JP_80

I’ve got a Chet Herbert cam (RPM Range: 3500-7800, Advertised Duration: 300/312, Duration @ .050: 264/276, Lift: .645-.630, Lobe Center: 106c) and can’t get it to idle down. Idles fine when it’s cold but jumps around when hot. Will idle as low as 800 and as high as 1600-1700 (not in gear). I’ve been told that it because it’s not producing enough vacuum and that there is a way to fix this. But I haven’t found anything on it. Does anyone know?

Durango_Boy

On the other side of that it might be a vacuum leak.

A small leak might cause a rise in engine RPMs. Thoroughly check all rubber hose ends that are a part of the vacuum system.

JP_80

All new hoses...but will check them and make sure one didn't get pinched, coming off, etc.

fotyfobravo

How much vacuum you making? If you haven't read this...start here.....this fixed my LOW vacuum Big Block.

https://www.corvetteforum.com/techti...=115&TopicID=3

SH-60B

Good advice about the vacuum but check the mechanical advance too. A weak or missing spring can do the same thing. Sorry if this is already in the tech tip but that is just too much for me to read.

DRIVESHAFT

Thats a monster cam for a small block. Its never going to be happy idleing very low.
It will probably idle at 1000 rpm +, even when its running perfect.
My first check would be base timing. Its going to want a lot. I would bet it needs to be at least 20 btdc.
Usually, when a car has a vacuum leak, it idles worse when its cold than when its hot, so I doubt that is it.

lr172

If an enine runs well cold without choke, you should take a look at your mixture. This is a good sign that your idle mixture is fat. Given the Monster cam, I'll gamble that your transfer slots are exposed, causing a rich idle condition that cannot be adjusted with the idle circuit. Lot's of good data on this subject here. Exercise the search function.

Belgian1979vette

with the above.

There is a remedy to get the plates to close on the transfer slot. It involves drilling small holes in the throttle plates to compensate.

Your cam is indeed really big. My guess is that it is made for over 8000rpm max. Also with that kind of LCA, you have a lott of overlap at TDC which will reduce vacuum also.
Unless your engine is really up to the task, I assume the cam is overkill and reduces more power in the usefull range than you win in the top.

The Money Pit

This is something I've been wrestling with for some time,and still have not fixed perfectly. Big cams do need a lot of initial timing. I have mine set with 18 initial,36 total mechanical,and am using a VC-1862 vacuum advance can. That's a NAPA can that pulls 16 crank degrees advance with 8 inches of vacuum.This has gotten me very close,but still I can not idle in gear under 1200 rpms for long without toe tapping the gas to keep it running.

I have drilled 3/32 holes in all four throttle plates,replaced the power valve with a 4.5,(idle vacuum in gear is 7.5 inches),fooled with idle air bleeds,idle feed restrictors, and the transfer slots are square.

I have come to the conclusion as others before me have,it's just the nature of the beast.

My next trick is to try my old RPM intake to see if it'll tame this cam down some.

JP_80

What does it idle at when not in gear?

Can you please explain what you are talking about here:

I have drilled 3/32 holes in all four throttle plates,replaced the power valve with a 4.5,(idle vacuum in gear is 7.5 inches),fooled with idle air bleeds,idle feed restrictors, and the transfer slots are square.

I'm not sure I understand. Thanks

The Money Pit

The engine (406) will idle all day at 1200 rpms in park or neutral,but when I drop it into gear,the rpms drop to 1000. If I use the big toe,and give some gas, bring the rpms up around 1200, it'll stay running. If I let off,in a few seconds, it begins to stall. If I turn the idle up to 1200 in gear, when back in park it's 1500 or better, and of course bangs hard when you shift back into gear.Ideally I'd like the 1200 in park, 1000 in gear,without my big toe on the gas.

The steps I took to try to correct this were many,and all relate to the same problem I think you have. The idle circuit has ports in the throttle bore,that during idle,must be covered to some degree. This allows the vacuum available to in affect pull fuel into the carb,and allow the engine to run. With lumpy cams,the idle vacuum is low,engine wants to stall,so the tendancy is folks just crank up the idle speed. This allows more air/fuel in,and the engine usually runs,....but at a faster than desired idle speed.In my case,two feet on the brake,and the car still wants to drive through the garage wall.

In order to allow the engine enough air to idle,and still have the throttle plates cover the idle ports for proper idle control, you can either drill holes in the throttle plates, or spend lots of money on a new four corner idle carb. I elected to drill.

The idle circuit is nothing more than a carb within a carb. The difference being it only has to work at idle,and do to this,can be fairly small in jet size in comparison to the main carb circuit. The idle air bleeds, and idle feed restrictors are essentially the "jets" in the carbs idle circuit.Changing one or the other leans out or richens the idle mix,the idle mix screws fine tune this.

But wait... there's more. Ignition timing has a huge affect on the idle with big cams. The vacuum can I chose to run begins pulling in at 2-4 inches, and is fully deployed with only 8 inches of vacuum. This can will add 16 crank degrees of timing when fully deployed. The fact I can not maintain the 8 inches is why my engine is stalling out. When I plant my big toe on the gas,idle remains at 1200 rpms, vacuum is at 10 inches, engine runs great.Take away the toe, vacuum drops to 7.5, and falling. As the vacuum falls, so does the added timing, and the snowball gets bigger.Before I dug into the timing curve, I only had 6 inches of vacuum. This I felt was too close to the power valve rating of 6.5 inches. If the power valve opens at idle, this too could cause idle issues, so I changed to a 4.5 inch power valve.

It is known single plane intakes are better on the track, dual planes better on the street. This is why I'm going to try my RPM intake again. If I gain 1 or 2 inches vacuum at idle, I'm sure it'll idle at 1000 in gear.

I am no expert on this by any means,but have tried a bunch of tricks to try to fix this. Just thought I'd share.

OzzyTom

What stall speed do you have on converter ?
Surely it's a simple case of installing a higher stall unit?
I'd have thought with your combo that it would be 2500 at least.

The Money Pit

It's a 10" 3000 stall converter. Should be plenty. In fact the last rendition of this 406 ran the same carb prior to any mods with a 2200 stall. The only difference is the solid cam and intake. I was running the RPM intake, and a Comp XE-284 with 1.60 rockers, and could idle at 650 in gear.

After the intake/cam swap I started chasing the idle problem, and began the learning curve.I know it's right on the ragged edge of working out, but I just haven't found the right combination.

Here's a clip of the last time I had it idling at 1200 in park.
http://www.finitesite.com/markwallas/80vette.wmv

The part about the two feet on the brake was before I did any mods to the carb to try to fix the issue. I had the idle speed set up pretty high then.

This is what I meant about drilling the throttle plates.


This where it stands now after all the mods.

JP_80

Money Pit - Thanks for the explanation. After doing some extensive research I think my timing is off too (12 initial, 33 total @ 5000 rpm). It sounds like you’ve got me headed in the right direction now. I’ll start seeing what all I can break this weekend while trying to figure all this out. Good thing is its only money and beer that will be lost if I screw anything up!!!

Good luck on getting yours figured out.

427Hotrod

Well that is one nasty little small block! Can't wait to hear it someday now that I live in the Ft Worth area.

You're on the right track. You need LOTS of initial timing.

Not unusual to juggle things to have near 30* at idle.


JIM

JP_80

427Hotrod - Welcome to DFW. Hopefully we can meet up for a beer.

What would happen if I locked out the centrifugal advance? Someone mentioned this to me yesterday.

The Money Pit

I thought about trying the "lock out" option, but hear all kinds of horror stories about starter kickback,loss of mileage,pinging under light throttle,etc.

I'm convinced I can make this work, but the magic combination somehow eludes me.The warm weather is right around the corner,so tweaking will commence soon!

JP_80

He did mention that by locking it out it could make start up hard. Based off what I've read I think I'll take your approach and try and figure out a good combination.

I figured I would ask and see what other people's opinions where on locking out where.

Wisp

Hi...

I had same problem.... Only pulling 7 Hg at idle, and had a BIG stumble.

Engine would idle, but only shortly, until the vacuum droped and the vacuum can started to fluctate until the engine died.
Tried several things....

New carb.
New ignition.
New intake.
New plugs.
Several money on tuning.

Nothing helped.

What I did:
Use many evenings to read about timing, tuning and vacuum and then fix the problems.

I am running 7 Hg in idle, big cam and idle is 700 RPM and engine is very steady.

You will get your vacuum can 1862 to pull at 7 Hg, but not fully. Then it will fluctate and will course the timing to be very jumpy until engine stalls.
I change the spring in the vacuum can, so it will pull fully at 7 Hg. Afterwards I had a very little engine knock at cruise, but this was fixed with at bushing.

My timing is now:

18 base.
34 with vacuum connected.
and 38 at 2800 RPM.

Pulling very strong, will idle steady at 700 RPM and no engine knocking

Best regards Søren

The Money Pit

Wisp,
I'm interested in how you change the spring in the vacuum can. I'm certain what you did will solve this problem.Also where do you get new springs, or is this a stretch the old one till it works approach?