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MG TD TF 1500 - Diagnostic help required

A brief history of the last few weeks in chronological order.

About halfway through a 20 mile trip Mobius started to sputter. Upon acceleration there would be a hesitation and no firing of the engine. After a brief pause, while coasting, it would then kick in again. It seem to get progressively worse as I approached my destination. A few hours later the return trip was more of the same but a lot worse. Halfway home I had to pullover and I checked the float bowl for gas level and dirt. It all appeared okay. I managed to restart the car and limp home.

FYI I am running a 1950 MG TD with a supercharger with one H4 SU carburetor. The engine was rebuilt two years ago and has been running very very well.

Thinking that it was an ignition problem I removed the pertronix and installed my original Point system. The road test started okay but after a few miles it was more of the same problem.

I replaced my original point set with a completely new set and set the points and the timing. The road test started okay but after a few miles it was more of the same problem.

I next replaced the coil with one I have in the garage. This road tested very well for a short run so I took it to our car club meeting 20 miles away. I barely limped into the parking lot and had it towed home.

I bought a Pertronix Flame Thrower 3 ohm 40k volt capacity coil. Each of my road tests were getting progressively shorter before the problem occurred. This one was about a mile or two and I barely made it Home.

I replaced all of this poplar wires and the coil wire but the problem continues.

It has now become very difficult to even get the car started. While trying to set the timing I noticed something that may or may not be unusual while trying to rotate the distributor. I could get it started but it it was rough. If I put a little pressure on the distributor cap in a vertical downward direction it ran better. If I put a little pressure on the distributor cap towards the engine it would shutdown. If I put a little pressure on the distributor cap towards the fender it would run a little better.

I put a tie wire on the distributor cap to the radiator support as you can see in the photo.

I thought it was a bad distributor cap but I have replaced it with a brand-new one and it is more of the same. I have also replaced my red rotor with a new red rotor.

The first few incidents showed the problem after several miles and when the engine was fully warmed up. But that time window has shortened up considerably.

I have also check all of the fuel filters and they are clean. I have a facet fuel pump in line with the main fuel pump and I can switch between the two with a toggle switch in the glove box. Switching the fuel pumps has no effect on the performance.



Mort Resnicoff

A few other notes.

The TDC mark on the pully coincides precisely with the points just opening and the points are set at .012".

All of the plugs appear to be in good condition and are set to about .025"

The timing sequence has never changed.
Mort Resnicoff

BTW, I don't think it is a condenser problem since the first occurrence happened when I had the electronic ignition in the car. I have also had two different condensers with the two different point sets I have tried.
Mort Resnicoff

Sounds like the bowl of the distributor has worked loose. When the distributor is cold, there would be little movement. When the distributor gets hot, the bowl will wobble causing misfire. The bowl can be repaired, but I do not remember who can perform the repair.
Rick


Mort, check to see if you have a ground wire on the distributer body like is shown in the photo.

Richard Cameron

The zinc alloy bowl of my distributor was loose on the steel core and when gripped in a vice it could be moved either way. I sent it to Advance Distributors, who said it was scrap and sold me a new one, which works very well. Others have said the old one can be repaired by pinning the bowl to the core, but I think its only a DIY job if you have appropriate kit to do it. One day I'll give it a try. The short fix is get a new distributor.
Dave H
Dave Hill

When was the last time the distributor was overhauled? If the answer is "not since I owned the car", I would start with sending it to Jeff Schlemmer at Advanced Distributors. Best $150 (or so) you'll ever spend on the car.
Lew Palmer

Mort, after reading these last few posts I'd suggest that you beg, borrow or steal an early distributor and see if it clears the problem. If so, I'd go get one. I'd loan you mine, but it's the late type. Bud
Bud Krueger

I agree, it sounds like the distributor body has come loose from the shaft. Jerry Felper repairs them if Jeff won't. Jerry did one for me years ago and it's still on the road somewhere in a TF.
Steve Simmons

The distributor was overhauled by Jeff at Advanced Distributors about six or seven years ago. It may indeed have come loose. I will try to check it tomorrow but if I don't get a chance it will have to wait three weeks till I get back from a trip. Thank you all who have suggested it, it sounds like a possible cause.

Richard, I had that ground strap on the car since I bought it eight years ago. I recently removed it since the distributor is solidly mechanically connected to the engine block. I could not see I need for the extra strap. I do not remember if I took that strap off before or after the symptoms arose.

Could someone explain why that ground strap is there?

TIA,
Mort
Mort Resnicoff

To provide a good ground to the engine block. If there is dirst or corrosion between the distributor and the block, you will not get a good spark.
Lew Palmer

Is the drive belt on the SC slipping?
Ian Bowers

Nothing should change moving the dizzy, so it must be in that area. Hard to tell from your picture, but make sure the HT lead to the coil is fully seated in the cap, and that the LT terminal is not shorting to the tach cable or drive. Wiggle the dizzy cam side to side and recheck all of the simple stuff. I drove my car for 25 years with no ground strap, but it should have one with points. Petronix doesn't care if grounded or not. There were a bunch of bad condensors out there a while back, many failed new (search archives). When it fails or is barely running, pull a plug wire and see how far the spark jumps and if it is even, etc. Just random thoughts, good luck! George
George Butz III

I'm no expert, but I had similar symptoms. After rebuilding the carbs, spark, etc. still no fix! Then I simply loosened the dizzy nut, turned dizzy 1/8 turn counter clockwise, and the symptoms disappeared instantly! Wish you luck,
Ed
efh Haskell

I had this problem with a dizzy, I could see the marks where some hamfisted so an so had hit it to remove it from the block.
I used a hydraulic hose clamping tool to fix it back to the iron core. It was successful but only used it as a emergency backup.
That body was used on many British car so there should be many about.
As Richard says, check the earth strap, if it fires clean then you have a loose intermittently grounding body. The earth wire must go from the ally body to earth.
Good Luck.
Ray TF 2884
Ray Lee

I am headed for the garage to look at two specific things. The ground strap and a possible loose bowl on the distributor.

Ground strap
George you said "Petronix doesn't care if grounded or not."
I don't remember if I took the ground strap off before or after the symptoms started. However, when the symptoms started I had the Pertronix in the car and had been using it for many years. So that would rule out the ground strap as a problem. Since I now have the standard points I will put the ground strap back.

"There were a bunch of bad condensors out there a while back, many failed new (search archives)." Again when the symptoms first appear there was no condenser since I was running the Pertronix.

Loose Distributor bowl
I can envision a loose distributor bowl affecting the points gap and thus the performance.

My question is, how would a loose distributed bowl affect the Pertronix???

Mort
Mort Resnicoff

Mort did you have the lobe sensing Pertronix or the one with the collar that fits over the distributor cam? If it was the lobe sensing one the lose bowl shouldn't have affected the ignition. But if it was the one with the collar the movement of the bowl would throw the collar out of alignment and vary the space between the collar and the pick up module.

Regards

Tim
Timothy Burchfield


Mort, item #8. of the Pertronix installation instructions says to re attach the distributer ground wire securely.

http://www.pertronix.com/support/manuals/
Richard Cameron

If you changed the dizzy and the same problem persists, I'd bet on a fuel/air problem.

If you have the main jet "enrichener" hooked up, try it to see if running richer helps.

Got a vacuum/boost gauge to observe your vacuum? It won't really point to a bad manifold gasket, though. I've had numerous manifold gaskets fail with heavy supercharger cantilevered out on just the four studs.
Try starter fluid around the intake areas for vacuum leak.

JIM N

I have been struggling for days trying to get it to run right.

Summary:
5 Speed
Moss Supercharger
H4 SU Carb
Newly rebuilt
Pertronix
Flame Thrower Coil, 3 Ohm, 40K volts
Plugs clean

I tried to get it tuned up properly using points to no avail. It must be me.
I put my Pertronix back in and it idles well and I set the advance to 30 degrees at 3,000 RPM.

Results of a short test run:
If I go through the gears at slow to moderate speed, shifting at about 3000 RPM it runs well.
If I go through the gears with rapid acceleration, shifting at about 4000 RPM, I get a complete hesitation as I take my foot off the clutch. With my foot on the clutch I pull to the side of the road and stop. When I take it out of gear and release the clutch the engine goes back to normal idle.
The same thing happens through several trials.

At the lower acceleration the vacuum gauge appears normal. At the higher acceleration the vacuum gauge shows about 15 inches of Hg which would be normal without load and sitting at idle.

I thought about Jim Northrup's supercharger suggestions.

The fan belt is a little bit slack and I cannot move the current idler pulley to make it any tighter because of an interference with the radiators Y connection. It may or not be on the borderline of slipping. I don't know any way to check that.

Does anyone know where I can get a larger idler pulley for this setup?

I tightened the nuts on the four studs holding the supercharger in place. I was able to get about a half to three quarters of a turn on each. I tend to think if the manifold was the problem it would run poorly all the time.
Mort Resnicoff

Tim,
It has a magnetic collar that fits over my asymmetric cam and snaps into place.
Mort


All,
I did replace the distributor ground wire.
Mort Resnicoff

I agree it sounds like a dizzy problem .... but it is weird that the clutch seems related.
However, if you find it is not the dizzy/clutch .... I had a slightly similar problem with my road TC about 30 years ago. I eventually found some jelly-like stuff in the bottom of the carby jets. It was ok idling and revving up while stationary, but under load the jelly would move up the needle and block the jet. When there was high fuel demand AND the carby piston rose, so did the jelly. Perhaps a long shot but worth a try if you can't find the problem.

Good luck,
Bob Schapel
R L Schapel

A loose blower belt would likely only result on lower boost and maybe a telltale squeal.

Rather than look for replacement idler, go one size down on the belt length.

You could try wraps of electrical tape across the pulley surface building up layers so the center is higher, crowned. I've done this to some idlers and my belt grinder just so the belts track and stay centered. You might wrap the tape in the direction that would keep it on rather than peel it off.

To me, the symptoms fit fuel starvation. It sounds like your fuel pumps are in series (our TD has a pair in parallel). You checked your fuel filters, but maybe doublecheck that you don't have a plugged strainer in the fuel tank, on the firewall pump or at the carb inlet. I've discovered rust plugged strainers where I least expected to find a screen. You might even gut the check valves from the firewall pump and disconnect the wire for the test. You could have crud settled in the fuel line, too. Remove the fuel line from the carb and pump fuel into a container to visually watch the flow. Every year at this time, I go through many neighbors' lawn mowers and occasionally remove a gelatinous crud that results from our modern gas after months of setting, the ire of Briggs & Stratton.
JIM N

Thanks for the 'jelly' story Jim & Bob. My daughter's lawn mower has had a couple of refusals to start in the past year. Dad-to-the-rescue dropped the float bowl and found this creamy, gelatinous goop in there both times. After clearing the carburetor passages and putting fresh fuel in the machine fired right up.
Saturday I took Lazarus for a 25 mile (each way) trip on an afternoon with the temp at 92F. Got a bit of sputtering a couple times at 60'ish mph. At one point My speed dropped down to about 50 and I decided to get off of the Interstate. Within the next mile it cleared up, but I still got off. It still acted up a few times on the way home. Happily, my low-fuel lamp had come on earlier in the trip. This'll give me the opportunity to drain the tank and fill it with non-ethanol fuel. I plan to check out the various fuel filters and float bowl while I'm at it. I wonder if I'll find any jelly? Bud
Bud Krueger

Jim Northrup,
Back in May you responded to this posting. You said:
“I’ve had numerous manifold gaskets fail with heavy supercharger cantilevered out on just the four studs."
I think you have put me on the right track. I have taken the supercharger and exhaust manifold off the car and discovered a few things.
I had previously used a non-fuel resistant gasketing material. The gasket was in poor condition and showed possible exhaust leaks into the intake. I have cleaned and re-assembled everything.
I do not recall any specifications for the torque on the nuts on the studs.
20 foot pounds has been suggested but it seems light to me.
Do you have any suggestions?
Thanks,
Mort
Mort Resnicoff


Attached chart might help.

Jim B.

JA Benjamin

Update

The engine is still not running correctly.

On the ignition side everything has been replace, repaired, substituted and tuned. I have the same sputtering issue and lots up power with Pertronix or points. New flamethrower coil, two different sets of points and the condensers and a new distributor. Different sets of spark plug and coil wires. Three different red rotors.
Currently I have the new distributor with the Pertronix installed. About a 32° advanced at 3000 RPM. The car idles smoothly from 1000 RPM to 4000 RPM.
When I try to drive and step on the gas for acceleration I get no response to the throttle, the car idles poorly at low rpm and I need to pull to the side of the road and in a minute or two I get a smooth idle.

On the fuel side I have the fuel pump converted to electronic and I have a second Facet pump in line with the primary pump. I have a toggle switch in the glove box and get the same results with either pump. I have checked all fuel filters from the tank through to the carburetor. After a discussion with Joe Curto I removed and disassembled the carburetor. There was no dirt or obstructions on any ports and all the standard checks showed the carburetor to be functioning normally.

I have removed the belt from the supercharger. Car idles and revs up normally. The test drive was the same issue.

The engine was rebuilt two years ago and had been running excellently. The sputtering just appeared while driving one day.

Thoughts?
Mort Resnicoff

Is it something simple like the fuel tank not venting? I see no reference to checking filler cap.

It would account for no load ruuning OK but full load not getting sufficient fuel.
Ray TF 2884
Ray Lee

Had exactly the same problem last week with my super charged TD. Changed the whole ignition system a number of times. Sometimes it worked and sometimes not.
Eventually I heard a clonk after a stop. It was the piston in the SU that could get stuck sometimes. Polished piston and cylinder carefully and changed the needle at the same time; probably not necessary. Now it is running perfectly
Y Strom

Mort:
Do you still have the exhaust gas analyzer on your car?

When this happens, if its ignition your exhaust should go rich. If its fuel it should go lean?

Jim B.
JA Benjamin

In my case I had all sorts of CO readings, obviously depending on where and when the piston didnīt move freely. Now I have set it to 5 - 6 % depending on load.
Y Strom

I think you have ruled out ignition. Have you checked to make sure you have at least 12V at the coil when it misfires? Not sure of your set up, have you tried without air cleaner? With air cleaner off, verify carb piston not stuck or sticky when it mis-behaves. Also kill engine, take top off of float bowl and eyeball actual fuel level in float chamber, possible there is a fuel delivery restriction (float needle area?) and you suck it nearly dry. Nothing intermittent mechanical I can think of, when even one valve sticks, it rattles like crazy and you would know it. A local TF racer had to rubber mount his carbs or float chambers due to vibration, which somehow affected fuel flow, I don't remember if too much or too little. Just some thoughts. George
George Butz III

You might have already checked this, but make sure you are not pulling any air in between the pump and tank, it doesn't take much. I had this on our MGB and found it was a cracked suction line between tank and pump. The engine would run great and idle very smooth, but after starting a drive it would run normal for a period, but not enough fuel was being pumped to keep the carbs filled and it would shudder and die, turned the key back on, the pump would fill the carbs and the process would be repeated. Because the air came in in front of the pump, it would fill the carbs, pressure would build and shut off. PJ
PJ Jennings

Thank you all for your comments. They are very good suggestions. I think I have localized the problem as the carburetor.

First remember I have a single H4 1-1/2 inch, SU carburetor on the end of the supercharger.

As per Joe Curto's suggestion I removed the damper and put a rod in so that I could watch the movement of the piston while driving.

If I accelerate very slowly and steadily, everything appears normal. The rod moves up and down with the application of the throttle. It runs smooth and powerful.

If I accelerate quickly the piston does not react at all. The butterfly is open and the engine is drawing more air but the needle does not rise to give me more fuel. The car hesitates due to fuel starvation.

All parts in the carburetor are in their proper orientation and in very good condition. I have checked for obstructions in all pathways and they are clear. The needle is centered in the jet. I can lift the piston and it drops steadily with a resounding clunk.

When apart I did the suction chamber drop test. I taped off the vent holes and let the suctioning chamber Drop off the piston and it takes 3 to 4 seconds.

What could be causing the piston not to rise upon rapid demand?
Mort Resnicoff

The first thing that comes to mind- "oops! I dropped the bell!" It deforms very easily and piston will bind up to the point where piston won't even go back in.

I've used layout dye (can use a thin lacquer or even magic marker) inside the bell to find interference and sand it out.

Aluminum components have a tendency to stick so a light rubbing of graphite on the mating surfaces wouldn't hurt. A pencil will work if you don't have powdered graphite on hand.

JIM N

Piston rises from vacuum above. Is the hole in the piston clear, or could something block it intermittently? Is the throttle plate really opening- any chance of the linkage slipping? Could the piston be jamming- look for any bright or scuff marks on the shaft and faces of the housing and piston. Try lightly lubricating everything, including housing. Engine torque moves something- linkage, pushes on something and causes air leak. Just guessing. George
George Butz III

A few weeks ago I had another problem with fuel supply. It is not the same problem as you have, but I think it is worth mentioning here. Fuel passed freely down thru the float chamber outlet when disconnected and also thru the hose to the jet. However, when the hose was connected to the float chamber, the seal became misplaced, so it formed a restriction. Therefore max speed was only 50 kph. The problem with this in combination with the piston that didnīt move freely, was a hard job to locate. But now it works perfectly.
Y Strom

"If I accelerate quickly the piston does not react at all. The butterfly is open and the engine is drawing more air but the needle does not rise to give me more fuel. The car hesitates due to fuel starvation."

Mort,

The piston is acting correctly. The dampener fluid prevents the rise of the piston, and although it is at first counter-intuitive, with the throttle opened, the increase in the speed of the air through the gap of the piston and the bridge drops the pressure at the bridge. The pressure at the float bowl vent is higher, and the fuel is drawn into the airstream in excess of what it would if the piston rose and increased the area annulus of the jet and needle.

This is the acceleration pump feature of your carburetor. People 'play' with the dampener fluid (motor oil of same weight as engine; ATF; 20 wt. light oil, etc.) to 'adjust' the period of the accelerator pump effect. As the pressure decreases above the piston in the suction chamber, the piston rises and eventually reaches equilibrium between the pressure at the bridge and the pressure above the piston.

Of course, the fuel won't easily rise if it is too low in the bridge. I would check the float height by removing the suction chamber and the piston, and lowering the jet until it is just level with the fuel. This takes some practice. Then, measure the depth of the jet, it should be lower than .120 down from the bridge, but not lower than .200. With the enrichment lever off, your jet should be about .065 +/-.003 down from the bridge.

But honestly? I'm thinking you are sucking air somewhere into the carburetor, leaning the whole thing out. Either too little fuel, or too much air, depending on the air/fuel side of the leak.

Warmly,
Dave

Dave Braun

But Dave, Mort's testing was carried out with the chamber cap and dampner completely removed, so there should be no fluid dampening action on the piston at all.

I've heard from Mort that he has spoken with the expert at Moss to discuss the issue, but I'll leave it to him to discuss the why's and wherefore's.

Tom Lange
MGT Repair
t lange

As Dave says you must be going suddenly weak.
Try disconnecting the fuel line from the carb. Put the supply into a large container and switch on only the Lucas pump and leave running for at least 5 minutes. If it is a fuel flow problem the pump will slow down as the fuel suction is obstructed by either a vent problem or a moving blockage.
If it slows down open the filler cap and see if it makes a difference if not you have a moving blockage.
I have seen a tank (not T Type) collapse with a mechanical fuel pump.
PS a full tank will cause a vacuum well before a near empty one.
My apologies if you have already done this.
Ray TF 2884



Ray Lee

You're right Tom, I missed that part. In fact, taking the cap and dampener off and having the piston rise over a steady state, but not when quickly accelerating could only be caused by a very heavy rod, and not a light stiff piece of wire or light hollow rod.

How heavy was that rod for the test, Mort?

Warmly,
dave
Dave Braun

Once again thank you all for your comments and your help.

Every component of the ignition and fuel supply was carefully checked and re-checked and are in excellent condition. To make a very, very long story short, I spoke to a tech at Moss Motors who had experience with the very same issue.
With a demand for rapid acceleration there would be a total loss of power and sputtering almost to the point of stalling.

Solution:
Increase the diameter of the two small holes in the bottom of the piston.
With number drills I determined the hole size to be 0.0635. In increments I went up two numbers to 0.070. The first increase of 0.0035 virtually eliminated the problem. With the next increase of 0.0030 the car is running better then it has ever run before.
I still need to do some careful carburetor adjustments to see if that is the ultimate diameter.
I have no idea why after running this supercharger/ carburetor combination for more than five years it suddenly became undrivable. In retrospect I believe I always had a slight hesitation on rapid acceleration but nothing that would detract substantially from the cars performance.

Moss tech support claims that a sudden high vacuum occurs just above the two portholes in the face of the piston forcing that side of the piston up rapidly and un-balancing the piston and causing it to tilt and jam.
My floor was that the holes were too small to allow adequate vacuum to form above the face of the piston. I find several holes in both explanations.
The bottom line is that it works for me and has work for others.

Hours and hours of time spent over many, many weeks and the expense of quite a number of new replacement parts, only to have the solution be an increase in diameter of 0.0065.

Thanks again,
Mort
:))
Mort Resnicoff

Mort - your posts are very helpful because they get us all thinking. Sooner or later we will come across a similar situation, and will now have a better handle on the process of where to start to diagnose.

Glad it runs fine!

Tom Lange
MGT Repair
t lange

Excellent feedback Mort...something that is too often neglected!
Gene Gillam

Thanks for letting us know, Mort. Who was the singer who had the song with lyrics '...little things mean a lot..'? Bud
Bud Krueger

For Bud,


Little Things Mean A Lot

Patty Duke

Blow me a kiss from across the room
Say I look nice when I'm not
Touch my hair as you pass my chair
Little things mean a lot
Give me your arm as we cross the street
Call me at six on the dot
A line a day when you're far away
Little things mean a lot
Don't have to buy me diamonds and pearls
Champagne, sables and such
I never cared much for diamonds and pearls
But honestly honey, they just cost money
Give me your hand when I've lost the way
Give me your shoulder to cry on
Whether the day is bright or gray give me your heart to rely on
Send me the warmth of a secret smile
To show me you haven't forgot
Now and forever , that always and ever
Little things mean a lot
Blow me a kiss from across the room
Say I look nice when I'm not
Touch my hair as you pass my chair
Little things mean a lot
Give me your arm as we cross the street
Call me at six on the dot
A line a day when you're far away
Little things mean a lot
Don't have to buy me diamonds and pearls
Champagne, sables and such
I never cared much for diamonds and pearls
But honestly honey, they just cost money
Give me your hand when I've lost the way
Give me your shoulder to cry on
Whether the day is bright or gray give me your heart to rely on
Send me the warmth of a secret smile
To show me you haven't forgot
Now and forever , that always and ever
Little things mean a lot

Songwriters: Carl Stutz / Edith L Calisch
Mort Resnicoff

This thread was discussed between 11/05/2018 and 12/06/2018

MG TD TF 1500 index

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