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MG MGB GT V8 Factory Originals Technical - Wont Start

My 75 BGT Rover 3.5 conversion has been off the road for nearly a year since the conversion was done due to ill health. Has been started fairly regularly, up to about two months ago when i got it going but it would run for a minute or so then cut dead the tacho needle went straight to zero.I have rewired the main feed to the coil but was getting a weakish spark from the coil to the distributor lead so have changed the module on the side of the distributor, still refuses to start, fuel pump is working ok and carb smells of petrol if it is turned over for too long so it still appears to be electrical. Any suggestions please.
Kevin jackson.
K.R Jackson

If it cranks but won't start the first thing I do is to connect a timing light to the coil lead and each plug lead and watch the flashes while cranking. Flashing on the coil lead but not the plug leads indicates a bad cap and/or rotor which I have had. If it flashes on the plug leads just point the light at the crank pulley and check it is at about the right time.

What ignition do you have? A weak spark if you have points is probably a bad condenser. You mention a module on the side, on a 4-cylinder car this would be the 45DE4 which was notoriously unreliable. Electronic units of any type are difficult to diagnose, unlike points, and the only thing you can do is keep substituting bits until it works.

As well as the module the pickup can also be a problem, as well as the connections between them, once you have verified you have a 12v supply to the ignition system. The DE unit also had two additional supplies to it as well as the ballasted connection from the coil +ve.

Paul Hunt

Paul

Thanks for your reply, to clarify my engine is a post 86 3.5 Rover with the amplifier mounted on the side of the distributor I believe its a 35DLM8, I have replaced this module and swapped the coil just in case it was faulty.The plug leads were replaced when we did the engine rebuild albeit standard grade and could probably do with an upgrade. The new feed to the coil is direct with no ballast resistance which I believe to be correct. You mentioned the pick up inside the distributor, what should I look out for there? I would really like to get it running properly before the winter sets in, only managed to do 300 miles in it to bed in the engine before I had my health problems and having spent 3 months 1 day a week building the car it would be nice to get the oil and filter changed and start getting a bit of fun out of it.I spent a lot of time whilst I was recuperating following the BBS and it really kept me going.

Many thanks,

Kevin Jackson.
K.R Jackson

It realy sounds like a fuel problem if it actualy runs for a minute or two. If it were the electrical system I wouldn't expect it to run at all. A faint possibility occures to me. Is your coil a 12V coil?. With a ballast resistor you run a 6 volt coil. If (like me) you've not bothered with the ballast resistor you need a 12V coil. If the coil is a 6V'er then it may be overheating and shorting out somehow, very unlikly though as a 6 Volt coild should be producing a big spark (unless dammaged). If it is a 12 V coil are you sure you've elliminated the ballast resistor?, it's often hidden in the loom as a length of pink and white wire.
You say it runs for a minute, does it run well?
If you rev it up a little and it runs for a much shorter time then it's likley to be the fuel supply, ie it's empying the float bowl and the fuel pump is't working properly. have you tried pulling the fuel line off the float bowl and letting the fuel pump run for a minute or so into a container (don't blow your self up by the way). Perhaps the float/valve is sticking shut. ie when the engine is cool it opens and allows the float chamber to fill, when hot it sticks shut.Wild guessess I'm afraid.
Peter

Just went through a series of problems that started out like yours. Check the ignition relay (seems like that is your problem)if bad you don't need to replace it with the $29.00 Lucas item but can get a $6.00 Radio shack one that works as well.

Radio Shack (p/n 275-226 only $6.29 and made in England to boot)There is a 40 amp one available too.
Michael S. Domanowski

Here are the 40 amp relay p/n's at Radio Shack.



They also have, for special order:
40A
SPST
Part # RSU 12131421
$7.69

40A*
SPDT
Part # RSU 1213147
$8.19

70A
SPST
Part # RSU 12131477
$9.99

Good site for electrical help is:

http://www.mgcars.org.uk/electrical/index.html
Michael S. Domanowski

MGBs didn't get an ignition relay until the 77 model in June 76 so I wouldn't expect a 75 conversion to have one. However a 75 4-cylinder *would* have had a 6v coil with ballasted ignition feed - the pink wire contained within the loom that Peter mentions, with a white wire at the fusebox end and a white/light-green at the coil end - and supplying a full 12v feed to whatever ignition system you are using, with a 6v coil, would not be good. It all depends on what coil you replaced it with. A 12v coil generally has a primary resistance of about 3 ohms, a 6v coil about 1.4 ohms. By comparison a 12v Sport coil I have measures about 2.4 ohms. You should not use a coil with a primary resistance lower than this without a ballasted feed, either through the loom resistance wire or an external resistor.

35DLM8 seems to be a later version of the improved (from the very unreliable DE8 system) Lucas system where the external amplifier of the 35DM8 was miniaturised enough to mount on the distributor body instead of located remotely on the inner wing. Nevertheless, it comprises an electronic sensor as well as the electronic amplifier, and the only way you can check these is by substitution. *If* you can find someone with a 35D8 points type distributor, and *if* it has the correct 'end' to mate with your oil pump (I believe there were two types - one with the 'spade' on the distributor and the slot on the oil pump and the other the other way round) then that would be the best way forward to diagnose problems with the ignition. Short of that you have to try and borrow an SD1 unit from somewhere, or shell-out for a new one without knowing whether it will clear the problem.

The timing light test is the crucial thing when it won't start, and if that flashes OK on the coil lead and all plug leads then you need to do the fuel supply delivery test. The pump should deliver at least 1 Imperial pint per minute into a container with the feed pipe removed from the carbs.
Paul Hunt

Paul

Many thanks for your suggestions. Having replaced the amplifier module if it is not in the wiring then possibly the coil I swapped for the original SD1 coil is also not to good, I will get this checked and if necessary relace it with a new Lucas 12v sports unit which appears to have as much performance as many of the expensive alternatives.
I don't think I've got a fuel delivery problem but will check it to on the safe side,I did notice a slight leak from the fuel pump area when i was going to put the jump leads on to give a bit of exra oomph,
once I saw the leak I gave up on that idea. The fuel pump is new so probably/hopefully only a clip or union that needs tightening up.Wont have time to do any work on it this weekend,going to Goodwood on Sunday to drool over all the beautiful machinery only the dangerously wealthy can afford.Will keep you posted of any positive progress, but I think as it needs some minor fettling it may as well go to my mechanic for sorting prior to getting on a rolling road to aort out the Edelbrock/Weber and fine tune the timing. It certainly wasn't runnig to badly before
this problem emerged, a couple of times I accidently got it on the second two chokes and it almost took off.
just as well I put the SD1 four pots and some new drilled V8 discs on so I know it will also stop, I appreciate they are a bit heavy but will bolt straight on either with sleeves over the original bolts or if you can get them i believe TR8 bolts will do the job. I obviously had them reconditioned 60.00 per caliper but as 2 sets came free with the donor SDI
it was not an expensive upgrade even got the discs half price because Moss had made a mistake in their
price list!Thanks again Paul and also MIchael and Peter in Melbourne Incidentally I got the BGT from my nephew Jonathan Jackson who has just moved back to Melbourne so it is indeed a very small world these days.

Kevin Jackson.
K.R Jackson

This thread was discussed between 30/08/2004 and 03/09/2004

MG MGB GT V8 Factory Originals Technical index

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