Hello I-sixers
My Chevy 250 with a rebuilt head, offy intake, DEMON 525cfm 4brl, and headers. Stock cam w/ over 145,000 miles. Runs very rough at all rpms, like a misfire but all the plugs seem to be firing. I had it running good and driving good for 2 weeks and then all of the sudden it runs very rough. I just cant seem to figure it out and advice would be very appreciated.

I would check the fuel filter and/or fuel line at carb and see if there is any clogs or something. Did you do any tune up procedures to it recently? It sounds like it may be something to do with the distributer components. If it is running rough at all rpms, I heard that it could also mean low cylinder compression pressure, but it would also lack power probably too. ~John~

a 525cfm 4bbl carb seems a bit big for a pretty much stock 250...did it bog in the low end? how did it run...did u re-jet it at all? take a reading of the plugs (color and condition check)and let us know what color the plugs are...the spark plugs are your first source of tuning to know whether your running to rich or too lean. Its possible that youve aquired a vacuum leak in that time, but not too likely. i cant see a stock setup 4bbl carb running well on the engine without a re-jetting, but u said it ran fine for 2 weeks. i think a good place to start would be the color and condition of the spark plugs. then u can take a spray bottle or even a propane torch (from what I've heard...although i would stick to the spray bottle) and spray down various parts of the engine and carb where a vacuum leak could be possible and if the engine changes idle speed when u spray a certain spot then its a vaccum leak that u have to fix. also..how does it run at idle..just as crappy? or is it just in gear?

Any chance, with 145K miles, that the timing gears are starting to go away? If all else fails, it might be worthwhile to pull the front cover.

Well if it is a fiber Gear there is not starting to Go It will just Up and Go Now on the other hand a Alum set may??????? But i dout it. as for a 525 being to much hummmm I ran a 600 on a stock Block/motor No issues. But anyway something to check if your running points Is to check the Points and the condencer. If the condencer is going bad It will run funny/brake up at all rpms But seem to idle fine. Been there Long time ago Before Changeing to a HEI
hope this helps }[oooooo]

hello
I can not seem to find any vacuum leeks. the plugs all look fine excepted for #2 it was fowled up and the vacuum gage bounces for 20 to about 17 inches. I wonder if it could be some thing in the valve train. And it pops in the exhaust .

Yes you may????? have a sticking valve? a burnt valve? a bent push rod??? a Lose Rocker, or maybe ? to tight? many things to look at and check. Let us know what you find. Hope this helps
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mann1977, I recently pulled the valve cover off of a friends 250 for the same reason, it ran terribly at all RPM ranges, had lost power and had 0 compression on the #1 cyl. The culpret? Just as as Twisted6 said, the intake rocker on the #1 cyl. had for some reason worked itself loose. We pulled the head, checked the valve, push rod, head gasket, etc but never found what caused the problem. We re-seated the valves and bolted her back up. That was about three months ago, still running like a top. The old advice guy at NAPA said that the rocker arm nuts on 250's were bad for backing off? I don't know if that's true or not, but I still don't know what caused that nut to back off. My buddy swears that it was a new nut when he had the motor rebuilt about 7,000 miles ago.

i have had both the nuts back off and the studs pull out some.