I just installed my 350 back into my Z28, it was a good running engine prior with a K1102 Summit cam. I decided to change the cam when it was out and went with an XE256H on 112 LSA. I just run the 25 minute break in, now it only wants to run with 2-4 degrees advance at idle, and seems a little rough / lopey, is this normal? I thought it would be a smooth idling cam. Most my other small blocks liked around 8-10 but this will diesel when shut off and start hard at 8 degrees. Throttle response seems a little sluggish too. I installed the cam straight up. The engine is around 8.5:1, stock "Hencho" heads...any ideas or anyone else have a similar combo? Maybe I need to recheck my valve lash / clearance?

 

Should have mentioned, I'm running a Holley 600, RPM intake, regular HEI

 

Run on or dieseling most of the times comes from throttle being opened too far at idle, and just enough air gets in to keep the engine running sort of..

I fixed many of these probs with a new return spring, or adjusting the idle mixture screws, timing and idle speed

Or anther common cause for it, is carbon build up on pistons/valves/chambers causing hot spots and they stay hot long enough to burn all of the extra air/fuel in the chambers.

The XE256 is not much bigger than the K1102. Idle should be about the same with both cams.

IIRC the XE256 is a 212/218 cam where the K1102 is a 204/214. Both on 112 LSA. So the xe256 should idle pretty smooth.

IMHO 2, 4, even 8 degrees timing is not enough for a carb'ed small block. Most likes 10, 12, 14* BTDC at idle the best

 

Looking like I should take my carb off and do an overhaul. The more I think about it there is a chance it could be out of whack, it has been sitting for quite awhile. I rechecked my valves last night they were all set right.

 

Had some progress tonight, got my timing at about 14* at idle and it seems to like that. I cannot get my vacuum gauge to read steady though, it bounces around 15 and my idle air screws don't seem to change anything. I think it should have better vacuum that, starting to think maybe I got the cam timing a tooth or two off? Would that cause my symptoms?

 

15" hg vac sounds about right really. 15-16" with that cam and a low compression engine is about all you will get out of it. If you had more compression you would maybe get 17" vac from it.

It should have a little twitch to the vac gauge. Not rock solid, but not bouncing all over the place. It is a slightly bigger cam and you will notice that on the vac gauge.

Being out of the idle circuit and on the transition circuit would cause the screws not to effect anything... Would also cause the run on.

Bad power valve, wrong size power valve, vac leak, dirt/trash in the idle mixture screw holes will all cause the no effect deal.

With 15" vac you will want a 7.5" power valve

 

Thanks Night Rider327 for your suggestions...I think I fixed it once and for all. I was doing some reading via google searches and found a Holley article that said "if you turn your idle air screws all the way in and the engine doesn't die, you've got a bad power valve." Sure enough I took the float and metering block off, switched out the power valve, cleaned metering passages, and it idles like a champ. It was sucking fuel through the power valve and running waaaay rich. I didn't tinker too long but in a few minutes I had it idling smooth and pulling from 17-18" of vacuum @ 12-14 BTDC. Awesome, installed my cam correctly after all! Thanks again...

 

Your welcome... Cool deal, I'm glad you got it fixed man