Mystery Ignition Poblem.

[CastleMGBV8]

I'm having a problem with the engine inmy MGB V8 conversion cutting out under heavy acceleration, it doesn't do it under normal brisk acceleration even when using a lot of revs 5000+. I initially thought it
was a fuel starvation problem as I have yet to fit my new Red Top Facet and regulator but have now noticed that when this happens the tacho drops to zero indicating that it's ignition related.

The rover 3.5 has the 35DLM8 dizzy and the original ballast was by-passed with new wiring, the ignition switch is new and so are the wires and connectors to the amplifier on the dizzy, any suggestions welcme.

Kevin

[Chorlton]

When I had my bog standard MGBGT (ive had 4 of them) I was always having trouble with earthing. I had problems on all of them and know many other who had similar probs.
Its those stupid twin 6 volts uner the rear seat and the earth on them followed by the earth leads from body to engine
I dont know if its relevant to the V8 model but its worth a look

[CastleMGBV8]

Thanks but 12v battery, all new earths and upgraded loom so if not component failure, coil or amp it could be in the ignition circuit so need to do more checking.

Kevin.

[ramon alban]

I'm having a problem with the engine inmy MGB V8 conversion cutting out under heavy acceleration, it doesn't do it under normal brisk acceleration even when using a lot of revs 5000+. I initially thought it
was a fuel starvation problem as I have yet to fit my new Red Top Facet and regulator but have now noticed that when this happens the tacho drops to zero indicating that it's ignition related.

The rover 3.5 has the 35DLM8 dizzy and the original ballast was by-passed with new wiring, the ignition switch is new and so are the wires and connectors to the amplifier on the dizzy, any suggestions welcme.

Kevin

Kevin, my own experiences show that the ign amplifier has a typical finite life of around 100,000 miles and usually goes on the blink before total failure.

try subsitution of the amplifier, coil and rotor arm, as they can all display the problems described.

Also I believe the innards of the dizzy has a "hall effect" device that may be breaking down at that speed.

Just one point to clarify. My SD1 TP Vitesse with 35DLM8 dissy does not have a ballast resistor, so if your system had one that has been bypassed, is your coil a 9 volt unit running continuously on 12 volts. (see page 12 engine tuning data in rover workshop manual) and the coil simply fails at the heaviest usage.

On the other hand If you car is an Efi, did you mean the Efi trigger or inline resistor (sometime called a ballast resistor) has been bypassed??

If so I'm not sure the ECU is supposed to be subjected to that situation.

See the full description and purpose of that particular component here:

http://www.vintagemodelairplane.com/pag ... ger01.html

[CastleMGBV8]

Ramon,

Thanks for your response. To clarify the engine is on an Edelbrock 500 carb. the ballast was by passed, the coil is the original 12v one from the SD1 the distributor has new cap and rotor arm and the amp is original but I do have a couple of spares, one new, one used from a on a R/R 3.9 I bought as a spare to have recurved.

The odd thing is if you drive the car briskly using revs it does not cut out but give a bit more throttle and after a couple of seconds it does a sudden cut with the tach going to zero but the engine will restart immediately!!

I think it's the old trial and error to try to eliminate the problem by changing one thing at a time mainly the amp and coil.

If you have any other suggestions I'd be pleased to here them.

Kevin.

[mgbv8]

Kevin
The fact that you lose the tach signal points in the right direction. You now know you lose the pulse to the coil I suppose. I'm not familiar with your ignition setup, but I would be looking at the trigger mechanism first.

[ramon alban]

Ramon,

The odd thing is if you drive the car briskly using revs it does not cut out but give a bit more throttle and after a couple of seconds it does a sudden cut with the tach going to zero but the engine will restart immediately!!

Kevin.

Kevin, completely out of left field here, but if the car revs freely in (say) lower gears and does not cut out but does when you give it a heavy bootful, is it possible an ignition wire or connector is being physically trapped somewhere in the accelerator pedal/mechanism and breaking contact?

[CastleMGBV8]

Ramon

It did cross my mind that the torque reaction of the engine could be pulling a wire somewhere so will have a look to see if anything looks like the culprit.

Thanks,

Kevin.

[x.l.r.8]

2 things spring to mind, change the amp on the side of the dizzy, also think about the coil as it sounds like it's breaking down under heavier loads. Also pop the cap off and see it the inner shaft of the dizzy (after you have taken the rotor off) moves up and down, it's held in place with a fiber washer and if it's worn or has beed disturbed sometimes the inner shaft walks up and down making a bad connection on the kinglead. I would ensure your coil is a 12v also. and what coil is it. The rover system doesn't like any old 12v coil thrown in the system. Change the rotor also as they have been known to break down or track and cause all sorts of problems, if you have a soare one you may as well use it. Just do the amp first, then the rotor and then look at the coil. If the torque of the motor is pulling a wire then either you need new mounts, or you have a wire that's obviously to tight and would be easy to spot. If the amp gets hot (if the thing is close to the thermostat housing) then you need to take the dizzy out and relocate it further around to get the thing as far away from heat as possible. I drove to work once at low revs because everytime I tried to rev the engine it died, I called my wife but in the mean time I put an OLD rotor in jsut to see, and it still has the same rotor in now, I threw the NEW one away and have a spare as it;s one of the things that costs nothing but can leave you stuck in double quick time.
While your playing with the dizzy it may be worth checking the air gap at the same time adn if you have play in the shaft you should address that aswell.

[CastleMGBV8]

x.l.r.8

Thanks for the for the additional pointers I will be working on the car this weekend so will check the points you raised.

Kevin.

[CastleMGBV8]

x.l.r.8

Forgot to ask.

What should the air gap be and how is it adjusted.

Kevin.

[x.l.r.8]

Oh now your asking, anyone with a late SDi manual or late rangie? I don't have anything that new, I guessed mine at around 14 thou close enough to ensure good pick up but far enough away to miss if any play. 0.014-0.016 would be close enough, make sure there is no play, unlikely as it would miss all over the place. Adjustment comes from the 2 screws holding the horseshoe down, one of the screws is also the post to the clear cover so be carefull you don't damage the thread. Not sure where the other screw is but you adjust the air gap like you would a points gap. It's not going to be your problem but it's worth optimising the system. You could also straight mount the amp away from the dizzy. All you need to do is make some fly leads up from screw down Y shaped lucas connectors with 1 of the tangs cut off, or spade terminals cut in half. then extend the leads to mount the amp elseware. If your going down that route then the conversion in the sticky would be easier, however if your problem lies in the rotor/cap/coil it won't help. I designed that system as I was fed up of the standard system not performing on slightly tuned cars (higher comp, 214 cams and the odd squirt of nitrous) and it worked flawless, I still have the same system in my road car some 7-8 years ago. But the car was running with the external amp first.
As you get a compleate cut out not a fireing on a few and a lot of missing, I would also look at the kinglead and the coil, how hot does your coil get when it cuts out, it's going to be hot but you should not blister your fingers. my other thought is to take out the rev counter (discontinue the wire from the coil that goes to the tach, usually green), if you have a short in the revcounter, this may be earthing the -ive side of the coil.
Adie

[CastleMGBV8]

Thanks Adie, will go through all the points mentioned and hopefully find the culprit.

The car is kept at a friends in Bromley so unfortunately it's not just a case of popping out to the garage and testing one thing or another, I am going to drive the beast this weekend anyway as have just swopped in a 3.07 axle which makes a world of difference to the drivabilty, on the original MGB 3.9 it ran out of gears almost as soon as you changed up.

I v'e also decided to build a 4.6 engine using a pair of Buick 300 alloy heads, see my post under pistons.I know insanities creeping in!

Kevin.