Engine won't idle

[87mcss]

I have a 400 sbc engine. I put a 17059508 Q-Jet on the motor as I upgraded the cam heads and intake. I went through the carb and made some adjustments per Cliff's book. The engine runs ok and the cam break-in was fairly uneventful. The engine revs up and the high idle seems fine, but the idle is nonexistent. I can't seem to dial in the idle mixture screws whatsoever. It doesn't seem to matter if they are 3 turns out or 12 turns out. It BARELY want to idle, somewhere between 400-500 rpm. Cranking idle speed screw does very little as well until you finally get it so far open that the main metering kicks in and it's humming along at 1300 rpm.

Here is what I have done to the idle circuits:
Idle tubes: .039
Idle down channel: .059
Idle bypass air: I forgot to write down but I'm thinking it's between .106 and .110
Mixture screws holes: same here but I'm thinking about .090
Upper and lower idle air bleeds: .070

I've tried adjusting it every which way I can think. I've tried to add timing and that made no difference, overall the engine seemed best with about 15° initial advance. Since it is just stumbling along I'm concerned that the power piston is up and making this even more complicated. Now, the cam in this motor is a little wild... okay it's probably too wild, admittedly. It's 235/243 at .050" w/ 109LSA. I know Cliff will hate this cam. So a rough, smelly idle is expected but I'm just trying to get it to idle at all. As I said as of right now no matter of adjusting the idle screws seems to make a lick of difference once they're open enough to keep the motor running. Where should I be looking?

Thanks!

[Mudsport96]

I'm not going to be much help as I'm still learning myself, buy some of the guys will want to know what compression you are running and maybe what intake. Do you know what the camshaft part/grind number is?

[87mcss]

About 9.4:1 compression. Edelbrock Performer RPM intake. Cam is Howards 118041-09.

[77cruiser]

Have you tried more initial timing? Probably doesn't have much vac. Just keep upping the initial & see if it helps.

[87mcss]

Yeah, adding timing seems to have little to no effect either. I got as far as making it hard to start because it had so much timing but it still wouldn't idle with a lick.

[Cliff Ruggles]

What heads/piston used  on the 400 SBC and what is the compression ratio?....

[87mcss]

Stock pistons, 64cc chamber heads. approx. 9.4:1 compression.

[87mcss]

I'm not sure if it's my whole problem but I definitely think the power piston spring is too strong. It is fully lifted while the enginet is trying to idle. Is modifying the spring advisable or do I need to purchase a selection?

[Mudsport96]

Just for giggles you could pull the power piston spring out and put the piston and metering rods back in. Then try running it. That would tell you if the spring is the problem, but I'm thinking you need less camshaft, wider LSA or more compression.
I looked the cam up and its 297 duration seat to seat, I think it would be happier with closer to 10.5:1

[77cruiser]

Shouldn't affect the idle at all unless fuel is pulling out of the boosters. How much vacuum are you getting?

[87mcss]

Not much. About 5" but again, it's struggling to idle at 400 RPM. At an 800 RPM idle it would obviously be better. At high idle, while warming up, it's humming along at 15"-16" or so.

[77cruiser]

The cam you have will need about 8-900 to idle decent.

[87mcss]

Which is what I was shooting for.

[87mcss]

I decided the engine was starving for more air and fuel. I opened up the idle down restrictions from .059 to .064, the idle mixture holes from .099 to .104, and the idle bypass from .101 to .109. It is MUCH closer now. I can get it to idle out of gear around 750 but that is still with too much throttle angle. At this point it really seems like it wants more bypass air. That's gonna be my next step.

[77cruiser]

Still think it needs timing. How high have you had it?