Rough running 200 2v head

First up peeps, my car was running ok last weekend, apart from taking time to warm up, bit longer than i thought i just persisted, drove to a car show and swap meet in town, then another car show. Then on Monday i had her booked in for exhaust mods. The extractors were cut back at the merge where the 3 go into 2, there was a large overlap in the pipe there, pipe size i think we went up qtr inch over what it was, then ran twin into a 3” collector, single 2.5” exhaust and a custom spiral auger muffler that was made by my mate at the muffler shop. When i picked the car up it was loud, even louder than my 2.5” collector with no exhaust and this was with the exhaust, being a race muffler this was a trade off because the improvement in performance was substantial. Bu it ran rough, like the whole tune was out. I am running a slightly worked 250 2V head, factory manifold thats been modified and a 350 Holley, the Holley seemed to like being jetted up and ran its best with 70 jets and a 6.5PV, not sure why but it went well. Running so roughly i pulled out the plugs, all fouled, cleaned and regaled them one was closed right up like it had been dropped, gapped at 40. Reset the mixture screws and idle, i decided to go back to a 8.5 PV and dropped some jet so down to a 68’s. Back to the muffler shop and changed this muffler, to loud, and got a regular performance one (Redbak), sounds so much better, rough running is still there and i am scratching my head. Any thoughts?

Are you running a pcv valve? I was having a similar problem and put in a new one which cured the it. But, I am still running too rich.

There are more fuel circuits than just the main jets.
I modify the metering block to install changeable jets below the power valve.
I also lean the idle circuit from a fixed orifice to a changeable jet & relocate the idle jet to a different part of the metering block for a much improved idle to main jet transfer metering ratio.

No PCV as no provision for it, i did consider the metering block, and making mods to it or buying a modded one with spare air bleeds, another guy mentioned taking to it with a drill.

Seems my problem may be a bigger one, i have been through most of what i can do at home. Number 1 plug is white, tip, insulator etc, all others are normal, heat range is normal, i changed the plug, it ran smoother, like 5 minutes running, then went to same condition as other plug i had removed, white, changed the lead on the number 1 also, nothing, removed lead, didn’t run any different also. Can the dissy just do this to one plug? I don’t think so, I’m thinking its kissed a valve or done a ring, i removed the rocker cover and ran motor, all valve train looks to be fine visually, rockers, pushrods, maybe a valve isn’t sealed. Im slowly ruling things out but cant get a compression test till tomorow. Its not head gasket, I’ve changed fuel, i have clean and re gapped all plugs. This happened coincidently when i picked the car up from the exhaust shop last Monday. Ran rough ever since.

Have you hooked up a vacuum guage?

I have seemed to have fixed the problem, by habit i always put 98 octane premium fuel in, with not driving it to much and mainly tuning i have been either cleaning, gapping or buying new plugs when i put the 2V head on. The head has lowered the comp to where the 98 octane isn’t liked. I emptied the tank. Put some 91 in, gapped the plugs at 35 instead of 50 like the dissy people say and cleaned them. Instant smoother running. Still had a rough spot down low off idle to about 1800 revs so without a timing light i undid the dissy clamp and turned it anti clockwise, i could hear it instantly improve. That took the stumble out. Did some driving and highway driving, still has one spot higher up the rev range where its roughish. Im changing he power valve to a 8.5 from the 6.5, that may help.

Just wondering what plugs you are running. I think my plugs are too cold. I am running the stock NGK 7510 / Autolite 46.

Last week when i was half asleep thinking about my rough running monster and thought i may have had one range colder, i woke up went out and checked them there in the standard 5 heat range i use NGK’s, I’ve used Iridiums before and indexed them with no real noticeable improvement at the track. One range colder would have caused more issues i reckon.