Need Help With My Truck - Stalls When Stopping

[Gearheadtom]

Okay, so as the title says, I've been having trouble with my truck dieing when I'm slowing down and stopping.
First off, I'll give a little background on the truck, and what I've done so far. It's an '86 GMC with the 305 and rochester 4bbl carb.
A few months ago, they starting putting 10% ethanol in the regular gas here, and after I started running that, the truck started dieing when I was slowing down, so I switched to supreme, and all the problems went away.
About a month after that, my truck started running overall horrible, determined it was a carb problem, and dropped on a rebuilt carb since I had no time to rebuild mine, had the carb tuned by the shop I got it from for free, and my truck worked perfect, until about a week ago...
I was comming back from the camp, needed gas, and had to put in $15 of the 10% ethanol gas to get home, and my problem of the truck stalling when I slow down came back.
I've put over $50 of supreme in since then and the problem is still there.
So, I've checked for vaccum leaks, found none. Once the truck dies, I can give it one pump, it'll fire back up and stay idling in gear fine, and if I give it a bit of gas when slowing down it won't die. I've tried slowing down in neutral, and it doesn't die, but the rpms drop, the truck spudders, then picks back up. Plugs, fuel pump, cap and rotor only a few months old, wires only about a year. Idle speed (when my tach was working) is about 1000-1100 in neutral, and about 900 in gear.

I don't know what I should check next. Anyone have any ideas?

[SWIGIN]

I drive a 79 bronco and when we first got that 10% crap it ate my rubber diaphram in my mechanical fuel pump.

[blue27]

also check rubber fuel lines, it eats them up too

[RIDE-RED 250r]

I have seen a bad distributor (the whole thing) do that on an '88 TBI 350 in a Blazer. Dropped in a distributor that was known to be in good shape, problem went away. But your situation does sound fuel related.

Is there an in-line fuel filter? My inclination is yes. If so, I would replace it. Should only be about $10

[coolpool]

I'm going with Swigin on this one. I'd replace the fuel pump. Probably the cheapest fix to try.

[Gearheadtom]

I replaced the filter just a day or two ago, and I don't think it's the pump because it runs great other than it dieing. It revs fine, and never dies when I'm just driving along or accelerating, even when I've got it on the floor with all 4 barrels of the carb wide open haha.

[blue27]

get a gauge on it and see.... if you have pressure then you will need to try something else. you have to start somewhere. It could be ignition box that is getting weak or a pick coil in the distributor that dosent like to be hot and losing voltage when hot.

[rjs89ia]

does it sound like its missing or coughing at idle, because if so id look into changing the intake gaskets my engine was doing the same thing it would always chug and stutter and it always seem to be the intake gaskets. unless you've got a set of small chamber 305 heads i wouldnt see why the fuel would be an issue, if your engines not built or you dont have a hi-po cam you shouldnt be idling so high either ive got mine set around 800 and thats with a medium performance comp cams xtreme 4x4 cam and an estimated 9:1 cr

[big red in iowa]

id check for a vac leak at the carb

[Gearheadtom]

Once I'm stopped the truck idles fine, no spuddering or anything. I'll check my vacuum lines for any leaking again, and wd-40 test the carb gasket for any leaks.

[Gearheadtom]

I checked for vacuum leaks, found none around the carb. I checked the vaccum advance on the distributer by just sucking on the hose to see if I could build vaccum and I couldn't. Started the truck up and sprayed some carb cleaner (the same stuff I was spraying around trying to find any leaks) into what looks like a vent hole on the vac advance, and the truck died down and picked back up.
I don't know if that means there's a hole in the diphram for the vac advance, or if it's supposed to suck a bit of air through there anyway. Other than that, I found no other leaks.

[big red in iowa]

id say u just found your problem, replace replace the vac advanceand keep on truckin!

[Thorpe]

Advancer is shot...

[just ben]

plug the vac line that goes to the advancer and see how it runs.Not advancing wont cause it to stall like that but the vacuum leak caused by the bad diaphram will. Did you replace the quadrajunk with the oem emissions type? if so a bad o2 sensor will cause stalling when stopping. Is all the emissions junk still there? if part of the system is removed it will cause problems like you are having. When you push the brake the the engine still push? like if you were to brake with a manual trans without pushing the clutch? If it does that would be a bad torque converter.

[Gearheadtom]

it's got basicaly the same carb that was on it stock. Most of the emmisions stuff was unhooked. The only vacuum lines now run to the distrubuter straight from the manifold, one from the carb to the choke pull off, one to a switch on the firewall, and a 3rd from the carb to the e-vap canister, along with some vent hoses and stuff. The couple other vacuum ports on the carb have been plugged off.
When it was acting up before, all the emmisions stuff was still hooked up.
I think all the trans and stuff is good. It'll almost die sometimes if stop in neutral (spudders, rpms drop, then picks back up)
I'll pull the hose off the advance and plug it to see if that makes any difference
Thanks for the help so far everyone!

[Gearheadtom]

Plugged the vac line to the advance off and took it for a drive to a buddies place. The truck deffinitly worked better, but still died stopping for one red light. I'm thinking maybe since the idle is rougher with no vac advance, that it'll be less likely to die if I replace the vac advance to get the idle a little smoother, I know at least a new vac advance wouldn't hurt.

[HondaHarry]

Did you change the paper fuel filter element in the carb itself? There is a tiny rubber spring loaded check valve inside of it and they get stuck sometimes.
My 76 chevy did the same thing and the check valve was coming apart inside of it. they make a short filter and long filter, make sure you use the correct one.
http://quadrajetparts.com/rochester-quadrajet-parts-fuel-filters-short-paper-fuel-filter-with-check-valve-p-79.html HH

[Gearheadtom]

Did you change the paper fuel filter element in the carb itself? There is a tiny rubber spring loaded check valve inside of it and they get stuck sometimes.
My 76 chevy did the same thing and the check valve was coming apart inside of it. they make a short filter and long filter, make sure you use the correct one.
http://quadrajetparts.com/rochester-quadrajet-parts-fuel-filters-short-paper-fuel-filter-with-check-valve-p-79.html HH

Ya, that's the only filter on my truck that I can find. The new filter came with the new check valve, and I compared the old one to the new one and I'm sure I got the right one

[Gearheadtom]

I think I may have found the problem with that leaking vac advance. Drove to work and back today with the hose plugged, and the truck never died once.