MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG TD TF 1500 - Phantom misfire on Judson TD - need help

Hi everyone:

Today was going to be the day that the TD was going to come home to stay - it was five years ago this weekend that I started the project, and it would have been nice to get on to driving the wheels off the little monster.

But it didn't happen that way. The transmission problem is solved - we found a T9 5-speed at a wreckers in Vancouver and built one good trans from two tired ones, so we've got five gears and five synchros forward.

But I don't know if the next problem is carburetion or ignition. We've got the HS4 1.5" running into the Judson with a K and N air filter, pertronix ignition, NAPA low-pressure pusher fuel pump mounted at the tank, flowed head, big valves, Crane fast road cam, 30-over pistons, header. But we've got a chronic misfire, and here are the symptoms: No matter what needle we use, it will intermittently idle on three, then four. If we change the needle [richer or leaner, it doesn't matter] it'll idle nicely on four, but when we pull it out of the shop and run it through the gears and put a load on it, the car immediately stumbles - then we limp back to the shop on three cylinders and start the whole thing all over again.

We changed the coil, the plugs look good - I'm not sure where we should go from here. In Road & Track a few months ago, Peter Egan wrote about a misfire in one of his old pieces of English iron that was only cured when he tossed the electronic ignition [presumably Pertronix] and went back to a set of points and condenser. I don't know if we should try that or not.

Any suggestions? I'm still not sure if we're dealing with fuel or spark - or both.

Dave


Dave Jorgensen

Dave,

A couple of thoughts:

1. Is the piston rising and falling like it should in your carburetor or could it possibly be sticking at times? Try running the car with the damper out...it may stumble on acceleration but, if it is interfering with the piston rise and fall, the miss might disappear.

2. The low pressure NAPA fuel pump...what is the pressure out? The TF's with the rear mounted fuel pump had a higher pressure out than the bulkhead mounted pumps. If you're not getting enough gas you'll misfire at higher rpm's (not sure why it would do so at idle though).

3. Is your distributor advance mechanism working correctly?


Both of my TC's are blown...one with a Marshall-Nordec clone and the other with an Eaton. I have the Pertronic ignition on both and swear by it (I was having condenser problems and those totally disappeared with the electronic ignition). I'm also using the same K&N filter so I know you're getting enough air.


Gene
Gene Gillam

Dave -
I think missing at idle is pretty well independent of most of this stuff. All needles are essentially the same at the idle set point (1st 2 or 3 stations); "richer" or "leaner" only comes in off idle. The only mixture factor is in how you set it at idle.

Fuel and air requirements are both at absolute minimum at idle, so again if it runs at all it ought to idle OK. The carb piston should not be moving at all at steady idle/steady load. Low float levels might cause lean misfire, but the fuel flow through the carb above idle can compensate, depending on needles. We used to have a lot of VWs and a few other cars that would stall on decel and show signs of lean at idle because the float levels were too low, but they would run fine otherwise.

That leaves sparkles, and so, has the distributor been rebuilt correctly? Sloppy dizzys are most sloppy at low speed, above that flywheel inertial effects tend to smooth things out. Certainly changing back to points is easy, but I'd ask Mr Egan if he has some explanation as to why electronic ignition would cause difficulties, given he has access to a lot of "experts". I have no doubt he'd just love playing with your car! (and he could write off the trip) I know more sophisticated systems have in built dwell compensation correction, which implies that lesser ones do not; maybe a shaky coil and a basic electronic system interact badly?

FRM
Fletcher R Millmore

Dave, I'd try another Dizzy cap if you have one. The caps available to us now aren't top quality. I had a dead cylinder(2) on my B a while back that I finnally figured out was a bad cap. Cap had maybe 800 miles on it. Same goes for the rotor unless you got a good one from Jeff. Also have a look and see if your rotor is riding a little high. There have been issues with the Pertronix and the rotors riding to high. Your description leads me to believe it's a spark issue. Put your strobe in the seat with you and see if you can trace a "dead" cylinder under load.
LaVerne Downey

Dave ,

If you were to do a short run with the car when it is running on 3 cylinders, then look at the plugs, you might be able to estabish if a particule cylinder is the source of the misfire. This could point you in the direction of the problem. I have a Pertronix in my car and have had no issues with it.

George
George Raham

An infrared thermometer can isolate one chronic cylinder.
The timing light works to watch one cylinder at time, as mentioned.
With fresh rebuild, you'd assume compression is up on all 4, but a compression test might disclose a problem, like valve lash waaaay too tight on an exhaust valve.
Just missing, right? Not backfiring and blowing out the relief? It craps out under load, right? ...not just idle?
A vacuum gauge may give a hint towards an intake leak anywhere from the manifold gasket to internal issue with the blower.
How's your voltage? Try your lights and see if it gets worse.
Fuel pressure has been questioned, above. If you push the tickler down does gas squirt out when it is acting up?
If it runs OK for a while, then craps out, it is probably ignition. What gap are you using in your plugs? I realize your electronic ignition eliminates the issue "point gap" but these called for a really tight plug gap originally. Inspect cap with magnifying glass for hairline fractures or carbon tracks inside if older.
I really like the single carb for tuning idle.
We went to check out a TD that had a Pertronix- uunfortunately, it had not started in days, after they washed it. I checked it out and informed them there was no spark. My buddy told me it did run the folllowing day, but my wife insists we're sticking with points.
Hey George, from Alberta also, this is one of the reasons I like those extra gadgets on the dash!
Jim Northrup

You have to figure out what cylinder is missing -if it is the same one-and that will narrow it way down. Simple to loosen the plug wires, and then while missing pull them off one at a time and listen to RPM drop- should be the same. Or dsfollow Jim's suggestion- purchase IR thermometer and shoot each header outlet- that will tell the story also. I agree with all of the above possibilities. When you find the missing cylinder, in order I would make sure the plug wire is correctly seated in the cap, change the plug (forget about trying to clean), the recent Petronix we put on my buddy's TC had the issue with the rotor being too high, which would trash the cap, change the cap and wires anyway. If that same cylinder still misses, I would shut it down and remove the float bowl top and visually check the fuel level. Then check for air leak, lastly compression. Wow- that motor looks seriously fast!! Gerge
George Butz

Ditto what everyone says.

The fact that it idles nicely when cold and poorly after running in could be a heat soak on the carburetor causing either vapor lock in the bridge or (not likely) altering the shape of the suction chamber hanging up the needle and piston. By the time you fiddle with new needles the heat soak is gone, and the car runs well until you heat it up again. Put a wet rag around the carb without fouling the linkage and run it. Or... you could be running the fuel too low in the bridge. Measure it at .16 down and set the carb and see what happens. Or... when you get the stumble shut off the ignition (which will kill the fuel pump) and see if you are using fuel faster than the pump can supply.

But I always check compression and valves, and electrical before the carb, unless I know the fuel is dirty, old or the carb isn't set up properly.

So that leaves intermitant wires, are you running solid core? or bad coil, yeah I know it is new, but I would swap one out just to be sure, or bad rotor and cap. Check with Jeff Schlemmer on a new rotor, his are really solid. Could be pertronix... I would try points just for fun.

Good luck,
dave
Dave Braun

Dave,

I had a similar problem with my Marshall-blown TD.

Compressing the gasses into an engine puts a lot of pressure (no pun intended) on the ignition system, and compression pressures can be very high and will tend to snuff any spark if the spark isn't sufficiently big.

All the symptons you describe were similar to mine. It was only when I fitted a large oil-filled coil, that the probmens went away. It seems that a paltry 25,000 volts just wasn't enough to fire the plugs. 40,000 volts did the job and I was using standard points and a condenser.

Pertronix themselves offer a 'meaty', low-resistence, 45,000 volt coil, designed specifically to work with their own solid state ignition. Probably provides for a quick rise at low primary voltages. Also, a big coil willl often overcome any inequities in fuel/air mixture. Try to borrow one from a hot rod shop, before you buy one.

There are lots of good suggestions in here. Let us know how it works out.

Gord Clark
Rockburn, Qué.


Gordon A Clark

Thanks, all of you. When we track this thing down I'll let you know what it is.

Dave
Dave Jorgensen

Gordon may very well be on to something there...I do have the hi-output coils installed.
Gene Gillam

Jim from Michigan,

I understand the importance of all those gadgets Jim. The "cluster" just looked out of place on the facia.

George from Alberta
George Raham

George,
I can appreciate simplicity and original appearance.

I like lots of gauges, information and idiot lights to call attention to important functions. My wife & friends like the cluster of gauges/lights/switches. I want stuff to play with while my honey drives me all over the country!

The idea of adding the voltmeter, boost/vacuum and air fuel ratio appeals to me, along with knonk LED display, since adding Marshall Nordec blower.

Keep us informed Dave!

Jim from MI!
Jim Northrup

Jim from MI,

Thanks for your comments re guages. It's great that each of us can enjoy our cars in our own way. My wife likes to be driven when we go out in the MG. It's good we are not all the same.

George from AB.
George Raham

This thread was discussed between 04/04/2010 and 05/04/2010

MG TD TF 1500 index

This thread is from the archive. The Live MG TD TF 1500 BBS is active now.