New Carb Choice?

[Omie01]

I haven't posted in a while because I have been enjoying my Mustang this summer. No doubt it is crazy fast, ( I have never seen a speedometer move this fast!) But one little hiccup now, I just can't seem to find the perfect idle/cruising setting for my Pro Systems carb. It either has high idle, or I get a bit of a hesitation upon initial acceleration. I have a perfect 15Hg vacuum, the timing is spot-on, everything is working perfectly, plugs are even the perfect color. I have been back and forth many times with timing/curb idle/mixture tuning and it seems like if I change anything it runs one way, then I re-tune it runs another, no in between, its either high idle, or slight hesitation upon initial acceleration. Any idea's?

 

[will e]

Can you describe your timing? What is you initial? Are you running ported or manifold?

 

[Don C]

I had the same issue many years ago on a Ford pickup I took in trade for installing a sbc in a Land Cruiser for a neighbor. It turned out to be the bushing in the distributor breaker plate was worn oblong and every time the vacuum moved the breaker plate the point gap, dwell and timing changed. Whenever I have a driveability issue now that is one of the first thing I check.

 

[c9zx]

What are you adjusting to transition between the two conditions?

 

[Omie01]

I am sure the dizzy is in good shape, I checked everything. I am mostly adjusting curb idle and timing, I have set the timing everywhere from 16deg to 21deg, that does not seem to have much bearing other than idle speed, it seems to run well anywhere in that window for timing, if I bump up the advance and drop the idle to its lowest point where it still runs smooth the idle is around 1100rpm, if I drop the advance to the lowest point it wants to run at, and set the idle down to around 800rpm, I get the hesitation in transition from idle to acceleration. Its very driveable either way, and the hesitation is minimal but I would like to get rid of it. This carb is just very finicky to the smallest adjustments.

 

[c9zx]

I don't recall your cam specs but I seem to remember it is a healthy cam. I'd set the initial timing to 18 BTDC and leave it there (limit total mechanical to 36). Depending on the cam and compression 800 rpm may just be too slow. I'm assuming it is a DP with 4 corner "mixture" adjustments. If so, remove the carb, drain it, and turn it upside-down, and measure how much of the transfer slot is exposed (a very narrow vertical slot in the base plate at the edge of the throttle blades). Set both primary and secondary idle screws so that about .025 of the transfer slot is exposed. Set the mixture screws to 1 turn CCW as a starting point. Tune for best vacuum with the mixture screws, leave the idle screws where they are. Record the vacuum, mixture screw position, and idle RPM. Drive the car. If the slight hesitation is gone, it may have been caused by not having enough of the transfer slot exposed (there is a reason it is called a transfer slot). If the idle speed is still to high, the idle channel restrictor and/or the idle air bleeds will need to be changed (this assumes they are changeable since it is a Pro-Systems carb). The direction of the changes depend on what readings are recorded. Having written all of this, it occurs to me that I should have just said have all your information ready and call Pro-Systems to get their recommendations. Let us know what you find out. Chuck

 

[Hemikiller]

What engine and which carb do you have? Have you tried increasing the acc. pump discharge nozzle size, or swapping in a more aggressive pump cam?

 

[Stanglover]

I have no idea what your carb is, but may I offer this bit of information.

I have literally just returned from a local performance shop after having them look at my Holley Street Avenger 670. I was experiencing much the same problems. I just could not get it to idle right, it was sooting up the plugs running around town, but okay on the highway. I just got to the end of my tether and decided it was time for the pros to look at it.

The carb is only 2 summers old, The engine is a 351C 4V 4 speed with a slightly upgraded cam, nothing wild by any stretch and a fresh rebuild. My timing was a bear to get right, but I have it spot on now. (I'll post on this later) The problem was I was messing with it way too much. I had the idle transfer slots too far open and had no control on the mixture screws. I was trying to get idle by using the throttle stop screw, wrong thing to do as I've learned. Anyway in this case, what it really needed was the drilling of a 3/32nd" hole in each throttle plate. It needed more air. Now the tech was able to reset the idle screws, get the transfer balance exactly right and bingo. It now runs better than it has ever ran. No smell of gas or black smoke. The response is seamless and it pulls like a bear. Now I'm a happy camper!! Perhaps this is what your carb needs especially as you have a non-stock cam.

All the best with it and let us know when you get it sorted.

Geoff.

 

[Omie01]

c9zx, thanks for the info, I am going to try that. This carb already has the holes drilled in the primary plates, I was actually thinking that may be part of the problem, maybe a little TOO much carb for the engine, BUT......it runs so well once off idle, and as I said I have never seen a car accelerate like this thing does!!! To refresh, cam is Lunati HR, .594In, .611ex with 112Lsa if I remember correctly. I will post results soon! Thanks for the responses everybody!!