This has been an ongoing issue for literally 3 years now and has stumped me and everyone else whos given me advice. My car, when cold, idles just the way I think it should ...its nice and clean/healthy sounding lopey/racy idle but as soon as the temp guage rises above 110* the idle will change and begin to sound for lack of a better word sloppy. The mean chest-thumping lope goes away and turns into a real flat sounding crappy idle... Almost like the cam was magically replaced with something much smaller and the compression dropped down to 8:1. Now, I know what you're thinking... its simply the idle mixture and/or initial timing but I assure you that no matter what I do to this car it ALWAYS does this! I've been through 2 different short blocks, 2 intakes, 3 cams, 2 ignition systems, 2 torque converters, and 3 carbs in the past 3 years and each configuration did this! The only things that remained the same during all this is the heads and accessories. For the past 1.5 years I've been running a 750 Speed Demon mechanical secondary and it runs great. This carb never skips a beat. I couldnt get this carb to backfire, bog, or stumble even if I tried. Before I bought it I had a 750 VS Holley and then an old 4150 750 Holley. These carbs ran pretty good too but not as good as the Demon.

Timing is at about 18-19* initial and my Demon has the transfer slots "squared" Setting the 4 mixture screws about 1.25 turns out the car will produce the highest idle RPM when warm (but it smells very stinky and idles like I described above). If you turn the screws in any more the car will actually begin to idle a bit more lopey like it does when cold BUT its still pretty sloppy and it'll also make the RPM's rise up high around 1300RPM out of gear and then drop down to like 750rpm in gear. The car will also want to stall when slowing down to idle or turning the steering wheel (power steering) all the way to lock. Putting on my old 4150 Holley and playing with the idle the car still idles crappy though it doesn't stink nearly as bad as the Demon nor does it seem to idle as lopey.

Another thing to note is that when you initally start the car it'll idle perfectly for about 5 seconds, even when warm, then go back to its usual sloppy self. Also it idles nicely when you first come to a stop or when cruising at a low speed through like a parking lot (just above idle). As soon as you come to a complete stop though the car will idle crappy again after a few seconds.

Anyone got an idea on this? Keep in mind I've tried just about EVERYTHING and have replaced a lot of parts over the years. I've played with the idle on this car until I couldnt see straight any more... The motor runs strong everywhere else I just wish it would idle like it did when cold.

valves adjusted too tight or a burned valve??

valves are definately adjusted properly. I dunno about a burnt valve... wouldnt that cause other problems? The car runs strong and runs about what I think it should down the drag strip...

Though I forgot to mention that I do think a little oil may be getting into the combustion chamber. When you pull the plugs out there is oil on the threads (valve covers arent leaking)... my guess is the valve guides a little worn out but I plan on taking the heads off very soon and checking all this. Could this be causing the idle problems? It doesn't make sense that it'd only run like crap when warm though?

It sounds alot like a vacuum problem, possibly the diaphram for the power brakes, if it has them. Are the brakes hard? Do you hear a whistle? Does the tranny shift late? Run a vacuum line into the car, moniter vacuum throughout the various conditions.

If it's the car in your signature, that would open the door for all kinds of sh!t if you are still running any or all of the emission stuff. Burnt or stuck EGR valve, Broke, Cracked, Defective CTO(thermal vacuum switch). Maybe even a plugged catalytic converter.

The valves being too tight? It is a definite possibility, since your talking about problems developing with heat. As far as a burnt valve, do a compression test.

The info is kinda vague, makes diagnosis a little tough.

my car has zero emissions equipment. Its just a regular ol' carbed small block. I've capped off all vacuum sources many times in the past. I mean, I'm starting to think that I've done everything anyone will ever think of doing about 5 times.

I really am starting to believe that its something wrong with the heads...

It's going to idle rougher cold than when warmed up. If you are running vacuum advance that can be some of your problem. Check you manifold vacuum at idle in gear and compare it to what it takes to pull in your advance can. If no can, what happens when you set the initial higher? Also oil getting on the threads doesn't usually come from inside the engine. It's normally burnt before it reaches there. Check the plugs for richness. Idle circuits could be too rich.

I second the vacuum advance. As the engine warms it will idle smoother, vacuum increases, playing havoc with trying to get it set right. I fought my sons car when it had the 355 in it, ended up disconnecting the vacuum advance and all was well. He might have lost a little fuel mileage, but the idle stabled, both cold and hot. Just add a little more initial after disconnecting vacuum advance.

[QUOTE=broke]

Timing is at about 18-19* initial and my Demon has the transfer slots "squared" Setting the 4 mixture screws about 1.25 turns out the car will produce the highest idle RPM when warm (but it smells very stinky and idles like I described above). If you turn the screws in any more the car will actually begin to idle a bit more lopey like it does when cold BUT its still pretty sloppy and it'll also make the RPM's rise up high around 1300RPM out of gear and then drop down to like 750rpm in gear.


Describe "the Demon has the transfer slots squared" . Exactly how are you setting your idle speed. It sounds to me you are getting beyond idle circuit area. Where are your throttle plates in relation to the transfer slots on the Demon carb? Primary and secondaries. IS it a road DEmon? These carbs were not designed for a lopey cam at all, and are very hard to get a decent idle out of although it is possible. This very reason is why Demon came out with their "idle ease" base plate which is basically an adjustable air leak. But it can be accomplished with out it.

That lopey sound at idle - engine cold - is from the motor running fat. Had similiar issue here. Running a 870 Holley, I had to drill my primaries, you have the idle eaze to do the same. I also run ported vac advance with about 20 initial for a stable idle. You also need to check the RPM the mec advance comes in. Your mech advance may be coming in right around your "idle speed" so you are getting and advanced timing at idle (more so when cold at fast idle), then drops off significantly when idle rpm drops. This can often be 4-6*. I had to go to tighter mec advance springs to reduce mec advance below 1000 rpm. Make sure you have steady timing at idle, not jumping around.

Here is something i have seen happen many times over the yrs on my personal motors and other friends of mine too which i think could very well be happening in your case.

The long story short is,the cam your running may be on the edge as far having just enough durstion @ .05 to produce a choppy idle esp when oil & motor are cold.

Then when the oil heats up a smidge it can allow the lifters to bleed off just enough duration at idle/low speed that it smooths the idle because it's acting like a milder cam due to a couple less deg dur with warmed oil.

This is a very common thing to happen when there is just enough dur to give a coppy idle when cold and then it smooths out a fair amount when hot changing from what sounded like a fairly nast hot cam idle to a mild idle when warmed up.

My 396 with with a 284 adv dur cam(yours is less with 274 avd dur and almost same cubes) with 228 deg dur @.05 on a 112deg L/S used to do the same thing. your 383 and my 396 close enough in cube to compair and your runing a smidge less cam than i was .

Most motors with perf cams experience this to varying deg depending on setup & cam profile but when you have just barely enough dur by a couple/few deg to produce a choppy idle when cold the idle will ALWAYS NOTICEABLY SMOOTH OUT WHEN THE MOTOR & OIL HEAT UP which is what your experiencing IMHO.

You can tune it to produce a choppy idle when warmed up as is mentiuoned above posts but if the motor runs good dont sweat it. If its a choppy/nasty idle you want the only way to get that will be a new cam with approx 6-10 deg dur @.05 which should be just enough to leave you with enoug int dur for a choppy idle after the oil/motor heat up some but the idle will be nastyer when cold with the additional dur.

Scott

I may make a video tomorrow showing how dramatic the difference in idle is. The idle isnt "choppy" when cold, it is downright rough ...but when I say rough I mean that in a good way. Its a very nice clean sounding melodic rumpity-rumpity. I know of a few other people running the exact cam as me and their motors idle rough at all times just like mine does when cold. I'm not really concerned about making the car sound "cool" like it does when cold, It just doesn't sound healthy at all once warm and like I said just as soon as the car drops to idle the rumpityness comes back then you can just about tell when the idle circuit fully takes over and the engine begins idling like a old pickup truck... it actually sounds like its not even running on all 8 cylinders at times (though it doesnt seem to be missing, doesnt sputter, nothing)

My Speed Demon was bought just before they started using the Idle-Eze baseplates so I dont have that. I do have the vacuum advance hooked up and today I will try to disconnect it. I'm gonna go out now and play with this puppy until I get some kind of result... I'm tired of it.

Broke,again,been there done that many times in 35+ yrs exp with SBC & BBC.

The cam your running is just too mild/short enough on duration that it can sound ratty/mean/tough when cold and then sound almost like a pussy cat when warmed up when the oils warms up and a couple deg dur are lost from slight lifter bleed down.

The comp 274 your running with 383 cubes can act just like that,believe me i have seen this happen many times over the yrs.

The 274 has 230 deg dur @.05 which is exactly on the fenc IMHO for your motor to idle choppy/nasty cold and settle down a lot when warmed up.

Yes if you play with the tuning as mentioned in above posts you can make the idle sound A little more ratty when warmed up giving an illusion of a hotter cam but no matter what you do the idle you have warmed up is what your going to have unless you get more int dur @ .05 like a min of 236-240 deg which is what you need with a 110 deg L/s for a choppy idle when hot with your 383 SBC IMHO.

Scott

I dont have a 383... just a plain ol' 350. The cam is 230/244 @ 050 with a 113LSA it idles MUCH rougher than my old XE274 which was 230/236 @ 050 on 110LSA. I know someone with a 10.5:1 350, AFR 180's, victor jr intake, and a 750 HP series Holley running this exact same nitroushp 274 cam and it idles at all times just like mine does when cold. Very racy sounding. As I said before, I am not really concerned with the motor sounding mean at idle, I just know that it shouldnt be idling as it does now and it bothers the crap out of me.

I did some tinkering today and noticed that when I put my finger over one of the idle air bleeds (specifically the secondary ones) the motor will begin to clean up and idle better! To get more specific with the way my Demon is setup when I say the transfer slots are squared I mean they are exposed to the point of where they are "square" shaped - just as they were out of the box. Idle RPM is around 750-800RPM in gear like this.

oh yeah, when I first got this motor running about 3 years ago I had a pretty big cam in it: 238/246 @ 050 on 112LSA. The motor did the same thing! Put this exact cam in a friend's car (similar engine specs) and it idled like a pro-stocker.

Do the vacuum advance check. Set the carb and base timing with the vacuum advance plugged and see how it idles and performs. Your can may need too much vacuum to advance it compared to what the engine is making at idle. The 274 is a rough idle cam but the vacuum advance will smooth it and idle it up. Then you have to idle it down and once you lose the vacuum the engine wants to die. May have to go with a lighter can vacuum and if you search the forums there is a can that will work with X amount of vacuum.

ever check your intake manifold for leaks?

I've had like 2 different intakes on this car. Literally everything that can possibly be changed has been at one point or another - except the Vortec heads. I'm 98% sure there are no vacuum leaks. I plugged off the vacuum advance and it makes no difference whatsoever to the idle... I've pretty much tried everything I can think of yesterday and it still idles horribly.

I noticed something else funny today though... While messing with the car I accidently unplugged a plug wire and didnt even notice it because the car idles exactly the same on 7 cylinders (regardless of cylinder) as it does on 8!

I've had like 2 different intakes on this car. Literally everything that can possibly be changed has been at one point or another - except the Vortec heads. I'm 98% sure there are no vacuum leaks. I plugged off the vacuum advance and it makes no difference whatsoever to the idle... I've pretty much tried everything I can think of yesterday and it still idles horribly.

I noticed something else funny today though... While messing with the car I accidently unplugged a plug wire and didnt even notice it because the car idles exactly the same on 7 cylinders (regardless of cylinder) as it does on 8!

If you unplugged the vacuum advance and the idle didn't change, there's your problem. You're not making enough vacuum to actuate the can at idle. Try capping the vacuum advance and change the timing and carb setting until the car runs good. What is the timing set at without vacuum advance? What is total set at? The can diaphram may be bad.

well I just think I made a little progress in this battle. I began going against popular "know how" when tuning a demon's idle... I've been keeping both primary and secondary throttle blades open the exact same amount and have also been adjusting the 4 mixture screws exactly the same. Well, I went ahead and started closing off the secondary idle mixture screws and the idle got better! primary at 1.25 turns out and now the secondary are about 3/4 turn out. I also opened up the secondary throttle blades just a tad bit more than the primary.

I think I'm onto something here... but now that it seems to run better with things set unequally its going to probably be a nightmare to get perfectly right but I believe I am definately onto something...

Camaro_fever68: initial is around 18-19* with 34* total (with vacuum advance unplugged, of course). I did also notice that even with the advance plugged in the car's initial timing does not change unless you rev the car slightly then it kicks in and the timing goes over 40* when it goes back to idle the timing stays up around 25* for a few seconds then drops back down to normal.

You cannot "square" a demon with a low vacuum cam, they take alot of messing with without the "idle ease" but it can be done. Worst case you can drill some holes in the primary throttle plates, but I would use that as a last resort. Keep trying and don't worry about keeping the primary and secondary adjustments equal. You need to get the engine more air and less fuel. But you can't get too far up the transfer slot or you will begin drawing off the intermediate circuit. That is when you get the idle changes and terrible stink. Also If it were mine I would disconnect the vacuum advance for good.

Broke, for the heck of it have you tried another carb? If your close enough you could stop by and try mine out. But it sounds like you narrowed it down to your carb anyway.