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MG TD TF 1500 - Intermittent Power Loss

I've got most of the kinks out of Mobius and it is running fairly well.
There remains one problem I need some suggestions for.

Remember these basic modifications:
5 speed Skyhook transmission
New Moss supercharger
One SU 1-½" Carb
4.55 rear
4 core radiator

At highway speeds in the 60 to 70 mph range the power will suddenly decrease and I get some back firing. Probably in the intake manifold, but I'm not sure. The car slows down to the 50's and the supercharger pressure gauge flutters like crazy. If I ease up on the gas and the gauge goes to vacuum it still flutters like crazy but now in the vacuum range. Usually after a few moments at the lower speed the gauge will settle down and the power returns and all is fine until it happens again. Sometimes it is so bad I need to pull over and let me and Mobius settle down. Then we are off till the next time it happens. If I stay under 60 mph it will run all day without a problem.

So I have this loss of power at high speed.

I don't think it's ignition because it runs so well at 60 mph and under.

I've had the carb adjusting nut turned all the way down and also at a proper adjustment with no change in this event.

The vacuum gauge is located in the intake manifold.

I've checked and cleaned all the fuel filters. At the float bowl, the pump and the tank.

It sounds like fuel starvation so I put a T in the gas line between the fuel pump and carb to monitor fuel pressure when the event occurs. The fuel pressure gauge is right in front of me on the dash board. I get a steady 1-½ lbs. of pressure before and during the event from the main Lucas electronic fuel pump. I have a switch to flip to my auxiliary Facet pump but it holds steady at 2-½ lbs. before and during also.

TIA,
Mort




Mort Resnicoff (50 TD-Mobius)

Perhaps ignition crossfire or other ignition breakdown - cracked cap or coil breakdown.

A sticking intake valve will give manifold pressure fluctuations but I'd guess ignition first.
J E Carroll

I wonder if you are getting some Valve float at the higher speeds. Did you ever put in higher rate springs when you did your engine work. Just a thought.
C.R. Tyrell

Mort,

Did you ever get this issue with the air filter removed
Just a thought.


R D Jones

Mort,
How long does it run 60-70 before the gremlins attack? Sounds like you're carb is running short of fuel, even with positive fuel feed.

I'm going to make a suggestion that most would ignore, but I welded an O2 sensor bung in the exhaust pipe right below the manifold and installed an elcheapo O2 sensor & air/fuel ratio meter. The feedback is amazing, ...priceless.

We've run the older Moss Magnacharger for about a decade, but only added the gauge for the last 3 years. I discovered we STARTED running out of fuel at about 60mph, with the single 1 1/2" SU, and by 80, the gauge would blip so lean, it wouldn't register. This is with 2 fuel pumps, too. Power flattened right out, but it didn't stumble like yours.

I experimented with numerous changes, but the simplest adjustment that finally made all the difference in the world... I adjusted the SU fuel level according to Dave Braun! We now cruise 80+ with the air/fuel ratio running absolutely rock solid, just a smidge rich, perfecto!

Gotta ask, though- what did you set your valve lash too? I erred to .019" for our '50, but have a roller lifter cam waiting in the wings. Hate to think that the lash could disappear at higher temps and lift valves off the seats, inviting wiredrawing if uncorrected.

Also would like to know where you ignition advance climbs up to. A clubmember with the new Moss blower said they recommend 30 BTDC, but I'm more conservative winding up at 25 degrees. We run regular, too. Knocklite and my ear agree there's no pinging.

By the way, that clubmember set his idle way up around 1200rpm, as he said he was instructed, as the engine needed the high idle to compensate for the blower energy requirements. Bullroar! I can idle ours down to 500 if I try. There is no power draw from the blower at idle/vacuum, in fact, you can take the belt right off and run the engine- it'll suck the blower around demonstrating friction is negligible. I find 700-800 works fine, even though the generator light comes on.

Have fun with your blown MG!
JIM NORTHRUP SR

Jim,
The gremlins come at various times. Sometimes in a few minutes sometimes much longer.
I've noted your mention of the O2 sensor in the past with much interest. How can I learn more about it and can you give me more details on the parts you used? You can send to my e-mail posted above.

"I experimented with numerous changes, but the simplest adjustment that finally made all the difference in the world... I adjusted the SU fuel level according to Dave Braun! We now cruise 80+ with the air/fuel ratio running absolutely rock solid, just a smidge rich, perfecto! "
I have Dave's article on tuning the SU Carbs. I have great difficulty in step 14, noting a change in the engine rpm. That makes step 15 difficult also. In step 16 I don't perceive any change so I left it alone.
At idle and lower speeds it runs great.

I'll have to recheck the valve clearance but I believe I set it at .015"

Timing at 30 degrees at 3,000 rpm.

Idle at 1,000. Can go lower but it is rough till the engine gets hot. When hot can idle in the 500 to 800 range.



Mort Resnicoff (50 TD-Mobius)

"R D Jones, Maine, USA

Mort,

Did you ever get this issue with the air filter removed
Just a thought."


It's acting like it is fuel starved. Wouldn't supplying more air make it worse?
Mort
Mort Resnicoff (50 TD-Mobius)

Jim,
Do you have any pictures of where you connected the bung in the exhaust and your gauge in the dash?
Thanks,
Mort
Mort Resnicoff (50 TD-Mobius)

gauges in stainless sheetmetal instrument cluster


JIM NORTHRUP SR

inconspicuous O2 sensor


JIM NORTHRUP SR

O2 closeup-
as I mentioned, it is way off-center to minimize obstruction and there's no loss of sensitivity.


JIM NORTHRUP SR

What I find interesting is that it keeps running rough even after the demand is reduced. I would think that fuel starvation, which can give back firing (air looking for fuel), would end and it would go back to running smooth as soon as the throttle is lifted. Perhaps the blower is inducing something else into the equation that I don't understand.

The reason I suspect ignition is that higher cylinder pressures expose ignition weaknesses - it takes more energy to jump the spark gap and the high tension electricity will find an easier path. Once started on that path it may continue for a while.

J E Carroll

Jim N.
Thank you for the pics. Very helpful. Just out of curiosity what are the two gauges on the left? Fuel? Water Temp? Oil Pressure?

Jim C.
You said "high tension electricity will find an easier path. Once started on that path it may continue for a while." What other path comes to mind?

Thanks,
Mort
Mort Resnicoff (50 TD-Mobius)

Mort

Sorry that I cannot help with your problem but what are the two chrome strips around the starter motor?
Sandy
DARNOC31

Sandy,
Are you referring to the strips that are on Jim's car??
SPW
STEVE WINCZE

They look like hose clamps holding the O2 wire away from the manifold.

Tom Lange
MGT Repair
t lange

The two gauges on the left are water temp & oil pressure. The little dots on the cluster are LED idiots lights. There's an obnoxious green one for low fuel.

Distributors can develop carbon tracks internally that can eventually short out a cylinder spark. These aftermarket caps have a history of developing hairline fractures which can provide a leak path, too.

Those hose clamps on the starter clamp the old Ford 6V starter solenoid onto the starter. It has a button on it so I can start it while tickling the carb and opening the throttle.
JIM NORTHRUP SR

Jim, what sort of readout do you get from the O2 sensor?
Bud
Bud Krueger

Mort,

By easier path I mean a crossfire to another cylinder through a carbon track or cracked cap, wire to wire, wire to ground, internal coil breakdown, etc.

Usually when you get backfiring due to fuel starvation it stops as soon as you lift the throttle. If I correctly read what you wrote, the engine continued to run rough after you eased the throttle and that's what doesn't compute for me as fuel starvation.

As you increase the manifold pressure/cylinder pressure the plug gap gets harder to jump so maybe the spark is jumping elsewhere. Once the air is ionized that path may stay the easier gap to jump for a while even when the plug gap is getting easier due to the lower pressure when you back off on the throttle.

Just a possibility, of course.

Jim

J E Carroll

This one has 10 LEDs in an arc (about 270 degrees); 3 Green ones on the right for rich, 4 yellow center for "ideal" and 3 red on the left for lean. When they disappear past red, I know it is really starved.
Our's runs full rich (call it 10) from idle up to about 20mph, it rises to 9 on the rich-ter scale from about 20-40mph, and 8 from about 40-60, and by 80mph is is running an "ideal" 7, still a touch on the side of rich. I'm happy with this, as trying to lean the main jet out makes takeoff from idle suffer. As it runs rich at idle, it starts great without the enrichener, too.
The response of the LEDs is virtually instantaneous with the throttle. Backing off on the gas makes the LEDs dance in the ideal to lean range, and then it jumps right back up hitting the gas.
Although it wouldn't be categorized as a "scientific instrument," I have to believe this cheap gauge gives a better feedback than gauges with a needle or digital display. The only difference I've read about wideband is they have just a slightly expanded voltage range to read thru, but those are on the extreme fringes of rich and lean.
It keeps me entertained while my wife drives. I have not had any luck running a cable to the enrichener (carb is mounted 90 degrees from conventional and contempoary cables suck), but it'd be neat to be able to adjust the mainjet on the fly.
This also came in handy when I TRIED in vain to adapt Harley carbs to the blower, but that's another story.
JIM NORTHRUP SR

Jim, you've got me intrigued. I don't want to steal Mort's thread, so would you mind dropping me an email at the above address. There are a number of questions I'd like to ask. TIA. Bud
Bud Krueger

Bud, Jim N,
Don't hesitate to post on this thread instead of private e-mail. I am interested in anything to do with this project and appreciate all the input.

Jim C,
I understand and will look into the ignition system. I will be doing a chassis dyno and hopefully it will give me a lot more information.

Mort
Mort Resnicoff (50 TD-Mobius)

Are you running points now Mort?

Bronze valve guides?

What do the plugs look like?

Evidence of carbon around the pop off valve?

What kind of boost pressure are you seeing when it goes south?

Whats the sustained rpm .

Next time it happens, pull the choke full on and see what if any effect it has on it.

L E D LaVerne

Bud, Mort, et al...
I just added my email address to my heading. Let's see if it shows up. Anyway, it is jrnorthrupsr@Yahoo.com

Just reread symptoms- "the supercharger pressure gauge flutters like crazy."

That is mighty strange! LaVerne is on the right track. Obviously, your boost/vacuum gauge is between the blower and head.

You wouldn't think the pop off valve is chattering, a new manifold would show the black carbon if it is letting go.

Make sure your dogs are still snug. With all that extra weight hanging, I have to doubleheck ours constantly. Have contemplated robot cutting new ones out of 1/2" plate to get more threads. A bad intake gasket shouldn't generate that flutter, but when we had one go, the steel acted like a reed and created a shrill screech like a bearing bound up.

Sure hope it isn't new bronze valve guides binding. IF it is running real lean, exhaust valves might be getting really hot and expanding. I like the idea of runnning with enrichener out for a while (or crank main jet down a few flats) and check plug colors.
JIM NORTHRUP SR

Mort,
I just recalled that last year and the year before (since we put the '50 back on the road), we'd run into episodes of backfiring from the blower intake. It'd happen around 60 or 70 mph and up, in hot weather, with the water temp over 200F, closer to 230F. We'd slow down or pull over to let it cool down a bit, then take off like it never happened. This was also when the air/fuel ratio meter was disappearing off scale lean at highway speed.

Since then, the valve lash was opened up to .019" (it is a '50 XPAG with unknown cam), the carb was straightened out to run right, and last winter, the radiator was recored so it runs much cooler.

I also call into question running 30 degrees advance. As long as it doesn't spark knock...
JIM NORTHRUP SR

Mort, you have mail. Bud
Bud Krueger

Thoughts:
valve float or bounce?

Too Lean: Does it happens every time you hit the RPM, or only after the RPM is there for a few minutes, it might be a lean issue. creating slop in valves. (I know you have an O2 sensor...)










Dean E

OK, proving again that I don't know anything about superchargers, I looked at the picture of one in the Moss catelog and started wondering. The standard XPAG comes with two 1-1/4 inch SUs. The XPAG in the Mark II has two 1-1/2 inch SUs and the supercharge appears to have just one 1-1/2 SU. The combined bore (fully open throttle) with two 1-1/4 SUs is 2.454 square inches. Combined for two 1-1/2 is 3.534. ChemEs learn a lot about liquid fluid dynamics but not really much about gas fluid dynamics except the gasses are pretty compressible and liquids aren't but it seems to me that you are drawing a whole lot more air through that single 1-1/2 carb than you would through naturally aspirated 1-1/4s or 1-1/2s. It also seems to me that unless you have a lot of fuel available and a pretty big jet orifice to get that fuel into a very rapidly moving air stream you are going to lean way out and get fuel starvation.

Could it be that at higher RPM or power demand you are on the ragged edge of sufficient fuel flow through the jet? I don't know if you can put bigger jets in an SU or if just adjusting the needle does the same thing. I also don't know if any of this equates at all to reallity but, as I said, I was just wondering.

Jud
J K Chapin

Jud , at first dam... good thinking. I follow this tread with great interest . Your remark !!,,brings me to the idea : wrong needle. Or need one with more aggressive steps perhaps to solve his problem ??
I don't know about superchargers on British engines also . But I think the problem is fuel related also.

Cheers
Gerard Hengeveld

I can't speak for the Moss blower, but my TC and TD/TF carbs from Joe Curto use an RA needle with good results, the same as a Judson uses. A single 1-1/2 is perfect for my blower (and others like Arnott, Shorrock, Ital, Marshall, Judson). I dissuade purchasers of my blowers from even trying 1-3/4" carbs; one potential purchaser insisted that he was going to use a 1-1/4" carb "for better fuel economy," and I dissuaded him from buying a kit from me.

Tom Lange
MGT Repair
t lange

Dean and everyone else, too,
"Thoughts:
valve float or bounce?
Too Lean: Does it happens every time you hit the RPM, or only after the RPM is there for a few minutes, it might be a lean issue. creating slop in valves. (I know you have an O2 sensor...)"
If you ask me, that's about what I feel was our issue. Running hot weather on the highway for quite a while with a boost, 230F water and fuel dropping lean, combustion and valve temps were astronomical. My inclination is an intake valve wasn't closing, causing the backfire. I worried .015" lash might not be enough, so went back to .019", thinking MAYBE the stem grew and took up all the gap. It may be more related to stem width binding in a guide. At 80mph the engine is only turning 4 grand, so it wouldn't be float from too much rpm.

Jud, Gerard & Tom,
I had my concerns that a single carb just might be at its limits to pass enough fuel, as it seemed to be maxed out at 60 and get leaner up to 80. I tried a different needle, main jet and steel float needle from Joe Curto, but they didn't resolve it. As mentioned, after the float level was adjusted via Dave's method, the air fuel ratio has been great. Hard to imagine a slight adjustment made all the difference, but the meter says the engines happy now.

Having the radiator recored last winter has made a great improvement on the water temp, and the backfiring hasn't occurred at all this year. As I recollect, there were maybe 3 episodes that transpired in the previous years, so it was fairly rare.

Good luck Mort!
JIM NORTHRUP SR

L E D LaVerne, USA

Are you running points now Mort?
Yes. Petronix is off the car.
Bronze valve guides?
Not sure of the metal of the guides
What do the plugs look like?
Last time I checked they were ok but I will do the "shut off while running test again".
Evidence of carbon around the pop off valve?
There is a slight discoloration but the o-ring is properly seated.
What kind of boost pressure are you seeing when it goes south?
On demand I get 5 lbs. of boost. When things go south the gauge is fluttering all over the place.
Whats the sustained rpm .
In the 60-70 mph range the tach is at 3200 -3800
Next time it happens, pull the choke full on and see what if any effect it has on it.
I will try to reproduce the event again and try this test.
Mort Resnicoff (50 TD-Mobius)

To all,
First thank you all for sticking with me on this.
I am favoring the cause to be running lean at speed.
I will proceed with installing an O2 sensor ala Jim Northrup.
If it does drop to extremely lean at speed and during the event that will be a good starting point. The next thing will be to figure out the cause.
Carb? Valve guide? Valve lash? etc.
Mort
Mort Resnicoff (50 TD-Mobius)

Dean E, Colorado, USA

Thoughts:
valve float or bounce?

Too Lean: Does it happens every time you hit the RPM, or only after the RPM is there for a few minutes, it might be a lean issue. creating slop in valves. (I know you have an O2 sensor...)

Dean,
I am not sure of the rpm when it happens. I know it happens in the mid 60 mph range which means the rpm's are about 3500-3600.

Jim has the O2 sensor. I am investigating one for my car.
Thanks,
Mort


Mort Resnicoff (50 TD-Mobius)

Jim,
You seem to have found the solution in here somewhere.

"Since then, the valve lash was opened up to .019" (it is a '50 XPAG with unknown cam), the carb was straightened out to run right, and last winter, the radiator was recored so it runs much cooler.

I also call into question running 30 degrees advance. As long as it doesn't spark knock... "


Tappets to .019"

Carb straightened out??? New needle? Which one?

I am running a 4 core radiator now and heat is not an issue.

What is it you don't like about the 30 degree advance at 3000 rpm?

Thanks,
Mort
Mort Resnicoff (50 TD-Mobius)

I know John Twist says .015" clearance, but I'd go with whatever was recommended for cam. Unfortunately, I don't know the cam in this one as it was rebuilt from John's University Motors. As it is a '50, I upped the lash to the old .019"

I spent a great deal of time trying to get the air/fuel ratio under control by numerous changes to the SU, and also a failed attempt to adapt Harley carbs. At highway speeds, about 60mph, the meter would read rich, and then drop off scale lean momentarily, then pop right back up; by 80mph, it was off scale lean, and pop back up to rich momentarily. It looked as if fuel feed maxed out at 60. I'd switched needle, mainjet and to a steel float needle, but that didn't change anything. Cranking the mainjet way down didn't resolve it either. I've since gone back to the original needle & main jet. RA needle was one of the two, I think the original. I followed Dave's directions on removing the bell and slide to tweak the float level with respect to the bridge; before that, I'd set it via MG's procedure using a drill bit. After that adjustment, the air/fuel ratio has been rock steady.

The engine would run upwards of 230F in hot weather at highway speeds, running lean as described. The backfiring only occurred as few times in hot weather, so it was just an intermittent transient and I can't say that any one change resolved it.

Running much cooler this year with the recored radiator and enjoying a steady gas feed has proved problem free. We ran to GOF Central, 500+ miles, and back, in temps close to 100F both ways, and if that problem didn't raise its ugly head then, I doubt it'll recur.

As for 30 degrees advance, I searched the archives for supercharged ignition timing and found the consensus of opinion was 25 degrees maximum. Another 5 degrees will up the temps & pressures a bit. I point this out as it seemed temp was the criteria of main concern, but pinging is the critical factor in advance; I really do respect maximum advance for power and gas mileage. I printed out a degree wheel to stamp marks 5, 10, 15, 20, 25 & 30 BTDC, and use a timing light to set timing to 25, revving the engine until the advance stops climbing.- I don't care what static is.

Next year, hopefully, I'll start tuning all over again on the 50's original 1466cc XPAG with Len's roller lifter cam. Might venture up to 30 BTDC if all runs well and I feel brave.

I also installed a Knocklite on the car, and it has never signaled pinging. I have a feeling it wouldn't anyway, as striking the engine with a hammer doesn't give an indication. My ears tell me we've haven't encountered detonation... knock on wood!

You might trying running around with main jet cranked down/enricheneer pulled, in these cooler fall temps.
JIM NORTHRUP SR

With a scan tool on my pickup truck I can see the knock sensor become active and the timing retard without ever hearing it. Of course my wife would admonish me to turn my hearing aids up but I can still hear my dump truck ping.

The the computer in the pickup will run the advance approaching 40 degrees, if I recall correctly. It's always moving depending on the load. The wonder of computers, I guess.
J E Carroll

Just a thought Mort:

You had an issue with the engine stabilizer.
While you were in there did you loosen the belt driving the Supercharger?
Could the belt be slipping just a bit now and then?

Jim B.
JA Benjamin

Jim,
I did not loosen the belt but I will put it on the list of things to check. It is hard to remember to only make one change at a time so when I cure the problem I will know the cause.

Today I went for a test run to pull the choke all the way when it started to sputter. The choke made no difference. Although it is not enough evidence to dismiss fuel supply I am going to concentrate on other engine functions first.

Today I set the tappets at .019 (up from .015).
My plug gape was around .030 so I reset them at .025".

Tomorrow, after I put the hood back on, I will take another test ride.

List of things left to check;
Inspect points and other aspects of the dizzy.
Lower advance from 30 to 25 degrees at 3,000 rpm.
Recheck fuel and jet level in Carb.
I have a chassis dyno scheduled for next week if all else fails.

Thanks,
Mort
Mort Resnicoff (50 TD-Mobius)

Update - Wednesday
I put the hood back on.

Since my last test run I have done the following:
Set tappets to .019". (Was .015")
Set plug gap to .025" (The circular gap gauge I used was very unreliable and the gap was actually in excess of .030. I reset it with the feeler gauges.)
Lowered the advance from 30 to 25 degrees at 3,000RPM.

I know I should have done one at a time but with the hood off I was on a roll.

The test run was great. Sustained 70-74 mph and could not reproduce the problem. I hope the issue is resolved. I will be making long weekend trips for the next three weeks and that should really test the issue. For now I am hopefully optimistic.

Once again I want to thank you all for your help. Especially Jim N. for all the e-mails and info.

This is truly a great forum.

Mort
Mort Resnicoff (50 TD-Mobius)

This thread was discussed between 25/09/2013 and 02/10/2013

MG TD TF 1500 index

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