Need help with carb issues

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downsr

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Had a 750 holley with vac secondaries ran and idled good.Saw raw gas in intake when i removed carb.Bought new 650 double pumper.I have to turn the mixture screws out 3 turns to get it to idle right but still has hesitation.Why do i have to turn the screws out that far opposed to 1 1/4 on the 750.Lastly how can you check needle valve with carb off.Thinking about putting the 750 back on if i can figure out where the raw gas came from.The engine is a 340 stroked to 416 solid lift cam with 235-240 duration cam
 
Does the 650 have screw in air bleeds? How much initial timing do you have?

On the 750 were the floats set correctly? Sometimes after the engine is shut off the carb will heat soak and percolate and push raw gas out of the boosters. Needles are pretty cheap and todays gas is hard on everything. If you buy new needles go ahead and buy the gaskets too, there is one under the nut and one under the screw head.
 
Was there two small holes drilled in the throttle blades of the 750?. What power valve # does each have?
 
problem solved went back to my 750 and took the 650 back. I found out the 650 would have to be modified to make it work correctly.Did not want to pay 500 dollars for something i had to modify to make work
 
Do you have a heat shield under the 750 carb? Just wondering if your fuel was boiling.....
 
nm9stheham said:
Do you have a heat shield under the 750 carb? Just wondering if your fuel was boiling.....
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Hope you get that carb setup working properly. I am kinda in the same boat. I have a 67 Barracuda Fastback with mid 1971 year 340 4-Spd that I just purchased a few months ago. I know that there are all kinds of variables used to choose a carburetor CFM properly depending on CID, RPM, Cam Grind, Heads, headers, Tranny and Intake etc but can anyone tell me what carb/cfm bore size came factory on these engines?
I read somewhere about some dyno runs that were being done on one of these little monsters and the article mentioned using a 800 CFM carburetor and according to the HP/Torque curve data the little engine was letting none go to waste. They actually printed that they were getting just over 400Hp and 440Ft-Lb of torque at 3800 rpm with no changes to the lower end. I was thinking of installing a 280H Comp cam (around .480 lift), headers, Edlebrock Performer Air Gap Dual Plane intake and 800-850 Holley Dual-line twin pump square bore carburetor. Would this bee too big of a carburetor? I ran a 750 Holley (Vac Secondary) square bore on a little 350/.040"+ Nova and it seemed to make good use of the flow with a similar setup.
I hate to pay $600.00+ for the new 850 cfm Holley and it be too much. I hear these are high winding little motors with stock 4.020" bore and 3.35" stroke, forged crank/ beefy rods. This is my first Mopar car, so I really don't know much about them. Would appreciate any insight. I will drive this car on occasions to cruise town just me and the wife. I'm sending her to paint in a few weeks (The car), haha.
Thanks
 
downsr said:
Had a 750 holley with vac secondaries ran and idled good.Saw raw gas in intake when i removed carb.Bought new 650 double pumper.I have to turn the mixture screws out 3 turns to get it to idle right but still has hesitation.Why do i have to turn the screws out that far opposed to 1 1/4 on the 750.Lastly how can you check needle valve with carb off.Thinking about putting the 750 back on if i can figure out where the raw gas came from.The engine is a 340 stroked to 416 solid lift cam with 235-240 duration cam
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FWIW;
The amount of turns out suggest to me it was lean dispite how it ran.
Needle and valve as in for the float height?
Normally the fuel level and the screw in plug on the side of the bowl. A sight plug is a normal thing now I think and you should see the fuel level at the bottom of the sight plug.
Checking the needle and seat would be a disassemble.
 
1badfish67 said:
Hope you get that carb setup working properly. I am kinda in the same boat. I have a 67 Barracuda Fastback with mid 1971 year 340 4-Spd that I just purchased a few months ago. I know that there are all kinds of variables used to choose a carburetor CFM properly depending on CID, RPM, Cam Grind, Heads, headers, Tranny and Intake etc but can anyone tell me what carb/cfm bore size came factory on these engines?
I read somewhere about some dyno runs that were being done on one of these little monsters and the article mentioned using a 800 CFM carburetor and according to the HP/Torque curve data the little engine was letting none go to waste. They actually printed that they were getting just over 400Hp and 440Ft-Lb of torque at 3800 rpm with no changes to the lower end. I was thinking of installing a 280H Comp cam (around .480 lift), headers, Edlebrock Performer Air Gap Dual Plane intake and 800-850 Holley Dual-line twin pump square bore carburetor. Would this bee too big of a carburetor? I ran a 750 Holley (Vac Secondary) square bore on a little 350/.040"+ Nova and it seemed to make good use of the flow with a similar setup.
I hate to pay $600.00+ for the new 850 cfm Holley and it be too much. I hear these are high winding little motors with stock 4.020" bore and 3.35" stroke, forged crank/ beefy rods. This is my first Mopar car, so I really don't know much about them. Would appreciate any insight. I will drive this car on occasions to cruise town just me and the wife. I'm sending her to paint in a few weeks (The car), haha.
Thanks
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That would be a 3.31 stroke on a 340 with a 4.04 bore. Just saying..... But to address your question in with a short answer, a 750 would be fine.

The stock carb for 340's were a 650 AVS (early) and both size primaries on the (later) spreadbore TQ.

While the dyno made use of an 800cfm carb, I'd hesitate to use it on the street. (On just 340 cubes)
 
rumblefish360 said:
That would be a 3.31 stroke on a 340 with a 4.04 bore. Just saying..... But to address your question in with a short answer, a 750 would be fine.

The stock carb for 340's were a 650 AVS (early) and both size primaries on the (later) spreadbore TQ.

While the dyno made use of an 800cfm carb, I'd hesitate to use it on the street. (On just 340 cubes)
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Thanks rumblefish360 for the correction on bore and stroke. Your numbers are even better. I am going to try to get around 500hp out of the 340, if possible with a 125 shot of N02. May not be do-able. Have a guy with a Camaro 383 stroker says he can beat the little fish. He is claiming 500hp but seen no dyno numbers.
 
Any rule of thumb for choosing the correct cfm? Using a 4.040 bore, 3.31 stroke and 6500 rpm get 1276.85 cfm of airflow assuming 100 efficiency. If we assume a Volumetric Efficiency of 0.8, we get 1021.48 cfm. I have read that a window of 0.8-0.85 (VE) is plausible for these LA engines? Thank for the info.
 
That number seems a bit high. A 340 needing 1021cfm to make that power at 6500 doesn't need that much carb.

1Badfish. To make 400hp with a 280* cam is possible and a 100/125 shot on top will add the needed power to get the 500hp no problem. It is certainly doable. Getting 500 N/A, no power adder out of a 340 would require a bit more work and a stouter combo than you may want for the street. The engine would become a bit of a nasty running mill.
 
1badfish67 said:
Any rule of thumb for choosing the correct cfm? Using a 4.040 bore, 3.31 stroke and 6500 rpm get 1276.85 cfm of airflow assuming 100 efficiency. If we assume a Volumetric Efficiency of 0.8, we get 1021.48 cfm. I have read that a window of 0.8-0.85 (VE) is plausible for these LA engines? Thank for the info.
Click to expand...

My 340 made in the 450 hp range with a 750 on it. I wouldn't put a 850 double pumper on a 340 unless it was strictly a high winding drag race only. The 850 has larger venturiis which slows down the air speed at lower rpms which can cause a couple issues, sluggish throttle response and poor air and fuel mixing at low rpms.
 
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Exactly! I know some guys run a 850 on there small block but as you said, there really only track only.
Of course, strikers are another story and don't apply.
 
3 turns on the screws usually means you have a good grasp on the T slot exposure but could be border line lean or too big a PCV. It really depends on where you are getting your idle air bypass. Depending on where the hesitation is, pump cam profile and squirter nozzle size along with float level and power valve selection usually are the things that cure it. If you are monitoring the AF with a wide band then you can consider fine tuning with air bleeds too.
 
mguner said:
3 turns on the screws usually means you have a good grasp on the T slot exposure but could be border line lean or too big a PCV. It really depends on where you are getting your idle air bypass. Depending on where the hesitation is, pump cam profile and squirter nozzle size along with float level and power valve selection usually are the things that cure it. If you are monitoring the AF with a wide band then you can consider fine tuning with air bleeds too.
Click to expand...
Sorry so long in getting back o the question. I have been looking at ordering the Holley 770 cfm vacuum secondary carb. Holley # 0-83770. You guys think this would be a good fit for my setup? I have not pulled the rear-end to verify gear ratio. I found out that the 489 has some kind of posi or Sure Grip in it or at least I assume it does since while backing it uphill into my shop it slipped both rear tires on the concrete. Anyway, was going to order the Holley today. Didn't think 20 more cfm would hurt anything?
 
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