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MG TD TF 1500 - Carb piston difference

Does Anyone know anything about the difference in the 2 carburetor pistons shown out of an SU H carburetor? As you can see in the photo there are small extensions on the bottom of each piston. The one of left only has 1 and the one on right has 2. The extensions are spring loaded and I assume they are there to soften the shock of the piston hitting the bridge. But does anyone know when this change occurred? Also I believe I've seen the same type pistons with no buttons on the bottom. Just curious. BTW they both have 3370 stamped in the top rim.

Tim


Timothy Burchfield


Recommend you ask Joe Curto Inc. phone(718) 762-7878. He will know if anyone does.
Richard Cameron

Tim, if you find an answer off board, would you please update this post? Regards, Tom
tm peterson

Richard, Thanks.

Tom, of course. I guess I'll have to contact Joe Curto since no one else has any information. I dislike contacting professionals with non profit making inquiries. But I understand Joe is glad to answer questions. I'll send him and email.

Tim
Timothy Burchfield

Here's what Joe Curto said about the extensions.

"The little buttons would leave the piston up off the bridge, the small gap allows a flow of air into the carb on starting allowing it to suck up a small amount of fuel and start the car , later cars used a plastic button , they would protrude 015 to 020 from the face of the piston . The ones you have would have a little plug in the depth of the piston , you may be able to tap down on the plug and push out the button."

Tim
Timothy Burchfield


I'm confused? Seems like to me Joe didn't address the question you posed about two buttons vs a single one.
Richard Cameron

Thank you Tim.
Regards, Tom
tm peterson

Some SU pistons have the vacuum transfer port in the bottom and some in the side but carry the same part number. Maybe in the one with 2 buttons the extra button is to block a drillway to the side port.
Ray TF 2884
Ray Lee

Ray, thanks. But if the extra button was just to block a drillway it probably wouldn't be spring loaded. Seems like extra expense and labor for just a plug.

Tim
Timothy Burchfield

Does anyone have an HF carb piston with no buttons? The problem (minor) with the buttons (especially the two button one) is that you don't get that satisfying click when dropping the piston while centering the jet. I wonder if the purpose of the buttons was like the poppet valve in the butterflys of later SUs on the MGBs. They were there to prevent excessive fuel being drawn in when the throttle closed rapidly.

Tim
Timothy Burchfield

I thought the valves were to prevent running on when the ignition was switched off. Some car had a solenoid air valve in the inlet manifold to do the same.
Ray
Ray Lee

Ray, Joe Curto says the buttons were there to allow air entry during starting not to prevent run on. He also said he doesn't know why or when SU went to two buttons.

Tim
Timothy Burchfield

Burlen confirmed what Joe has stated. I asked before seeing the early post. They are indeed so that the piston will always allow air in and whether you've one two or indeed none should make no difference to the carbs performance although I take Joes' point they may assist during start up.
JK Mazgaj

JK, thanks. Now if we only knew why they switched from one to two :-)

Tim
Timothy Burchfield

I wonder whether weight balance would/was as issue the piston would possible need to slide evenly.
JK Mazgaj

JK, common sense tells me that they went from one to two buttons for performance results. But practicality tell me they went from two to one for economic reasons. The weight of the spring and the plugs is miniscule so I doubt weight was an issue. I still haven't heard from anyone with pistons that don't have any buttons. I modified two pistons by removing the single button. I'm going to try them soon in my 51 TD. After days of trying to get a lean mixture I figured I'd try pistons without the buttons. And yes, I have been through every system check known to man, to include using three different sets of carbs, to try and get a lean mixture. My next step will probably be bigger carbs. With a roller cam, larger valves and a .120 over bore perhaps the 1 1/4" carbs just aren't big enough.

Tim
Timothy Burchfield

Sounds correct to me. I think the best way to get a leaner mix is probably to lower the fuel level in the float bowls. My engine is 0.080" over and has various other mods, with 1.5" carbs it tends to run rich with the fuel level set by the book. I now prefer to set it by measuring the level of the fuel below the top of the jet. By the book would be equivalent to 1/8" to 3/16", but maybe 1/4" is better. Modern fuel is one cause I guess, not necessarily the small percent ethanol (because that should have the opposite effect), but the higher proportion of branched, cyclic and aromatic isomers, which gives higher octane and other different burn characteristics.
Dave H
Dave Hill

This thread was discussed between 24/06/2019 and 29/06/2019

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