Weird questions

[chrisnsarah]

I know you guys are tired of hearing about the bad idle but here goes anyway.

It's still there even after the new manifold. We can tune the carb mixture easily but no matter what we do, the lumpy idle is stil there.
Next, we did a cylinder balance test and all cylinders dropped evenly by 50 rpms, so we assume the engine is ok and all plugs are firing evenly?- it should be after only 4k on it.
Hooked up a vacuum gauge and it was vibrating between 15 and 20 at idle but would smooth out with increased RPM's. I've heard this is worn valve guides but I don't have any smoke and this wouldn't cause a bad idle, would it? Also read somewhere, maybe on this board that dual-plane manifolds will give this reading. If so, can I connect the 2 banks to a tee for the gauge for a better reading? On the Edelbrock, all the vacuum ports are on one plane and the only place I can tap into the other bank is the full vacuum port on the edelbrock carb.
My next theory is if there are any weak valve springs or loose rocker arm bolts. All the rocker arms, fulcrums, pushrods, lifters, and cam are new. Do some RV cams have a slight roughness to them? I'm wondering if it's the cam now. Or maybe the shop put in a slightly wild cam by mistake.

[J4GRAND]

Does sound like a cam lope. I have the Summit K8600 cam and have a slight lope at idle.

[chrisnsarah]

Some progress today- I got a new vac gauge today and connected the 2 planes and tee'd it to the new gauge. It shows 19-20 lbs at idle and flutters only 1 lb. I give it a little throttle and it smooths out. the vac chart shows if it is valve guides, the variance would be greater, so I'm almost ready to rule out the guides so now I'm focusing on the carb.
Also I seem to have a flat spot at part-throttle and it pings. If I go full throttle, the knock goes away and I take off like a rocket. My edelbrock has the stock calibration and from reading the book I'm thinking of going 1 or 2 stages richer in the cruise mode. Just wondered if this is the way to go for this senario.
Finally, if I start my truck in the morning after presetting the choke and if I start it within a few minutes after shutting it down while hot, it starts fine and catches instantly. If I leave it for more than 5 minutes, I have to crank it almost a dozen times before starting and it takes a second for the motor to get up to RPM's, so this means what? Fuel return line is working again.
Edelbrock 1406 carb w/ spacer insulator
Edelbrock 2131 manifold
401 w/4k miles.
TFI w/ MSD

[marcd]

CNS -
Although I have the stock 2150, I seem to be having a similar problem - rough idle and the restarting problem (although mine has to sit for several hours). This problem is pretty common and I have heard a lot of fixes, most of which I have done. I am wondering if a bad fuel pump my be related to both problems - i.e. at low rpms there is not enough fuel pressure until the pump really gets going. That being said, since the fuel goes into the carb and sits in the bowl , would it even make a difference?

I might just put in a new pump anyway.

[nograin]

Marc, Unlikely its the pump for the reason you gave. Pull the top of the carb off to take a look and see if there is about the right amount of fuel in the bowl. Bet there is.

Chris,
What is your initial timing set at? Since you are no longer running a stock cam, stock specs may not be what is best. It would help us if you can find out what cam they put in. Either the specs, or the brand and model number - someone will then be able to look up the grind specs.

My guess is that you ought to try retarding the initial timing a little. Try 2 degrees less. That will not only make starting easier, but may reduce pinging if that was the cause. However, it may not make the idle any smoother...

If you back the timing off, it is equivelent of richening up the whole curve in terms of emmissions, efficiency and exhaust temperatures. It is possible you will have to lean out (turn in) the idle mixtures a bit (using your vacuum gage is best).

What do the spark plugs look like? Very lean or too much advance will look pretty darn white. Any real rich points - including too much choke during start up will cover them in black carbon and they must be cleaned or replaced.

[AMX factor]

I changed cam/etc and fought the rough idle for months. One day while tuning I pulled the vac advance hose off and plugged the open carb port. Idle smoothed right out. Turns out I have a hole in the diaphram in my vac advance on my "new" for the 3rd time dizzy. I went to lighter springs and new weights for mechanical instead of vac advance. Runs great now.