305 engine issues

[trac209]

Im absolutely stumped here,I have a 305 in my winter driver and it's giving me fits to no end and I'm out of ideas. In short all of a sudden without any changes made by myself it has been bogging and stumbling and rough idle,and it seems intermittent. I first looked at carb all seemed fine fuel filter,choke etc, then took a look at hei cap and the carbon button was broken off and just fell out when I lifted the cap. So I installed a different cap rotor and After a short ride it cleared up and was running super smooth and without a hiccup.

That was yesterday,went ice fishing today about an hours drive away and it ran like new the whole way there. Then on way home it started doing it all over again. Bucking and surging on takeoff and only ran decent if I could stay on the gas and even then it was bad.

I'm completely lost at what this could be due to its intermittent nature,the only thing I could think of ice in the fuel line but once you can step on it hard it will accelerate hard, once you try to cruise its crappy. Any thoughts or ideas would be super helpful,thank you

[Warrens69GMC]

sounds like a vacuum leak. When you are on the gas vacuum is lower, hence it running "better". With cruising or idling, vacuum could be its highest, giving the leak somewhere to go and causing the bucking.

[trac209]

Yeah I've tracked down all the lines and they are good plus if that's what it was I don't think it would be intermittent

[Warrens69GMC]

Areas i've found vacuum leaks when all new rubber vacuum line were new-

pcv valve grommet
valve cover gaskets
egr valve
vacuum advance canister
vacuum reserve tanks
booster
carb mounting gasket

[kwmech]

Did you retrieve all the pieces of the carbon button?

[James the III]

Check for sticking egr valve..

[trac209]

Yes I got the carbon button out and egr valve has been blocked off for years. After some research it seems the power piston could sticking and causing issues with the primary metering jets which would have symptoms I'm describing. I'll check this out and report back

[James the III]

Originally Posted by trac209

Yes I got the carbon button out and egr valve has been blocked off for years. After some research it seems the power piston could sticking and causing issues with the primary metering jets which would have symptoms I'm describing. I'll check this out and report back

Blocked off welded or plate bolted down..?
If the latter I'd check for leak .

I'd also check for vacuum leak at the throttle shaft..
Is the 2 secondaries vacuum diaphragm and linkage that controls them working .
It could be the base plate plugs in the carb bottom leaking..

But you replaced the cap and rotor and it cleared up.. Did you do the lightning test on the plug wires (at night spray the wires with spray bottle and look for a lightning show..)
You moving them to do the cap might have moved a grounding plug wire away from the spot it was jumping spark to, and the wire(s) slowly made it's way back to the form/shape it was in before you did the cap and now is grounding out the spark.
I'd also think if the cap carbon button was broken.. there most likely carbon dust inside the distributor housing and on the parts.. the spinning action could put that in the air and you might find carbon tracking on your new cap or rotor or both.

[trac209]

Every issue you've described has been addressed,wires are new and not arcing out,egr is blocked off with plate has been for years. A vacuum leak is not the issue here. Carb sprayed at idle and not leaks are detected anywhere including throttle shafts,vac leaks don't just come and go. Distributor was all cleaned and that as well. Ive done everything you'd look for at first that's why this is so perplexing. Checked the power piston as well and it's working but doesn't mean it can't stick. This is the only thing I can think of that has these exact same running symptoms that can come and go. I'll check distributor again though

[James the III]

Originally Posted by trac209

Every issue you've described has been addressed,wires are new and not arcing out,egr is blocked off with plate has been for years. A vacuum leak is not the issue here. Carb sprayed at idle and not leaks are detected anywhere including throttle shafts,vac leaks don't just come and go. Distributor was all cleaned and that as well. Ive done everything you'd look for at first that's why this is so perplexing. Checked the power piston as well and it's working but doesn't mean it can't stick. This is the only thing I can think of that has these exact same running symptoms that can come and go. I'll check distributor again though

The air door for the 2nd flapping in the breeze will do this.. failing diaphragm or busted spring..

Sticking advance weights,,

a die'n coil intermittent short

heat stove air housing door flapping in the breeze
exhaust block off door to heat the heat stove when cold if at exit of exhaust manifold.. and flapping in the breeze..
cyl can't suck in fuel/air mix if the exhaust is blocked..

Thinking you got all the carbon out of the housing.

New wires don't mean not faulty ,
sunk or sinking float allowing fuel to push out the vent.. once the fuel pump catches up..

loose batt power connection at the distributor connector ..
tach wire short intermittent short
Sticking pcv valve

The fun of an issue that comes and goes..

[trac209]

Update,Well ffw to today without touching anything first I wanted to see if there was any change from cold to warm engine. Started it up and it ran with a bit of a stumble as it burned out the lubricant I added to the power piston to see if it was indeed sticking. Engine was running good and strong but as the engine warmed up enough for the heat out the heater to be hot the engine started running worse and worse. It should have occurred to me the times this started happening was at higher speed on the highway that this might have been a heat issue but it hadn't.
This time and not wearing gloves I noticed the air cleaner was super hot and also the thermostatic switch for the air cleaner damper door was still closed,therefore all the air coming into the carb was super heated. I let the engine cool down all day and with the thermal switch disconnected I went for a little ride and it was good the whole time,in fact the more I drove it the better it got. I would guess the issue was the carb boiling the fuel at high speed and it would take quite awhile to cool down hence the intermittent issue since most of the test drives and errands didn't get the fuel hot enough to start causing issue,plus the various outside temps we have been having could mask the symptoms. Weather or not this is still just another coincidental fix still remains to be seen as it needs a good 30 min higher speed trial. Thanks James you were on to something with the door idea and all who chimed in with ideas,much appreciated

[James the III]

I sadly have been there done that..
Was running bracket race at the 2002-3 super shevy show and was 2 rounds from getting money and a new set of BFG'S and the car fell on it's face.. on drive home it ran like crap.. hunted for the issue for a week.. after trying yet another idea, I drove it with no air cleaner housing, ok drove great,, power back.. drove an hours, good to go, get home put air cleaner and hoses back and left for a cruise /show and it in 15 minutes was back falling on it's face..
took the air cleaner off and just happened to be holding it the right way to see the flapper door closed.. sure enough.. drove to show/cruise with the air cleaner lid upside down and no issue.. put lid on correctly and the issue was back..
The flapper door left the building and happily ever after..

[trac209]

Well back to square one it's doing it again,was good today for a bit then bad. I completely give up this thing is being retired lol

[68gmsee]

Hey, trac. Can you give us a little more info on this 305? The year, is it stock or modified, mileage, carb, etc.. Sometimes it helps knowing that info to try to isolate a stubborn problem.

[trac209]

1985,bone stock with qjet. I'm thinking whatever is going on has to be carb related although the carb isn't old. I changed fuel pump,all plugs again today. Even did a compression test and all cyl were above 150 psi. It was drivable as long as you didn't get stuck at lights in traffic and had to slow down. Once at 50 plus mph it was ok but unresponsive and has huge flat spots in the 750-2800 rpm range. Must be a piece of trash somewhere in the carb causing havoc.

[68gmsee]

I'm sure you've checked to insure flooding is not the problem by checking for exhaust smoke when failure occurs but the ignition problem is somewhat harder to detect if it's intermittent.

The ignition problems could be a loose wire or connector terminal at the starter, internal ignition switch wiring (burned/corroded/loose) and even defective battery cables. The battery cables can corrode internally and the corrosion is not visible on the outside.

The things to check on the distributor is also loose corroded connectors or even a faulty module.

I don't know if your engine has the ESC (electronic spark control module) but it's also been known to cause intermittent stalling, rough idling, etc.

[trac209]

Distributor has been changed I even by passed the starter positive by connecting distributor to battery. Not running rich, plugs are lean if anything. I'm thinking it is the carb and I will be cleaning it. And trying that.

[whitedog76]

I changed a cam in a 305 a few years ago. Several lobes flattened. The new oil out isn't lifter friendly.

[James the III]

Is the tank sock gettting sucked up and blocking the fuel from the tank..

[trac209]

Originally Posted by James the III

Is the tank sock gettting sucked up and blocking the fuel from the tank..

Well I thought that too,fuel flows freely from tank. I used some fuel system cleaner,then blew out the passages out and it's running excellent again. There must be some form of build up that was lodged in there. I added another inline filter and we will see how long it holds.

[YoungPup1977]

sounds really weird...but check compression please...not going to tell my 305 secret quite yet....

[James the III]

next time it does this, don't shut it off,, keep it running, and remove the fuel/tank cap.. and drive, does it go away?

[68gmsee]

Originally Posted by James the III

next time it does this, don't shut it off,, keep it running, and remove the fuel/tank cap.. and drive, does it go away?

I thought about that possibility also, but trac says in one of the posts above that it runs okay when holding steady at 50 mph. It would certainly fail then if it was a gas tank vacuum problem.

[James the III]

Originally Posted by 68gmsee

I thought about that possibility also, but trac says in one of the posts above that it runs okay when holding steady at 50 mph. It would certainly fail then if it was a gas tank vacuum problem.

Don't be so sure of that.. the pump at that engine rpm might be able to suck hard enough to have the vent seal leak.. or suck through the sucked up the line tank filter sock

[68gmsee]

Originally Posted by James the III

Don't be so sure of that.. the pump at that engine rpm might be able to suck hard enough to have the vent seal leak.. or suck through the sucked up the line tank filter sock

That certainly is a possibility. Wouldn't hurt to try it.