head swap

[Bad67]

Just put on a new set of dart pro 1 heads on my small block chevy 355 needless to say its been nothing but a nightmare everything that prob could have went wrong did.1st off every 1 of the intake pushrods were hitting the head and had to be clearanced,keeping in mind these heads were purchased brand new from dart.finally got everything straightned out and got it running now its really hard to start,it will fire right up but runs real rough until engine reaches temp then it runs fine any clues as to what may be causing this?it has a demon carb msd ignition with msd dist timing is set to 38 carb has been tuned exactly how demon told me to do it.car ran fine before i changed heads never had this prob.any help appreciated thanks

[rustbucket]

valves too tight?

[rustbucket]

also when I changes heads I had to use .100 long push rods. just some thoughts.

[Bad67]

i went over valve lash 3 times to make sure it was correct mine is .018intake .020 exh. When i checked pushrod length i came up with stock length

[rustbucket]

Man, Im not sure. Does it pop through the carb when the motor is cold? Possibly a lean condition?
Vacuum leak? You could try richening it up a little and see if it makes any difference when cold. Maybe some of the smarter gearheads will think of some suggestions. Sorry if Im not much help.

[Bad67]

no dont be sorry any input is appreciative.no it dont pop back through the carb when its cold it just has a rough idle and wants to die i have to keep feathering the gas until it warms up a little

[SS8UUP]

Im curious about this one, I have a similar problem with mine.

[rustbucket]

My quick fuel 950 acts very similar when its cold, but when the motor warms up it seems jetted perfectly. I had a different carb on it that would crank and idle fine when cold but it was too rich when the motor was warm. You could try choking
the the carb with something that wouldnt be sucked
into the carb and see if it runs better when cold.
Does it crank easier if you pump the throttle prior to cranking the engine? This carb may act totally different with the new heads.

[79 Camaro]

What type of plugs are you running and what is the heat range? I always thought that is was just the nature of carberated motors to run rough and want to die until the warmed up a little?

[Bad67]

they are autolite ar3933 spark plugs these are what dart recomended

[sunsation540]

when cold they need extra fuel to stay running IE choke.. i find that many combos need to much throtle screw to idle if you are more than 2.5 turns off base idle you have uncoverd the transition slot that will make it run rough and lazy until you open it to 1/2 throtle runs ok but won't idle if this is the case remove the carb turn it over. the set screw for the rear throtle stop add 1/2 turn in this will make a big diff on the front as well if you have rear adj dom and hp pro form etc close all blades and turn 1 turn at a time both ends also a side note if you have to go more than 1.5 turns out on idle adj screws air bleeds you need to re jet.. add 2 numbers try to stay with 1 turn just as a base line until strip jetting can be done... if you have putzed with the air bleeds in the top of the carb start from scratch.

[Bad67]

throttle blades are set correctly so i know the transfer slot is covered,i set the carb up with the help of a demon or barry grant tech.but it is possible that im more than 1.5 turns out on the idle mixture screws i will have to recheck them. thanks for ya help

[John_Heard]

AR3933's are way to cold for a N/A street car, those are equivalent to NGK 5671A-9, a plug most people will use up to around a 350 hp nitrous hit.

Go down to the parts store and get you a set of NGK R5671A-7 (Stock #4554 plugs), or if you prefer Autolite pn 393. That will probably help with the rough idle.

You didn't say if your driving this around town, or getting ready to go to the track and spray the snot out of it, but for on the motor those -7's should work real good. You'll need colder when spraying it much.

[Bad67]

i cant find a 393 plug, according to the autolite website the ar3935 is a cross of the ngk r5671a-7.This is a street strip car and no im not spraying it at all,im sorry i should have mentioned this before.do you really think the hotter plug will correct this prob?

[Necessary Evil]

Sounds like you have a bad carb. base gasket. Spray some carb clean around base area and see if idle picks up. I had this same thing happen to my 71 Torino , ended up being the carb mounting gasket.