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3.9 v8 ignition probs

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Ricey30 Avatar
Ricey30 Phil R
Chessington, Surrey, UK   GBR
Hi all

In the process of trying to start my conversion and whilst fuel and oil pressure are good, i dont think i am getting a decent spark.

With a plug out and laid on the rocker cover I get a couple of sparks which then fade to nothing. I have done the same with a plug in the coil lead and again get v week to fading sparks.

It has the original 35dlm dizzy I have replaced the side mounted module with a lucas one but no joy. I am running 1404 carb and am using two wire connection from the module to + &-. I checked the coil which is a lucas dlb198 and it shows 12v on ignition and 9v on cranking. When I checked the dizzy coil pickup by spinning the dizzy shaft and checking I got a pulse from thr pick up coil - I did.

I am also confused over which coil I should use as I believe being a rubber bumper model i have a inbuilt resistance wire, could this be the problem?

If anyone is running a carbed 3.9 I'd love to know what ignition set up you are running and if you altered the wiring loom?

Any advice greatly appreciated

Phil

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Denis Avatar
Denis Denis Hill
Bearii, Nth Victoria, Australia   AUS
Phil I have a 3.9 in my 73 GT chrome bumper with similar ignition and run 12V . If you are using the OE coil supply on a rubber bumper you will be light on for volts. Run a wire from the switched side of the fuse box and it should work. Tape up the old wire. Denis



68 B roadster, Daffodil yellow, supercharger, Burgess SC head, SC cam, Mikuni HSR 48 carburetor and engine built for supercharging.

73 BGT V8 conversion starting with a bare shell. Built the engine early in 2016 with high comp pistons and a few other nice bits. Started on the body late 2016 and found Its a lot of work and expense starting from scratch. Did the work myself, mechanical, body. paint etc all except the interior trim.
Finished and going well, great to drive and quick. Future mods, maybe an increase in gearing but not for a while.

Ricey30 Avatar
Ricey30 Phil R
Chessington, Surrey, UK   GBR
Hi Denis

I’ll give that a try this week, also I have noticed that the new location for the coil prob isn’t a good earth. I was going to run a separate earth lead from the coil clamp to ground to see if this is another possible reason?

Cheers

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B-racer Avatar
B-racer Jeff Schlemmer
Shakopee, MN, USA   USA
1950 Willys Jeep Pickup "Ratrod"
1971 MG MGB
2014 Dodge Charger
The coil housing doesn't need to ground, but the distributor does need to have an exceptional ground, so use a new engine ground strap and run a wire from the distributor to that strap.

Use a 1.5 Ohm coil powered by full alternator voltage. Make sure your plug wires are no higher than 5000 Ohms for the longest wire. Gap your spark plugs at .040".

Flaky spark on the DM distributors are usually poor grounding and too much HT resistance. Run non-resistor spark plugs and plug wires under 5K Ohms each!!!!!!!!



jeff@advanceddistributors.com

Denis Avatar
Denis Denis Hill
Bearii, Nth Victoria, Australia   AUS
Jeff I have noticed that some of the diagrams on Rover sights show a condenser in the electronic ignition circuit. Seems strange to me, have you noticed this ? Denis



68 B roadster, Daffodil yellow, supercharger, Burgess SC head, SC cam, Mikuni HSR 48 carburetor and engine built for supercharging.

73 BGT V8 conversion starting with a bare shell. Built the engine early in 2016 with high comp pistons and a few other nice bits. Started on the body late 2016 and found Its a lot of work and expense starting from scratch. Did the work myself, mechanical, body. paint etc all except the interior trim.
Finished and going well, great to drive and quick. Future mods, maybe an increase in gearing but not for a while.

Ricey30 Avatar
Ricey30 Phil R
Chessington, Surrey, UK   GBR
Right but if progress from last week taking on board all that’s been said.

At the moment I now have a fat juicy spark at the spark plug.

To get this I fed a 12v supply from ignition on side of box to bypass the resistor wire in the Mgb loom. I changed the distributor completely and added a small gauge ground wire to the spade supplied on the side of the dizzy. I did ground the coil bracket as well but I had heard like Jeff said if the dizzy is not grounded it is no good.

So all that said now I have the spark solved which I think is combo of module and grounding issues,
It try’s to catch when turning but won’t fire up!

Been over TDC 3o before on static had the rockets off to check that I am on the compression stroke, which I believe both valves are static and then the exhaust opens after tdc?

Fitted dizzy as per book and the plugs are wet with fuel.

I gave up today and hope that I had flooded the engine whilst sorting all the above?

Will try tomorrow and see if it caches when dry

Otherwise it’s check the compressions to make sure my cam timing wasn’t out, (spent a day on that)

Any other thoughts or tips greatly received !!

Ricey30 Avatar
Ricey30 Phil R
Chessington, Surrey, UK   GBR
Right wasn’t flooded and now my juicy spark has allbut disappeared !

Jeff I’m going to check all the bits you mentioned and fit s new earth strap in addition to the existing and make sure my dizzy clamp is tight.

But that’s next weekend now

Thanks fir input I’ll report back ....

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Jim Blackwood Avatar
Jim Blackwood * BlownMGB-V8
Gunpowder Rd., USA   USA
I respectfully disagree with not grounding the coil, but not in all cases. Essentially there are at least a couple of different wiring schemes internally, most specifically one where the ground for the high voltage winding is connected to the case and one where it is not. If resistance checks show continuity from the spark tower to the case you will get better coil performance if you ground it. Otherwise the spark has to go through the points/condenser and the battery and any other electrical components that are running to complete the circuit and finish the trip back to the coil tower. Even 50KV needs to return to the point of origin in some way. If it's through your radio that's where it's going. I consider this poor design.

If there is no continuity from the case to the tower you have no choice, but in that instance it might be worth considering a different coil unless it is a racing only application.

Jim

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