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MG TD TF 1500 - More unresolved ign/fuel issues. TD-4834

I have been fighting this engine since being put together for the third time. Don't ask... I now have 58 total miles on the car and the engine finally idles well. Highly modified 10.5:1 comp. radical Fanelli race roller cam, header, MK II H4 carbs etc. the engine is stumbling between 1500-2000 rpm.

Fuel system checks:
I started by checking all six full screens multiple times and the float bowls have no sediment beyond the filters as well. New fuel lines new electronic pumps. Good flow into a catch can. Have ruled out fuel delivery. Floats are set correctly. Originally started with GJ needles and then LS1 needles with no effect. Throttle shafts and bushings are new as well. When I pull the enrichener cable there is quite a lag before the engine picks up so I'm not really convinced that it is fuel problem it may just be the electrical coming back on strong the car does rev above 3000 but I have to really play with the throttle and clutch to get it above that mark. It will rev under load freely above that mark once I've overcome the stumbling. I have also change oil wts from the SU 20 wt all the way up to 80wt with no or neglegable effect

Electrical:
I am running a new Mallory dual point distributor with the curve set for 24° total advance. Externally set at 8° initial with a total of 32° full advance at ~2800-3000rpm. I have advance the intitial to as far as 14° initial with no effect. I am running a new sports coil and the wiring has been changed to Neg earth. Question #1. Which terminal on the coil goes to the points with Neg ground with a +/- terminal marking? Same question when using an original coil with letter markings? I want to insure I have it wired for best efficiency. I am going to swap a stock coil in place next to see if it may be a coil issue. If no change I will throw a rebuilt 40162a distributor in place to see if there is a
change.



I'm really at my wits end trying to get this sorted out. The season is nearing the end and I really want to enjoy this car


Thanks

Bill Chasser
TD-4834

W. A. Chasser

Bill,
It helps me at times to see what cannot be seen - use a timing light to see if each and every cylinder is firing - also use it to check the coil output.

I had the same symptoms & traced it down to the condenser, but found a dud plug in #4 and dud plug lead in #2 in the process.

All I had was a timing light and spares to swap in/out to narrow it down. No fancy multi meters required.

Good Luck

Tony
A L SLATTERY

Bill - with the car changed to negative ground and a coil marked +/- the - side goes to the points and the + side to the ignition. With a coil marked CB and SW, you would have to know if the coil was designed for a given polarity. If the coil is designed for positive ground, then in a negative ground vehicle, the SW goes to the points and CB goes to ignition. Cheers - Dave
DW DuBois

Maybe you need a distributor ground wire? Those dual point dist's have been known to go through condensers.
D mckellar

Bill

Statement "the engine is stumbling between 1500-2000 rpm" does not provide sufficient information. Is the engine missing at those RPMs or does it have a dead spot when under acceleration?

Be very aware of over advancing the timing, this is one of the primary causes of blown engines and was why many mechanics had a strong edict to isolate the vacuum advance mechanisms on BMC engines.

Graeme

G Evans

To follow up on Graeme's question: is the stumble under no load, part throttle, full throttle, under load in gear, etc? If in gear does it pull strongly once past 3K? George
George Butz

Not sure what you mean by checking six full screens multiple times, and by 'enricher cable' I assume you mean the choke cable, which normally has some slack. Are you doing this with a hot engine, when pulling out the choke should cause the engine to stall. It sounds like a weak mixture - with a hot engine and the choke connecting rod between the carbs disconnected, have you tried raising each damper piston, with the 'tickler' pin, a small amount - the engine should increase revs a slight amount, then start to stall. If the engine dies straight away, that carb is set too weak; if the engine runs rough, that carb is too rich, which I suspect will not happen with your engine.
You mention 'radical race cam' - are you using the correct needles.
R A WILSON

When checking carbs, remove any air filter system.
R A WILSON

Sorry, should have said, if there are no 'tickler' pins, then each damper should be raised, in turn, about one eigth of an inch with a small screwdriver. The removal of any filters will allow this.
R A WILSON

Thank you for your responses I will try to answer them all

A L Slattery. I have not tried using my induction light to try to diagnose ignition issues, only to set the timing but at least in #1 the impulses have been consistent. I will try your method in deducing the other cylinders. I do have a diagnostic scope but the issue seems to occur under load. Free revving in the driveway doesn't appear to produce the same result

Dave Dubois. Thank you. I get confused on current direction since I've gone to negative ground and the coils. I don't think the coil is the issue but I'm going to switch it out to be sure. If there is no change I will put the sport coil back in place. Do you know if the Sport coil Moss sells requires a ballast? I don't recall any literature that came
with it.

D Mckellar. Yes I have a ground wire from the external distributor clamp to the block which is grounded to the frame.

Graeme. It acts like the ignition is cutting out. It isn't coughing through the carbs as if a lean condition is occurring. I was bumping the timing in 2° increments to see if maybe it was an advance issue causing it at 8° intitial set at 1250rpm idle speed the throttle response was slow. At 14° the response was much better but it didn't change the stumbling issue. My total advance has been reset and kept at 32°-34° degrees with each initial advance change.

George. The issue initially occurs under load at partial and full throttle. Once it occurs even with the clutch disengaged it is difficult to get it over the rpm mentioned but once done the engine revs easily

R A Wilson. I am running dual pumps. There are filters in the tank bungs, the pumps and the carbs. Six screens total filtering the fuel system. I do not have chokes ( I don't think SUs have chokes?). I have enricheners that lower the jet to fatten the fuel mixture to start the engine. I have not played with the ticklers. The Pistons fall freely and are not binding the needles. With GJ needles (std MK II) the plug readings were good. But I then changed to the LS1 needles ( rich MK II) to see if it would help if indeed it was a lean condition causing the problem. There was no change in performance. I might add that I have changed plugs as well in the event it was a bad plug or plugs. Again no change. I am running std heat range NGKs

Bill Chasser
TD-4834
W. A. Chasser

Bill,

You mention you're using Len's "radical race roller cam"...it's my impression that it doesn't really come "on" until you're above 2500 rpm. Have you checked with Len?

Gene
Gene Gillam

If you measure the resistance of the coil primary it should read about 3ohm for a non ballast coil.
Dave H
Dave Hill

Gene. I'm convince this is a fuel or ignition issue and not the camshaft specs. Yes the cam is Definately comes in above the stumbling point when the engine isn't stumbling I have a diagnostic machine that I haven't hooked up to the engine yet but it only appears to do this under load but iit is not consistent every time that the throttle is applied. Which I believe also takes the cam out of the equation. I don't think the machine is going to give me much if any usable data as it doesn't occur while free revving

Dave. Unfortunately my dad was the electronics guru when it came to multimeters. I never learned how to use one though there must be a half dozen of them here at the house. This is one of those times I wish he were still here to help figure this out.

Thanks everyone for your continued input

I hope to get back on it in several hours.

Bill Chasser
TD-4834
W. A. Chasser

Bill,

My engine doesn't really start running/pulling hard until it warms up completely...before that it stumbles a bit too. I have one of Len's cams for the supercharger and I do feel that contributes.

Gene
Gene Gillam

The car starts easy balks a bit when cold but it's when it is at operating temps that all the real problems exist. I still think this is an ignition issue. Unfortunately I got nowhere today I the car and likely won't be able to get back to it until Monday.

Bill Chasser
TD-4834
W. A. Chasser

Bill-
Just throwing ideas at you. Are you running a generator or alternator?

Dave brought up the coil likely being (should be) 3 Ohms. Your "sport" coil could be 3, 1.5 or even 0.6 Ohm. Ohm is a measure of resistance. Too little resistance in a coil circuit can make the spark ridiculously intense. The RPM range you are having troubles is right where a generator would start throwing some heavy current/voltage at the coil. This is only really a problem with Points, not pertronix types, so you don't hear too much about it.

Those electrical testers you mentioned have batteries that need to be replaced before they start leaking. The Ohm range you would select is the lowest, or likely 200 Ohms, meaning it can measure up to 200 Ohms at that setting. The symbol for Ohm is an upside down horseshoe, same as Greek capitol letter "O"-omega. Touch the red and black probe together and you will get a number like .2 or 1.2 or maybe zero. Write that # down. Disconnect the coil wires, and put either probe on either of the two side connectors--one probe on each terminal, and you will get a number like 4. 4 minus the first number is the coils primary Ohms.

Even if you are running an alternator, the coil still could be a problem, now or later, with the wrong coil.






D mckellar

Bill

I think you could chase your proverbial for a long time with this problem. I would be using the modern technology that is available today, put the vehicle on a Dyno where they can monitor all ignition and fuel parameters with the vehicle under load throughout its revs range. You have a lot going on which is outside original engine specifications.

Going to cost you a few dollars however cheaper than swapping bits that may or may not provide a positive outcome.

Be very keen to know what the diagnostics reveal.

Graeme
G Evans

Early carbs reduced, or choked, the air supply to provide a richer mixture for starting, and in the UK the name 'choke' has remained, but I refer to the system you mention. However, I am curious as to how you set the mixture for both carbs, as simply screwing the brass nuts down six flats is normally only the starting point. Both my workshop manuals mention using the tickler or screwdriver approach for the mixture, as well as matching the intake 'hissing' for the throttle setting, although mechanical air pressure measuring devices are available.
R A WILSON

This thread was discussed between 10/09/2016 and 11/09/2016

MG TD TF 1500 index

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