Compression problem or carb problem?

[tpwebb]

On my 35hp merc. I have replaced the rings on the two pistons. I had no compression and felt this was the reason that had me to replace the rings. Now after putting the motor back together I still can not get it to run. I have compression at the plug holes but it will not draw gas into the chamber. I have fire at the plug. The carbs have recently been rebuilt and I have reset all port screws back to spec according to my book. What could my problem be for it not drawing gas into the motor. The valves appear to be fine. I need some advice on what to check next. If i should feel suction on the carb intake air port and I dont, what should I check. Thanks TIm

 

[nate_evans79]

Re: Compression problem or carb problem?

I'm new to the boat motor world, but I would guess that maybe the fuel pump would be suspect. Not sure what kind your motor has, but mine is a "mechanical" type, and if the diaphram in it is ruptured, then it won't pump the fuel up to the carbs. <br /><br />Like I said, its just a guess, and correct me if I'm wrong, just thought I would throw it out there.<br /><br />Nate.

 

[Laddies]

Re: Compression problem or carb problem?

Did you ck the reed valves when the engine was apart, they act as intake valves on a 4 cycle and if one is chipped or broken on a 2 cyl engine it won't start

 

[Silvertip]

Re: Compression problem or carb problem?

I must assume when you reassembled the engine you installed the flywheel key. Without it the timing is likely wrong. Put a little pre-mixed fuel in a spray bottle and squirt some directly into the carb. If the engine tries to fire, you have a fuel delivery problem likely due to a bad fuel pump, restricted line, clogged filter, restriction in the pickup in the tank.

 

[budman8330]

Re: Compression problem or carb problem?

I agree with laddies. even if the motor is off time, or the fuel pump was wacked, you sould still feel vacuum if you put your hand over a carb. <br /><br />i believe the reed valves are suspect. I know its a stupid question but are you sure the throttle shudders were wide open when you checked for vacuum at the carbs ? Or when you ran the compression test ? <br /><br />Also dont forget, a compression test's results are priceless, but what it does not show is a lack of primary compression. The crankcase has to be alternatly under high and low pressure. after the intake port is closed by the piston, further downward travel of said piston causes the trapped mixture to be pressurized so it can rush quickly into the cylinder when the scavenging ports are opened. <br /><br />Up piston movement creates a lower pressure in the crankcase, enabling the mixture to pass in from the carb. <br /><br />when crankcase seals or gaskets fail, the crankcase cant hold pressure anymore. Another source of leakage, such as base gaskets, or a cracked casting will yeild the same results <br /><br />I remember my teacher telling me this about 3 years ago after i read this post. Thought i would relay it, not too sure how much it helps, but i hope you get this resolved without having to tear into it too much. Like i mentioned though, You should still feel vacuum whether or not your fuel pump is out. Only other explanation I can give is the same one laddies mentioned, and that is reed valves and or you just had a "mind slip" and left the throttle closed when you were checking for vacuum <br /><br />Good luck !

 

[tpwebb]

Re: Compression problem or carb problem?

Thanks for the advice, but if it is a lower compression problemm in the crankcase how can I verify, I dont feel air pass or hear any air pass. To replace the bearings on the crank shaft would mean a total breakdown of the engine? I would like to know for sure that what it is first before going to that extreme. THe reed valves you can see with the carb off and They apear to be fine. I'm not giving up yet.<br />Tim

 

[Laddies]

Re: Compression problem or carb problem?

If you have spark and compression and you squirt fuel in the carb the engine has to sputter or something, try pulling the plugs and squirting fuel in the cyls, if it fires then it's a reed problem, the crankcase has a vacume as the pistion goes up and then the reeds close and the fuel is presserized and goes thru the transfer ports into the cyl and is compressed and fired a bad reed will stop the process to both cyl on a 2 cyl engine a the carb will have no vacume to induct fuel, You should be able to ck for pulse by removing the fuel pump and turnig the engine over you should have pulse from both crankcase holes