Need help with engine running rough

[chicklin]

I've got a weird problem and this is my first 350, so I'm learning all kinds of new stuff.

1986 K10 Suburban 350 4bbl Quadrajet 700R4 auto

When I first got the truck, it idled really rough when cold, it would die unless you stayed on the gas. Once it warmed up, it wouldn't die but still idled rough. However, at all other RPMs it runs smooth as butter.

My first thought was vacuum leak, but I sprayed around everywhere and couldn't find anything. The carb was really gunked up and I discovered the choke was fried. So, since then, I've put in new plugs, a newly rebuilt Q-jet, and new fuel pump. I have the cap, coil and rotor, but haven't put them in yet. Has good oil pressure at idle (about 30 psi)

The truck is still doing the same d*mn thing. Pump it twice, it fires up, revs up and dies unless you hold it above idle. Once warm, it will idle, but rough. I've set the hot idle screw about where I want it. I'm not sure about the cold idle. I fiddled with it when it was cold and hot and it didn't seem to do anything, and anyway, it was doing the same thing with the old carb. I believe the choke is set correctly, about 1/8" from closed when it's dead cold. And, it's operating correctly b/c I can watch it slowly open to vertical. How do you adjust the cold idle and choke pull off (or do you)?

The other thing I noticed is that when it's idling it surges: vrooom..........vrooom........vrooom, all on its own.

Any ideas? What about the ignition module? Is it all self-contained in the dizzy on these, or is there an external component, too? It sure feels like a vacuum leak, but I just can't find anything. HELP!!!

[gchemist]

Close the choke completely. It should stay closed longer. Turn the choke adjustment a litle more (clockwise)The cold idle screw is on the opposite side of the carb. Carbs are picky. Carbs hate cold temps.

Do have a tach? Tachs are useful for trouble shooting.

Connect a vacuum gauge to it. Tune the carb with it.
How much timing are you running?
http://www.centuryperformance.com/tu...e-spg-148.html
http://www.secondchancegarage.com/public/186.cfm

[chicklin]

Originally Posted by gchemist

Close the choke completely. It should stay closed longer. Turn the choke adjustment a litle more (clockwise)The cold idle screw is on the opposite side of the carb. Carbs are picky. Carbs hate cold temps.

Do have a tach? Tachs are useful for trouble shooting.

Connect a vacuum gauge to it. Tune the carb with it.
How much timing are you running?
http://www.centuryperformance.com/tu...e-spg-148.html
http://www.secondchancegarage.com/public/186.cfm

Okay, I'll check the choke and timing tonight. With the choke, though, when the engine is hot and the choke is vertical it still idles rough, so it wouldn't seem that is the problem. As far as the fast idle screw goes, I know where it is, but it didn't seem to make an difference on idle speed. What is the proper way to adjust this?

[chicklin]

A couple other things:
- There is a definite miss at idle, you can hear it in the exhaust. It seems to go away when accelerating, but maybe it's just that I can't hear it anymore.
- Once in a while, I can hear a light tapping noise coming from passenger side of the motor. I've never heard a noisy lifter, so I'm not sure if that's it, but that's the impression I get. It doesn't do it at idle, only when I goose the throttle and seems to go away under load. Not sure if this has anything to do with the idle issue.

[chicklin]

What about the EGR valve, how do I test that? I reached back there and was able to squeeze the valve and it returned, so it doesn't seem frozen. Also, with the engine idling (roughly), I unhooked the vacuum hose to it and plugged the hose and the port on the EGR and that didn't seem to make a difference. Gasket, maybe?

[chicklin]

One other thing, on the new carb, I set the screws to 1 1/2 turns out from closed. Not sure if that's right or not, but that's always what I've done with carbs. It seems to accelerate fine, no backfiring or anything.

[Libertad]

I know it sounds dumb but check and make sure your PCV valve is all the way into your valve cover. I had the same systems as you are having and after pulling the carb off and checking for vacum leaks and no results, I finally noticed the worst vacum leak of all my PCV valve was sticking out.

[chicklin]

Originally Posted by Libertad

I know it sounds dumb but check and make sure your PCV valve is all the way into your valve cover. I had the same systems as you are having and after pulling the carb off and checking for vacum leaks and no results, I finally noticed the worst vacum leak of all my PCV valve was sticking out.

Checked the PCV valve (I had replaced it already). It's seated firmly and all the hoses going to that vacuum circuit are tight.

I replaced the coil, cap and rotor this afternoon. No dice, but probably needed done, anyway.

I'm leaning towards the EGR valve. It's pretty crusty and, while I can get it move when I reach around there and press on it, I don't seem to be getting much if any vacuum from it when I hold my finger over the port and goose the throttle.

I'm starting to run out of ideas here. The only thing in the ignition or fuel delivery system that I haven't replaced is the condenser and ignition module in the dizzy.

Will try and check the timing this afternoon, maybe get a vacuum gauge.

[donut]

Do you have power brakes?
A leaky booster had me chasing my tail until I thought to pull the vacuum line off that.

[chicklin]

Originally Posted by donut

Do you have power brakes?
A leaky booster had me chasing my tail until I thought to pull the vacuum line off that.

I do and I haven't checked that yet. The brakes work fine and hold when pressed. I haven't noticed any extra hissing noises other than the whoooosh when you press them, which I thought was normal. I'll give it a check.

[chicklin]

Picked up a vacuum gauge tonight and read both those links above. Very interesting. I think this is going to help a lot, plus it'll be good to know how to do a little more technical troubleshooting, rather than the "tinker, listen, tinker" method I usually use.

[chicklin]

Okay, hooked up the vacuum gauge and here's what I found. My vacuum averaged about 17 inches, which is probably a little high since I had the idle turned up. Anyway, it would intermittently jump around between 16-18 inches, sometimes dip as low as 15. It wasn't a steady drift back and forth, it was more of a ticking or jumping around in that range.

When I would slowly bring the RPM's up, it would smooth out and hold solid at around 20-22 inches (can't remember exactly). When I goosed it, it would quickly drop about 5-6 inches and then jump to the aforementioned 20-22 inches. Then, if I would goose it again and drop it off, it would jump to 25-26 and then back down to idle range.

So, the part where I played with the throttle sounds correct, according to the links above. But, the intermittent jumping around at idle sounds bad (Ignition miss, sticking valves, lifter bleeding off) and the fact that it smooths out with a little throttle sounds like maybe worn guides. Maybe it's a combination of things.

So, is my diagnosis close? There is definitely a miss, as I said before, but the whole ignition system has been replaced except for the module. But, it's a solid state module, right? Seems like it would either work or not.

Does this all sound like I need a valve job? How hard is that to do or how much would it cost? Any chance a little Marvel Mystery oil might help?

Thanks for all the help. I'm learning. I'm used to my 1954 Chevy 235, it's quite a bit simpler

[chicklin]

One other question: am I going to do any further damage to this thing if I continue to drive it until I fix whatever the problem is? I've got good oil pressure, so I wouldn't think it would just explode on me, but who knows.

[billnorman]

It's easy for me to give advice from here in my recliner. So here goes. First, you must know your compression. Chevy V-8's can mask cylinder misses because they run so smooth without load applied. Chevrolets are known for camshaft lobe failure, which probably won't show up on compression. Also, rocker arms have been known to fail at the valve and swallow the valve stem, but that's rare. I don't see anything about you changing the plug wires. They should be in the factory location and as far away from the exhaust manifold as practical. Mixture screws ought to be adjusted to where the motor runs best, one and a half turns won't run mine. A change in speed usually comes from air. Usually. If it's changing RPM at idle, you should be able to check that the timing does not change, disconnect the vacuum advance, idle speed is too low for mechanical advance to come in. Timing should remain at a constant setting. If it wanders around I would be suspicious of your timing chain. My nephew once brought a piece of test equipment to help me that counted the number of times each plug fired and compared them, it helped me find a shorted distributor cap that was fooling me. Your vacuum gage is a valuable tool, sounds like it's pointing out a valve issue, maybe a weak or broken spring or lifter. If you are near oil change, some Sea Foam added to your oil can really clean lifters and valve stems. Anyhow, the choke suggestion is right on. These motors need a lot of choke when cold weather sets in. Also, the secondaries must be firmly closed. No air past the butterflies at idle. Well there is my free advice. It's probably worth what you paid for it, but, heck, it's the internet. Hope some of this helps. Keep us informed.

[chicklin]

Originally Posted by billnorman

It's easy for me to give advice from here in my recliner. So here goes. First, you must know your compression. Chevy V-8's can mask cylinder misses because they run so smooth without load applied. Chevrolets are known for camshaft lobe failure, which probably won't show up on compression. Also, rocker arms have been known to fail at the valve and swallow the valve stem, but that's rare. I don't see anything about you changing the plug wires. They should be in the factory location and as far away from the exhaust manifold as practical. Mixture screws ought to be adjusted to where the motor runs best, one and a half turns won't run mine. A change in speed usually comes from air. Usually. If it's changing RPM at idle, you should be able to check that the timing does not change, disconnect the vacuum advance, idle speed is too low for mechanical advance to come in. Timing should remain at a constant setting. If it wanders around I would be suspicious of your timing chain. My nephew once brought a piece of test equipment to help me that counted the number of times each plug fired and compared them, it helped me find a shorted distributor cap that was fooling me. Your vacuum gage is a valuable tool, sounds like it's pointing out a valve issue, maybe a weak or broken spring or lifter. If you are near oil change, some Sea Foam added to your oil can really clean lifters and valve stems. Anyhow, the choke suggestion is right on. These motors need a lot of choke when cold weather sets in. Also, the secondaries must be firmly closed. No air past the butterflies at idle. Well there is my free advice. It's probably worth what you paid for it, but, heck, it's the internet. Hope some of this helps. Keep us informed.

Lots of good things to check, thanks. I will try and check the timing and compression tonight. Plug wires look good, but I suppose it couldn't hurt to replace them. Choke is set correctly now (i.e. it snaps all the way closed when dead cold). I'm also going to try some Sea-Foam or kerosene just before my next oil change and then run some synthetic and change it on short intervals for a while. Probably ought to pull the pan and clean it up, as well as the oil pickup screen.

[chicklin]

Changed plug wires today. No change.

[vtwinsport]

Youl'll want the choke to stay open about an 1/8 th of an inch or it won't idle upon cold startup. FYI.
Had that problem with mine. Had to readjust it as it was closing completely.
definately chack the vac advance on the dizzy. next weould be a vac leak.

[chicklin]

Originally Posted by vtwinsport

Youl'll want the choke to stay open about an 1/8 th of an inch or it won't idle upon cold startup. FYI.
Had that problem with mine. Had to readjust it as it was closing completely.
definately chack the vac advance on the dizzy. next weould be a vac leak.

If I'm reading the linked pages above correctly, I don't believe I have a vacuum leak, at least not one significant enough to cause it to idle this rough. If it had a vacuum leak, it should read consistently low on the scale, but mine is right around 16-17. The fact that it's dancing around in that range seems to indicate some sort of valve issue, doesn't it?

I will check the advance anyway. Thanks.

[chicklin]

Success!!

Well, pretty much. So, I found a couple of things. First, I think I may have a vacuum leak in the distributor advance. Need to test it some more.

Second, the timing was WAY off. The timing mark was way over on the passenger side of the water pump. I set it to 8 degrees BTDC, per the sticker.

Third, my carb was WAY lean. You guys were right, 1 1/2 turns, even on a freshly rebuilt carb, was not enough. I think I landed somewhere around 4.

So, after getting all this straightened out, it smoothed out and purrs like a kitten. Holds about 17 or 18 inches vacuum rock steady and no more miss in the exhaust.

I do have a couple issues remaining. First, the idle is still too high. The hot idle screw is almost completely backed out and it will not drop any more. Second, I seem to have lost all my low end power. Acceleration is terrible until the secondaries kick in and then it picks right up, but off idle it's terrible.

One thing I need to go back and look at is the idle needles. I pretty much set them off of the vacuum gauge (turn rich until highest vacuum, back off a quarter). I used this method with both needles, but I kind of forgot to make sure they were both turned the same amount...ooops. However, this shouldn't have anything to do with the acceleration, should it? Any other thoughts would be appreciated.

Thanks for all your help!

[chicklin]

Did some more fiddling and figured out the acceleration issue. The plug I was using to cap off the advance vacuum line was not tight enough, so when I set the timing it was way too far retarded. Once I got that figured out and back to 8 degrees BTDC, the acceleration was back to normal. I could probably advance it some more but it would bump the idle up....

Which leads to the problem I have left. Everything is running pretty smoothly at this point, holding 17 inches pretty steady. However, I cannot get the idle down to where it needs to be. To be honest, I don't have a tach (I will get one tomorrrow), but it sounds awfully fast, maybe 1000 RPMS. And, it's still clunking a little putting it into gear, which is generally due to high RPMS. The problem is that the idle screw is backed completely off the throttle.

So, am I still looking for a vacuum leak? I think so. The idle mixture screws are out 4 full turns, which seems like a lot. So, I thinking I'm getting too much air and I'm giving it too much fuel to match and the idle is high. I can turn the mixture screws down to drop it a little but that's really masking the problem and I lose vacuum and it stumbles more.

So, back to chasing vacuum leaks. At least it's running much better, now.

[donut]

Check to see if your coming off the high idle, (pretty sure some one mentioned it) should be on the choke side of carb. This could keep the regular idle screw from bring it down far enough.

4 turns on the mixture does seem a bit much, mine runs decent about 2 turns out. IMO try to get the idle down first, you may be transitioning into the run circuit instead of the idle.
By chance, does the idle go higher with more choke? If so it seems to me air is feeding it also.

[chicklin]

Originally Posted by donut

Check to see if your coming off the high idle, (pretty sure some one mentioned it) should be on the choke side of carb. This could keep the regular idle screw from bring it down far enough.

4 turns on the mixture does seem a bit much, mine runs decent about 2 turns out. IMO try to get the idle down first, you may be transitioning into the run circuit instead of the idle.
By chance, does the idle go higher with more choke? If so it seems to me air is feeding it also.

Definitely not hitting the high idle screw. In fact, I had to bump it up a couple turns this morning when the truck was cold.

Short of retarding the timing or turning the idle mixture screws in, I don't know of any way to get the idle down. I agree, 2-3 turns ought to be plenty for a freshly rebuilt carb, but it really stumbles at that setting, so I think vacuum leak is going to be the culprit.

[vtwinsport]

Try sprayin carb cleaner around the edges of your carb base and intake. Spray a little at a time in each spot, it the idle changes in any of those spots, you have a vaccum leak.

[chicklin]

Originally Posted by vtwinsport

Try sprayin carb cleaner around the edges of your carb base and intake. Spray a little at a time in each spot, it the idle changes in any of those spots, you have a vaccum leak.

Yeah, I've tried that and didn't find anything. I just need to get some time to go through each connection methodically and narrow it down.

What do you think about unhooking all of the vacuum ports on the carb and plugging them. Then, if there's still a leak, it's the carb (not likely b/c it's new with a new base gasket) or the intake. If no leak, hook up each vacuum circuit one-by-one until I figure out which one is causing the problem.

I am tempted to just rip out all of the emissions stuff that accounts for most of these vac lines. I don't think we need that stuff in Missouri.

[chicklin]

Got some more time to look at this today while the kids were sleeping. I definitely have a vacuum leak on the intake manifold and on the EGR, as well. Ugh, looks like lots of stuff to remove to get those fixed. If I'm going to the trouble, is there a standard replacement intake that would give me a little power boost but not lose any torque? I'd like a bolt-in replacement, no adapters, etc. and I want to keep my Q-Jet since I just bought it.

Any tips for pulling the intake? Can I leave the carb attached? How about installation, should I coat both sides of the gasket? Thanks!