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MG TD TF 1500 - MG TF 1500 Uneven Carb Piston Movement

You can see in the video the uneven response to the throttle. The left carb is slower. Seems sticky. It doesn't return all the way down. Cab is running rich

Right carb is responsive. Still a little rich - but leaner than the rear.

I have cleaned with plastic scrub pad, carb cleaner fluid and finished off with a micro fiber towel. The piston will move freely and about the same speed when 'drop tested'. Carb body inverted. both pistons at top and released to fall.

I thought perhaps the jet wasn't centered. Looks ok and there are no marks on the needle indicating wear where it connects with the sides.

Any suggestions?

Polish the inside of the chamber with jewlers rouge ?


Dave

http://www.youtube.com/watch?v=yF6CyZd8CQg&feature=youtu.be
Dave Moore

My guess is that the needle isn't perfectly centered. With the damper caps removed, can you lift the piston with your finger and hear it drop with a sharp click? if not, then the jet needs to be re-centered. Plenty in the archives.

Altering the chamber is a LAST resort! Have you perhaps switched pistons?

Tom Lange
MGT Repair
t lange

Dave,
You might want to take off the air cleaners and put the dashpots back in as they are the none vented top carbs (no hole in the dashpot - right?)

Did you test the air balance on both carbs and make it the same? Loosed the clamp between the carbs and measure the flow or match the hiss with a rubber hose in your ear :) adjust the throttle stop to balance the air flow through both. re clamp and re test to check.

No matter what - without the springs and dash pots fitted, the piston and needle should drop with a satisfying clunk when you let them drop under gravity. They have to be totally free.

Dave Braun's write up is the way to go to set them up correctly.

http://www.dbraun99.com/Setting_SU_Carburetors.pdf

I am no expert but it sure helped me.

Rod

R D Jones


The carbs are vented though a hole at the top


I re-read the 'drop test' part of Dave Braun's guide. Realised I was doing it wrong.

Here is a video of me doing it right ( I hope )

http://www.youtube.com/watch?v=SLD_7pNRfbk&feature=youtu.be

Both appear to be working smoothly and dropping at the same rate.
Took a while to figure out you have to hold your finger over the vent hole in the piston...Was getting really random results until I realised that sometimes my finger was in the right place and others not...

Obvious with hind sight of course. I had thought the instructions referred to blocking the small vent in the chamber body at the top... I did this too with some blue tack

Here also is a picture of the relevant jet. Does look a little off. Closer on top right to edge. Fuel is high too ?


Dave




Dave Moore

And here is the needle. Looks ok to my untrained eye. There were no obvious wear marks on the sides.



Dave Moore

Dave - the drop test compares the fit of the two pistons in the chambers, and has no bearing on needle centering. It is done with the dashpots and pistons off the engine; needle centering is determined with the carbs assembled, on the engine.

Your two dashpot caps should have no holes in them, I believe, since your dashpots are the later, ventilated type.

You don't mention trying what Rod and I suggest: check the needle centering. With the carbs together but with the caps removed, stick your finger into the carb throat (engine off) and lift the piston as far as you can. When you let go and it drops you should hear, as Rod notes, "a satisfying clunk," or as I say, "a sharp click." If you hear a dull sound or if the piston makes little sound, the jet must be re-centered.

Let us know.

Tom Lange
MGT Repair
t lange

The only other concern I see is the level of the fuel in the jet bearing - you show it from above? It looks like the fuel level is rather high - which will make it run rich. Dave Braun maybe the one to confirm that?

In Dave's write up he explains how to adjust the fuel level [by adjusting the forks on top of the floats] Fuel in the jet bearing should be around 0.160" to 0.200" below the Bridge. As the float rises it cuts off the incoming fuel and that sets the fuel level in the jet bearings.

Quote 'Each carburetor should have the same fuel depth
within ± 0.010 of each other, and both fuel depths should be 0.160 ± 0.040. - It takes some practice to measure the depth of the jet below the bridge."

In your picture the fuel looks to be almost the same height as the top of the jet bearing? - On my carbs it is further down. Difficult to say without perspective but it sure looks too high {I.E. rich}

I do have to say your carbs are different form mine as I have the older style that have the vented tops [small hole in the top of the dash pot] Your carbs as Tom has said, are a later version and are vented internally - hence you have the small tube that angles up to the top of the chambers.

Fuel level should be similar I believe as the principle is the same for both.

Rod

R D Jones

Dave,

Your first video (revving the engine) indicted to me that your carbs aren't balanced...you're not getting a equal amount of air thru them.

Rod's second paragraph described the process...try it and I think you'll eliminate your problem. Once you loosen the corrugated looking joint that ties the carburetors together, each carburetor will be working individually and you can adjust the airflow thru each by using a hose to listen to the airflow thru the throat. You'll be adjusting the throttle dsk opening.

Gene
Gene Gillam


When I take off the air filter on the left carb and stick my finger in to lift the piston it is very sticky.

once I get it going it moves freely enough - but does not drop to the bottom with a "a satisfying clunk," or "a sharp click."

So jet centering required. Odd as there is no marking on the needle but their you go.

I agree the fuel looks high on the bridge... that will be my next problem to look at.

Thanks for advice gentlemen.

Having read the instructions for centering the jet... is it realistic to do this on the car or will I have to take the carbs off ?

Dave
Dave Moore

On is fine. If you you don't have a centering tool, trial and error using the loosen-at-the-bottom works, it just takes longer and is a bit fiddlier.

Tom Lange
t lange

dave, do NOT use any abrasives on the piston/chamber combination. as you may know, they are matched sets, matched clearances.
the method of drop test you used works..i put a piece of tape over the hole in the piston and my thumb over the damper boss on the chamber...flip them over and start counting..catching the piston with the finger tips extended past the chamber edges..john twist has a good video on you tube on this.
if your drop tests are the same and the jet is centered..and piston drop when installed gives you the nice "thunk" sound and you still have uneven response...have you swapped the damper rod? the slower response followed the damper rod. i had two different washers on each damper rod which gave two different response times to the piston. not sure if this is helpful for you or not.

regards, tom
tom peterson

I'm really grateful that people find my setting up instructions useful. Thanks for the kind words.

Centering the jets in place on the car is possible by working in increments. Raise the jet full up, the jet orifice is 0.090, and the jet needle at its first station is 0.089.

Remove the lower attachment for the jet return spring to provide wrenching room. Loosen the jet bearing nut a turn. Note that this also effectively lowers the jet, that's where the incremental part comers in when using just a jet needle, rather than a centering tool. Now push the piston all the way down by inserting a rod, punch or even a screwdriver so the piston is full against the bridge. Slowly tighten the jet bearing nut and check for the satisfying clunk several times as you snug it. Finally, reset the jet to the initial setting of 0.070 below the bridge. Note also that it is possible to cause the upper jet bearing and the lower jet bearing to be askew relative to each other while making this adjustment, so sliding the jet up and down as you incrementally tighten the jet bearing nut as you perform this adjustment will help alleviate this condition.

Hope this helps,
Dave
Dave Braun

Been away in Naples ( got to see Pompeii and Herculanium - put them on your bucket lists..) so haven't had a chance to work on the car.

Will start this weekend with the shop manual, Dave - your original original guide and your extra comments to hand guys...

Wish me luck.

Dave
Dave Moore

I found the problem !

I was taking the backing plate for the air cleaner off to provide access to the jet for centering...

As I pulled it away it was stuck.

The top of the gasket was stuck between the dashpot and its seat.

Basically gave the dashpot a small tilt that only mattered at the start of the piston travel.

Car has always run well except the idle was 'unstable' a little lumpy but mostly with an big variation in revs - anywhere between 500 and 1200

You would never see it with the backing plate for the air cleaner in the way...

Probably been that way since the PO had it restored.

Trimmed it off.

Nice solid clunk as the piston hits the bridge :-)

Fuel being high is coming from one of the floats in the bowls having a small hole and filling with petrol... slowly sinking and adjusting the fuel cut off.


Smallest things can make such a big difference.


Thanks for all your help and advice.

I am moving on to the next stage of tuning using Dave's guide as soon as I get a new float...


See the pic below for the gasket interference.


Cheers.

Dave


Dave Moore

This thread was discussed between 04/08/2013 and 13/08/2013

MG TD TF 1500 index

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