740 carb review also known as DFT DFTA weber

[joedrum]

Many of us are going single carb ... There have been two carbs use here on this forum ... The Solex one barrel and the 740 carb Weber this is about the DFT DFTA and 740 Holley and Rochester and Motorcraft licence copy of weber

This pic shows the first reason I picked this carb. You can see 24-25 ..printed on the carb body it represents air flow ..This rang my bell cause it was easy for me to see the flow of air was close ...I must say here the 740 comes in other sizes and the 22-22 740 is dang near spot on for "oldwings in airflow at full throttle.

The two stage Weber is a perfect design single carb for a full rpm motor like a "oldwing ...Providing super charge control over entire rpm range if jetted and manifold is setup to match carb air flow needs.

 

[joedrum]

The Weber carb is set up to be adjusted in four different areas in the rpm running range of the "oldwing ...This is a huge advantaged to owners of "oldwings ...

Here as things went it became obvious that some things had to go.To mount on the "oldwing .The electric choke wasn't worth cutting bike frame for so it was removed.There's a power valve that on these carbs holds a gas valve closed. And when vacuum drops like in opening throttle it dumps gas in, enriching the mixture. These things are not carb parts really they're epa regulation parts so carbs can be set up lean. "Oldwings don't have a vacuum that actually matches this valve and there is no adjusting it. So it is removed. Gone also is anti dieseling device solenoid. I don't know much about the altitude device but it too is removed.So essentially all things except accelerator pump was removed. With all this done it fits in the "oldwing just right. Throttle hook up is easy as the pic above shows I spent no time rigging it and it still there now.

 

[joedrum]

This shows the the carb body and the gas and air metering jets between barrels. The top barrel is primary and has the accelerator pump shower nozzle in the barrel. In this pic if you look at the bottom barrel you can see where the barrel chokes down to correlate to the numbers on the side of the carb body that represents air flow. Coming from the middle between the barrels you can see where the gas charge mixed with some air goes to middle of barrel where more air is mixed for final charge to motor.

As for the metering of the charge there is an idle jet and main jet tower for each barrel. In the picture. The two on left is primary and the two on right are secondary barrel.

 

[Ansimp]

Joe I put my glasses on and I swear I can read 24-25 on your carby not 22-25 as you suggested :yes: :doh: :hihihi:

 

[joedrum]

sheesh past the edit time too darn


(I edited that post to say 24-25 DF)

 

[joedrum]

Ahh the heart of the Weber carb. Lets start with the main jet ,tube and air bleed. The two longer deals in the pic one for each barrel. The bottom jet is a gas jet. It is in the emulsion tube and above that is the air bleed jet. This is a neat setup. Every part of making a perfect charge of gas and air is all done with these parts and carb body set up. Through out rpm range and different airflow rates. It has no moving parts at all. The way the carb works the idle jets get gas from the main tubes at about the same height the pic shows them. That puts the feed rather high and certainly past most clogging issues. At this point it is time to say that gas and air get mixed here. The main gas jets run down in the carb body to where the bowl feed is. And from there air and gas is mixed or juggled to get a perfect charge for motor .

 

[joedrum]

The idle circuit. Sheesh this is big for us "oldwingers. 80% of the mpg is in the idle circuit. It is never out of the loop of operation. It seems to me that the air bleed on the main jet tube on the primary and the idle jet both play a part in the feed to the idle mix screw. When the idle mix is right you will know it on a Weber. Here's where it gets dicey and set up all starts. This carb was set up to run a 1600 cc motor of rather low rpm as compared to "oldwing motors. So for us "oldwingers our motors could be 600ccs short to 500 or 400 like me with my 1200. So at idle the air flow is low to get the carb to atomize good and run ...this makes manifold design a big deal. To be brief the manifold needs to be fit and trim. Not much fat as in volume to hinder air speed at carb for it to work good. Okay I need to takes pics to continue on. More coming. It is very interesting how this carb delivers the idle circuit charge.

 

[slabghost]

I'm sure glad someone knows these carbs.

 

[Dusterdude]

No kidding


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[joedrum]

Okay this is where most really drop the ball. Right at this point. The Weber carb is a great carb and so much better designed than stock carbs. It doesn't handle the air as in a manifold design. When talking way smaller "oldwing motors tricking the carb to work through air control is everything to get this carb to work and work good through out the rpm zone.

Unlike what is commonly said to discount singles. " They run out punch at high rpm" ....This is simply not true from a too big carb on a motor. As one who has over carbureted monster motors in cars I have first hand experience in what goes on and the problems you have. They are never at high rpm. The high revving "oldwing will be at its biggest here. This is where a too big carb would shine not fall on its face. So I always knew that belief was just BS .When BS is in the room. Smart thinking people know there are huge gains to be had. But, no one was finding it.

 

[joedrum]

With all this experience on carbs on cars I was quite confident I could trick it to work good on hooch. There was seemingly no way to get the stock Honda racks to work without reinventing the whole package. The Weber was what I figure the best for my bike. One thing was for sure in my mind and most don't think like me. You don't make a big carb work on a too small motor by building a too big manifold to go along with it. This only compounds the problem. I knew from stories. People couldn't get the to big carbs to idle or hardly run at all. I knew that to bring more flow from the "oldwing so that the carb could use it. It had to be choked down to get any hope of being rideable. I had watch videos of singles very few actually ran well. All seemed to have flat spots ,and none seemed to even come close to the poorly designed stock set up. I'm not going into what's best or debate. Plain and simple is this. If you want to influence what going on at the too big carb, you don't do it at the head. You do it close to the carb.

Enter the VW type 4 plenum. It was easy for me to see this was the best carb mount available in a no waste set up. As in volume space, this was good in my mind. But people were still having trouble getting them to perform good . Most were using carbs that were just way too big and really had no hope. Like having a 750 double pumper mounted on a 289 ford or 283 chevy there is no chance at all. I don't care what you do. The DFTA 740 carbs don't even bolt up to type 4 plenum without modifying. Sheesh that was good sign. Carb is smaller than type 4 plenum is designed for. Well, to me this means that the outlets on the type 4 are probably too big for "oldwing and this DFT 740 weber carb could benefit from smaller outlet too. As the design was for a much bigger carb. For me and the way I look at it. The plenum is part of the carb like a jet sized right it will draw what it needs out of the carb. Too small and it wont be able to use all the carb has. Too big and it's not capable to draw anything out except high rpm maybe. Personally I have the plenum outlets choked down to 1" .
Am I positive this is perfect? No..I can tell you that on my 1200 modified motor I have no off idle problems at all. No bogging at all in low rpm .In fact the complete opposite. I have tractor like power and torque there. I know that on Dan's bike had similar things happen. Not exactly like mine but close. When he went down in runner size at the plenum outlets to 1" ID all the way to the heads. In his case when he had bigger runners like what most would consider for more flow. His bike was almost too dangerous to ride in busy California. Might get ran over. Lots of bogging problems from wide open type 4 runners to the max. Okay enough about this part in the set up . I'm sure its clear I have made good power from tight plenum set up ... this is what I think is required to get all you can from a DFT Weber carb. Again I'm not saying I'm totally spot on . But yes I will say I'm close and this info is super valuable to people thinking of trying this . More coming.

 

[joedrum]

This pic shows the idle jet outlets in the top barrel the very bottom hole is where the mix screw is and with it you adjust the final mix rate with the air available for good charge ...84% air and 16% gas..for idle.

To me this represents a bad plate adjustment .This carb fast idle screw is backed all the way off and the plate has exposed transition idle outlets and is not set right .I like the set up .It seems once set right, hard to go wrong .

This shows what I think is set up right .With the fast idle back screw backed off. The plate closes in barrel and only the mix screw outlet of idle jet circuit is exposed .In this position the motor should never run. Only when fast idle screw is used to get a little air to idle .Weber says if it takes more than 2 turns to get it to idle once fast idle screw hits the linkage then it's time to change idle jet as this will bring in transition outlets not to be in play at idle.

 

[joedrum]

So here we are. Lets say plenum flow is good and now its carb set up. The Weber idle circuit accounts for a lot of mpg figure. Right now I am not totally clear if the idle jet handles just gas or air and gas together. So everybody knows I'm not sure . I do know that when idle jet is screwed in all the way it seems the bottom hole is about the level of the gas level in the bowl .Not saying any thing to that either just is what it is .It really makes no difference in set up and how it works .Right now my bike which is 1200 stroker, over an 1100 .I'm using a 45 idle jet. When the mix screw is adjusted to where it runs best. It is only a shade more than 1/2 turn out from bottom. According to Weber, if you are less than one turn out .You should go lower in jet size .Well the carb comes stock with a 50 jet .This indicates I should try a 40 jet .This is the lowest of the Weber idle jets .I haven't done this yet .I don't have one or I would. This is a good sign this carb has all an "oldwing can handle .But this is just me .I know right now Dan is using a 50 on an 1100 and so is everyone else with stock jetting .Though I heard of one rebuild carb having a 45 jet also. Having this jet right is huge for idle and lets say 25% throttle. This probably takes all the fancy clutch work you have to learn when you own a "oldwing off the table .

From here the primary main well takes over. The long tower jet combo of gas and air mix is wide open In adjusting to suit the motor just right for "oldwing. Personally I've barely scratched the surface doing this. It takes parts more than I have up to this moment but I have learned a lot juggling around the stuff I have. It seems the air bleed jets have huge impact. Basically the bigger the air bleed jet is, the less gas gets sucked in. The smaller the jet the more gas gets sucked in. I'm not going into why all this is applies except to say it's all about making the gas charge right in every demand. Here on the primary tower I am basically stock as carb is. Hmmm. I wish I had parts. I'd play here some but I must say it's hard to argue too much fault here it works good.

 

[Ansimp]

Joe I am a little confused about the Weber idle circuit, I was under the impression that once off idle (1500 > rpm usually) the main jet circuit would be supplying the majority of fuel available to the engine and would be the circuit used for cruising and responsible for best fuel consumption.

 

[Dusterdude]

Fwiw,my idle jet atm is a 45


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[joedrum]

Now here we go. We are moving along now to where the progressive linkage starts to engage the secondary throttle plate. Up until now it's been closed and even more so than the primary is that this plate be closed good. The idle outlets in the secondary are all above throttle plate and should be inactive if the throttle plate is closed tight. The idle jet in secondary is for part open type cruising or highway touring. This is cool. This is where I was having problems on my bike. Long before I had most of what I am typing figured out it puzzled me. I wasn't getting anything out of the too big carb in the secondary. As I looked at things it also puzzled me why the air bleed was so huge from the primary air bleed. I finally decided the carb wasn't suffering any air flow problem it was getting too much air. And I had a tower jet, tube and bleed that was almost like the primary it is 165 air bleed and 105 gas jet as opposed to the stock 220 air bleed and 95 gas jet. This instantly gave me a secondary on first test ride.
The idle jet I am running right now is 65. Stock is 60. I am still here at this jetting. It seems hooch can still use more gas. But again you have to have parts. The biggest discount I can say about the Weber is the tubes seem expensive at $36 a piece. Sheesh! When I relayed what I thought about the air jet as I think in time order Dan tried this idea first. Sheesh, cant remember.
But I am certain he didn't agree so I'm pretty sure I hadn't proved it yet. I finally got Dan to try it. It was hard to find anything out cause he was out testing here. Hmmm testing again. Well I knew it passed the test and he like it. He wouldn't quit riding it. Before he didn't even want to ride. So this is in no way final answer to jetting. But it has my total backing of in this carb. This carb is capable of beating the finest stock carb rack, with all its adjustability. Its execution speed in response is almost enough to do that. The torque down low is huge over stock and when the jetting is dialed in the best it can be I can see high rpm not only equal but hugely better like the low end torque. There are other things here too. Like manifold heat too and its effects and solutions on jetting and such.

All I can say is hooch is running as good as any two stage system I've ever had. No bogging at all. Smooth transition and punchy secondary. I'm totally excited about the fact I can feel power to come.

This is really not a comment thread but more a thread of what the forum wants people to know about Weber carb. And with all the lets say, not very substantial claims made about this carb. The things that need to be stripped for best reliability and running.

I've got more to add but ill stop here. Taking a break a bit. I'm sure its okay to comment right now. Don't let my words stop you. There's people around here who know how to take posts down if they want. I know I've had my fingers cut off several times. I am sure glad they they grow back :smilie_happy: :mrgreen:

 

[joedrum]

Ansimp » Wed Nov 27, 2013 5:09 pm[/url]":14uxrcpi]
Joe I am a little confused about the Weber idle circuit, I was under the impression that once off idle (1500 > rpm usually) the main jet circuit would be supplying the majority of fuel available to the engine and would be the circuit used for cruising and responsible for best fuel consumption.

Well Tony some of what I say is assumption as to percent of throttle but one thing wasn't. Weber said 80% of mpg is in idle circuit. Idle jet in secondary kind of makes this clearer. There's no idle going on over there but its still has idle jet for transition purpose and steady feed part open.

 

[joedrum]

Dusterdude » Wed Nov 27, 2013 5:37 pm[/url]":3j8xxma2]
Fwiw,my idle jet atm is a 45


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Yep yours was the one I heard about I'd call that good. :mrgreen:

 

[Dusterdude]

[SMILING FACE WITH OPEN MOUTH AND SMILING EYES]


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[joedrum]

Well I hope everyone is starting to catch on. How the Weber works and the system. I made one misstatement. The low speed circuit as Weber calls the idle circuit is 80% of the operation not mpg. Somehow I didn't translate the quote good. Okay I really like the low speed circuit set up and the adjusting to get it right. Involving fast idle screw, mix screw and motor air to get jet size just right.

I think this one setting takes care of most of the "oldwings problems they have with stock carbs and the spit and sputters crap ..good feeling to know the bike has all kinds of power to take off with carburetion.

Now for the main well tube and jets. The adjustability is huge. Lets talk tube first. The tube has air bleed jet on top and the gas jet on bottom. The tube has holes in it. The reason is when open throttle on straight siphon type carb like a primed well it really starts to flow from the air speed and volume. Too much so really, so the tube has holes in it and it draws air in though the air bleed jet. This slows the gas siphoning down to the level the motor can run good with. It also does a premixing of air and gas to some degree before entering barrel. The amount of holes and their location is the adjusting factor. Lower holes start the braking action on the gas siphon quicker the higher the less braking and also the size of air jet too. This whole set up makes it to where the jets are bigger and less likely to clog. Complete adjustability and no moving parts to do this.

There's like no excuse not to be able to dial in the Weber in and it seems like perfect mod for these bikes.