140 Mercruiser Fires Back, Sputters

[Peaceful Pirate]

Installing a 140 Mercruiser (mid 70s) that has been sitting up for some time. Pulled it apart ? head, manifold, pan and all was well. Rebuilt carb. New gaskets and put is back together. Had 90psi +/- 5 cold on all four sitting on the floor of the shop. Got it together and installed. Finally got it started and runs strong after timing and carb adjustments. Set timing and rechecked valve adjustment after it warmed up ? back up to chatter then down slow 3/4 turn. Idle smooth. BUT (there is ALWAYS a but!) on acceleration fires back through carb, sputters and tries to die ? will if I don?t back off. If throttle advance is very slow, once she gets going she will wind up and run smooth. Again rechecked valves; thought about fuel so added an inline electric fuel pump just to rule that out. Still sputtering so changed out distributor to rule out advance problems. Thinking carb; however, thought I would quit for the and go to the ?Mountain of Wisdom? and get some food for thought. Any suggestions or comments? Thanks in advance!!

 

[Don S]

Re: 140 Mercruiser Fires Back, Sputters

Run it, warm it up and redo the compression check. 90 is not good.

Compression Test

 

[Peaceful Pirate]

Re: 140 Mercruiser Fires Back, Sputters

Don...Understand. 90 was cold and dry in the shop. Intended on doing a check today after it warmed up, but got off on other things. Well run tomorrow. Thanks.

 

[DMZ]

Re: 140 Mercruiser Fires Back, Sputters

Make sure accelerator pump circuit in carb is working properly. With the engine off look down carb throat and move throttle linkage slowly open. There should be fuel seen pumped into the venturi area from the pump circuit.Without adequate fuel you could be experiencing a lean condition causing the backfire.There is a rod connecting the throttle plate with the accelerator pump arm that can be bent somewhat to correct the problem. Another cause could be a low fuel level in the float bowl. I'm assuming your timing was set correctly and the ignition system has points. If possible get an electronic ignition system. Good Luck

 

[Peaceful Pirate]

Re: 140 Mercruiser Fires Back, Sputters

Don/DMZ?

Pulled carb and replaced the accelerator pump and then verified 1 1/8" from top of horn on linkage. Also changed out venturi while I had it open. Put carb back on this morning and unit runs great. After warm up, verified timing then ran compression check 122/125/124/125.

Thanks again for your help!