1978 250 running rough

"Hi all, ok here is what I am

"Hi all, ok here is what I am dealing with. 1978 with a GM staight 6 250 ci. New plugs wires, cap, rotor, and plugs. Rebuilt the carb and it starts up no problem. The problem is when I come off of idle it wants to spudder. I can back of just a touch and it will rev right up. I messed with the timing thinking that might help. I did some but now it has a slight miss. At idle and higher. I took it out this weekend and that is when I could feel and hear a cyc not working right. I has the electronic converson in it. Is there a way to check the air gap and if so what should it be. My next thought is if the valves are not adjusted properly will that cause this problem as well. It has a newer fuel pump, brand new fuel water sep as well. When I got the boat a few months ago it did not even run. I was running pretty good till I took it out. The spudder off of idle has been there since I rebuilt the carb. Any sugestion would be great."

"Hi Allen, my favorite engine

"Hi Allen, my favorite engine !! the sputter off idle means the accelerator pump circuit is not working or not working properly(did you remove the ball & spring from under the primary venturis? and clean the passage? with the engine and the flame arrestor off, disconnect the throttle linkage so you can work the carb by hand, look inside the carb with the choke plate open, when you move the throttle quickly you should see a steady stream of fuel from 2 places shoot down into the carb, now move if slowly(kinda like when you throttle up), you should now see just a little, but you should see something unless you move it extremely slowly. The accell pump rod should be in the hole closest to the fulcrum of the lever or towards the center of the carb (if it has 2 or 3 holes, the inner most) if only then disregard.

As for the slight miss with the electronic version, in the package should have been a clear peice of plastic about have the hight of a business card and a little shorter, this is what is used to measure the air gap drom the sensor and the collar. Before you play with the lifter adjustment do a compression check. The miss could also be from a spark plug gap(should be .035) if you didn't set the gap before installation that's most likely your problem plugs aren't gapped from the factory, bad cap, bad coil(arcing) or a loose wire.

hope this helps"

"Ayuh,... Have you done a Com

"Ayuh,... Have you done a Compression Test,..??"

"The spark plugs were gapped b

"The spark plugs were gapped before install. The accel circuit was working as well. It has a Rochester 2br carb. I did remove the check ball and spring and installed new ones from the rebuild kit. I have not done a compression test yet. I was thinking that as well. When it started with this problem of the spudder at higher rpms, I pulled the plugs to see how they looked, and #4 cyc had fuel on the plug and the rest were perfect."

"I have the same engine in my

"I have the same engine in my rig, I would have another look at the accelerator pump. I rebuilt my carb some years back and it began hesitating the same way. So naturally the first seat of pants trick is to look down the carb and see if it is getting the "happy" squirt, which it was. I had to take the carb apart for other reasons and found that the seal on accelerator pump plunger was folded a little, causing the hesitation. My point is just because you see the happy juice, doesn't necessarily mean it is functioning perfectly.

Eric"

id also do a continuity test o

id also do a continuity test on theat #4 wire . that wont tell you a whole lot other than its got thru passage but it sounds like your not getting the right fire under load to the number 4 . i would pull that #4 wire while it is ideling and see if the miss gets worse or stays the same

did you correct the timing aft

did you correct the timing after you messed with it ?

"I have not put a timing light

"I have not put a timing light on it yet. I plan to this weekend. I am suspecting the distbutor might be a tooh off. If I move the dist just 1 or 2 deg either way it runs way worse. I tdc # cyc and the rotor and the cap lined up perfect, but then I remembered I advanced it a few deg to get it to run. If I retard the timing just a hair it is very hard to start. I am going to do the compression test tomorrow and see what I get. I am not putting anything past the carb either. It was plugged up good when I bought the boat in Dec. The guy I got it from also put in the pentronix igniton so who knows the dist could be a tooth off. I should be getting more timing advance than what I am getting."

"I forgot to mention this, whe

"I forgot to mention this, when I come off of idle and it is stumbling, you hear what sounds like a large vacume from the carb."

"the distributor being a tooth

"the distributor being a tooth or 2 off will only affect where 1 lines up, once you turn the dist it wont matter, put the light on at idle @ 750-800 see what it is, should be 4 degrees. if you have a light that has an adjustment on the back bring to 3k and see if you are getting 30 degrees of advance, could pull the rotor and pertronics plate and check the mechanical advance to.. weights could be missing, sticking or just shot.

an air leak around the base of the carb? or adapter plate?

good luck"

"Ok compresion test is done an

"Ok compresion test is done and here are the results, #1 110, #2 135, #3 135, #4 135, #5 125, and #6 120. Not too bad for an engine with about 520 hours. So as for the # 4 cyc, the spark plug was broken. When I pulled the plug wire to see if it made a difference on how it ran, the plug came with it. Replaced the plug gapped at .035. Timing was off by a lot. Rest at 4 deg idling at 760 rpm. Runs smooth as can be now. Still has the stummble when I come off of idle, but seems to go away after I get it good and warm. I can deal with that. I did pull the top of the carb off and checked the accel pump. Looked good and it is spraying like it should. Thanks for all the input. You all saved me a huge headache.
Allen"

"Great News !!! The stumble y

"Great News !!! The stumble you have left is probably a choke adjustment, it's needs will change with outside air temp.

Safe boating !!"

"after checking all the above,

"after checking all the above,,,, i have 2 of these engines and had the same prob, i found a water leak in #1 cylinder from the water jacket, behind the thermostat area , around the # 1 valve seat the heads are very thin, in this area and are common to corrode there, remove the plugs and check for water leaking in"