Holley Carb problem

[skycuda67]

Im so close to getting my 67 barracuda back on the road after six years sitting, but im struggling with the carb now. My setup is a .030 over 360, rv cam, strip dominator intake, rebuilt 3310 holley 750, headers, full msd with plugs gapped .045. The msd manual says .065, tried it, ran terrible, after further reading went to .045 and its much better. the carb is jetted 71's or 72's I think, stock 6.5 PV. It starts immediatly idles awesome cold or hot, has great throttle response, but it chokes a little at just under half throttle and burns black smoke while revving. I don't have a vacuum gauge but I was looking at the vacuum powered idle/performance gauge on the dash and I got it tuned up to the 18 mark which is the highest point I found while adjusting the mixture screws, but still blew black smoke and studdered a little during acceleration. I don't think its too much carb for my set up, I was hoping it might just need a different PV? what should I try next?

 

[d55dave]

First, you need to understand that the idle mixture screws are just that - they only control the mixture at idle. Once the throttle blades are open off idle the idle circuit is out of the picture, so the drive-ability problems are not affected by the idle mixture screws.

Driving fuel mixture is controlled solely by the jets in the metering block, and is augmented by the power valve when required (you open the throttle, the vacuum drops and the power valve opens to richen the mixture).

Float level is also important and can have an effect on the driving mixture.

Your 3310 should be a vacuum secondary carb. Your part throttle hesitation will likely be one of three things - not enough or too much accelerator pump shot, secondaries opening too soon, or a power valve that is not opening.

Out of curiosity, what is your initial and total timing set at? It can have a big affect on drive-ability.

The black smoke during acceleration suggests an overly rich condition, likely the result of too much accelerator pump, or a power valve opening too early.

I would suggest the following;

1 - ensure that the front and rear float levels are set properly. The level should be just at the bottom of the hole in the fuel bowl (your holley will have either a sight glass or brass plug that you remove to check this). If you need instructions on how to set the level, ask and we will help you. Be careful checking it on a running engine as fuel leakage will likely occur and there are sparks going on in the distributor!

2 - ensure that the accelerator pump is working - with the car shut off look down the throat of the carb and open the throttle - there should be a good stream of fuel coming out of the pump discharge nozzle when you open the throttle.

3 - try disconnecting the vacuum pot from the secondaries (or wire them shut) and see if the hesitation goes away.


try these things and let us know what you find.

 

[RustyRatRod]

You also have a terribly mismatched intake manifold. BUt I don't think that's what the trouble is.

Get a real vacuum gauge and get some readings and go from there. You might not have the vacuum that "mystery gauge" is showing.

Oh and "RV cam" covers a lot of territory. Specifics would be helpful. Even a description of the idle.....a video with good audio perhaps? This IS 2015 after all.

 

[skycuda67]

You also have a terribly mismatched intake manifold. BUt I don't think that's what the trouble is.

Get a real vacuum gauge and get some readings and go from there. You might not have the vacuum that "mystery gauge" is showing.

Oh and "RV cam" covers a lot of territory. Specifics would be helpful. Even a description of the idle.....a video with good audio perhaps? This IS 2015 after all.

I will be picking up a vacuum gauge ASAP, and i would have included more details if i had them sorry, I ended up buying this rolling block from a well known mopar guy in the area to get the car on the road this summer while i spend time building a 360 how i want, all he knew is it had an aftermarket cam, got it from a buddy, i took the pan off before installing the motor to give it a look over and saw it had aftermarket pistons but dont know specs, just .030 over. i decided to just bolt on the parts i already had with plans of keeping an eye out for better heads to swap on this summer, then move over to my future build when ready. anyways i remember that when doing timing it backfired through the carb a couple times and I just tested the PV, in park at idle i could turn the a/f screws all the way in and the motor did not die, if i read things right that would indicate a blown PV yes?

 

[d55dave]

That could also (and more likely) indicate not enough initial timing, so the idle speed screw is turned in too far causing it to idle on the main metering circuit. You can verify this by looking down the throat of the carb while it is idling. If you see fuel being pulled through the Venturi's it is idling on the main circuit.

Increasing the initial timing speeds the engine up, you can then turn the idle speed screw out to slow it down and it will start idling on the idle circuit. Once this happens your idle mixture screws will be effective.

 

[rednesss]

Or that your butterflies are open so far, that it is actually still running on the transition slots and not the idle.

 

[Locomotion]

While the intake may not be the main cause of the problem, it can make it difficult to tune the carb because of the mismatch with everything else. A good dual plane intake will make it more responsive. In theory, the big intake will reduce the vacuum signal to the carb and can require bigger jets, squirters, lower power valve and lighter secondary spring than with a dual plane.

OEM primary jets were 72 and the secondary plate is equal to a 75 or 76 jet. That's what you would start with if you have/get the secondary metering block conversion. OEM power valve was a 6.5 and squirter was a .031. Maybe the pump cam is too big and make sure the power valve isn't blown. I don't recall what pump cam they came with.

 

[thesiren74]

Check the power valve my had the same problem and turned out to be bad.