Carb Question

henry g

New User
What are the differences in a DLTX 33 vs a DLTX 71. I know the choke mechanism is different, my question is about performance on a enhanced engine. Im looking for the carb that will perform better and why.
 
I heard the 33 and 53 is the same all fuel carb just different numbers. 71 would be a strait gas carb and 72 is a all fuel. They say a gas carb running gas will have better fuel economy than a all fuel carb running gas and better on plugs. 33 is all fuel and 71 is a gas carb.
 
33 went to a 53 real quickly in s/n sequence and then quite quickly to 71. All have same size throttle plate and same load nozzle. 33 has an economizer plug which is only in use at intrmediate throttle to lean up mixture for economy. . Doesn't affect WOT or idle. JD was more interested in fuel effeciency back then than they were top HP, hence all the numbers for same CFM carb. A 63 that feeds a 500 cid late "D" has no more air flow than a 71/53. There are some idle port differences as to the number of them. If you have the same size bore/throttle plate and the exact same load nozzle then WOT you will have same hp. Roberts Carb has ONE test tractor,a '44 "A" that they test everything from an "H" to a 730 on. They made an adapter to narrow down the duplex to bolt to original single manifold runner. They have it hooked to a dyno. His own statement " all will run/idle/load just fine, only difference is HP".And he proves it. I have had a 63 (500 cu in) and 71 (321 ci) and 18 and even 67 (190 ci "B") apart and the fuel/air passages in the castings are the same size. Adj. needles are all same. Your question is about max hp on an enhanced engine. You are going to get same air flow/blade dia. and same fuel ( passages and nozzle are same) with both AT WOT. Why does a "big nut "G" carb bring so much?? Same size as 71. Low availability is my opinion. Also just a note , the carb isn't the "weak link" in the air flow into a JD 2 cyl. Some are modifying internal throat of carbs. Better have a dyno and a few spare castings. If one is looking at feeding 700-800 cubes or more then why would one use one of these carbs anyway? Nothing else on one of these is still stock anyway. Block/crank/head/valves/manifold/pistons...all custom made so why does the carb have to play by "stock appearing" rules? never understand it.
 
I will give you that all the fuel passages are the same but the main throats on the A B G carbs are different ID's . The throttle disk on the A/G are the same size but the B carbs are smaller.
 
That is true, I probably shouldn't have put the 67 "B" in that sentence. was only to compare castings/fuel/air passages. "H" is smallest,then US "B' (10) then styled "B' then A/D/G.
 
Thanks for the analysis. Now, the 33, 53, 71 and 72 have the same size venturi, but the 51 is an 1/8" bigger I believe. With the throttle and choke blades all being the same size, do you think that increase in venturi size helps all that much?
 
A chain is only as strong as it's weakest link. Venturi creates "air velocity" to pull and atomize liquid fuel. The "signal of demand" from the engine still has to come through 1 11/16 bore. If you are pulling then you are inevitably going to be lugging down in rpm sometimes to get traction and/or speed limits. I believe the recovery/response would be a bit crisper with 71 size venturi. My opinion is to look somewhere else for your HP. I think you are splitting hairs when it comes to carbs on these. A stock late A at around 975 rpm only requires around 70 cfm. 89-90 or so at 100 % fill.Figure out your cubes X intake strokes per min and even that presumes 100% fill (which we know it doesn't). I don't know if anyone in tractor pull forum has tested these for cfm but just size comparing to a car, I would say around 150 cfm anyway.I applaud you for doing your resaerch, though, as apposed to just doing what you hear around the pulling track. I only know what I've experienced. Might even want to check with Josh Blackburn, he's on the pulling forum and lives here in MI and pretty much knows the JD 2 cyl pulling inside and out. Thanks RB
 

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