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MG MGB Technical - Odd engine failure.
|An odd thing happened yesterday when I was driving home from work. I drive in occasionally when someone else is away and I can use their carpark. I'd offered to give a workmate a lift to his ferry. Car started fine and we got down the driveway at work and started up a steep road in front of the office.|
Then the engine just lost power and started spluttering and wouldn't rev. It was as if several cylinders weren't firing. I was able to roll backwards down the hill in neutral and off the road to pop the bonnet.
All the plug leads were good. I checked the distributor and that all seemed fine too. Everything is still almost new. The car only has 3100 km on her since rebuild.
I tried to start her with my workmate listening. It sounded terrible! He said it sounded like I had lost a valve then promptly disappeared off to go catch his ferry! Not very helpful.
So I removed the rocker cover to have a look. Nothing obviously broken. Amazingly I managed to keep my white shirt and french cuffs clean! I had noticed on the previous start attempt smoke came from the front air filter and also it also seemed to be a little inside the rocker cover too.
Since nothing was wrong I put it back on (people were walking past - busy street - and still complimenting me on the car despite it obviously being broken).
Turned it over just on the starter so it wouldn't try firing and couldn't hear anything mechanically broken. So plug leads back on and tried again and she started fine. She then ran fine all the way home (a nervous drive though).
I have no idea what could cause that but a friend suggested maybe a carb dash-pot getting stuck up? This morning I will go check the oil in the dampers.
My opinion is she is just a jealous car and she gets upset when I have passengers. Like the time I took a friend for a long drive then when I got home the front suspension fell off.
Anyone have a less supernatural suggestion? Anything I should check? The loss of powerer was just as if several (not just one) cylinders weren't firing properly.
I am still amazed that every single time I take that car out I get compliments from people. Even when I am fixing it on the side of the road. Last time it was a tram driver, a schoolgirl and a chap who pulled alongside in a Triumph Herald!
|Forgot to say it's a 78 B with HIF carbs. I just checked the dashpot oil and it was fine. I went for a drive and no problems. With the air filters off I moved the pistons and they both moove freely however the rear one doesn't move as freely as the front. It's hard to tell how long the pistons take to drop as it's quite fast but seems to be about 3/4 second for them to fall back down. With the rear one taking longer. I swapped the dampers around and it made it more even but the rear still falls more slowly. With no dampers in they both move freely. No indication they could stick.|
How freely should those pistons fall when damped? I am guessing they should at least both fall at the same rate!
|If the oil was present in both carbs, and the car was fine before the "incident", it seems a slight difference in damping is unlikely to be the cause. Smoke I'd guess is hot oil vapour from the rocker box. For it to suddenly get so bad would suggest ignition or possibly bad fuel or fuel filter blockage. had you recently filled with fuel? As it is now running ok I'd be inclined NOT to fiddle! If it does it again, what did/does the tacho do as it died?|
|Tach is the critical thing as Michael says, this should always be the first thing to look at in any loss of power - before you stop, knock it out of gear or dip the clutch. With the momentum of the car (hopefully) still spinning the engine if the tach has dropped to zero, or is leaping about in time with the misfire, then it is an ignition LT problem. If also the ignition warning light is on, or flickering, then it is a loss of 12v to the ignition system.|
If the tach is only dropping steadily as the car slows then it is HT, fuel or possibly condenser.
As it seemed OK on restarting then I'd suspect fuel starvation. If it was the pump then when turning on the ignition before a successful restart you would hear the pump chattering away - assuming a standard SU.
|My guess is that the points, in the fuel pump, are glazed over just enough to give you sporadic fuel delivery. Do you know the history of the fuel pump? My '67 was doing this 2 years ago and it turned out to be a faulty fuel pump that was less than 1 year old. I replaced my stock SU point type fuel pump with one of the transistorized SU units and have had no trouble since. RAY|
|Since it happened on a steep incline, I would suspect a carby piston sticking. If it happens again, try the lifting pins an each carby before you disturb anything else.That should tell you if one of them is stuck or not.It might be worth checking the float chambers for residue also. Barrie E|
|I had a similar occurance last summer in my MGA-after a 200 mile perfect drive the last mile was dreadfull backfiring- loss of power thought the worse -turned out to be fuel starvation to the front carb|
|P D Camp|
|Hi Simon, could it be shortage of air getting into the petrol tank to replace petrol being drawn out this could slow or stop delivery to the carbs. When you were stood at the roadside it could right itself.|
Just a thought. Johnny Cook,UK.
|Maybee a simple combination of too steep a road on a not fully warmed up engine - If you still had some choke on it it could have simply fuelled up under load or if no choke just gasped for mercy|
|I had a motor bike that gave similar symptoms, that proved to be a sticking advance retard mechanism. It would spit back when attempting to start while fully advanced. That was a Triumph Speed Twin, single Amal carb. I'm not saying that happened here, just that popping the dissi cap if it happens again would check it out.|
|Hi chaps, some answers to questions and suggestions. The tach didn't drop when it was spluttering. Well, it dropps in revs but it didn't drop to zero. It wasn't like it was electrically cutting out. The fuel pump was definitely going. I have one of those small square Facet ones so no points to go wrong. I do have a cut off switch but that was on. And it didn't splutter in the same way it does when I forget to turn it on and it does run out of fuel. Then it dies more evenly. This failure was very rough. So it was really shaking and spluttering. |
I didn't notice what the ignition light was doing sorry.
I did remember to check the fuel tank. I have a vented Monza style cap but when I opened that there was no rushing of air or anything so I don't think that was the problem. The street is quite steep and I did have some choke on since I'd just started the car.
I made sure the dash pots were full of oil and have made sure the dampers rods were clean (I took them out and cleaned them) so will leave it at that for now and see if it happens again. I don't drive every day but I have driven a number fo times before and never had a problem on that particular hill. Will keep an eye on it.
|This is the street in question. I come down the road/driveway to the left then go up the hill. There are lights at the top onto a main road. They are quick. You often get stuck when the Noddy at the top isn't ready to go and by the time they get moving the lights change!|
|I recently had a similar problem with my Suzuki Samurai. The engine started to run rough and backfire, then stalled. There was a strong smell of gas, but something told me to check under the distributor cap. It turned out that the rotor was almost completely black where it conducted high tension electricity. I cleaned the rotor on the sidewalk and reinstalled it. The car started right up and ran flawlessly. The rotor is less than a year old. That was almost 2 months ago and the problem hasn't returned. RAY|
|One of the jets sticking? I used to have the same problem on the MGA...it wasn't only just after starting, but often in the middle of a long run. I got used to it after a while, just tapped the fuel bowl when it happened, and hey presto, the fuel started flowing again. I eventually changed the jets back to the original brass type, and have never had the problem again...the newer jets have a neoprene or similar seal, and I think this tends to swell and stick in the shut position.|
|Simon, unfortunately your Google maps show Wakefield SI (the Mainland), near Nelson.|
|Did you roll back to a flat part of the road? Could be a float valve sticking when at an angle, as well as other carb parts. Unfortunately with a 'woodpecker' pump you have lost the diagnostics function of the SU.|
|Hi Paul, was on the flat when it restarted. My car was kind of a bitser when I got it and there was no sign of the original SU pump unfortunately.|
Gary, odd about the link. Was looking for Whitaker place in Auckland actually. It's on the intersection with Wakefield street.
Did it clatter with the problem like loose tappets? How long had the engine been running? How long before it started again?
|My apologies to the members for touting for trade, but if Simon would like an original SU pump for his car he can contact me on|
|Hi Peter, I didn't hear anything like that but my work colleague who had his head under the bonnet though it sounded mechanical.|
Car had only been running a few minutes. Literally started, down the driveway then it conked out.
I guess it was about 15 minutes when it restarted again. Was long enough for me to check the dizzy, remove the rocker cover to look for anything broken, retrieve the rocker cover bolt that fell under the car(!) then put it all back together.
Hi Tony, thanks for the offer. My car is so unoriginal I am not so worried about having the Facet pump. In fact there is very little that is original on it now. I just rebuilt it how I wanted it.
|Did it keep running on two cylinders?|
|Hi Peter, it was running but really roughly and it wouldn't rev at all. Pushing the throttle seemed to make it want to die.|
if you used the choke to start...I guess it is warm over there? Maybe stuck choke or faulty choke...I am lead to beieve modern fuels can affect the integrity of the rubber seals on the choke mechanism and make it run rich and maybe flood and clog up. Just a guess. If no mechanical clatter then not siezed valve. Rotor arms can also short when hot then restrat when cool.
|If the car wants to die when you open the throttle it is often fuel starvation. The mixture leans out as the level drops, but when you are not demanding power it will still run, ask it to do some work when I cant richen up and it will falter and sometimes even stall.|
|A similar thing happened to me a few weeks ago. I started the car and drove into town. When I went to re-start the engine, it wouldn't start but turned over fine and occasionally backfired. I pulled the cap and checked it over. I then removed the rotor and cleaned the contacts on the curb. Upon re-assembly, the car started right up and ran fine. I was going to replace the cap and rotor, but I forgot all about it. Well, I drove into town this morning and the same thing happened. I tried cleaning the rotor again, but it didn't work this time. I went to a local parts store and purchased a new cap and rotor. The car started right up and runs fine. Something to think about. Caps and rotors should be replaced every year because they absorb moisture and eventually short out. RAY|
|I've only ever once replaced a cap or rotor when I didn't need to, and thought afterwards "why did I do that?". I've only had one cap fail in 40 years, and a cap or rotor (changed both) on my daughters car, both of which had done many years and tens of thousands of miles. Changing parts like these as a routine is *more* likely to cause problems with the quality of parts we are getting these days. Your intermittent problems are simply saying there is quite likely something else wrong, and like cleaning the rotor first time but not the second, what you did and appearing to fix it was nothing more than coincidental.|
we have had good results and feedback with the red rotor arms made by The Distributor Doctor. These should last for years.
|Peter, Actually the vehicle that that the cap and rotor failed on was a Suzuki Samurai. This is my daily driver. The cap and rotor were replaced 4 years ago, according to my records. The carbon brush, inside the center of the cap, was literally falling apart. The distributer on my B was made by a company called Davis Unified Ignitions. They base their distributors on the GM systems of the late '70s, which used a large distributor cap with the coil mounted inside the top of the cap. The stock coil was good for 20,000 volts, but their custom coil is good for 50,000 volts. They recommend that you open your spark plug gap to .055". They use the highest quality material when manufacturing their caps and rotors. They even supply you with Nylon hold down bolts, for the rotor, instead of the normal metal ones to reduce the possibility of a short to ground. When I worked for an Esso service station, for 7 years, we stocked just about every conceivable part imaginable. A standard tune up consisted of points, condenser, rotor, cap, PCV valve, fuel filter, air filter and sometimes a set of ignition wires. Most of these cars were driven to New York city every day and having the car fail was not an option, so the customers preferred that we erred on the side of caution. I went on many a service call where bringing along a cap and rotor got the car running on the first try. I've replaced a lot of caps, rotors and coils with evidence of carbon tracking that caused the spark to short out. RAY|
I had forgotten you mentioned Samutai. We tend to fing all the red/brown based arms seem good, the black ones seem iffy and the blue ones that seem to originate in China are not good at all.
We had the metal post of a 45D dizzy cap loose and wobbly the other day. the rotor arm was smacking it now and again and causing a hiccough! Nevcer seen that one before.
|Opening up the plug gap causes the effective HT voltage to rise. Go too far and you can get problems with other HT components breaking down if they aren't designed to operate at that voltage. As with so many things, there simply shouldn't be a need to go to high voltage coils and massive gaps except possibly for specialised engines.|
|Paul, On my ignition setup, everything is designed to work with the higher demand being put out by the modified HEI coil. The company, that manufactured the distributor for me, makes their own caps and rotors, as well as high tension wires, for a specific application. The quality, and price, compared to standard ignition parts is immediately obvious. For a standard application car, this would be massive overkill, but on a supercharger engine it works out quite well.|
Peter, When I installed my supercharger kit, Moss recommended one of their new distributors to go along with it. The distributor had no country of origin stamped on it or even a label on the box. The rotor was hitting one of the distributor cap terminals so badly that the whole assembly needed to be replaced in less than a month. I can only guess that it came from China. RAY
|Hi Ray |
Lasted a whole month eh? That must be a good one :) We have, to be honest, only had troubles with the ones with a blue rotor arm...first one, the vac advance module fell off when we used it to rotate the dizzy to play with the timing, it also missed on the rotor arm and the condensor later expired. The second one came in with a horrendous points miss. Change of rotor arm, points and condensor and it ran much better. Don't forget, these O2 sensors and ecu thingies are all made in the same place too....probably? Now where did I put my key to wind up my clockwork F1 from 1902?
See thread on MG Midgets efi for 1275 :)
|Saw someone selling blue rotor arms at Stoneleigh this morning, as well as several people selling red. Looks like the red are getting more generally available, which is a good thing - until the Chinese think that would be a good colour for theirs.|
|Good thought Paul, we will have to get Monsieur Le Distributor Doctor to mark them somehow :)|
|The Moss Chinese distributor was so far off that the gap, being jumped inside of the distributor cap, actually burned a hole in the cap. I've never encountered that before. RAY|
|Funny you guys are talking about distributors. In the weekend I found that one of the clips holding the cap down on mine wasn't clipped properly. So one side of the cap wasn't being held firmly. You could lift one side up completely. Funny thing is I had been driving all weekend and hadn't noticed any problems. There were no misses or stuttering or anything like that. I think the plug leads were enough to press it into position.|
When the car died last I didn't notice anything wrong with the clips when I pulled the cap off but it could explain the loss of power I guess if the cap had lifted on one side?
|Lifting one side far enough would cause missing on one or more cylinders near that clip. One clip does usually hold the cap relatively level, which isn't a recommendation!|
This thread was discussed between 11/02/2011 and 22/02/2011
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