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howdy folks! well ive got a question for any of you who know old carbed big blocks. especially mopar 400’s. my 400 2 barrel is an all original engine. it sets in a 74 Newport, and has roughly 107k original miles. I have had the car for about 4 years now. and it has always ran pretty good. but ive spent the whole time always trying to tweak it to run a bit better. its issues are it has always idled a bit rough (not bad, and sometimes it comes and goes) and from time to time it will have a small hesitation from a stop, when giving it half throttle, but when you put your foot in it a little, it will go. I have done carb swaps/rebuilds, tune up, after tune up. put better than stock ignition parts on. done every test possible… it has decent compression. in the 130’s-140. (book says service limit is 100psi. and up to 40 psi difference between cylinders. and im not even close to that… anyways heres the down and dirty of the question. I often change plugs and take the old ones out to read them. as I have been slowly working the bugs out I have been finding the plugs to be looking gradually a lot better. especially after I put a new set of valve stem seals in it last year. the plugs used to look fuel fouled, and also somewhat oil fouled. NOW by the looks of the plus, things are still a bit rich. but not too far off. the ONLY problem is now my number 8 plug still appears to have some oil on it. not much. but I believe it is just enough to make it have that slightly rough idle. this is the only plug to have any sort of oily appearance. for instance the other day I threw a set of brand new autolite 85’s in. it started up GREAT. and ran the best it ever has(I think it was just the icing on the cake, after all what else I have done to it lately) I drove the car about 50-60 miles. mostly all highway driving. the car ran really really nice. I brought it home and parked it. 2 days later I started it up and it was back to its old ways. running a little rough… but how could this have happened with it just setting in the driveway…? this is what I think. I think that I may have put the valve steam seals on reverse of where they should have gone, on cylinder 8 last year…the intake on the exhaust, and the exhaust on the intake. but only on that cylinder… that would explain why after running fine, then having it set, it wouldn’t. I imagine the oil would have slipped past the stem seal and into the cylinder. therefore now fouling that brand new plug again… what do you guys think about this? also is it possible for plugs to make things appear rich (having the porcelain look a little brown…) but actually be running lean? because when running lean, you would have unburned fuel. and that would make it look the same as rich? right? and also ethanol is supposed to make carbed enginse run a bit lean… because ethanol has less energy within it. so you have to burn more of it, to get the same energy.. and carbs are supposed to be jetted a size or two up, to compensate… I know most of you reading this probably hardly know what a carburetor even looks like. but for those who can remember, what do you think? would installing the valve stem seals opposite from each other make that cylinder get oil in it? (not much oil, very small amount.)because the intake and exhaust seals are different. im just not sure if it would be enough to matter. im hoping its not the guides that are the problem. but do guides normally go only on 1 cylinder??? THIS IS VERY LONG I KNOW!!! HAHAHAHA I guess my plan is to get some more seals, re install them on that cylinder and make sure they are correct in their placement, try to check guide clearance, and maybe put a step hotter plug in either just #8, or else the whole engine… im not sure if it would be good to run a different plug in only 1 cylinder though? what do you guys think? AND BOY, I SURE WISH I HAD AN EXHAUST ANALYZER AT MY DISPOSLE. its hard to tune an old engine, especially with modern fuels….
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I would think The valve seals are the issue.When you start
the engine any smoke or oil usage? I believe mopar used umbrella
valve seals. with the #8 fouling. That would be your misfire and
rough idle.Fix the #8 fouling then see where the motor is. If your
still rich.You may need to drop back on the carb jetting or adjust
the mixture screws. As for the plugs run all the same. see its a stock
motor run a stock plug. Does the engine have headers? What is timing at?
vacuum advance work? I would set everything to stock as a base line.http://www.mopar.com/assets/pdf/performance/catalog/Big_Block.pdf
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