you why. If the linkages are tight (and the holes in the lever arms are not oval), there should be no slack anyway.
dominic clancy

To Dominic, Les et al

Thanks for the time spent on this interesting (to me anyway) issue of carb balancing:

I would say this:

1) the only carb balancing tool sold by Moss UK seems to be the Carb Synchroniser, MMM386-200, price at 24+ pounds! As I live in Malaysia, and don't have access to informed SU people here, and was rather uncertain about my own ability to balance the carbs (AND usually order everything I MIGHT need due to shipment time & cost, convenience etc etc), I ordered one anyway, not quite knowing how it would be used. MEANWHILE I tried to balance using the standard tube in ear. I seemed to be a reasonable way to do it, but I was not convinced I could really detect a pitch of sound closely comparable for each carb - it just sounded "ok". Probably was too, but anyway my synch tool arrived in due course, and after a few attempts, was able to use it - I think - quite successfully. It was expensive for what it is (although a tad more that bent wire) - but in fact it is very simple & quick, & personally I would use it every time over the tube in ear method. That of course presupposes that a) you have the tool, otherwise a tube will probably be close enough (note, I am more confident with this now, I think), and b) you have the air cleaners off. I do for now, because I am finetuning the mixture by checking the plugs & tweaking the mixture nuts appropriately after a good run - and you can't get at it very easily if at all, if the cleaners are on - although I accept that when I do finally put the cleaners on again, it may richen? the mixture slightly.
However, once they are installed, I intend to fabricate the piston movement tools that Les recommends, because it sounds like a very simple but good idea.

Now, to my progress, and recurring issues - went for a good run today (75km), and going there the car was great after all my distr. and balancing work, running quite sweetly and smoothly (timing probably needed a slight vacuum unit advancement, but that was all). I am running without aircleaners temporarily, but not sure this would necessarily cause problems.
On the return, it started to occasionally "hesitate" ever so slightly, developing into a mild misfire (although nothing like my previous experience with it), then one or two mild spitbacks, but didn't get any worse, and I got home okay, - a lot of hills around where I live. Also the idle had increased to fast idle - can't figure that one. Arriving home, the engine really started to sound just a bit laboured, almost as if it was a little short of oil and tightening up....or perhaps a bit retarded? in fact it was running at its normal 160 degrees on the open road, and I have a Kenlow fan for slow traffic. After it cooled, I checked oil, water etc, everything good. Checked the plugs, very slightly rich, but almost correct (and comparable, so my balancing attempts weren't too bad!), cleaned them, and tweaked the carb mixtures just a tad weaker, and re-adjusted the idle...Checked the distributor, found the points had closed slightly so reset them to 0.0016" - Checked ALL the HT connections. Everything looked good, so went for a quick run. STILL the slight hesitation, occasional misfiring. Tweaked the Vaccum first retarded, car ran worse, then advanced in steps, car ran (pulled up hills) progressively better - but still the hesitation/occasional missing. So what is left? I am fairly sure it is not the carbs, they are in better tune than they have been for a long time. The points etc look okay, but as my previous experience showed, refurbishing these completely cured my misfiring problem, but here it is again, although in a much muted form. I have new points & condenser on the way from Moss, but not sure this is the problem either. But of course it has to be either carbs or points. Any suggestions would be welcome at this point, as I think I have covered most things...

Cheers
Andy
Andy Drinkwater

Andy

I spotted an interesting diagram in an old 1950s book called "The Motor Repair Manual". The diagram is called Tracing Engine Faults.

I have reproduced it on my website. Have a look at item 2 on this link: http://www.mgaroadster.co.uk/technical_information.htm

Steve
Steve Gyles

First swap the Condensor (very often they fail quickly or run for years with no trouble) secondly adjust your timing, and third make sure that your vacuum tube is connected at both ends and is no leaking.

Timing is not usually anywhere near the "book" settings, as unleaded fuels mean that timing has to be adjusted to suit the fuel you are using. If you advance the timing to the point that the car pinks a bit under load and then back off a fraction, then you will have it about right. Maybe others with a standard setup can give some ballpark figures here.

Make sure that the choke is also shutting off (it needs a spring) and that the the car is not running rich (it will stumble if it is too rich)
dominic clancy

Steve
Thanks for the link - I checked out the picture, and it is indeed interesting! The "gradual fadeout" top right seems to describe my symptoms perfectly, so I shall have another good look around the carbs & distributor in case I have missed something - and I did wonder about water in the fuel. Meanshile, when I get my new parts from Moss, I will of course change the points & condenser etc.

And Dominic, appreciate your advice and comments too, especially about the practical way to advance ignition to get it more or less right, I will do that!

Cheers

Andy
Andy Drinkwater

This thread was discussed between 01/01/2007 and 08/01/2007

MG MGA index

This thread is from the archive. The Live MG MGA BBS is active now.