17057253 carb not idling or revving

[Seff]

Refurbished a 17057253 carb with a HP kit and new jets/rods, story outlined here, yet it's not working very well for me.

Now, the way I've understood that a carb should work (bear with me, this is the first carbed car I have known) is that you prime the carb at least once to set the choke, crank and the engine springs to life, quickly gaining idle speed until you touch the pedal and the choke pull-off and/or choke coil allows the thingy to come off the steps of the fast idle cam, partially or totally, depending on vacuum/heat in the exhaust crossover.

Now, what the 17057253 carb does, is that I prime and crank, and the thing ROARS to life, running way above even fast idle, and doesn't kick down the steps of the fast idle cam when manipulating the accellerator, even after several minutes. I can see the choke coil works and expands, because the choke cam/lever/flap opens further than the choke pull-off would make it. I can get it to drop off the fast idle steps just fine on the bench, but not on the car.

When I manually push the thing all the way off the fast idle cam, the idle surges (low for 1 second, high for 1 second, ad nauseum), and a ticking sound emerges (or the engine got quiet enough that I heard it) that doesn't seem to exist with my '71 4MC.

When I rev the 17057253 hard, it spits what looks like a mist of fuel up the top of the carb.

I'm perplexed.

[omaha]

...I am thinking that the choke needs to be taken out of the equation so you can determine what is going on.
Loosen the choke housing and turn it so the choke is in the open position. Also back off the chokes fast idle screw.
Now start it up, you may have to pump the accellerator a bit till it warms up. If it still has issues  you need to deal with them before you go farther. It is possible that there is some air bypassing the throttle plates or maybe the vac advance is in the wrong vac source (you can block that off too temporarily). Since the choke is only needed during warm up, it will be advantageous to not have it on during the initial idle adjustments of the carb. Then after the engine can idle on it's own,(while it is warm) then you can go back and do all the choke adjustments (which by the way, are numerous). There are more adjustments for the choke than any other adjustment on the outside part of the carburetor. Use a vacuum gauge to find the best idle, timing must be right also. The popping has me thinking that somehow the timing is off, like too far advanced. If the vac advance line was in the wrong vac source(on the carb) it could possibly put too much advance in it at idle which WOULD raise the idle speed (somewhat). A good vacuum leak could also cause some strange things to happen. Also, one mistake I did one time was I put the choke flap actuating rod in upside down. That really screws up the function  of the choke. (don't work at all almost). Basically, you are trying to start out with the fewest possible variables and once you get one dialed in (mostly vacuum at idle and the timing at idle. Both go hand in hand.)Then you can go on to the next tuning variable. If you have the screws so that they function and you don't have any significant vacuum leaks, the thing should idle. Plug the vac advance while you are finding the right idle with the screws and the timing. After, reconnect the vac advance to the correct ported source. Basically you need to eliminate any variables one at a time before you go on to the next. Best of luck and keep plugging away at it....good luck!!

[Seff]

Alright, I'm setting timing again, might as well. Doing all of these steps, might as well.

The vac advance plugs into the transmission vacuum advance, which is also hooked up to manifold and to carb. I'll take a picture.

[Seff]

Timing read at 50 BTDC with HEI and vacuum advance disconnected and plugged, idling. Set it down to 38, runs much smoother now. This is all with the old carb, mind you. Haven't had the time to put the refurbed one on again.

The DVCS module has front driver's side of carb connected to the top, back of carb connected to the upper side port, distributor attached to the middle, and manifold attached to the bottom. Transmission is connected directly to manifold.

[Shark Racer]

If your idle RPM is less than 2500, you're probably running a bit too much advance.

Are you running "locked timing" or do you have mechanical advance? If you're running mech advance, your timing should be quite a bit lower than that...

[Seff]

Running an aftermarket HEI, it has mech and vac advance. If I turn the dist in either direction any considerable amount, idle drops and/ior becomes choppy. Don't have a tach, so I don't know how high it's running.

[Seff]

It was running 50 degrees at idle, so I took it down to proper levels and the carb runs much better. Needs dialing in once I get back to my actual workshop, including tip-in. Maybe I'll get above that 14 MPG I've been getting so far.

[Cliff Ruggles]

Initial timing MUST be set at idle speed with the vacuum advance unhooked and line plugged.  Also verify that the mechanical advance is NOT adding any timing at idle speed.  Base timing for most engines will be around 8-12 degrees.

The HEI may add a LOT of timing at idle if a manifold vacuum source is used to apply it.  This will increase engine rpm's, and may cause difficulty in setting the idle speed at normal rpm's.

I use and recommend ported vacuum for most set-ups.  Using manifold vacuum often requires that the advance be modified so it doesn't add a lot of timing at idle speed.

For most well thought out engine combinations, we limit the advance unit to 10-14 degrees, and almost always apply it with a ported source.....Cliff

[Seff]

Currently the HEI vacuum is connected to the DVCS.

[Seff]

Is it better to use the DVCS or to use ported vacuum? I have a spare ported vacuum port left over from the secondary choke pulloff.

[Cliff Ruggles]

A good ported source directly to the vacuum advance would always do exactly what it needs to.  No timing at idle/coasting, and FULL timing from the advance right off idle, light throttle cruising, etc.....Cliff

[Seff]

I'll try that. Got it idling lower by adjusting the right screw. >.>

[Seff]

I checked, the DVCS connects to ported vacuum, so the result is the same, I think.

She starts and idles very well now - doesn't smell fat like she used to when idling and warming up. Tried to adjust the ATP via tip-in, but without a tach I'm having a hard time hearing those 50-100 RPM rises or drops. I'll see if I can get one.

[Seff]

Alright, I found a tach and got my idle speeds right. My choke works well, and it drops to lower revs when it should. Timing is 24 at idle for an aftermarket HEI with the vacuum disconnected.

I want to adjust my APT, but I'm unsure if tightening or loosening the screw will lean the part throttle mixture. Seems such a simple thing, but I'm not sure. Logic tells me that tightening the APT will lean it out, since that allows the main rods to go further into the jets. Is this correct? The reason I'm doubting it is that my mileage got worse after I tightened it the first time around.

[Cliff Ruggles]

24 degrees timing at idle is WAY to much.  Most engines will like around 8-12 degrees.  Some really low compressin engines with big cams in them may want a bit more, around 14-18 degrees, but 24 degrees is too much, and the total timing will be too high at full throttle.....Cliff