Ok, here's what I'm contending with. Got my L48 rebuilt and used the same L98 heads that I used for two years, no problems, until we had a wiped cam lobe and it still ran fine, so decided to trust a long time associate from the race scene, to rebuild it.

Long story short, it has been nothing but problems since we put it in. It's certainly faster, but it runs with a ragged feel to it. Noisy valve train, that we've adjusted a bunch of times, to no avail. The vacuum reading is pretty steady with only a mild flicker at about 12 hg. A quick acceleration, it goes to zero, then jumps to 23 or so, then quickly settles back, so that seems normal.

I've running an AEM air fuel ratio gauge with an O2 sensor and at idle, its moving all over the place and even going solid lean periodically. We tightened up everything, manifold, carb plate and even isolated the carb from all the external vacuum, with the same results.

I did a compression check and everything was at 150 or better, but that was a few months ago. We have gotten it to run smooth a few times, steady AF ratio, no valve train noise, but then it detoriates after a half day, the valve train gets louder and it picks up that ragged feel again. It accelerates hard, but has that sucky, rough feel, throughout the rpm range.

I'm not sure the engine builder reused my LT1 valve springs that I had before. In fact, except for the cam and balance specs, we didn't get a list at all, for what's in the motor. He used the cheapest cast pistons and I wonder how good the 'Custom' cam is as well.

Any input from the more experienced crowd here would be appreciated. We do a lot of work on a lot of cars at our shop, but building engines is something we haven't tried yet (though we can work with a short block). I know I got screwed, just looking to get it running right.

 

what caburetor?
really no details on the camshaft, solid, hydraulic, flat tappet, roller, etc.? Pretty low vacuum at idle.

 

Without you knowing what is in it there is really no way to tell you what to do to help the engine. To diagnose and to improve power it really is going to have to be torn down.

 

this may have absolutely nothing to do with it but I had the same symptoms except for the valve clatter. I had a pvc valve that was cracked that I couldn't see. I changed it out and my engine has been running smooth ever since.

 

I'll get details on the cam a little later. Carb is a Holley 650 mech do, purchased new two years ago. Flat tablet, hydraulic, with Comp Cams ultra pro magnum XD's, 1.5 rocker ratio.

 

Thinking the same thing, have another engine being put together as we speak, but it just got started.

 

Cam specs: Int 280, exh 230, 0.480 lift, 110 LSA

 

Assuming the 12 vacuum reading is not caused by a vacuum leak, does the carburetor have 6.5" or lower power valves?

 

It came with an 8.5 from Holley.

 

Just picked up a 6.5 and we're going to see how it works. The old power valve had wrinkles in it as well and we found fuel moisture behind it, in the chamber as well. Going to put it together and see how it works.

 

John went through the entire carb, cleaning all the passages and found a piece of FOD in one of the passages. Replaced the 8.5 power valve, that looked shot, with a smaller 6.5. With vacuum at 14 (I stated 12 on memory), Holley states that it should be half of what the vacuum is at idle, so the 6.5 is the correct fit.

Funny enough, that fixed every friggin' problem that this car had. Noisy valvetrain, fixed, now it's quiet (go figure). Rough running, AF ratio all over the place, fixed. Now the AF ratio is steady at steady throttle settings and idle. Engine runs strong and smooth (again, go figure). Had a ton of valve clatter from the whole valvetrain and that's gone.

It now runs smooth and hard. Needs some more advance, but the real test, will be, whether it deteriorates and slowly returns to a noisy, rough running little bitch. But right now, just finished shining it up. Starts instantly, revs easy and Sassy is happy. Hope it stays that way...

 

The valve train noise has returned, but it sounds like it's limited. The AF ratio is behaving and staying within a limited range, so that's good. Starts instantly and has no dieseling on shutoff.
We still need to put some advance into it.

I have motor #2, waiting to start ordering parts (once we know exactly what the block deck height is...). A lot of good advice here on a 383 thread and it's going to get done right next time.

 

That's odd about your valve train, but then you've figured that out without me

I put Rhoads lifters in when I did my heads and cam, they are noisy but are good lifters, probably not your issue though.