Carb experts Please help

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jefflock

jefflock

69 Dart 408 10.08 best pass so far
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Engine package is a 70 340 + .030, J heads, comp 284 cam 240/246 507/510, 750 Q series double pump. Engine is timed at 20 initial 34 total.
Carb seems to want to run rich all the time. Dropped the jets from 74/84 to 70/80. The idle mixture screws are barely open. I changed the air bleeds from 36/70 to 39/75. This brought the idle speed up and allowed me to get a better idle. But still the mixture screws are like 1/4 turn open. Does this sound right? Is there something else I should try to get the mixture screws farther open?
 
Check.........the..........power..........valve............

both for leakage and number (vacuum) rating. Check your engine vacuum.
 
Have you checked your throttle blades to make sure you are not uncovering to much of your idle transfer slots?

Here's a few questions I have for you: The Q Series is a Quick Fuel carb, correct? New carb? Part number? Does the carb have mechanical secondary, four corner idle mixture and front and rear idle speed adjustment? Are there holes drilled in the throttle blades?
 
Demon 408 said:
Have you checked your throttle blades to make sure you are not uncovering to much of your idle transfer slots?

Here's a few questions I have for you: The Q Series is a Quick Fuel carb, correct? New carb? Part number? Does the carb have mechanical secondary, four corner idle mixture and front and rear idle speed adjustment? Are there holes drilled in the throttle blades?
Click to expand...

I checked the throttle plates and they are almost completely closed. It is a new carb from Quick fuel I purchased last year. Mechanical secondaries, 4 corner mixture screws and front and rear idle adjustments. My car doesn't seem to develope much vacumn so I installed a 2.5 power valve. Also there are no holes in the blades. After changing the air bleeds and adjusting the idle down I had to loosen the throttle cable and allow it move forward because it was holding the throttle open a little.
 
agreed. power valves dont take much to pop. the engine just has to back fire once to pop it. my new engine back fired just once on initial start up and it ruptured the power valve.
 
that is a really small power valve. unless you have a super radical cam i think it is light. You need to get a manifold vacuum reading and cut the # in half. so if you have 9 in of vacuum you would use a 4.5 power valve. have you checked the float levels? what about fuel pressure?
 
jefflock said:
I checked the throttle plates and they are almost completely closed. It is a new carb from Quick fuel I purchased last year. Mechanical secondaries, 4 corner mixture screws and front and rear idle adjustments. My car doesn't seem to develope much vacumn so I installed a 2.5 power valve. Also there are no holes in the blades. After changing the air bleeds and adjusting the idle down I had to loosen the throttle cable and allow it move forward because it was holding the throttle open a little.
Click to expand...

So you have physically looked at the idle transfer slots (from the bottom of the carb) to make sure they look like a "square"? What is your idle vacuum and at what RPM? Float level is where? Sounds like you recently replaced the PV. Have you checked your plugs to see how they look?

I suppose it MAY want more initial timing at idle, Have you tried adding more timing? Just trying to work through possible issues.
 
I hooked up a vacuum gauge to full vacuum and at idle (900 rpm) shows about 8 to 9". The carb is dry inside at idle and I can see the idle slots above the closed throttle blades. The motor has 20 degrees initial timing in it. I opened the rear blades up 1/4 of a turn then readjusted the idle back down and reset the 4 mixture screws. They are about 1/2 turn open. That is where the motor runs best while adjusting them. The cam is pretty lumpy with 240/246 duration at 50. I'm going to run this setup a while to see how it works.
 
You install the power valve based on what WOT vacuum might be. That's why Holley says I think 3 sizes below whatever your vacuum reading is. If your vacuum reading is between 9-8 then you should have a power valve of about 5-6.
 
Power valve. Are you running an automatic transmission? Check your vacuum reading in gear at idle, divide that in half and that's the power valve you should be running. With a manual trans, just check your vacuum at idle and divide that in half.

Check your float level and make sure you don't have a bunch of rust holding the float needle open. This can cause your float level to creep up at idle.

Fuel pressure. 6-7 lbs. is fine with a Holley. Rust can cause a fuel pressure regulator to hang too.

QF carbs all have a check valve to prevent a backfire from blowing the power valve, so I doubt that's it.

Make sure TDC is truly TDC as marked on your harmonic balancer, you might not be running as much initial timing as you think.

How much compression are you running? That's a pretty big cam to be running stock pistons in. The quoted 10:1 CR, once truly measured is usually quite a bit lower.
 
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