Here are the specs for the cams in my 502's

Pace Performance PN#12353920 - Flat Tappet Hydraulic Camshaft & Lifter Kit (228,238- .540,.540)

All models big-block Chevrolet V8 1965-66 California, 1965-68 with federal emissions. All non-emissions truck and all 1966-92 for off-highway applications. This is also an excellent marine cam. Good idle, daily performance usage, mild bracket race, 9.5 - 10.75 - 1 compression ratio 3000 - 3400 cruise rpm. Basic rpm 2200 - 5700, 6500 rpm attainable with proper valve springs and lifters. Technical Notes: These are hydraulic flat tappet camshaft kits. The duration at .050 lift (intake/exhaust) is 228/238; valve lift is .540"/.540"; lobe centerline is 114. These new generation of camshaft kits are designed and manufactured by Crane Cam Co. for GM Parts Division. All kits contain one (1) camshaft and sixteen (16) tappets.

I'll be pressure testing my risers and checking valves.

Exhaust valve not sealing well or the exhaust valve not following the cam well (broken spring, weak spring, not enough spring pressure), or sticking exhaust valve can cause reversion.

dgood,
Do yourself a favor and just modify the tail pipes. I had the same problem when I built a 502 yrs ago. I ened up extending the inner pipe by 1.5"-2" and the problem stopped. At idle I could see the water climbing up the ID of the exhaust pipe. I had a solid roller cam in it. Not sure what it was. But those three little holes in the inner pipe introduce the water into the exhaust stream and bingo.. reversion.
I cut the outer pipes and then extended the inner one, put a ring aound it with three grooves (2-6-10 oclock). Used a piece of rod the size of the difference between the inner and outer pipe. The two pipes can grow at different rates so you dont want to weld the inner to the outer.
That would solve a reversion problem.. not the easiest, but it is final. Find someone in your area that can weld SS and go for it..

Hope this helps.
Dick

I could not agree more with Dick (mr. Gadgets). Extend the pipes a little and solve the problem.
That cam with 238* exhaust duration nsounds a little big for that exhaust. I have seen the exact problem that you are having....one engine reverts and one does not. We lenthened the risers and the problem went away. I would bet good money that you have a reversion problem. I can not explain why one would do it and not the other. I quit trying to figure it out and just fixed it Good luck.
Eddie

even with two idenical engines...they rarely are, theres always differnces here and there, the way the cams are ground, how far a valve is in the seat from a valve job, timing, carb adj and idle speed....all could effect it if you are on the edge....Rob

BUT ,,,,still wy isn't it in #8 only one side ??????????

Can that be ???????

Certainly. Depending on the exhaust, some do not have equal risers because of the offset of the engine. There are just to many factors....just like what was listed above. Just a half an inch in where the water enters the exhaust could be all the difference needed if you are on the edge. I think the cam is to big at 238 deg. to use with that exhaust. It doesn't really come as a surprise to me that it reverts. I'm surprised that both engines aren't doing it. Like was said, a little extension to the pipes should fix it.
Eddie

Thanks everyone for your input. I will try as suggested and update the post.

Just came from the machine shop and with the intake off saw signs of water (IE rusty) in the aft intake ports on both sides. Is it possible for reversion to suck into the intake side of the head??? Doesn't make sense does it?.

When I found the problem, I fixed it before rust could occur, on the engine test stand. I wouldnt think it would get to an intake port. Maybe someone else can share their experience.
If you have excessive water psi in the block, it could have damaged the intake gasket??? Any signs of that?