breaking up when accelerating

[69projectKAOS]

my 69 f-body is breaking up when i got to moderately accelerate, it sputters then sometimes accelerates fine. its a 355 sbc, edelbrock 650 performance series carb, msd street fire ignition. other than that it idles fine and when your rev it while not in motion its fine, but when its under load it begins to break up then clears itself up. maybe gaping on the plugs is too far and blowing out spark? i think it was .30 when i did them the other day. please all help is needed.-Dom

[John Wright]

start checking the ignition side of things.....and check the basics:

Make certain it is getting the full 12v.

Ohm out each of the plug wires and make sure they are good and have the correct resistance for the length.

check the dizzy cap and rotor...ect, ect...

[shmoov69]

Ditto, also check your grounds on the engine, frame, body and battery. A bad ground can do all sorts of goofy things.

[H2Ogbodies]

.030 is too small for a big spark for one...try .055 gap and go from there. Check your timing too-it sounds like it is slightly too far retarded. Another thing to check is whether you are running too lean...on your carb if it is stock out of the box, your pump shot is located in the middle slot on the pump arm for average street economy-placing the pump linkage in the lower hole richens the pump shot up while the upper hole serves to shorten it giving you an even leaner shot of fuel. Put it in the lower hole and see if the stumble gets better...if so, it's a fuel delivery issue. You can get calibration kits for these carbs which have different sized metering rods and jets-not a lot of adjustability but a little.

[69projectKAOS]

.030 is too small for a big spark for one...try .055 gap and go from there. Check your timing too-it sounds like it is slightly too far retarded. Another thing to check is whether you are running too lean...on your carb if it is stock out of the box, your pump shot is located in the middle slot on the pump arm for average street economy-placing the pump linkage in the lower hole richens the pump shot up while the upper hole serves to shorten it giving you an even leaner shot of fuel. Put it in the lower hole and see if the stumble gets better...if so, it's a fuel delivery issue. You can get calibration kits for these carbs which have different sized metering rods and jets-not a lot of adjustability but a little.

the day i did my plugs it was running even worse, which made me think it was the plugs. and i was running rich on the driver side but lean on the passenger side. but then i played with the timing a bit and it is running better but still continues to break up. so tomorrow after work im going to re-gap all the plugs. what degree timing do you think will be best ?

i never really had this problem until i installed the msd ignition, because i remember driving it after it came back from paint and it pulled strong. im going to try what everyone has said . thanks-Dom

[Scott Parkhurst]

18 initial/36 total should be real close to right on for you..

[John Wright]

I agree with H2O.....030" is too small of a gap, even for a points style ignition...HEI likes it up around .045" or so......my old 75 chevy straight six with HEI had a factory gap setting of .060".

[69projectKAOS]

thanks guys, i am gunna try it when the rain clears up here, its been pouring like crazy. thanks again --Dom