Hey guys I hope you can help me out with this. I just rebuilt my motor and got it running yesterday. The problem is its idle is staying around 1500-2000 rpm. If I turn down the idle it dies out. Also its not staying at the same rpm at idle it just keeps going from 1500 up to about 2200 then back down to 1500. I had to adv the timing to almost as far as I could to keep it from back firing through the carb. The motor is a 255 and has a motorcraft 2150 carb. So guys any ideas what I can do. If you need more info or pics of anything just let me know. This is my daily driver and really would like to give my wifes car back. I took the #5 cylinder plug wire off to see if it was getting spark and it was but it didnt run any different with the plug completely disconected as it did plugged up? The dizzy was never removed, I only rebuilt the top end due to low fundage and I had to pay a pretty penny for all the head work done to get my 255 to breath. How would I check to see if the vacume advance is working? I tried to unplug it to see if it ran different but couldn't really tell because the engine idle was going up and down. I did notice that you can see the top of the egr port on the head (the little wierd shaped hole in the center of the intake side of the head) with the intake installed. These were completely plugged up with carbon before the rebuild. I put a block off plate on the egr valve. Could it be that the intake dont match the heads? I know they are both for a 255 due to the ports are smaller oval shaped holes instead of the rectangle 302 ports. Any help I can get is great would be greatly appriceated.
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lil help please

 

Ok I will take a guess............... how about a vacuum leak???? Also since you had to advance the timing to keep it from back firing thru the carb that sounds a lot like a timing issue.............. right firing order??? not sure if it applies to a 255 engine but how about valves clearance adjusted incorrectly?????

 

You can start with leaving the vacuum advance disconnected and plugged at carb. Get it running and ideling that way to start. I'm not familiar with the carb, but if it is an electric choke the fast idle could be improperly adjusted not allowing to adjust reg idle screw down. Check for vacuum leaks as Sparky suggested. The vacuum advance can be checked best with a vacuum gun, but if you can suck on the hose hard enough you should see it move the plate with cap removed. Clogged PCV valve and hoses will mess with your ideling also. But if I had to bet on one thing, I'd say a vacuum leak between carb and intake or intake gasket. That gives you some more things to eliminate at the least. Make sure you capped everything off where you removed the EGR.

 

Thanks guys I'll check all of this and post back the results. When I installed the carb there was no gasket for the carb to spacer to intake and there wasnt one when I disassembled it. Should I find a gasket for that?

 

Yes, could very well be your problem. If you spray carb cleaner at that junction I'll bet rpm changes verifing leak.

 

Ok guys I took the carb off and put a gasket on it and noticed while I had it off that the fast idle lincage was missing some parts and was held together with a twist tie that broke. I have no idea what is missing or how to fix it. Any suggestions?