740 carb review also known as DFT DFTA weber

[tom foreman]

Awsum,joe

 

[joedrum]

been messing idle circuit a lot ... here a vid ....iran out of gas in the middle ... idle getting leaner and leaner .. red neck hippy cant keep gas in it ..... :BigGrin:
:music2: [video]https://www.youtube.com/watch?v=wxKz8z8qkYU&feature=youtu.be[/video]

 

[dan filipi]

Says sign in to watch video. Did you publish it?
Yeah,,,something new by YouTube.

 

[dan filipi]

Sounds like it's ready for a road trip.

 

[slabghost]

Sounds good as long as the gas lasts. :smilie_happy:

 

[joedrum]

ok ..ive been planning a trip to fla ....so been working hard on idle circuit on hooch to try and figure out the mpg thing and rich plugs ...even thought the bike runs well ..i could find ways to get it rich acting and loading up ....a big one around here is simple backing off throttle and going down hill ...when get to the bottom of long hill it would be loaded up and have to clear it with a few revvs ...wasnt even a big problem ...but the plugs were still dark and mpg is down ....so went back through things and really just consintrated on idle circuit ....

one of the first things i did was look at earlier versions of weber carbs ....after doing so ... seems earlier versions had no power valve and also had a metering screw that basiclly metered the flow of idle circuit ....dft dosnt have this ...it has the anti diesiling device fuel shut off ....most of us have taken this off ...on my bike i could see ...1 fuel flow was wide open 2 the riged plug i had put in has a rather big open spot ...so not only was flow wide open as earlier carbs could be metered back ...it had like a resivor of fuel that could gravity feed into idle circuit and flood things ....

so i changed that with a allen wrench set screw deal ...that could be bottom out to have no resivor like holding tank ...and i could meter fuel some ...this meaning plug is not perfect but it close enough to work ...

i could actually get it metered down to where i was lean... at idle but test rides proved that i could still make it load up ....so this wasnt the the big fix but it helped ...

so got to thinking ....on hooch the primary air bleed on main jet tower was 160 .... hooch has 220 now and performs great with it ... so it accured to me that maybe idle circuit needs same thing going on and this will make idle charge better ... it was seeming to.... at idle the motor and its air was at its most low point to make a difference in bad charge make up ....

so i tested the fix air on hooches carb 32-34 dft ..it was 130 size ...i also check 32-32 carb it was 115 in fixed air so this is different between the two carbs ....

ok the thought here was to add so much air to the gas charged from the starting point it wasnt a matter of cutting back to much flow... but to stop it from ever happening in the first place ....so i took a big swing.... the idle gas jet is 45 i cant go lower there ... so i went from 130 fix air bleed to 180 fixed air bleed by drilling it ..while carb was still on bike with only the top removed ....

did the drill by hand with a pefect size small vise grips as drill bit holder ...and cleaning drill filing off bit constantly this eventually got all the way through .....work great love this carb .....

okay the results are in ive done two test rides...this seems to get as good as stock idling ...bike fires right up ...plugs are way cleaner looking ... that was huge... on ride sheesh bike got better as ride went

my bike is sort of riged at moment but is ready for trip ...my assesment is the biggest problem with dft is idle circuit fixed air bleed and also creating a huge gravity feed fuel dump at soleniod fuel shut off spot ...thats my opinion ...the trip south ought to prove weather hooch is in the mpg game for road work .... :builder: :thanks:

 

[dan filipi]

No load up coasting down hill IS a big deal.

Joe and I have chatted for many hours getting a good understanding of this dft carb.
Many mods have been tried to lean out the idle, this latest one seems to be getting there if it isn't already.

I'm posting a picture with some explanation of what Joe is talking about.
I may have more to add to this but this is the jist of it....

Idle mixture screw #44 (in blue) was removed from the dft carb by some stupid EPA requirement for the car it was used on.
This screw metered both fuel and air (emulsified gas/air mix) into the passage which leads to the lower mix screw #40.
The raw gas from the idle jet next to the top air bleed #36 (in red) gets air added by the #36 air bleed to the raw gas as well as the other red circled arrow below.

What Joe has done is modified a simple set screw to meter the fuel/air mix at #44 to cut the flow speed/volume going down through the rest of the circuit as well as drilled the top air bleed #36 larger to allow more air into the mix at it's starting point at the top of the carb.

 

[slabghost]

Looks like a logical fix.

 

[sgq700]

Now THAT is a cool mod Joe!! the 60's weren't hard on you at all!! Nice Work Man! and Dan, Great work on making it clear enough for those of us that the 60's was hard on !! :thanks:

 

[mcgovern61]

.ive been planning a trip to fla .

When are you trying to head out Joe? I am leaving NJ on June 27 to head to Fort Lauderdale. Camping on the way down. opcorn:

 

[joedrum]

im dam near gone gerry ...with one music stop maybe ... or just pretty much strait through ...if mother allows it.....should be great break in ride ....

 

[mjanderson]

joedrum » Tue Nov 26, 2013 4:36 pm[/url]":2b5sqyvw]

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.

Hey Joe,
I was working through this topic in an effort to set the jetting on my own 32/34. In the picture above, you have the primary jets on the left. The 740 manual shows them on the right. Maybe this is straightened out later in the posts, I don't know...

Thanks, Mike

holley 740 rebuild manual Page 16

• dan filipi
• Mar 22, 2015

 

[dan filipi]

Right is primary, left is secondary. Both inline with its Venturi tube.

 

[mjanderson]

Thanks Dan. That is a great way to remember which is which. I had mine backwards based on the top picture. I had a 60 as the idle jet instead of the 45 I thought it was. It would idle but was still rich. This change should make a difference. :BigGrin:
Mike

 

[joedrum]

oops got that backwards

 

[mjanderson]

First mistake all year, right Joe? :hihihi:

 

[ekvh]

No, he was right, he was just standing on the other side when he typed it.

 

[joedrum]

possition is everything .. when i sit at a computer anything can happen :BigGrin:

 

[mjanderson]

I've been following this thread with great interest because I am using the 32-34 on my bike. Joe has blazed a trail like a pioneer on modifying this carb and I have already used much of the info. I was considering blocking off the idle feeds as described in the thread. Before doing so, I took some time to look at another carb body I have and I've come to the conclusion that those two outer holes in the primary bore are for above-the-throttle-plate vacuum feeds instead of idle gas ports. You can trace the tubes up the outside of the carb body to the two vacuum ports on the side. To verify this, I took an eyedropper and forced water through the two vacuum ports. As I expected, the water exited into the primary bore via those two outer holes. So it seems to me that the only idle gas feeds are the one for the idle mixture screw and those three other holes directly above it. Am I barking up the wrong tree?

Mike

photo 1

• mjanderson
• Sep 1, 2015

photo 2

• mjanderson
• Sep 1, 2015

 

[dan filipi]

I found the same as you on the dft.