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MG MGB GT V8 Factory Originals Technical - Ignition Problem?
|I am having a problem with the engine cutting out under hard acceleration, I initially thought that it was a fuel starvation when the carb was on the secondary chokes, but have now noticed that the tacho drops to zero which would indicate electrical. It's very odd because if I accelerate briskly through the rev range it doesn't do it, only on heavy load.|
|Disconect the wire from the coil to the tach. and drive and see what happens.|
Also check try a new coil, next check the coindenser.
Wiring can be difficult, but sometimes wires go to ground, so check your wiring to the coil and from the coil.
Not saying that this is one of your problems, but also it could be. Hard to tell from my laptop.
|Neglected to say it's got a Lucas DLM8 distributor with the ballast by-passed, new wiring and connectors to the the original SD1 coil and amplifier.|
if the tach drops, it is a indication of poor conections at the ballast resitor. I had this on my installstion too and cleaning the spade terminals and new plugs solved the problem on my car.
Hope this helps
|Thanks for comments, I'm going to go through all the connections again but am mystified as why it only does it on full throttle, there is no ballast resistor, and new connection from ignion to coil and new wiring from coil to amplifier on the side of the distributor,ignition switch is also new.|
The engine will rev cleanly to high rpm on a light throttle and I can't understand why engine load could have any effect.
may be there are two problems, one electrical and one with fuel.
The tach indicates that there is an electrical problem. The 'cutting out' under heavy excelleration means that there is not enough fuel delivered.
Flooring the pedal means injecting ~30ccm into the manifold, with a priority to refill the exellerator pump, there is not enough fuel available for a moment. You can try to upgrade the pump and fit a large filter close to the carb.
Several jears ago i fitted twin Mikumi pumps and a Filter King (with a large bowl) to my GT and it helped to solve this part of the problem i had on my caar too.
The electrical mall function can also depend upon the load on the wires and plugs when the engine is moving to one side under heavy excelleration.
I think I have the same electrical issue as you, I have the same distributor. When I floor it the rev counter gets to maybe 4000 rpm plummets to zero then rockets back up.
I'll be keen to see if you solve it.
I still have the orginal 4 cyl rev counter, adjusted to it's lowest setting, I wonder if it's just too much for the poor thing.
I wonder if I should just go the all electrical route with new modern gauges for the rev counter and speedo, I'm getting sick of them not reading accurately.
Does the engine cut out when the tach zeroes? as thats the problem I have, the engine will immediately restart without difficulty within a few seconds, if I allow it to do it on the overrun, still mystified!
No it doesn't. So looking at my problem as a control, Which suggest you have a fuel problem as well as electrical.
Having said that, on mine the white +ve to coil runs through the tach - I wonder if in your case the wire/tach is dodgy and causing both?
Could be a weird coincidence, but there is obviously an electrical problem to cause the sudden cut in ignition, my tacho is new V8 one but I will check the contacts to make sure, what I can't get my head round is that unless I am asking for full power or close to it the engine performs perfectly normally.
Will fit the new pump and filter/regulator set to 5.5 lbs psi as required by the Edelbrock 500 to hopefully take fuel supply out of the equation.
I will then go through all of the electrics again and consider replacing the amp on the dizzy with a stand alone type which is avaliable to replace the on-body amp of the DLM8.
|Strange thing is, it reads fine at high revs, it's just when I want to get there quickly it goes off.|
It's as though the tach can't cope with a sudden change in impulses, rather than the frequency of them.
Does your tach use the white circuit?
|Jusat to add, I had a full electrical test of the whole ignition system done when my carb was set up, and it was spot on, even under extreme load.|
The coil was fine.
Mines a 75/76 spec so has a Black/white feed to the tach and the ballast resistor was by-passed with a direct feed to the 12v coil.
I think I'll try another coil on it and if no change will swap the amp for a spare I have.
I hope you dont mind me jumping in here, but I'm just setting up my 3.5, to bench test it. Can you tell me where the two wires from the amp go to (Red & blue if my memory serves me right??), as I've not been able to track down a wiring diagram for this dizzy.
|C J Bond|
Which dizzy do you have the DLM8 with the amp on the side which should have a plug in connector or the dizzy with the seperate amp usualy by the coil.
If you know the donor vehicle it would make it easier, but if the connections to the coil are in place you need to work out which is the positive and the negative for connecting to the dizzy.
Sorry, yes its the DLM8 with the amp on the side and the wires are red & black. I dont know the donor vehicle though.
Is it as simple as red to + and black to - on the coil?
|C J Bond|
|If the coil lead does not have a plug to the amp then someone (like me) has made up a pair of wires to do the job but without looking at the car which is not at my home, I can't tell you which is negative or positive, the wires you have could well be red/positive and black negative which would be logical, but you can't be sure, what I can tell you is the original Rover plug in lead has a white wire with black tracer which goes to the terminal at the outer left edge of the amp and the other wire is plain white. If your in a hurry for an answer try posting the question on the forum below theres a lot of Rover guys who are very helpful.|
|You've still got tyhe original 4 cylinder Tacho, I hope you've had it re-calibrated! Otherwise it's useless!|
Get on to John Ostick at JDO Instruments, and have a talk to him. Tele; 01535 662701.
He's a star at sorting any instrument.
This thread was discussed between 31/08/2007 and 12/09/2007
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