The Stovebolt Site Forums The Shop Area Engine Shop 305V6 Idle problems...what else is new?

For the past couple of months, I have been trying to figure out the problem with My 305 V6.

The previous owner had rebuilt the distributor, with a new cap, rotor, points, but no new condenser. I got a hand-held tach/Dwell meter, and I adjusted my RPM to 525 like the book says. The dwell reading, according to the book is suppposed to be within 31-34 on dwell. When I checked it, it was 26! I adjusted the dwell to 32, and the engine runs a little better. At idle, the engine speeds up and slows down by about 25 RPM. I bought new plugs, new wires, and it still runs badly. Sometimes it backfires through the carb and out the tail pipe, so I adjusted the timing using a vaccum gauge and my hand-held tach. I loosened the hold down bolt for the distributor, and with the engine running, turned it until it started running rough. I then turned it the other way until it again ran rough. After monkeying with it, I finally got it to run a little better, but it is still unsteady at idle. According to the vaccum gauge, the vaccum reads 15, which is right on "Late valve timing" Could it be the valve timing or something else? I also adjusted the idle mixture, and it is at the best I could get it. I recently rebuilt the carb and put new intake manifold gaskets in.

I have to say, I am baffled, my dad thinks it needs a valve job, and Im starting to agree with him, unless you guys have other advice or suggestioons? Help? :mad: :mad: :mad: :confused:

gmcfan: Could be one of several problems, or possibly more than a couple going on at the same time. You will have to check to eliminate the sources. Trouble could be from late valve timing (chain), but unlikely. You can verify with a degree wheel. The most likely source of your troubles, are the items that have been adjusted/disturbed/repaired as of late.

Possible vacuum leaks, in either manifold gasket area, or in carb body/manifold mounting. This would cause the off idle"loaping",and the backfiring, and would make tuning impossible because of air leak, leaning mixture at idle, then richen as RPM increase (as vacuum lessens). So the mixture "bounces" all over the place, as the air to fuel ratio cannot balance.

You stated that PO had distributor repaired, possible troubles here also. Need to check advance weights for sticking/binding, and condition of springs. Check vacuum diaphragm for leaks and function. Also check breaker plate for free movement with advance. Check all electrical connections also.

With cylinder leakdown test, and vacuum tests, you should be able to determine if you, in fact, have vacuum leaks, and be able to locate them.

Need to set dwell, then time with timing light.

If you have vacuum leaks in carb body, can/will throw off balance idle mix, both ratio and volume and this will make run lean, and hard to idle. Check fit of horn gasket to float bowl housing for nicks, burrs. Need a good seal. May have to disassemble, place on sandpaper/on glass, lightly smooth burrs/irregularities. Coat gasket with bootgrease, or chapstick for better sealing.

Also if you can get to shop with scope, can set up plug firing in parade/stack patterns to verify spark firing/voltage amounts and check condition of ignition components, to verify all are good working order.

Repost your findings. Good Luck!

Think Chevy! :rolleyes:

What is your exhaust like? If it is plugged up...

While you're checking the distributor, also check for slop in the shaft. For some reason, the V-6's I drove were known for shaft wear. Idle kept changing, it would run then quit, backfire. One that had been shimmed one morning while we waited for a new one put a rod out the block when it backfired wide open on the highway.

Erractic idle and backfire sound like a bad set of points to me. If owner went to the trouble to rebuild distributor and not install a condensor, I have to wonder about the quality of the points and the rebuild of the dist.

A set of GM points and Condensor are cheap enough. Don't cheap out and buy a bargin brand. I used to work in several discount Auto repair places (national chains) and the points we sold were labeled "DELCO" but were low quality crap, If you really want to have a dependable Truck go to NAPA and buy a set of Echlin or other qualiy set of points and condenser. It sounds like you know how to adjust them. installing them is a snap. Dont forget to lubricate the point rubbing block, or dont bother. special lube should come with points.

Set your tiiming with a lignt. It is the only way to guarentee the timing is set corret. By the way, incase you dont know, changing point adjustment (dwell) changes timing. but not the other way around. Be sure to set point dwell first.

If still idling ruough, try choking carb with your hand or rag, If idle speeds up and smooths out, you are lean or have vacuum leak. Back fire thru carb could be vacuum leak, timing, condensor/points problem, advanced timing.

Let us know how it goes.

okay, heres an update. It is still running badly, backfiring, and missing.

I took the distributer apart, and it turns out the condenser is new. From the angle I was looking at it looks dirty. The points are Echlin, and are in good shape. I cleaned the scale off of the points, reinstalled and adjusted. It still runs rough. I adjusted the the fuel/are mixture to the best I could. I listened to the carb, and I can hear the gas spraying out of the jets, but it doesnt sound uniform. Everyonce in a while it sounds like air is escaping from the jets. It will spray, then make a small popping sound. It is still idling up and down, and I opened the throttle and let it run at about 1200 R.P.M, and the rpm is still wavering a bit. I put a peice of card-board over the throat of the carb, and it didnt die until there was only a little tiny bit the air could come through, so I figure I got the mixture right, but it sound erratic. What can I do now? I cant do anything major right now like getting the valves reground or anything, because I am out of work and money is slim. Please help, this thing is really ticking me off.

If you are blocking off the carburetor with cardboard until only a small sliver is showing, is the engine speeding up as you block off more and more of the throat? If so, you are lean. If not, then the mix is correct or rich. Easy to tell with that test if it is lean, not so easy to test for rich.

The sounds you describe (hiccup in sound of air entering carb) sounds like a intake valee leaking. Go by a cheap vacuum gage from Auto zone. Around $10. It will come with a paper detailing about 30 tests you can run with one simple hookup. It will even test your fuel pump.

Anyway,. hook it to the intake at a vacuum source. (Must have vacuum at idle) Needle should read around 18-20 inches of vacuum at warm idle/sea level. If needle has a constant pulse downward you are seeing the results of compression leaking past a valve into the maniifold. Quck, easy, accurate test. Wont fix the valves but will confirm or disprove the idea.

Curly, After I read your reply, I remembered I have a vaccum gauge, and I hooked it up. I started the engine and made sure it was at idle, and I checked the gauge. The gauge sits rock steady except for when the engine idles up and down by itself. The gauge registers 15, which the gauge indicates as late ignition timing. is this right? or could I have a vaccum leak that is making my reading inacurate? I hooked the guage up to the vaccum port on the intake manifold. no matter which way I turn the distributor when I have it running to change the timing, the gauge stays at 15. :mad:

Well, 15 does sound low. It could be ignition timing, but what sounds strange is the fact that the vacuum doesnt change with timing. Advancing timing will raise vacuum, retared will lower it. Is it possible you are hooked into a "ported" or "timed" vacuuum source that is affected by throttle position? Also, higher altitudes affect vacuum, but not that much.

Are you sure the gauge is hooked to intake manifold, which is the correct spot? . Ported vacuum is found on the carb, not intake manifold. Hard to give long distance help.

Can you give me some more clues? What happens to vacuum if you snap throttle? You say vacuum is rock steady except when idle changes. What does vacuum do at that time? Increase? Decrease? Choke engine by hand, what happens to vacuum? Lets try and figure this out.

I definatly have the vaccumn hooked up to the intake manifold. I tried what you suggested, I snapped the throttle open. The needle on the gauge shot to zero, the engine almost stalled, and then settled back into its usual irregular idling. I tried the snapping the throttle again, and it again almost stalled, and then it backfired through the carb, which scared the crap out of me. When I slowly rev up the engine, the gauge needle slowly heads towards zero, and when I stop at any certain throttle position, it settles back to 15. I didnt get around to choking the carb by hand, it was getting late and it was hard to see the gauge in the dark.

oh yeah, I almost forgot. I keep forgetting this, but what position does the exaust bypass valve sit in when the engine is warm-does the weight point up or down? The spring is broken on it, and the thing is pretty stiff, and it barely moved when I try to push it down with my hand.

Sounds to me like your butterfly valve is rusted in place, possibly shut or darn near. A friend of mine had a Pontiac motor that was running like crap. They pulled it to replace it, and discovered the valve was rusted shut. All their exhuast from the passenger side had to travel through the motor and out the drivers side. When they freed up the valve the motor ran like it had been rebuilt. If your spring is broken you should be able to move the valve by hand. Hope this helps?

(Their motor was a v-8 with single exhuast)

Hey, there is a post on page two of this forum by C1534 that asks about the heat riser thingy being stuck or what position is best. The general concensus is it ain't a good thing to have it stuck shut. Something about crappy running and detonation. Sound familiar?

okay, im going to add more of what I have done to my engine before it started running this way. I bought a carb rebuild kit and I rebuilt the carb with a new float needle, all the gaskets, and a new accelorator pump. Everything fit perfectly, except for the gaskets between the spacer and the carb/intake manifold. They were a lot thicker than the origionals, when this changed the spacing of the carb from the intake, could it change the way the engine runs?