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MG TD TF 1500 - Choke or No Choke at Startup

Hi,

Is there any reason to be concerned if I rarely need the choke to start my 51 MGTD? It seems if I just feather the gas pedal a bit it only takes a few times to get her going, then I just give enough gas to bring up the RPMs to warm up the engine and away I go.

Steve
SD Denham

Must be set too rich, otherwise you should need choke, even though you live in California, Pressing the accelerator does nothing useful with an SU at start-up until it starts running.
Dave H
Dave Hill

Steve,
Living up in Maine, I always had to pull the choke to start and even to pull away without splutter - if not waiting a while first.
Now I'm down here in Florida - Don't need any choke at all. Yet. Winter is not over... :)
One thing I would stress is make sure the jets are going back up fully to the normal position after you have pulled the choke out and returned it. Easy to check.
The jet return spring is not all that strong and if linkage is worn or stiff the jets may well stay down a little and make you run rich.
I soldered the last 6" of my choke cable inner, making it stiff, to help push the linkage and assist the spring return the jet to its full up position. Seem to help. The jets now go back up against the Jet nuts.

With luck, I may never need it now.....
Rod

Rod Jones - Ex Pat

It isn't a problem so long as your carbs are tuned properly for normal running.
Steve Simmons

Steve,

Try adjusting for a leaner mixture until the choke become necessary. If you can not achieve a leaner mixture, remember all the rebuild kits that had jet gland washers that were too thick. These brass washers are just thick enough to prevent ever getting rich running to stop. Took a couple of days and would not have found the problem if not for this BBS and the knowledge of others.


Jim Haskins 1953 TD
J M Haskins

Second the opinions here. Your carbs are running too rich.
Regards, Tom
tm peterson

The other cause of chronic rich running is the deteriorating of the vacuum chambers in our SU carbs. Over time they loose their ability to hold a vacuum, due to wear between the piston and the bell chamber, and this causes the needle to not pull up high enough, which thereby causes too-high a vacuum over the fuel orifice and a subsequent rich mixture at all mixture adjustment positions. The cure, sadly, is only found with a new bell and piston.
Kevin McLemore

I wouldn't blindly lean it out just so choke becomes necessary. You could end up way too lean under load and burn a valve. Set idle mixture and test from there. If you don't know what your needle profiles are, test with an AFR gauge to make sure you're set correctly at higher RPMs on the road.

I'm working on a car now that is way rich at idle. If I set it lean enough to require choke, it would be running extremely lean at speed. Being supercharged, it would probably blow itself apart. Obviously the solution will be different needles.
Steve Simmons

I agree. A So Cal car that is tuned properly should need a choke at startup. Mine spent 25 years down there and I used the choke first start of the day always. Once it gets warmed up you should not need it.
Christopher Couper

I recently had my carbs refurbed after a short period of ownership and having covered about 200 miles. The car started relatively easily without the choke. THe list of faults on the carbs are as follows
No return springs on the main shaft
Choke mechanism seized
different needles in each carb
worn shaft
one flooded float, on the same carb the inlet at the base of the float chamber was completely silted up.

Once heated up the car ran relatively well until I changed the plugs for the correct type, the plugs then started to soot up
As you can imagine I;m looking forward to see the difference

E I Buckley

Kevin. I wonder if you have hit the nail on the head with that comment about worn pistons and bell chambers. Lots of people complain of rich running with std needles and jets, which seems contrary to what you might expect with current fuel, especially if it contains ethanol. However, the slight doubt I have about it is how that wear takes place as there is supposed to be a clearance. Maybe its frequent dismantling and assembly.
Dave H
Dave Hill

Kevin & Dave. Been a while since I rebuilt my carbies but I seem to recall a drop test that can be performed to evaluate the condition of the piston & bell. Would this test eliminate wear here as a possible cause of running rich? Cheers
Peter TD 5801
P Hehir

I'm not entirely sure, but agree its a good question. Ideally I think it should, but as friction can arise for various reasons I doubt its a sure fire method. The total clearance between the piston and the suction chamber side is meant to be 5thou (that is 2.5 thou per side) and a total of up to 7thou is OK I have read. I will check mine tomorrrow because I have the same issue.
Dave H
Dave Hill

I think a good question here is what do your spark plugs look like? That is how you tell if your engine is running lean, rich, or just right. Then there are the usual checks and lifting the carb piston just about 1/16 to 1/8" and seeing what the engine does.

If the carbs are way too rich it will wash away all the lube on the pistons with gas, and damage the bores.
Ben Stevens

I think Peter is remembering a John Twist video on YouTube covering the carb piston drop test.

Kirk
Kirk Trigg

Hopefully this will give the link to the Twist video. Hope it helps.

https://youtu.be/IfU47Oqq9wA

Kirk
Kirk Trigg

It appears that you'll have to copy/paste to make it work.
Kirk Trigg

When you drop test the pistons, they should drop slowly. If they drop fast then someone probably mixed up pistons and covers. They are matched at the factory and should never be interchanged.

If they drop fast then that means you will never get a suitable vacuum to lift them on acceleration.
Steve Simmons

That is the test that we know and is being discussed. However is it infallible? Obviously it will work well on a shiny new carb, but after 60years few of us have shiny new carbs. All I am suggesting is that wear might give false results. I am going to measure the clearance between piston and suction chamber on my carbs to see what I find, even though they pass the drop test.
As said, spark plugs give an indication of rich running, and that is what many have reported finding regardless of what adjustments they make.
Dave H
Dave Hill

I was told years ago that the drop should be 5 to 7 seconds. I googled John twist and got "matching SU air pistons which seems to confirm this.
I also think that the high volatility of modern petrol makes for a rich mix at tick-over. But don't take a chance and lean off, do a plug colour check.
In winter my TF needs choke but after spring I can just catch it on the throttle and drive off. A lot better than my modern car which spews out fumes until it warms up.
Ray TF 2884
Ray Lee

The drop test is infallible. Regards, Tom
tm peterson

Ok Tom I'll believe you, I have now measured the clearance between the piston and suction chamber on my carbs. Despite being old they are broadly in the right range, being around 2.5 to 3thou clearance on average, however I found spots that were tighter than that, right down to 1.5thou. Rotating the piston moved the tight spot around. This surprised me because I thought that if anything was out of round it would be the suction chamber. There are obvious wear marks and scratches on both the piston and the inside of the suction chamber, but nothing severe. The central bearings look perfect but when pressed there is slight movement, which translates to just visible movement at the periphery of the pistons, maybe explaining the slight variability in clearance. I guess its self centralising though. A better result than I feared and I am ruling it out as a reason for rich running. Glad I checked though.
Dave H
Dave Hill

If there is wear, then suction will be reduced and the drop time will be very fast. The only thing that matters is the drop time, because this is a direct indication of vacuum above the piston. Therefore a false reading is impossible! There is either vacuum controlling the descent of the piston or there isn't.

Regarding plug color, this is a bit tricky. The only way to get an accurate reading this way is to run at normal highway speeds and shut off the ignition, then coast to the side of the road and check the plug. If you slow down and drive into your garage then you will get the color based upon low speeds and idle. This may be a rough indication of overall mixture, but it will not be an accurate measurement of normal driving mixture unless you have managed to get the A/F ratio completely even from idle to high speed. And if you've done that then may I request you come tune up my car! :)
Steve Simmons

This thread was discussed between 09/01/2017 and 11/01/2017

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