340 with som problems

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duster340

street racer
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my 340 drives mee crasy, today it has started to knock some havent found out from where yet but i will probably find it, it has started to run real poor and fires thru the exhaust during medium throtle checked all the sparkplugs to get an idee about what is going on and found them all too look ok a litle rich but nothing seriusly wrong except for the #6cylinder that is almost white wich is realy scary to see since i know that means that cylinder is running way too lean.. but i cant get why it is just that cylinder.... i have tried checking for a vackum leak att that port on my stripdominatormanifold and it seems to bee ok..i have also checked that i got ignition and my timing light flashes just like it should at all plugs and the ignition is set just as it has always been att 10degres at idle and 38degres at around 2500-3000rpms i have plenty of fuelpresure also cant find anything that seems to aply to what is wrong, any idees?
my backup plan is to put in my old 318 just to bee able to drive some more before it is to cold outside..
 
The first thing I would do is pull the valve covers and have a careful look to see if you punched a pushrod through a rocker, or maybe bent a pushrod. Next, I'd take a long screwdriver or piece of wood and hold it against various locations on the engine to determine where the knock is originating. The other thing to check, is the sound at camshaft speed or crankshaft speed. The answer to that should narrow down your search for the noise. Good luck.
 
If the one plug looks different, I would a compression test on the entire thing, see if there is any compression in that bad one, and where the other ones are. If you find it's low compared to the rest, then have a "leak down" test done. That will tell you where the pressure is going, and tell you what should be done next..For now, I'd say you're looking for a good rebuilder.
 
ok the powerloss and fireing in the exhaust seems to come from something wiht the iginition on closer inspection using my timing light it seems to forget to give a spark from time to time but it seems to turn over 3-4 revolutins with proper spark and then it does not spark for one turn and then it sparks 3-4turns again very strange must bee some loose conection somewhere just havent found where its a complete MSD ignition system and basicly everything exept for the distributor "base" is new for this season so it does not have so many hours of run time on it, havent had the time to check the compresion yet but i have found three pushrods that used to bee straight that now is slightly bent those are now replaced with some other spare ones with the correct lengt not worried about holes in the rockers since they are crane goldrace roller rockers forgot to give some spec on this engine that is a pretty wild streetengine with only 4"hg of vacum in gear with a lose converter, somthing that came to my mind on that single white sparkplugg is that it may bee a vacumleak somewhere around that cylinders intake port, have too check more things tomorow
i used to have 135psi compresion preasure measured turning the engine over with the starter...
thanks for the help i will also try to listen to where it knocks from!
 
if you connect a vacuum gauge to the manifold and the gauge isn't stabel you have a leak in a port....

Check compression and do the leak down test too.
 
thanks Buschi would never have thought about that way of checking for the lockation of a vackum leak! this site and the people visiting realy helps!
 
I had a similar problem once, and it turned out to be an intake lifter collapsing.It drove me nuts trying to find it way back then.
 
MtNemoMopar said:
I had a similar problem once, and it turned out to be an intake lifter collapsing.It drove me nuts trying to find it way back then.
Click to expand...
that also seems like good information but i sure hope that my solid roller lifters havent colapsed, think i have learnt now that i should realy give you guys the complete spec on my engine before asking a somewhat complicated qustion regarding poor performance! i will look more into the things you guys have sugested tomorow if the wheater admits it:)
 
ok now i have checked most things, compresion was fine on all cylinders it had 135psi solidly on them all, dont realy know if that is good but its defetly a sign that there isnt any big damages inside any of the cylinders..
after changing those crappy pushrods it is alot less strange sounds going on,
and the missfiring drives mee nuts its ok now i found that all the posts in my distributor had coroded and the where green some quick sanding made them shiney and i also changed all my sparkplugs just for the fun of it (delco R43xls)
and it now runs nice and strong again.... my theory at this point is that the sparkplug that was white and shiny was like that because it may not have fired at all under seirtan loads because of something gone wrong inside it, and that may have made it cleaned by those big quantitys of unburnt fuel going thru that cylinder wish i also gues is that that unburnt fuel ignited in my exhaust when it came into my headercolectors where it is hotter? difficult to say if iam right but it is my theory right now..
engine also pulls the usual 5" hg idelvacum in gear now;)


does 135 psi seam somewhat good turning the engine over with the starter with throtle plates wide open?
i know it has a pretty big rollercam that bleads of preasure at low rpm
thanks for all the input guys!
 
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