I don't know what is causing this weird "problem" so I don't know where to put the topic at, so I chose here for the traffic......

Sometimes when I'm idling at a stop sign or stop light my car seems to lunge foward a little, like rocking foward and back a bit. I have power brakes, but I'm pulling about 15" at idle according to my gauge. Also, my cam, as far as I know is just a 260 ADVERTISED duration cam, something like 212@50 so I doubt this is a brake issue. The brakes don't get soft or hard or anything, they feel just fine, the car just rocks foward like its trying to move foward and when it can't it rocks back, etc.

I don't know what causes it, and it only happens at what seems like random times.

Combo is Rebuilt Smogger 355 w/ Dual Plane, 600cfm Holley, HEI w/ Blaster 2 External coil, Headers, Dual Exhaust and a CC260H cam. Idles at 700RPM when warm.

Stock Convertor, Stock TH350 and Stock 2.73 10 bolt.

Power Disc Brakes. 15" Idle, 20" Cruise, 25-30" on decelleration.

Any ideas?

Nothing odd, nothing special.

Sounds like you have a bit of idle surge. When the idle fluctuates like that you
need to check the obvious. Baseplate bolts tight ? All hoses on tight and not sucking
air ? Primary throttle shaft is tight and not leaking ?

Try putting your car in park at idle and observe any fluctuation you may have. Start
checking hoses for leaks. This is after you are sure that the carb idle and air fuel
mixture screws are set correctly. Also, if you have a power brake car, don't
forget to check the plastic grommet on the side of the booster. It could be leaking.

Hope this helps a little.

Idle screws, floats are adjusted, baseplate is torqued to specs.The Primary throttle shaft SHOULD be fine, its a brand new carb and I checked it before I installed it.

I'll check the hoses for the trans and the brakes, and that grommet. The only other hose is the vac advance and I know that one is nice and tight.

when you checked that idle vacuum, was it in park or was it in gear? If you checked it in park, that could be the reason, cause once you drop it in gear, the idle might be dropping enough to make the vacuum a little iffy. Plus, the cam is probably loping a little bit at 700rpm, so with that lope and low vacuum, the breaks arent all the way there and you feel it cause of the stock converter. That would be my guess. Well, my first guess.

It definitely can be an issue where you have a vacuum leak or even something with the choke acting up weird from time to time. Double check the floats as well and if you didnt set the idle mix in gear, block the wheels up, put the e brake on and set it while idling in gear. Check the initial timing to see where it is at. With the vacuum advance uhnooked and plugged off, you should be probably at least 6 degrees initial and something closer to 10 will probably give better results. Hook the vacuum advance up to a full manifold vacuum source, not ported. That will give a better idle as well. Make sure the throttle isnt sticking at all in any way. Just all the simple stuff really.

Oh, when does this happen.....all the time, or just when its warm or cold. And does the outside temp matter. Is it when its really good and warmed up on a hot day or? It could be that you or someone else tuned that sensitive holley on a good day and it was tuned just fine, but now your seeing hotter and maybe more humid weather and it just needs run through again.

Oh, hows the brake fluid all together? And do the brakes work any better if you sorta pump em up a bit before applying constant pressure? Try pumping on them just a bit and see if they hold any better. They might just have air in the system and need blead well and some fluid to top it off.

They might need a bleeding, but it doesn't feel like the brakes. It feels like the brakes are holding the car well, but the car is trying to pull through them. I can release the brake, put it back in and it'll keep doing it.

I really haven't been able to figure out what kind of condition causes it. Usually on my way to the track, it'll do it after I'm out of the Choke/High Idle. But sometimes it does it on the way back and sometimes it doesn't?

15" vacuum at idle in gear, 22" at idle in Park.

I'll try bleeding the brakes and checking out the hoses and stuff for vac leaks.

I wish I actually knew what cam was in it...... I've got an order form with one, a build form with a different one and a cam card for the first one.

Also check your timing at idle. If you have light enough springs you might be getting some centrifugal kicking in at idle. In that scenario your idle rises just a little (which isn't unusual), increasing advance, and then the increased advance causes a large increase in rpm. Same thing with a vacuum advance that's connected to manifold vacuum but the canister is designed for ported vacuum. A slight increase in idle speed may be enough for a big increase in advance.

Jerrys is giving you good advice. The distributor advance ( springs & weights ) would have been my next response.

There are a lot of variables that would cause an idle surge. You know after
looking at your specs, I think that running a stock convertor with the 260-H may
cause a surge due to lope that in turn would affect the timing. I feel that there
may be a combination of factors that are creating your problem.

As for the 260-H and the stock convertor, you may want to consider something
with a little more stall.

Hope you find the problem.

The first thing I've always done when trying to solve an issue like this is to unhook and plug the vacumm advance. Then set idle speed, check your mixture adjustments, and THEN take it for a drive with the vacumm advance unhooked. If it's gone away, make sure you find the "ported" vacumm source and hook up the advance. If it remains, then find the manifold source, reset idle and mixture, drive and compare. This gets the easy stuff out of the way before we get confused with something else.

Will try that out. I'm in the middle of a radiator swap right now though so it'll be a couple of days before I get her moving again.

The stock convertor is not the problem. I have built several nearly identical engines and used them with the stock convertor and they work just fine with no surging. I love the 260HE cam...it is definately one of my favorite torque cams. It sounds like it may be idling a bit high...my low compression 350's with that cam will idle at 600 rpms in gear no problem. Make sure your throttle blades are almost completely closed, and tune the timing and idle mix scres from there.