After a complete rebuild on my 74 LS4 ... I am unable to get the advertized "slightly rough idle" out of the new CompXE268 camshaft. I have a "fairly ROUGH idle". Actually too rough for the T400 trans. I definitely DO NOT think its carburation.....I beleive it's timing.

It makes 11" Hg (manifold) at 700 rpm. Does anyone know if that's OK? I am using a vacuum adv canister (B26) that "starts" advance at 5 to 7 inches and maxes 8 degrees at 11" to 13" Hg.

Would replacing the vacuum advance canister with a B28 possibly smooth out this camshaft idle.

Current timing is:
16 degrees at idle with vacuum plugged
24 degrees at idle with vacuum hooked up at idle
36 degrees at 2800 with vacuum plugged
50 degrees at 2800 with vacuum hooked up.

I have tuned and tuned and am about to just resort back to a STANDARD (crane) factory spec cam ... ... as much as I hate to dig back into this motor.

Thanks.........

 

Well idle vac is determined by both compression ratio and cam overlap which u failed to post so and i'm not going to lookup Comp's XE268 data for you to figure it out. But <12" idle vac is low for accessory operation. The XE268 is not a large (overlap) cam so u may have intake sealing troubles too. I would not change the cam for this as the XE268 is already a mild performance cam. What's your c.r.?

Sorry for the bad news but 16* initial adv w/o vac adv is too high. The reason for the vac adv existance is to allow low intitial adv for starting and allow the vac adv to pull in the adv once running.
Initial adv w/ vac plugged should be <10*. With vac hooked up the eng should idle with more adv but not enough to ping - use a stethiscope for this.
36* max @ 2800 w/o vac sounds ok - but more conservative would 36* @ 3000rpm.
If u get full vac adv with only 11" u have the correct vac can to match your idle and the 5"-7" u say is your WOT vac where no vac adv is produced. But it reads here like your vac can is at least close. And if vac is still adding adv @ 2800rpm u still have more than 5"-7" vac @ 2800rpm so the easyiest fix would be reduce the mechanical adv at 2800rpm so u have no ping (36* adv total) with vac hooked up. Your mech timing curve may be to "hot" - lighter weights or tighter springs Mr. 44. Greater than 36* total adv only causes pre-ignition and hurts the engine (pistons, brgs) - no need for >36* adv anytime.

BTW tuning is cheaper and easier than swaping cams - your getting edukated now and soon u'll be the expert. Save that cam, find the leak and fix the mech adv.

Hope this helps ya',
cardo0

 

No problem with 16 degress initial unless the starter suffers kick-back. Larger than stock cams usually need more timing due to reduced DCR.

11hg isn't bad at 700 RPM. Is the needle steady at 11hg, or does it bounce around?

 

That cam is small and should idle. Your timing should be ok. What carb are you using? Take it off and check your throttle plate in relationship to the transitional fuel slots. What stall and gears? Do you have a wide band? Don't give up. I'm running 243-251@.050 and it idles nice at 1000 Probably could set it lower if I wanted to. Ron B.

 

Shoot a little water (squirt bottle) at the gasketed areas on the intake manifold to see if you get any change in engine response. If you have a small gasket leak (too much air/loss of vacuum), the water in that leak area should provide a short-duration seal; engine RPM should change if a leak is there. You can also do it at the base of the carb for the same test.

 

My needle is bouncing a little, if you would mind to have a look at my vacuum readings from this video at youtube and make a comment I would be pleasured.

http://www.youtube.com/watch?v=ugZuA8ymxXI

 

My (vacuum gauge) bounces just like yours. I worked on mine more today. I re-installed the B1 vacuum can.

(Now) With the 268 cam I am getting 14" Hg at 700 rpm in PARK.....
drops tp 9" Hg at 500RPM in DRIVE. I really would like to get this 268 camshaft working right. It's getting closer.

 

OK, keep posting as I like to see how it works out for you.Right now I'm not doing anything to my car, it's parked in the garage for the winther season