Welcome to our resource for MG Car Information.
MG MGB GT V8 Factory Originals Technical - v8 misfire/backfire
|hello there. very new to all this, looking for some advice. just aquired (today!) a 1970 MGB which was restored 12 years ago with new heritage shell and professionally converted to a v8 with a refurbished SD1 engine (carb) and 5sp gearbox. it has only done about 3000 miles since then and has hardly been used at all in the last 2 years. (it did less than 200 miles in the last year). a quick test drive revealed no obvious issues (but I am no expert, thats for sure) and i had the car briefly checked by a local mg specialist. he had a slight concern that the car seemed to be 'back-fuming' and said it could just be as a result of lack of use, possibly a blocked breather pipe, possibly something more serious... Anyway, the price was pretty good so I thought i'd go for it anyway. |
driving it back today from droitwich to west yorks (approx 125 miles) all going fine until started to lose power/misfire at about 70mph on the motorway. it seemed to backfire and puffed some interesting looking stuff from the exhaust. i pulled off onto the next roundabout, revved it a little in 2nd gear and back up the slip road... stil misfiring. came off at next junction and let the car idle for a couple of minutes whilst telling my wife in the 'support' car that her new baby might be coming home on a tow truck.. set off again and.... no probs!!!! pulling fine!!! was absouloutely fine for the next hour at motorway speeds and the about 3 miles before leaving the M1 it lost power again going up a steady incline in 5th at about 75mph. over the top of the hill and down the other side, let it coast a bit, blipped the throttle a couple of times and it seemed to clear and be fine again until we (mercifully) made it home. think it might have backfired one more time going through local village. the engine pulls very strongly when its not mis-firing.
I am HOPING the car just needs a service/new oil/plugs/timing checking etc and then a few good long runs to 'clean' the engine a little, and that the misfiring may just be a reflection of lack of use.
Do these symptoms sound familiar to anyone..?? should i be worried i have invested in an old dog??? if the timing/plugs etc fails to bear fruit where would be the next most likely place to look at?? On a technical level i am able to change the oil and filter but beyond that...well, I intend to learn but please be gentle with me.
Any thoughts would be much appreciated, even if the news might be bad.
|Congratulations on joining the V8 gang, Mark.|
Don't panic ....it doesn't sound too serious...the probable cause is lack of use, these are simple engines to service, If you don't get good advice on this site, try the MGOC one (WWW.mgcars.org.uk) there are a lot of carb V8's there.
|thanks for that- i hope not too.. update is that overnight it has left a puddle of oil on my garage floor... looks to be coming from around the front of the engine under the distributor... oh dear.|
|Mine leaks a little bit from the front oil seal, that's quite common, and easily fixed...could be the N/S rocker cover gasket...another cheap & easy fix. |
Sometimes the valley gasket springs a leak at the front or back, it is held down at each end by a long black 'U'shaped pieceof metal on a single bolt in the middle, that might just take a nip up.
Keep us posted
|While you are tuning change out the fuel filter, and you may want to remove and clean out the carb bowls. Sitting that much it's likely there is gunk in the gas. A bottle or two of injector cleaner in the fuel tank might do wonders. Another possible cause could be the fuel pump. The OEM pump is pricey but repairable. The double ended pumps are even more pricey but also repairable. Both are replaceable at nominal cost. Your oil leak is fairly normal, it's just marking its new territory. Lay a pan under it and see how bad it is. Just a few drops is no cause for concern. And Welcome!|
|thanks for the info so far and the warm welcome! glad to hear it it probably isn't anything completely terminal. like the comment re 'marking its territory'!! being a vet and dealing with dogs all day this strikes a real chord!!! so used to modern cars and totally smooth running without any fluid leaks, i guess i need to adjust my way of thinking. One of the things we like a about the car is the way it seems so 'mechanical' and raw. the intake noise when you press the throttle is quite addictive!!!|
|Mine sits on ex-kitchen vinyl floor covering as it's a bit incontinent front, middle and rear...and I like to keep a track on just how much is leaking out of each bit....the diff's winning hands down at the moment .|
|good to hear not totally unexpected :-).. the sunday papers have come in handy as haven't has chance to paint the garage floor yet. think i might steal some of my 3 year old sons pampers incontinence bed liners if it carries on.|
|Sounds like you have a car running lean, mayby caused by low fuel preassure as already implied. What was coming out of the exhaust? Was is silencer isulating material? Anyway check your igniton advance setting. When my ignition advance setting was to low ie 0-2 dgr before top dead center. I had lots of fuel in my exhaust, especially on decelleration.|
|How about old gas (petrol) clogged filter/s?Water in the gas. I had a 89 TBI once that did what you described, bad power etc and it was a filter in the pressure fuel line.|
I dont suppose it was a cold rather damp maybe foggy day. If so could be carb iceing. Mine suffers with this occasionally (Edelbrock).
Which carb has it got?
|the weather was v cold as it happens, barely above zero for the whole journey. ashamed to say not sure which carb it has; i imagine whatever was standard on the SD1. I don't suppose there's much that can be done re the carb icing. i suspect the fuel in there was very old. i have filled it up with a fresh tank and treated it to a load of cleaning additive. what are the odds on the misfiring and the oil leak being related or i am i looking at 2 seperate problems? I read somewhere that a blocked breather pipe could cause both symptoms? anyone any experience of this?|
|Do you know what 'refurbished' meant in this case? It could be anything from steam clean and paint to a full rebuild. |
Your leak might be the engine breather system - blocked flame traps and hoses can cause oil leaks and fumes galore.
They are cheap to replace anyway and I would do it as part of your normal replacment regime when you buy a new car - plugs, points, dizzy cap, rotor, leads.
The leak from the front is most likely front crankshaft seal ? - only pence and easy to replace - however getting the crankshaft pulley wheel off to get to it with the engine in the car is another matter! Also check the timing chain cover on the front and oil pump base/cover.
You need to get yourself an MGBV8 workshop (or haynes) manual off ebay. You will end up knowing every little detail of the engine, so best get a workshop (or haynes) maual for the SD1 as well, just to deal with engine issues. Again cheap as chips on ebay - I've got three!
I'm glad to read you are a vet you should be used to sticking your hands horrible places where you can't see! You'll be doing a lot of it.
Half the fun of having something like this is figuring out how the whole thing works and fixing your probs yourself. I would try to do the work the people have recommended her yourself, and only go to your local garage as a last resort. You'll end up knowing more than them.
|Thanks Liam and others, the haynes manuals are winging their way to me in the post as we speak!!! Much bedtime reading beckons, me thinks. Have to say my mechanical experience is pretty crap. I am a little tempted to let a mechanic friend of mine have a play and get things straightened out, and after that start tinkering myself. A bit wussy i suppose although I soemtimes berate animal owners who have been arsing around with their animals problems for weeks by themselves when a quick trip to a 'professinal' (!) could have sorted them out striaght away. I am looking forward to having a bit of a tinker though.. will be a good excuse to get away from the kids and wife for a couple of hours at the weekend. (only a joke dear, if you happen to read this.)|
The breather system is looking like a prime suspect at the moment, but all the other things mentioned will be attended to. thanks for all the info.
|How do you sieve through so much good advice ?|
Backfire = ignition of unburnt fuel
misfire = some cylinders are not firing as they should.
Loss of power = fuel starvation or igniton circuits have a faulty connection
1. get a manual (very unbritish)
2. Conduct some tests
The chances are that the old girl needs a good shampoo and trim. Break and grease all contacts on coil and the bottom fuze and under the dash from the ignition switch. Particularly checks the cleanliness of the bottom fuze and its connectors. Then crawl underneath and check there are no constrictions in the rubber part of the fuel line.
Question 1. Are the plug leads on the second and third cylinders (from the front) touching (and causing a misfire) - if so, separate and test with a run. There may be other leads where the insulation has broken down but this duo is the most likely culpret when it comes to misfires.
Question 2. Is the misfire heat related ? If so, the most likely causes are (1) the coil (change) (2)The HT leads or the capacitor in the distributor(change) (3) A non-functioning spark plug (4) Fuel starvation (disconnect/replace the filter - disconnect fuel line and switch on - look for a good stream) (5) Another circuit taking all the juice (unlikely unless the engine is hot enough to start the fans).
Question 3. Is the backfire the result of fuel which has not ignited normally because there is a problem with the HT system ? ie; does it still occur when you know electrics are in good order ? Yes ?? think carbs or breathers.
Question 4. Is the vacuum advance still working ? or is the ignition timing very slow ? (disconnect the advance pipe at the carbs and suck - put tongue over end - if the vacuum can be felt - spit and go onto the next test. If you can't feel the vacuum - rebuild the dizzi with a new vacuum advance unit (spit anyway)).
Question 5. Do you have a breather problem ? The breathers all need to be secure (no airleaks) - remember the vent at the very back under the carbs - it should have a Fuel filter attached to stop garbage getting in. The flame traps can be cleaned by soaking them in parafin. Heavy oil consumption can also be a sign of blocked breathers (check the integrity of the carb connection - is it gunged up ?)
Question 6. Do you have dirty fuel or are the carbs knackered ? (- change filter jets and needles - clean out bowls - clean out vacuum chambers and check springs are intact - top up with SAE30).
Question 7. Is air leaking past the Carbs' butterfly valves ? (put it all back together, advertise on ebay and get a sensible politically-righteous dung-burning eurobox and thank your lucky stars for your narrow escape) Alternatively save up for a sensible new carb.
PS. Yes you will need to know quite a lot about the beastie or have an old fashioned mechanic living close by. .... and we haven't mentioned 'suspension' or rust at all !
|Lordie roger, that gives us a fair bit to go on. the local mechanic is a good mate and is a traditional type, he specialises in beemers, but long before everything went ECU/electronic and he has worked on rover v8s in the past. he is quite looking forward to having a play i think. i'll print off this thread before i see him so he has some ideas to work on. |
fortunately the shell is a heritage and has done no more than 3k miles since it was rebuilt with the reconditioned engine/box in 1993. it was only ever taken out in good weather. i know this for a fact as it had no hood!!
i assume with it standing around for so long it will be worth replacing all fluids ie gearbox/brake/coolant etc etc?
the front discs also need replacing. currently they are standard MGB issue, not upgraded. the car will not be doing any track days but even so will be driven fairly keenly (mainly by the missus..); any opinions on wether its worth upgrading discs/pads?
finally... speedo calibration is way out; reads 55mph when doing 75mph on motorway; an easy fix??
|That business about odd exhaust... Check for mouse nests perhaps? Definitely change the fluids, belts or hoses that show any signs of aging, that sort of thing. Your mechanic friend will know what to do. Don't let Roger scare you, in all likelihood you have a real peach on your hands. As for the brakes, new high tech pads at the very least. Run a search for brakes and you will find a wide assortment of upgrade options.|
|Check the rocker cover gaskets too, the later Disco ones are the best, they have a built in spacer which ensures that they tighten down evenly.|
|What's you oil pressure mark, at tickover hot and cold, at when running at 3000 rpm?|
The mention of it standing for so long could be an issue with the oil pump being able to do its thing. However, the amount of driving you have done it would have gone bang before now if it was.
The firgures would still be interesting to know on a reconditioned engine - the boys here will be able tell exactly how reconditioned it was if you provide them.
|Also, it's good to know that all of the leaks can be stopped if you like. First quality sealants like "Right Stuff" and modern gaskets such as used in the late model Rovers do an excellent job. It costs a bit to do of course, either in time or money but it's far from impossible.|
|Oil pressure looks fine the whole time; can't remember the exact figure (and its been a bit a bit cold the last few days to get her out to check) but the MG specialist in Droitwich who checked was happy with it. Had a look under her last night and she's also had a little dribble from the rear diff (not suprising after Mike's comments)... heigh ho. Anyway, she's in tomorrow to see my mechanic for a few days tinkering so i'll report back on the findings. Thanks again for all the info. |
This thread was discussed between 19/11/2005 and 24/11/2005
This thread is from the archive. The Live MG MGB GT V8 Factory Originals Technical BBS is active now.