88IROC350TPI

This problem has me stumped... Allow me to attempt to explain.

My "new" motor (in sig) runs absolutely great in every aspect except for an eye-watering rich and crappy sounding idle. When the motor is cold it wll idle nice and lopey (still rich) but once warmed up its twice as rich and idles horrible, sounds like a tired ol' truck motor. No matter how much you play with the idle mixture and initial timing it goes from bad to worse. Now first thought is its a carb or timing issue, and it very well may be BUT my old motor did EXACTLY the same thing!

Old motor was: stock L98 shortblock, same vortec heads I have now, hydraulic roller cam: 224/234 @ 050, .499/.502 lift, 112LSA, GM (rpm clone) dual plane vortec intake, reworked HEI, and a 750 VS Holley. This motor did the exact same thing, ran great but idled horribly and uncuribly rich when warm. I just lived with it until the day she gave up on me.

Step forward almost a year later and the new motor is doing exactly the same thing only worse. I started off using my brand new 750 Speed Demon. Completely out of the box settings, 18* initial timing, 1000RPM in park, 800RPM in gear. It will foul the plugs before it even warms up. Driving around the plugs stay clean (little on the lean side). I thought I was experiencing the infamous rich-idle Demon issue so I put on an old 750 Holley DP - same problem.

With the demon the car idles its best with the 4 mixture screws 1 1/4 turns out. Its still eye-watering rich but this is the best point. If you turn them in anymore the motor will idle worse and the RPM's will drop excessively when you put it into gear. I've played with EVERYTHING for months and cant figure this out.

Oh one last weird issue... I had the air cleaner off the other day and noticed when you open the hood, the idle jumps up like 200-300RPM. Whats up with that? Is that a sign of something?

2 completely different motors (with the exception of the heads) doing the same thing... Anyone have an idea of how to fix this? I'll be forever amazed if you guys can figure this out.

Thanks,
Chris

89Warbird

What kind of air cleaner and filter? Do you have a fuel pressure regulator? If no, get one, if yes, what's it set at. I'll bet your fuel pressure is too high and forces the needle off the seat once you pick up the load then it's okay. But, for this to be true you wouldn't be able to adjust the float level and get it right, can you get a correct float level at idle?

88IROC350TPI

I have a mechanical pump that pushes 5psi at idle. Floats fine, no fuel leaks into the carb at idle or when turned off, motor pulls like 15" of vacuum out of gear so I dont see how the 6.5" powervalve could be opening on both carbs. Valve lash is also fine and I've checked a zillion times for vacuum leaks everywhere imaginable.

Also, if you drive the car around for any amount of time then let it sit for 20-45 minutes, once you try to restart it it'll just turn over and over and over. It takes atleast 5 attempts before it catches. I thought the carb was leaking so I actually sat there staring into the carb for 15 minutes, no leaks.

This problem is very interesting, as you can see. Believe me I've tried everything imaginable.

I'm running out of things to replace... even though it doesnt quite make any sense, I'm beginning to think the heads have something to do with it. Its the only common link.

jbenge

iTrader: (2)

I'm surprised you are having these problems with 15in of vacuum at idle, sometimes with big cams and low intake manifold vacuum you will have a severly rich idle. I just went through the same problem once I changed cams, but my vacuum at idle is only about 10in. Chickenman35 helped me out with some great suggestions, you may want to send him a pm. Since you are sure the fuel pressure and float levels are set correctly I would look at the transistion slots on the carb. If the slots are exposed too much you will have a rich idle and you may have to drill holes in the throttle blades to compensate and get the proper amount of the transition slot exposed. The transition slot should only be exposed .030-.040, any more than that tends to cause a richness at idle. Good luck.

89Warbird

The other thing which you didn't answer was what type of air cleaner do you have? Does it run rich with it off? Some air cleaners block the bowl vents and cause problems. Is your choke butterfly opening?

88IROC350TPI

I have one of those drop-base K&N's, I cant even remember the measurements off the top of my head. When you take the air cleaner off and close the hood, the idle drops down like 200RPM. When you pop open the hood, it jumps up 200RPM. It doesn't do this with the air cleaner on though.

I have no choke whatsoever. My transfer slots are perfectly squared as well.

2 different shortblocks
2 different ignitions
3 different fuel pumps
3 different carbs
3 different cams
2 different intakes

...and still this damn rich/sloppy idling. Oh yeah, I've put both my Holley and Demon on friend's motors and they've run fine.

rolling-robert

vacuum advance.

89Warbird

Okay, running rich, possible causes.
1. Too little air
2. Too much fuel
3. Weak ignition/incomplete combustion.

Have you checked your ignition system good strong blue spark on all cylinders?

Chickenman35

I would suspect not enough clearance between the air cleaner top and the airhorn of the carb, thus choking off the carb and causing a rich running condition.

Are you running a Hoodscoop? A Performer RPM is a fairly tall manifold and can cause hood clearance issues, so much that you have to run a fairly short (3" ) air filter. This may not be enough with certain air cleaner top designs. Is your K&N a 14" diameter model? The Moroso design is superior to K&N's in low clearance situations.

The fact that your rpm drops 200rpm when you lower the hood ( with no air cleaner ) is interesting. First thought would be trapped heat.....but it should take a minute or two for the heat to build. Just how close to the top of the carb is the Hood? You do have the underhood sound isolation removed?

As a quick test to isolate matters is to remove the air cleaner and the hood. Reset the idle mix, drive it around a bit and see if the problem goes away. If so then you have some air restriction issues.

Are you running any kind of spacer below the carb? You should have at least a 1\4" Phenolic or plastic ( not aluminium ) to isolate heat. Without a Hood scoop 1\4" or 5\16" is about the maximum that you can run. Problems with hot start indicate heat soak issues. Phenolic spacer, heat insulation sleeve around fuel lines, hood scoop ( with cold air box ) and hood vents all are necessary to properly run a crabed motor in our cars.


BTW...vacuum advance issue is critical for street use. Are you running one? Cammed motors really need a lot of advance at idle...or they will foul plugs like crazy on the street. You've got more than enough cam to need some help with the vacuum advance.

If you haven't already done so.... get a Crane Adjustable Vacuum advance. Set the limiter cam to 15 degrees ( usually the maximum ). Loosen diaphram tension with the allen screw so that you have full pull in at least 2" below minimum vacuum at idle in gear. Use a Mighty Vac to set diaphram or jsut set spring to minimum tension.

Intial static timing with vacuum advance disconnected. Should not start before 1,000 rpm. If it does this can cause hunting. 10 to 12 degrees initial should be enough for Vortecs......14 degrees might be necessary. Regular ( non fast burn ) heads usually require a bit more timing than Vortecs.

Then connect vacuum advance to MANIFOLD vacuum. Cam overlap causes Dynamic compression loss. This requires a lot of advance to light off mixture correctly as charge DENSITY ( not to be confused with rich or lean mixture ) is very low. Vortecs may require vacuum limiter cam to be decreased to 12 or 15 degress....but try it at 15 degrees first.


Full mechanical advance ( vacuum advance disconnectedof 31 to 32 degrees total... all in by 3,500 to 3,800 should be sufficient for Vortecs. Do NOT bring in all the advance before 3,200 rpm. This will cause "pinging" particularily in Vortec heads.

Hope this gives you some ideas. Post back with results.

PS: Do a search for " Hood Vent" or "Chickenman35". I've got some pics of my cold air box, Hood Scoop and Hood Vents posted somewhere. The Hood vents REALLY help exhaust heat from the engine compartment.

84 Z28 5.0 HO

My guess about the rpm drop when the hood is closed is that the hood insulation in comming in contact with the top of the carb choking it.

Tech @ BG

Have you been using the same air cleaner with everything?