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MG TD TF 1500 - Adjusting MKII Carbs


My car has never seemed to run quite right to me. It does not run very smooth at an idle and not much power. It’s a real MKII with the correct carbs and running GJ needles.

The distributor was rebuild by Jeff at Advanced Distributors. Timing was set by turning distributor while engine is running (highest rpm then back some).Plugs, coil, condenser, wires, cap all changed, some several times.

I finally had some time to work on the car today. Here is what I did.

Installed an Innovate oxygen sensor at the tail pipe with an extension added to help eliminate a false reading.

I then adjusted the carbs equally to get a reading of 14.7 at an engine speed of 1000-1200 rpm.

The car did not seem to run that good, so I adjusted the carbs equally to get highest rpm and then readjusted the engine speed to 1000-1200 rpm. Oxygen sensor read 16.3 or so

I checked to see how many flats down I ended up. 12 (that seems normal)

As I finished this up a core/freeze plug started to leak :(

Any ideas what to do next?
Steve Averka

Have you checked compression? Run without air cleaner? Rotor? How does the engine sound- any excessive clattering, rattling, etc. How were the valves adjusted and to what setting?
George Butz

Rotor was change with one supplied by Jeff. Valves are set as follows intake .016 exhaust .018 hot per Crane

Compression #1 118 #2 120 #3 116 #4 111

Compression was kept low in anticipation of installing a Marshall supercharger

Engine sounds normal to me.
Steve Averka

Those are low compressions. I usually expect 135 in a standard head. But gauges vary.

It would be unusual for a car (of a TD's Vintage) to run at 14.7 air/fuel ratio. Even the SU guide says that 13.2 (slightly rich) at tickover will give the best running as the engine is under no load, the amount of air drawn in is very small, and dilution with burnt gases is very large. Under load the excess fuel cools the incoming charge and ensures all the air is burned. Running lean will cause high exhaust valve temperatures.

The 13.2 value can vary greatly from engine to engine, and perhaps you are reading more air at the tail pipe then just the exhaust, or your meter is off.

Instead of checking how many flats you are down, check to see how far below the bridge the jet is. I start at .070 and end up around .065 inches. Fuel should be .120 to .200 below the bridge. Adjust the float if necessary. These measurements can be made with a dial caliper with the piston and suction chamber (dome) off the carb.

An XPAG should idle at 500 rpm and idle smoothly at 800 rpm. If you need to have it 1000-1200 then something is amiss. If you can't get it to idle down, you have an induction leak somewhere.

Your timing should be 11-14 degrees BTDC measured with a strobe below 1000 rpm with Jeff's distributor.
Dave Braun

Dave

In the past I’ve tried to measure the height of the fuel in the jet but have not been too successful. My jets are down .092” . Carb shafts have been re-bushed / new shafts. What is the best way to check for other intake leaks?

I’m starting to think the ¾ race cam from Moss might have something to do with this, I read a post somewhere that said you need high compression to run that cam. Also in looking at the setup sheet that came with the cam it lists the recommended rpm range as 1800- 5200 .

Steve Averka

I'd say the 3/4 race cam most definately has an effect on the car's idle. You will likely never get it to run as smooth as a stock cam- but you will likely have great torque at higher rpms. Also, the timing. I'd set it specifically with a light. The turn to max rpm and then back a bit is too imprecise.
BRIAN WARMUTH

Steve,

With the float bowl covers off, carefully lower the jet until the fuel appears. Wiggle the jet to get a level, and then tickle the float just slightly. The idea is to overcome the surface tension of the fuel to determine its height in the bridge by bringing the jet to the exact same height. Then measure. Adjust as needed, replacing the fuel and float bowl lid each time.

Other leaks can be found by propane or starting fluid in the suspected area. If the engine speeds up from additional richness, you've found an intake leak.

.092 down would be more than I would expect for smooth running (the most I've seen is about .080 down.) Sometimes the shafts are new, but the throttle disks still leak because the edges of the carburetor are too worn, or the disks are worn.

The cam range probably describes the effective RPM range of driving, not idling. It means shift high enough to not drop below 1800 which is pretty easy to do on a TD as even shifting at 3000 rpm will keep you above 2000 on the next shift. I don't know what compression is recommended for that cam.

warmly,
dave
Dave Braun

I agree that the cam is much of the issue. I presume that it would have higher lift, more duration, etc., and would likely be rough and also gutless at lower RPMs. I also think the compression is really low to run decently with even a stock cam. I still think with float/fuel level correct, the proceedure in the NEMGTR's T-series book for carb tuning is the best of any (ie lifting the pistons listening to idle change, etc.) I don't know that a modern oxygen sensor would equate to decent running on these pre-WWII designed oil burning engines either. I would verify timing with the light as Dave says above also. If that checks out, better get going on the supercharger! George
George Butz

I agree that the 3/4 race cam and low compression are the problem here, as well as a possible air leak. In LA back in the 70's I drove a bugeye Sprite with a 3/4 race cam, a total slug below 3,000 RPM. Not a fun car to drive on the street, due to over-camming. A mild street cam completely changed the personality of the car.

XPAG's tend to wear out much more quickly when driven consistently at higher RPMs, and I think you will have to mostly run between 3-5,000 with your cam. I wouldn't consider using anything more than a fast road cam, if that.

Tom
t lange

I checked for leaks with propane – no leaks, so

I ran another compression check with the engine warm, no better maybe even a little lower, so

I did a wet compression check 145 –150 all cylinders, so

I’m figuring when I installed the new pistons after my air filter fell apart and the engine sucked a ¼” stud in and punched a hole in the #1 piston the new rings did not seat, rats
Steve Averka

Steve,

Take the car out and accelerate from 10 mph to 50 mph in top gear, several times in a row. The idea is to create an incredible amount of pressure in the combustion chambers (High load, high pressure) and drive the rings outward against the cylinder walls. Then drive at 40 mph in top gear for about 20 miles. Hopefully, that will seat them.

warmly,
dave

Dave Braun

I usually seat rings by driving overnight when it is coolest, and just the opposite of Dave B above. I warn the local police what I will be doing, then I drive all over town and on country roads accellerating to 50 then backing off until 10 mph. I do this over and over in one night until I have a couple of hundred miles on the engine, and the rings have always seated nicely for me. To me it's the high load and high pressure, followed by the high vacuum that sucks a lot of oil into the cylinders for good bottom end lube after the stress of accellerating. Plus it's a lot of fun!

Tom
t lange

Thank you for the help.

I’ve decided to buy a new standard camshaft and finally install a forged crankshaft in anticipation of installing my Marshall supercharger. While I'm at it I’ll replace the rings and bearing etc.

Steve Averka

I was always told that anything but a constant speed would seat the rings....up to 50, back down, etc... and that with a manual transmission you get that all through the gears.....and not to exceed 50 for the first 500 miles or so....
I do remember also that with new or rebuilt cars, you took them back in after 500 or 1000 (forget) and have the head re-torqued along with a change of oil and filter.....did that with the Wols a couple of months ago and suppose its time for the TD as well.....(now that I think of it, i think I sent one machinist's kids to summer camp this year)....
gblawson(gordon- TD27667)

This thread was discussed between 30/09/2010 and 01/10/2010

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